MySQL Reference Manual for version 4.1.7.

1 General Information

The MySQL (R) software delivers a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server. MySQL Server is intended for mission-critical, heavy-load production systems as well as for embedding into mass-deployed software. MySQL is a registered trademark of MySQL AB.

The MySQL software is Dual Licensed. Users can choose to use the MySQL software as an Open Source/Free Software product under the terms of the GNU General Public License (http://www.fsf.org/licenses/) or can purchase a standard commercial license from MySQL AB. See section 1.4 MySQL Support and Licensing.

The MySQL Web site (http://www.mysql.com/) provides the latest information about the MySQL software.

The following list describes some sections of particular interest in this manual:

Important:

Reports of errors (often called ``bugs''), as well as questions and comments, should be sent to the general MySQL mailing list. See section 1.7.1.1 The MySQL Mailing Lists. See section 1.7.1.3 How to Report Bugs or Problems.

The @command{mysqlbug} script should be used to generate bug reports on Unix. (Windows distributions contain a file named `mysqlbug.txt' in the base directory that can be used as a template for a bug report.)

For source distributions, the @command{mysqlbug} script can be found in the `scripts' directory. For binary distributions, @command{mysqlbug} can be found in the `bin' directory (`/usr/bin' for the MySQL-server RPM package).

If you have found a sensitive security bug in MySQL Server, please let us know immediately by sending an email message to [email protected].

1.1 About This Manual

This is the Reference Manual for the MySQL Database System. It documents MySQL up to Version 4.1.7, but is also applicable for older versions of the MySQL software (such as 3.23 or 4.0-production) because functional changes are indicated with reference to a version number.

Because this manual serves as a reference, it does not provide general instruction on SQL or relational database concepts. It also will not teach you how to use your operating system or command-line interpreter.

The MySQL Database Software is under constant development, and the Reference Manual is updated frequently as well. The most recent version of the manual is available online in searchable form at http://dev.mysql.com/doc/. Other formats also are available, including HTML, PDF, and Windows CHM versions.

The primary document is the Texinfo file. The HTML version is produced automatically using a modified version of @command{texi2html}. The plain text and Info versions are produced with @command{makeinfo}. The PostScript version is produced using @command{texi2dvi} and @command{dvips}. The PDF version is produced with @command{pdftex}.

If you have any suggestions concerning additions or corrections to this manual, please send them to the documentation team at [email protected].

This manual was initially written by David Axmark and Michael ``Monty'' Widenius. It is now maintained by the MySQL Documentation Team, consisting of Arjen Lentz, Paul DuBois, and Stefan Hinz. For the many other contributors, see section B Credits.

The copyright (2004) to this manual is owned by the Swedish company MySQL AB. See section 1.4.2 Copyrights and Licenses Used by MySQL. MySQL and the MySQL logo are (registered) trademarks of MySQL AB. Other trademarks and registered trademarks referred to in this manual are the property of their respective owners, and are used for identification purposes only.

1.1.1 Conventions Used in This Manual

This manual uses certain typographical conventions:

constant
Constant-width font is used for command names and options; SQL statements; database, table, and column names; C and Perl code; and environment variables. Example: ``To see how @command{mysqladmin} works, invoke it with the --help option.''
constant italic
Italic constant-width font is used to indicate variable input for which you should substitute a value of your own choosing.
`filename'
Constant-width font with surrounding quotes is used for filenames and pathnames. Example: ``The distribution is installed under the `/usr/local/' directory.''
`c'
Constant-width font with surrounding quotes is also used to indicate character sequences. Example: ``To specify a wildcard, use the `%' character.''
italic
Italic font is used for emphasis, like this.
boldface
Boldface font is used in table headings and to convey especially strong emphasis.

When commands are shown that are meant to be executed from within a particular program, the program is indicated by a prompt shown before the command. For example, shell> indicates a command that you execute from your login shell, and mysql> indicates a statement that you execute from the @command{mysql} client program: 

shell> type a shell command here
mysql> type a mysql statement here

The ``shell'' is your command interpreter. On Unix, this is typically a program such as @command{sh} or @command{csh}. On Windows, the equivalent program is @command{command.com} or @command{cmd.exe}, typically run in a console window.

When you enter a command or statement shown in an example, do not type the prompt shown in the example.

Database, table, and column names must often be substituted into statements. To indicate that such substitution is necessary, this manual uses db_name, tbl_name, and col_name. For example, you might see a statement like this: 

mysql> SELECT col_name FROM db_name.tbl_name;

This means that if you were to enter a similar statement, you would supply your own database, table, and column names, perhaps like this: 

mysql> SELECT author_name FROM biblio_db.author_list;

SQL keywords are not case sensitive and may be written in uppercase or lowercase. This manual uses uppercase.

In syntax descriptions, square brackets (`[' and `]') are used to indicate optional words or clauses. For example, in the following statement, IF EXISTS is optional: 

DROP TABLE [IF EXISTS] tbl_name

When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (`|'). When one member from a set of choices may be chosen, the alternatives are listed within square brackets (`[' and `]'): 

TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)

When one member from a set of choices must be chosen, the alternatives are listed within braces (`{' and `}'): 

{DESCRIBE | DESC} tbl_name [col_name | wild]

An ellipsis (...) indicates the omission of a section of a statement, typically to provide a shorter version of more complex syntax. For example, INSERT ... SELECT is shorthand for the form of INSERT statement that is followed by a SELECT statement.

An ellipsis can also indicate that the preceding syntax element of a statement may be repeated. In the following example, multiple reset_option values may be given, with each of those after the first preceded by commas: 

RESET reset_option [,reset_option] ...

Commands for setting shell variables are shown using Bourne shell syntax. For example, the sequence to set an environment variable and run a command looks like this in Bourne shell syntax: 

shell> VARNAME=value some_command

If you are using @command{csh} or @command{tcsh}, you must issue commands somewhat differently. You would execute the sequence just shown like this: 

shell> setenv VARNAME value
shell> some_command

1.2 Overview of the MySQL Database Management System

MySQL, the most popular Open Source SQL database management system, is developed, distributed, and supported by MySQL AB. MySQL AB is a commercial company, founded by the MySQL developers. It is a second generation open source company that unites open source values and methodology with a successful business model. See section 1.3 Overview of MySQL AB.

The MySQL Web site (http://www.mysql.com/) provides the latest information about MySQL software and MySQL AB.

MySQL is a database management system.
A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL Server. Since computers are very good at handling large amounts of data, database management systems play a central role in computing, as standalone utilities or as parts of other applications.
MySQL is a relational database management system.
A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The SQL part of ``MySQL'' stands for ``Structured Query Language.'' SQL is the most common standardized language used to access databases and is defined by the ANSI/ISO SQL Standard. The SQL standard has been evolving since 1986 and several versions exist. In this manual, ``SQL-92'' refers to the standard released in 1992, ``SQL:1999'' refers to the standard released in 1999, and ``SQL:2003'' refers to the current version of the standard. We use the phrase ``the SQL standard'' to mean the current version of the SQL Standard at any time.
MySQL software is Open Source.
Open Source means that it is possible for anyone to use and modify the software. Anybody can download the MySQL software from the Internet and use it without paying anything. If you wish, you may study the source code and change it to suit your needs. The MySQL software uses the GPL (GNU General Public License), http://www.fsf.org/licenses/, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL code into a commercial application, you can buy a commercially licensed version from us. See section 1.4.3 MySQL Licenses.
The MySQL Database Server is very fast, reliable, and easy to use.
If that is what you are looking for, you should give it a try. MySQL Server also has a practical set of features developed in close cooperation with our users. You can find a performance comparison of MySQL Server with other database managers on our benchmark page. See section 7.1.4 The MySQL Benchmark Suite. MySQL Server was originally developed to handle large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Although under constant development, MySQL Server today offers a rich and useful set of functions. Its connectivity, speed, and security make MySQL Server highly suited for accessing databases on the Internet.
MySQL Server works in client/server or embedded systems.
The MySQL Database Software is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and a wide range of application programming interfaces (APIs). We also provide MySQL Server as an embedded multi-threaded library that you can link into your application to get a smaller, faster, easier-to-manage product.
A large amount of contributed MySQL software is available.
It is very likely that you will find that your favorite application or language already supports the MySQL Database Server.

The official way to pronounce ``MySQL'' is ``My Ess Que Ell'' (not ``my sequel''), but we don't mind if you pronounce it as ``my sequel'' or in some other localized way.

1.2.1 History of MySQL

We started out with the intention of using mSQL to connect to our tables using our own fast low-level (ISAM) routines. However, after some testing, we came to the conclusion that mSQL was not fast enough or flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as mSQL. This API was designed to allow third-party code that was written for use with mSQL to be ported easily for use with MySQL.

The derivation of the name MySQL is not clear. Our base directory and a large number of our libraries and tools have had the prefix ``my'' for well over 10 years. However, co-founder Monty Widenius's daughter is also named My. Which of the two gave its name to MySQL is still a mystery, even for us.

The name of the MySQL Dolphin (our logo) is ``Sakila,'' which was chosen by the founders of MySQL AB from a huge list of names suggested by users in our ``Name the Dolphin'' contest. The winning name was submitted by Ambrose Twebaze, an Open Source software developer from Swaziland, Africa. According to Ambrose, the name Sakila has its roots in SiSwati, the local language of Swaziland. Sakila is also the name of a town in Arusha, Tanzania, near Ambrose's country of origin, Uganda.

1.2.2 The Main Features of MySQL

The following list describes some of the important characteristics of the MySQL Database Software. See also section 1.5 MySQL Development Roadmap for more information about current and upcoming features.

Internals and Portability
  • Written in C and C++.
  • Tested with a broad range of different compilers.
  • Works on many different platforms. See section 2.1.1 Operating Systems Supported by MySQL.
  • Uses GNU Automake, Autoconf, and Libtool for portability.
  • APIs for C, C++, Eiffel, Java, Perl, PHP, Python, Ruby, and Tcl are available. See section 21 MySQL APIs.
  • Fully multi-threaded using kernel threads. It can easily use multiple CPUs if they are available.
  • Provides transactional and non-transactional storage engines.
  • Uses very fast B-tree disk tables (MyISAM) with index compression.
  • Relatively easy to add another storage engine. This is useful if you want to add an SQL interface to an in-house database.
  • A very fast thread-based memory allocation system.
  • Very fast joins using an optimized one-sweep multi-join.
  • In-memory hash tables, which are used as temporary tables.
  • SQL functions are implemented using a highly optimized class library and should be as fast as possible. Usually there is no memory allocation at all after query initialization.
  • The MySQL code is tested with Purify (a commercial memory leakage detector) as well as with Valgrind, a GPL tool (http://developer.kde.org/~sewardj/).
  • The server is available as a separate program for use in a client/server networked environment. It is also available as a library that can be embedded (linked) into standalone applications. Such applications can be used in isolation or in environments where no network is available.
Column Types
  • Many column types: signed/unsigned integers 1, 2, 3, 4, and 8 bytes long, FLOAT, DOUBLE, CHAR, VARCHAR, TEXT, BLOB, DATE, TIME, DATETIME, TIMESTAMP, YEAR, SET, ENUM, and OpenGIS spatial types. See section 12 Column Types.
  • Fixed-length and variable-length records.
Statements and Functions
  • Full operator and function support in the SELECT and WHERE clauses of queries. For example: 
    mysql> SELECT CONCAT(first_name, ' ', last_name)
    -> FROM citizen
    -> WHERE income/dependents > 10000 AND age > 30;
  • Full support for SQL GROUP BY and ORDER BY clauses. Support for group functions (COUNT(), COUNT(DISTINCT ...), AVG(), STD(), SUM(), MAX(), MIN(), and GROUP_CONCAT()).
  • Support for LEFT OUTER JOIN and RIGHT OUTER JOIN with both standard SQL and ODBC syntax.
  • Support for aliases on tables and columns as required by standard SQL.
  • DELETE, INSERT, REPLACE, and UPDATE return the number of rows that were changed (affected). It is possible to return the number of rows matched instead by setting a flag when connecting to the server.
  • The MySQL-specific SHOW command can be used to retrieve information about databases, tables, and indexes. The EXPLAIN command can be used to determine how the optimizer resolves a query.
  • Function names do not clash with table or column names. For example, ABS is a valid column name. The only restriction is that for a function call, no spaces are allowed between the function name and the `(' that follows it. See section 10.6 Treatment of Reserved Words in MySQL.
  • You can mix tables from different databases in the same query (as of MySQL 3.22).
Security
  • A privilege and password system that is very flexible and secure, and that allows host-based verification. Passwords are secure because all password traffic is encrypted when you connect to a server.
Scalability and Limits
  • Handles large databases. We use MySQL Server with databases that contain 50 million records. We also know of users who use MySQL Server with 60,000 tables and about 5,000,000,000 rows.
  • Up to 64 indexes per table are allowed (32 before MySQL 4.1.2). Each index may consist of 1 to 16 columns or parts of columns. The maximum index width is 1000 bytes (500 before MySQL 4.1.2). An index may use a prefix of a column for CHAR, VARCHAR, BLOB, or TEXT column types.
Connectivity
  • Clients can connect to the MySQL server using TCP/IP sockets on any platform. On Windows systems in the NT family (NT, 2000, or XP), clients can connect using named pipes. On Unix systems, clients can connect using Unix domain socket files.
  • The Connector/ODBC (MyODBC) interface provides MySQL support for client programs that use ODBC (Open Database Connectivity) connections. For example, you can use MS Access to connect to your MySQL server. Clients can be run on Windows or Unix. MyODBC source is available. All ODBC 2.5 functions are supported, as are many others. See section 22.1 MySQL ODBC Support.
  • The Connector/J interface provides MySQL support for Java client programs that use JDBC connections. Clients can be run on Windows or Unix. Connector/J source is available. See section 22.2 MySQL Java Connectivity (JDBC).
Localization
  • The server can provide error messages to clients in many languages. See section 5.8.2 Setting the Error Message Language.
  • Full support for several different character sets, including latin1 (ISO-8859-1), german, big5, ujis, and more. For example, the Scandinavian characters `@^a', `@"a' and `@"o' are allowed in table and column names. Unicode support is available as of MySQL 4.1.
  • All data is saved in the chosen character set. All comparisons for normal string columns are case-insensitive.
  • Sorting is done according to the chosen character set (using Swedish collation by default). It is possible to change this when the MySQL server is started. To see an example of very advanced sorting, look at the Czech sorting code. MySQL Server supports many different character sets that can be specified at compile time and runtime.
Clients and Tools
  • The MySQL server has built-in support for SQL statements to check, optimize, and repair tables. These statements are available from the command line through the @command{mysqlcheck} client. MySQL also includes @command{myisamchk}, a very fast command-line utility for performing these operations on MyISAM tables. See section 5 Database Administration.
  • All MySQL programs can be invoked with the --help or -? options to obtain online assistance.

1.2.3 MySQL Stability

This section addresses the questions, ``How stable is MySQL Server?'' and, ``Can I depend on MySQL Server in this project?'' We will try to clarify these issues and answer some important questions that concern many potential users. The information in this section is based on data gathered from the mailing lists, which are very active in identifying problems as well as reporting types of use.

The original code stems back to the early 1980s. It provides a stable code base, and the ISAM table format used by the original storage engine remains backward-compatible. At TcX, the predecessor of MySQL AB, MySQL code has worked in projects since mid-1996, without any problems. When the MySQL Database Software initially was released to a wider public, our new users quickly found some pieces of untested code. Each new release since then has had fewer portability problems, even though each new release has also had many new features.

Each release of the MySQL Server has been usable. Problems have occurred only when users try code from the ``gray zones.'' Naturally, new users don't know what the gray zones are; this section therefore attempts to document those areas that are currently known. The descriptions mostly deal with Version 3.23, 4.0 and 4.1 of MySQL Server. All known and reported bugs are fixed in the latest version, with the exception of those listed in the bugs section, which are design-related. See section 1.8.7 Known Errors and Design Deficiencies in MySQL.

The MySQL Server design is multi-layered with independent modules. Some of the newer modules are listed here with an indication of how well-tested each of them is:

Replication (Stable)
Large groups of servers using replication are in production use, with good results. Work on enhanced replication features is continuing in MySQL 5.x.
InnoDB tables (Stable)
The InnoDB transactional storage engine has been declared stable in the MySQL 3.23 tree, starting from version 3.23.49. InnoDB is being used in large, heavy-load production systems.
BDB tables (Stable)
The Berkeley DB code is very stable, but we are still improving the BDB transactional storage engine interface in MySQL Server.
Full-text searches (Stable)
Full-text searching is widely used. Important feature enhancements were added in MySQL 4.0 and 4.1.
MyODBC 3.51 (Stable)
MyODBC 3.51 uses ODBC SDK 3.51 and is in wide production use. Some issues brought up appear to be application-related and independent of the ODBC driver or underlying database server.

1.2.4 How Big MySQL Tables Can Be

MySQL 3.22 had a 4GB (4 gigabyte) limit on table size. With the MyISAM storage engine in MySQL 3.23, the maximum table size was increased to 8 million terabytes (2 ^ 63 bytes). With this larger allowed table size, the maximum effective table size for MySQL databases now usually is determined by operating system constraints on file sizes, not by MySQL internal limits.

The InnoDB storage engine maintains InnoDB tables within a tablespace that can be created from several files. This allows a table to exceed the maximum individual file size. The tablespace can include raw disk partitions, which allows extremely large tables. The maximum tablespace size is 64TB.

The following table lists some examples of operating system file-size limits:

Operating System File-size Limit
Linux-Intel 32-bit 2GB, much more when using LFS
Linux-Alpha 8TB (?)
Solaris 2.5.1 2GB (4GB possible with patch)
Solaris 2.6 4GB (can be changed with flag)
Solaris 2.7 Intel 4GB
Solaris 2.7 UltraSPARC 512GB
NetWare w/NSS filesystem 8TB

On Linux 2.2, you can get MyISAM tables larger than 2GB in size by using the Large File Support (LFS) patch for the ext2 filesystem. On Linux 2.4, patches also exist for ReiserFS to get support for big files. Most current Linux distributions are based on kernel 2.4 and already include all the required LFS patches. However, the maximum available file size still depends on several factors, one of them being the filesystem used to store MySQL tables.

For a detailed overview about LFS in Linux, have a look at Andreas Jaeger's Large File Support in Linux page at http://www.suse.de/~aj/linux_lfs.html.

By default, MySQL creates MyISAM tables with an internal structure that allows a maximum size of about 4GB. You can check the maximum table size for a table with the SHOW TABLE STATUS statement or with @command{myisamchk -dv tbl_name}. See section 14.5.3 SET and SHOW Syntax.

If you need a MyISAM table that will be larger than 4GB in size (and your operating system supports large files), the CREATE TABLE statement allows AVG_ROW_LENGTH and MAX_ROWS options. See section 14.2.6 CREATE TABLE Syntax. You can also change these options with ALTER TABLE after the table has been created, to increase the table's maximum allowable size. See section 14.2.2 ALTER TABLE Syntax.

Other ways to work around file-size limits for MyISAM tables are as follows:

1.2.5 Year 2000 Compliance

The MySQL Server itself has no problems with Year 2000 (Y2K) compliance:

The following simple demonstration illustrates that MySQL Server has no problems with DATE or DATETIME values through the year 9999, and no problems with TIMESTAMP values until after the year 2030: 

mysql> DROP TABLE IF EXISTS y2k;
Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE y2k (date DATE,
    ->                   date_time DATETIME,
    ->                   time_stamp TIMESTAMP);
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO y2k VALUES
    -> ('1998-12-31','1998-12-31 23:59:59',19981231235959),
    -> ('1999-01-01','1999-01-01 00:00:00',19990101000000),
    -> ('1999-09-09','1999-09-09 23:59:59',19990909235959),
    -> ('2000-01-01','2000-01-01 00:00:00',20000101000000),
    -> ('2000-02-28','2000-02-28 00:00:00',20000228000000),
    -> ('2000-02-29','2000-02-29 00:00:00',20000229000000),
    -> ('2000-03-01','2000-03-01 00:00:00',20000301000000),
    -> ('2000-12-31','2000-12-31 23:59:59',20001231235959),
    -> ('2001-01-01','2001-01-01 00:00:00',20010101000000),
    -> ('2004-12-31','2004-12-31 23:59:59',20041231235959),
    -> ('2005-01-01','2005-01-01 00:00:00',20050101000000),
    -> ('2030-01-01','2030-01-01 00:00:00',20300101000000),
    -> ('2040-01-01','2040-01-01 00:00:00',20400101000000),
    -> ('9999-12-31','9999-12-31 23:59:59',99991231235959);
Query OK, 14 rows affected (0.01 sec)
Records: 14  Duplicates: 0  Warnings: 2

mysql> SELECT * FROM y2k;
+------------+---------------------+----------------+
| date       | date_time           | time_stamp     |
+------------+---------------------+----------------+
| 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 |
| 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 |
| 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 |
| 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 |
| 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 |
| 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 |
| 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 |
| 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 |
| 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 |
| 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 |
| 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 |
| 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 |
| 2040-01-01 | 2040-01-01 00:00:00 | 00000000000000 |
| 9999-12-31 | 9999-12-31 23:59:59 | 00000000000000 |
+------------+---------------------+----------------+
14 rows in set (0.00 sec)

The final two TIMESTAMP column values are zero because the year values (2040, 9999) exceed the TIMESTAMP maximum. The TIMESTAMP data type, which is used to store the current time, supports values that range from 19700101000000 to 20300101000000 on 32-bit machines (signed value). On 64-bit machines, TIMESTAMP handles values up to 2106 (unsigned value).

Although MySQL Server itself is Y2K-safe, you may run into problems if you use it with applications that are not Y2K-safe. For example, many old applications store or manipulate years using two-digit values (which are ambiguous) rather than four-digit values. This problem may be compounded by applications that use values such as 00 or 99 as ``missing'' value indicators. Unfortunately, these problems may be difficult to fix because different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.

Thus, even though MySQL Server has no Y2K problems, it is the application's responsibility to provide unambiguous input. See section 12.3.4 Y2K Issues and Date Types for MySQL Server's rules for dealing with ambiguous date input data that contains two-digit year values.

1.3 Overview of MySQL AB

MySQL AB is the company of the MySQL founders and main developers. MySQL AB was originally established in Sweden by David Axmark, Allan Larsson, and Michael ``Monty'' Widenius.

The developers of the MySQL server are all employed by the company. We are a virtual organization with people in a dozen countries around the world. We communicate extensively over the Internet every day with one another and with our users, supporters, and partners.

We are dedicated to developing the MySQL database software and promoting it to new users. MySQL AB owns the copyright to the MySQL source code, the MySQL logo and (registered) trademark, and this manual. See section 1.2 Overview of the MySQL Database Management System.

The MySQL core values show our dedication to MySQL and Open Source.

These core values direct how MySQL AB works with the MySQL server software:

These are the core values of the company MySQL AB and its employees:

The MySQL Web site (http://www.mysql.com/) provides the latest information about MySQL and MySQL AB.

By the way, the ``AB'' part of the company name is the acronym for the Swedish ``aktiebolag,'' or ``stock company.'' It translates to ``MySQL, Inc.'' In fact, MySQL, Inc. and MySQL GmbH are examples of MySQL AB subsidiaries. They are located in the US and Germany, respectively.

1.3.1 The Business Model and Services of MySQL AB

One of the most common questions we encounter is, ``How can you make a living from something you give away for free?'' This is how:

The company has been profitable since its inception. In October 2001, we accepted venture financing from leading Scandinavian investors and a handful of business angels. This investment is used to solidify our business model and build a basis for sustainable growth.

1.3.1.1 Support

MySQL AB is run and owned by the founders and main developers of the MySQL database. The developers are committed to providing support to customers and other users in order to stay in touch with their needs and problems. All our support is provided by qualified developers. Really tricky questions are answered by Michael ``Monty'' Widenius, principal author of the MySQL Server.

Paying customers receive high-quality support directly from MySQL AB. MySQL AB also provides the MySQL mailing lists as a community resource where anyone may ask questions.

For more information and ordering support at various levels, see section 1.4 MySQL Support and Licensing.

1.3.1.2 Training and Certification

MySQL AB delivers MySQL and related training worldwide. We offer both open courses and in-house courses tailored to the specific needs of your company. MySQL Training is also available through our partners, the Authorized MySQL Training Centers.

Our training material uses the same sample databases used in our documentation and our sample applications, and is always updated to reflect the latest MySQL version. Our trainers are backed by the development team to guarantee the quality of the training and the continuous development of the course material. This also ensures that no questions raised during the courses remain unanswered.

Attending our training courses will enable you to achieve your MySQL application goals. You will also:

If you are interested in our training as a potential participant or as a training partner, please visit the training section at http://www.mysql.com/training/, or send email to [email protected].

For details about the MySQL Certification Program, please see http://www.mysql.com/certification/.

1.3.1.3 Consulting

MySQL AB and its Authorized Partners offer consulting services to users of MySQL Server and to those who embed MySQL Server in their own software, all over the world.

Our consultants can help you design and tune your databases, construct efficient queries, tune your platform for optimal performance, resolve migration issues, set up replication, build robust transactional applications, and more. We also help customers embed MySQL Server in their products and applications for large-scale deployment.

Our consultants work in close collaboration with our development team, which ensures the technical quality of our professional services. Consulting assignments range from two-day power-start sessions to projects that span weeks and months. Our expertise covers not only MySQL Server, it also extends into programming and scripting languages such as PHP, Perl, and more.

If you are interested in our consulting services or want to become a consulting partner, please visit the consulting section of our Web site at http://www.mysql.com/consulting/ or contact our consulting staff at [email protected].

1.3.1.4 Commercial Licenses

The MySQL database is released under the GNU General Public License (GPL). This means that the MySQL software can be used free of charge under the GPL. If you do not want to be bound by the GPL terms (such as the requirement that your application must also be GPL), you may purchase a commercial license for the same product from MySQL AB; see https://order.mysql.com/. Since MySQL AB owns the copyright to the MySQL source code, we are able to employ Dual Licensing, which means that the same product is available under GPL and under a commercial license. This does not in any way affect the Open Source commitment of MySQL AB. For details about when a commercial license is required, please see section 1.4.3 MySQL Licenses.

We also sell commercial licenses of third-party Open Source GPL software that adds value to MySQL Server. A good example is the InnoDB transactional storage engine that offers ACID support, row-level locking, crash recovery, multi-versioning, foreign key support, and more. See section 16 The InnoDB Storage Engine.

1.3.1.5 Partnering

MySQL AB has a worldwide partner program that covers training courses, consulting and support, publications, plus reselling and distributing MySQL and related products. MySQL AB Partners get visibility on the http://www.mysql.com/ Web site and the right to use special versions of the MySQL (registered) trademarks to identify their products and promote their business.

If you are interested in becoming a MySQL AB Partner, please email [email protected].

The word MySQL and the MySQL dolphin logo are (registered) trademarks of MySQL AB. See section 1.4.4 MySQL AB Logos and Trademarks. These trademarks represent a significant value that the MySQL founders have built over the years.

The MySQL Web site (http://www.mysql.com/) is popular among developers and users. In December 2003, we served 16 million page views. Our visitors represent a group that makes purchase decisions and recommendations for both software and hardware. Twelve percent of our visitors authorize purchase decisions, and only nine percent have no involvement at all in purchase decisions. More than 65% have made one or more online business purchases within the last half-year, and 70% plan to make one in the next few months.

1.3.2 Contact Information

The MySQL Web site (http://www.mysql.com/) provides the latest information about MySQL and MySQL AB.

For press services and inquiries not covered in our news releases (http://www.mysql.com/news-and-events/), please send email to [email protected].

If you have a support contract with MySQL AB, you will get timely, precise answers to your technical questions about the MySQL software. For more information, see section 1.4.1 Support Offered by MySQL AB. On our Web site, see http://www.mysql.com/support/, or send email to [email protected].

For information about MySQL training, please visit the training section at http://www.mysql.com/training/, or send email to [email protected]. See section 1.3.1.2 Training and Certification.

For information on the MySQL Certification Program, please see http://www.mysql.com/certification/. See section 1.3.1.2 Training and Certification.

If you're interested in consulting, please visit the consulting section of our Web site at http://www.mysql.com/consulting/, or send email to [email protected]. See section 1.3.1.3 Consulting.

Commercial licenses may be purchased online at https://order.mysql.com/. There you will also find information on how to fax your purchase order to MySQL AB. More information about licensing can be found at http://www.mysql.com/products/licensing/. If you have questions regarding licensing or you want a quote for high-volume licensing, please fill in the contact form on our Web site (http://www.mysql.com/), or send email to [email protected] (for licensing questions) or to [email protected] (for sales inquiries). See section 1.4.3 MySQL Licenses.

If you represent a business that is interested in partnering with MySQL AB, please send email to [email protected]. See section 1.3.1.5 Partnering.

For more information on the MySQL trademark policy, refer to http://www.mysql.com/company/trademark.html, or send email to [email protected]. See section 1.4.4 MySQL AB Logos and Trademarks.

If you are interested in any of the MySQL AB jobs listed in our jobs section (http://www.mysql.com/company/jobs/), please send email to [email protected]. Please do not send your CV as an attachment, but rather as plain text at the end of your email message.

For general discussion among our many users, please direct your attention to the appropriate mailing list. See section 1.7.1 MySQL Mailing Lists.

Reports of errors (often called ``bugs''), as well as questions and comments, should be sent to the general MySQL mailing list. See section 1.7.1.1 The MySQL Mailing Lists. If you have found a sensitive security bug in MySQL Server, please let us know immediately by sending email to [email protected]. See section 1.7.1.3 How to Report Bugs or Problems.

If you have benchmark results that we can publish, please contact us via email at [email protected].

If you have suggestions concerning additions or corrections to this manual, please send them to the documentation team via email at [email protected].

For questions or comments about the workings or content of the MySQL Web site (http://www.mysql.com/), please send email to [email protected].

MySQL AB has a privacy policy, which can be read at http://www.mysql.com/company/privacy.html. For any queries regarding this policy, please send email to [email protected].

For all other inquiries, please send email to [email protected].

1.4 MySQL Support and Licensing

This section describes MySQL support and licensing arrangements.

1.4.1 Support Offered by MySQL AB

Technical support from MySQL AB means individualized answers to your unique problems direct from the software engineers who code the MySQL database engine.

We try to take a broad and inclusive view of technical support. Almost any problem involving MySQL software is important to us if it's important to you. Typically customers seek help on how to get different commands and utilities to work, remove performance bottlenecks, restore crashed systems, understand the impact of operating system or networking issues on MySQL, set up best practices for backup and recovery, utilize APIs, and so on. Our support covers only the MySQL server and our own utilities, not third-party products that access the MySQL server, although we try to help with these where we can.

Detailed information about our various support options is given at http://www.mysql.com/support/, where support contracts can also be ordered online. To contact our sales staff, send email to [email protected].

Technical support is like life insurance. You can live happily without it for years. However, when your hour arrives, it becomes critically important, but it's too late to buy it. If you use MySQL Server for important applications and encounter sudden difficulties, it may be too time-consuming to figure out all the answers yourself. You may need immediate access to the most experienced MySQL troubleshooters available, those employed by MySQL AB.

1.4.2 Copyrights and Licenses Used by MySQL

MySQL AB owns the copyright to the MySQL source code, the MySQL logos and (registered) trademarks, and this manual. See section 1.3 Overview of MySQL AB. Several different licenses are relevant to the MySQL distribution:

  1. All the MySQL-specific source in the server, the mysqlclient library and the client, as well as the GNU readline library, are covered by the GNU General Public License. See section G GNU General Public License. The text of this license can be found as the file `COPYING' in MySQL distributions.
  2. The GNU getopt library is covered by the GNU Lesser General Public License. See http://www.fsf.org/licenses/.
  3. Some parts of the source (the regexp library) are covered by a Berkeley-style copyright.
  4. Older versions of MySQL (3.22 and earlier) are subject to a stricter license (http://www.mysql.com/products/licensing/mypl.html). See the documentation of the specific version for information.
  5. The MySQL Reference Manual is not distributed under a GPL-style license. Use of the manual is subject to the following terms:
    • Conversion to other formats is allowed, but the actual content may not be altered or edited in any way.
    • You may create a printed copy for your own personal use.
    • For all other uses, such as selling printed copies or using (parts of) the manual in another publication, prior written agreement from MySQL AB is required.
    Please send an email message to [email protected] for more information or if you are interested in doing a translation.

For information about how the MySQL licenses work in practice, please refer to section 1.4.3 MySQL Licenses and section 1.4.4 MySQL AB Logos and Trademarks.

1.4.3 MySQL Licenses

The MySQL software is released under the GNU General Public License (GPL), which is probably the best known Open Source license. The formal terms of the GPL license can be found at http://www.fsf.org/licenses/. See also http://www.fsf.org/licenses/gpl-faq.html and http://www.gnu.org/philosophy/enforcing-gpl.html.

Our GPL licensing is supported by an optional license exception that enables many Free/Libre and Open Source Software (``FLOSS'') applications to include the GPL-licensed MySQL client libraries despite the fact that not all FLOSS licenses are compatible with the GPL. For details, see http://www.mysql.com/products/licensing/foss-exception.html.

Because the MySQL software is released under the GPL, it may often be used for free, but for certain uses you may want or need to buy commercial licenses from MySQL AB at https://order.mysql.com/. See http://www.mysql.com/products/licensing/ for more information.

Older versions of MySQL (3.22 and earlier) are subject to a stricter license (http://www.mysql.com/products/licensing/mypl.html). See the documentation of the specific version for information.

Please note that the use of the MySQL software under commercial license, GPL, or the old MySQL license does not automatically give you the right to use MySQL AB (registered) trademarks. See section 1.4.4 MySQL AB Logos and Trademarks.

1.4.3.1 Using the MySQL Software Under a Commercial License

The GPL license is contagious in the sense that when a program is linked to a GPL program, all the source code for all the parts of the resulting product must also be released under the GPL. If you do not follow this GPL requirement, you break the license terms and forfeit your right to use the GPL program altogether. You also risk damages.

You need a commercial license under these conditions:

Our GPL licensing is supported by an optional license exception that enables many Free/Libre and Open Source Software (``FLOSS'') applications to include the GPL-licensed MySQL client libraries despite the fact that not all FLOSS licenses are compatible with the GPL. For details, see http://www.mysql.com/products/licensing/foss-exception.html.

If you require a commercial license, you will need one for each installation of the MySQL software. This covers any number of CPUs on a machine, and there is no artificial limit on the number of clients that connect to the server in any way.

For commercial licenses, please visit our Web site at http://www.mysql.com/products/licensing/. For support contracts, see http://www.mysql.com/support/. If you have special needs, please contact our sales staff via email at [email protected].

1.4.3.2 Using the MySQL Software for Free Under GPL

You can use the MySQL software for free under the GPL if you adhere to the conditions of the GPL. For additional details about the GPL, including answers to common questions, see the generic FAQ from the Free Software Foundation at http://www.fsf.org/licenses/gpl-faq.html.

Our GPL licensing is supported by an optional license exception that enables many Free/Libre and Open Source Software (``FLOSS'') applications to include the GPL-licensed MySQL client libraries despite the fact that not all FLOSS licenses are compatible with the GPL. For details, see http://www.mysql.com/products/licensing/foss-exception.html.

Common uses of the GPL include:

If your use of MySQL database software does not require a commercial license, we encourage you to purchase support from MySQL AB anyway. This way you contribute toward MySQL development and also gain immediate advantages for yourself. See section 1.4.1 Support Offered by MySQL AB.

If you use the MySQL database software in a commercial context such that you profit by its use, we ask that you further the development of the MySQL software by purchasing some level of support. We feel that if the MySQL database helps your business, it is reasonable to ask that you help MySQL AB. (Otherwise, if you ask us support questions, you are not only using for free something into which we've put a lot a work, you're asking us to provide free support, too.)

1.4.4 MySQL AB Logos and Trademarks

Many users of the MySQL database want to display the MySQL AB dolphin logo on their Web sites, books, or boxed products. We welcome and encourage this, although it should be noted that the word MySQL and the MySQL dolphin logo are (registered) trademarks of MySQL AB and may only be used as stated in our trademark policy at http://www.mysql.com/company/trademark.html.

1.4.4.1 The Original MySQL Logo

The MySQL dolphin logo was designed by the Finnish advertising agency Priority in 2001. The dolphin was chosen as a suitable symbol for the MySQL database management system, which is like a smart, fast, and lean animal, effortlessly navigating oceans of data. We also happen to like dolphins.

The original MySQL logo may only be used by representatives of MySQL AB and by those having a written agreement allowing them to do so.

1.4.4.2 MySQL Logos That May Be Used Without Written Permission

We have designed a set of special Conditional Use logos that may be downloaded from our Web site at http://www.mysql.com/press/logos.html and used on third-party Web sites without written permission from MySQL AB. The use of these logos is not entirely unrestricted but, as the name implies, subject to our trademark policy that is also available on our Web site. You should read through the trademark policy if you plan to use them. The requirements are basically as follows:

Contact us via email at [email protected] to inquire about special arrangements to fit your needs.

1.4.4.3 When You Need Written Permission to Use MySQL Logos

You need written permission from MySQL AB before using MySQL logos in the following cases:

Due to legal and commercial reasons, we monitor the use of MySQL (registered) trademarks on products, books, and other items. We usually require a fee for displaying MySQL AB logos on commercial products, since we think it is reasonable that some of the revenue is returned to fund further development of the MySQL database.

1.4.4.4 MySQL AB Partnership Logos

MySQL partnership logos may be used only by companies and persons having a written partnership agreement with MySQL AB. Partnerships include certification as a MySQL trainer or consultant. For more information, please see section 1.3.1.5 Partnering.

1.4.4.5 Using the Word MySQL in Printed Text or Presentations

MySQL AB welcomes references to the MySQL database, but it should be noted that the word MySQL is a registered trademark of MySQL AB. Because of this, you must append the ``registered trademark'' notice symbol (R) to the first or most prominent use of the word MySQL in a text and, where appropriate, state that MySQL is a registered trademark of MySQL AB. For more information, please refer to our trademark policy at http://www.mysql.com/company/trademark.html.

1.4.4.6 Using the Word MySQL in Company and Product Names

Use of the word MySQL in company or product names or in Internet domain names is not allowed without written permission from MySQL AB. http://www.mysql.com/company/contact/.

1.5 MySQL Development Roadmap

This section provides a snapshot of the MySQL development roadmap, including major features implemented or planned for MySQL 4.0, 4.1, 5.0, and 5.1. The following sections provide information for each release series.

The production release series is MySQL 4.0, which was declared stable for production use as of Version 4.0.12, released in March 2003. This means that future 4.0 development is limited only to bugfixes. For the older MySQL 3.23 series, only critical bugfixes are made.

4.1 is available in gamma status and will soon move to production stage.

Active MySQL development currently is taking place in the MySQL 5.0 release series, this means that new features are being added there. MySQL 5.0 is available in alpha status.

Before upgrading from one release series to the next, please see the notes at section 2.5 Upgrading/Downgrading MySQL.

Plans for some of the most requested features are summarized in the following table.

Feature MySQL Series
Unions 4.0
Subqueries 4.1
R-trees 4.1 (for MyISAM tables)
Stored procedures 5.0
Views 5.0
Cursors 5.0
Foreign keys 5.1 (already implemented in 3.23 for InnoDB)
Triggers 5.0 and 5.1
Full outer join 5.1
Constraints 5.1

1.5.1 MySQL 4.0 in a Nutshell

MySQL Server 4.0 is available in production status.

MySQL 4.0 is available for download at http://dev.mysql.com/ and from our mirrors. MySQL 4.0 has been tested by a large number of users and is in production use at many large sites.

The major new features of MySQL Server 4.0 are geared toward our existing business and community users, enhancing the MySQL database software as the solution for mission-critical, heavy-load database systems. Other new features target the users of embedded databases.

1.5.1.1 Features Available in MySQL 4.0

Speed enhancements
  • MySQL 4.0 has a query cache that can give a huge speed boost to applications with repetitive queries. See section 5.11 The MySQL Query Cache.
  • Version 4.0 further increases the speed of MySQL Server in a number of areas, such as bulk INSERT statements, searching on packed indexes, full-text searching (using FULLTEXT indexes), and COUNT(DISTINCT).
Embedded MySQL Server introduced
  • The new Embedded Server library can easily be used to create standalone and embedded applications. The embedded server provides an alternative to using MySQL in a client/server environment. See section 1.5.1.2 The Embedded MySQL Server.
InnoDB storage engine as standard
  • The InnoDB storage engine is offered as a standard feature of the MySQL server. This means full support for ACID transactions, foreign keys with cascading UPDATE and DELETE, and row-level locking are now standard features. See section 16 The InnoDB Storage Engine.
New functionality
  • The enhanced FULLTEXT search properties of MySQL Server 4.0 enables FULLTEXT indexing of large text masses with both binary and natural-language searching logic. You can customize minimal word length and define your own stop word lists in any human language, enabling a new set of applications to be built with MySQL Server. See section 13.6 Full-Text Search Functions.
Standards compliance, portability, and migration
  • Many users will also be happy to learn that MySQL Server now supports the UNION statement, a long-awaited standard SQL feature.
  • MySQL runs natively on the Novell NetWare platform beginning with NetWare 6.0. See section 2.2.4 Installing MySQL on NetWare.
  • Features to simplify migration from other database systems to MySQL Server include TRUNCATE TABLE (as in Oracle).
Internationalization
  • Our German, Austrian, and Swiss users will note that MySQL 4.0 now supports a new character set, latin1_de, which ensures that the German sorting order sorts words with umlauts in the same order as do German telephone books.
Usability enhancements
In the process of implementing features for new users, we have not forgotten requests from our loyal community of existing users.
  • Most @command{mysqld} parameters (startup options) can be set without taking down the server. This is a convenient feature for database administrators (DBAs). See section 14.5.3.1 SET Syntax.
  • Multiple-table DELETE and UPDATE statements have been added.
  • On Windows, symbolic link handling at the database level is enabled by default. On Unix, the MyISAM storage engine supports symbolic linking at the table level (and not just the database level as before).
  • SQL_CALC_FOUND_ROWS and FOUND_ROWS() are new functions that make it possible to find out the number of rows a SELECT query that includes a LIMIT clause would have returned without that clause.

The news section of this manual includes a more in-depth list of features. See section C.3 Changes in release 4.0.x (Production).

1.5.1.2 The Embedded MySQL Server

The libmysqld embedded server library makes MySQL Server suitable for a vastly expanded realm of applications. By using this library, developers can embed MySQL Server into various applications and electronics devices, where the end user has no knowledge of there actually being an underlying database. Embedded MySQL Server is ideal for use behind the scenes in Internet appliances, public kiosks, turnkey hardware/software combination units, high performance Internet servers, self-contained databases distributed on CD-ROM, and so on.

Many users of libmysqld will benefit from the MySQL Dual Licensing. For those not wishing to be bound by the GPL, the software is also made available under a commercial license. The embedded MySQL library uses the same interface as the normal client library, so it is convenient and easy to use. See section 21.2.15 libmysqld, the Embedded MySQL Server Library.

On windows there are two different libraries:

libmysqld.lib Dynamic library for threaded applications.
mysqldemb.lib Static library for not threaded applications.

1.5.2 MySQL 4.1 in a Nutshell

MySQL Server 4.0 laid the foundation for new features implemented in MySQL 4.1, such as subqueries and Unicode support, and for the work on stored procedures being done in version 5.0. These features come at the top of the wish list of many of our customers. Already well-known for its stability, speed, and ease of use, MySQL Server is now able to fulfill the requirement checklists of very demanding buyers.

MySQL 4.1 is now available in gamma status, and will be production soon.

1.5.2.1 Features Available in MySQL 4.1

This section lists features implemented in MySQL 4.1.

New features currently being coded are or will be available in MySQL 5.0. See section 1.6.1 New Features Planned for 5.0.

Support for subqueries and derived tables
  • A ``subquery'' is a SELECT statement nested within another statement. A ``derived table'' (an unnamed view) is a subquery in the FROM clause of another statement. See section 14.1.8 Subquery Syntax.
Speed enhancements
  • Faster binary client/server protocol with support for prepared statements and parameter binding. See section 21.2.4 C API Prepared Statements.
  • BTREE indexing is supported for HEAP tables, significantly improving response time for non-exact searches.
New functionality
  • CREATE TABLE tbl_name2 LIKE tbl_name1 allows you to create, with a single statement, a new table with a structure exactly like that of an existing table.
  • The MyISAM storage engine supports OpenGIS spatial types for storing geographical data. See section 19 Spatial Extensions in MySQL.
  • Replication can be done over SSL connections.
Standards compliance, portability, and migration
  • The new client/server protocol adds the ability to pass multiple warnings to the client, rather than only a single result. This makes it much easier to track problems that occur in operations such as bulk data loading.
  • SHOW WARNINGS shows warnings for the last command. See section 14.5.3.21 SHOW WARNINGS Syntax.
Internationalization and Localization
  • To support applications that require the use of local languages, the MySQL software offers extensive Unicode support through the utf8 and ucs2 character sets.
  • Character sets can be defined per column, table, and database. This allows for a high degree of flexibility in application design, particularly for multi-language Web sites.
  • For documentation for this improved character set support, see section 11 Character Set Support.
  • Per-connection time zones are supported, allowing individual clients to select their own time zone when necessary.
Usability enhancements
  • In response to popular demand, we have added a server-based HELP command that can be used to get help information for SQL statements. The advantage of having this information on the server side is that the information is always applicable to the particular server version that you actually are using. Because this information is available by issuing an SQL statement, any client can be written to access it. For example, the help command of the @command{mysql} command-line client has been modified to have this capability.
  • In the new client/server protocol, multiple statements can be issued with a single call. See section 21.2.8 C API Handling of Multiple Query Execution.
  • The new client/server protocol also supports returning multiple result sets. This might occur as a result of sending multiple statements, for example.
  • A new INSERT ... ON DUPLICATE KEY UPDATE ... syntax has been implemented. This allows you to UPDATE an existing row if the INSERT would have caused a duplicate in a PRIMARY or UNIQUE index. See section 14.1.4 INSERT Syntax.
  • A new aggregate function, GROUP_CONCAT() adds the extremely useful capability of concatenating column values from grouped rows into a single result string. See section 13.9 Functions and Modifiers for Use with GROUP BY Clauses.

The news section of this manual includes a more in-depth list of features. See section C.2 Changes in release 4.1.x (Gamma).

1.5.2.2 Stepwise Rollout

The gamma version of MySQL 4.1 is available for download. See section 1.5.2.3 Ready for Immediate Development Use.

MySQL 4.1 is currently in the Gamma stage (indicating that a production release is just weeks ahead). At the end of this process, MySQL 4.1 will become the new production release.

Development is already ongoing for version 5.0.

1.5.2.3 Ready for Immediate Development Use

MySQL 4.1 is currently in the gamma stage, and binaries are available for download at http://dev.mysql.com/downloads/mysql/4.1.html. All binary releases pass our extensive test suite without any errors on the platforms on which we test. See section C.2 Changes in release 4.1.x (Gamma).

For those wishing to use the most recent development source for MySQL 4.1, we also make our BitKeeper repositories publicly available. See section 2.3.3 Installing from the Development Source Tree.

1.5.3 MySQL 5.0: The Next Development Release

New development for MySQL is focused on the 5.0 release, featuring stored procedures, updatable views, rudimentary triggers, and other new features. See section 1.6.1 New Features Planned for 5.0.

For those wishing to take a look at the bleeding edge of MySQL development, we make our BitKeeper repository for MySQL version 5.0 publicly available. See section 2.3.3 Installing from the Development Source Tree. As of December 2003, binary builds of version 5.0 have also been available.

1.6 MySQL and the Future (the TODO)

This section summarizes the features that we plan to implement in MySQL Server. The items are ordered by release series. Within a list, items are shown in approximately the order they will be done.

Note: If you are an enterprise-level user with an urgent need for a particular feature, please contact [email protected] to discuss sponsoring options. Targeted financing by sponsor companies allows us to allocate additional resources for specific purposes. One example of a feature sponsored in the past is replication.

1.6.1 New Features Planned for 5.0

The following features are planned for inclusion into MySQL 5.0. Some of the features such as stored procedures are complete and are included in MySQL 5.0 alpha, which is available now. Others such as cursors are only partially available. Expect these and other features to mature and be fully supported in upcoming releases.

Note that because we have many developers that are working on different projects, there will also be many additional features. There is also a small chance that some of these features will be added to MySQL 4.1. For a list what is already done in MySQL 4.1, see section 1.5.2.1 Features Available in MySQL 4.1.

For those wishing to take a look at the bleeding edge of MySQL development, we make our BitKeeper repository for MySQL version 5.0 publicly available. See section 2.3.3 Installing from the Development Source Tree. As of December 2003, binary builds of version 5.0 are also available.

Views
Stored Procedures
New functionality
  • Elementary cursor support. See section 20.1.8 Cursors.
  • The ability to specify explicitly for MyISAM tables that an index should be created as an RTREE index. (In MySQL 4.1, RTREE indexes are used internally for geometrical data that use GIS data types, but cannot be created on request.)
  • Dynamic length rows for MEMORY tables.
Standards compliance, portability and migration
  • Support for Data Dictionary / INFORMATION_SCHEMA.
  • Add true VARCHAR support (column lengths longer than 255, and no stripping of trailing whitespace). There is already support for this in the MyISAM storage engine, but it is not yet available at the user level.
Speed enhancements
  • SHOW COLUMNS FROM tbl_name (used by the @command{mysql} client to allow expansions of column names) should not open the table, only the definition file. This will require less memory and be much faster.
  • Allow DELETE on MyISAM tables to use the record cache. To do this, we need to update the threads record cache when we update the `.MYD' file.
  • Better support for MEMORY tables:
    • Dynamic length rows.
    • Faster row handling (less copying).
Usability enhancements
  • Resolving the issue of RENAME TABLE on a table used in an active MERGE table possibly corrupting the table.

The news section of this manual includes a more in-depth list of features. See section C.1 Changes in release 5.0.x (Development).

1.6.2 New Features Planned for 5.1

New functionality
  • FOREIGN KEY support for all table types, not just InnoDB.
  • Column-level constraints. See section 1.8.6 How MySQL Deals with Constraints.
  • Online backup with very low performance penalty. The online backup will make it easy to add a new replication slave without taking down the master.
Speed enhancements
  • New text based table definition file format (`.frm' files) and a table cache for table definitions. This will enable us to do faster queries of table structures and do more efficient foreign key support.
  • Optimize the BIT type to take one bit. (BIT now takes one byte; it is treated as a synonym for TINYINT.)
Usability enhancements
  • Add options to the client/server protocol to get progress notes for long running commands.
  • Implement RENAME DATABASE. To make this safe for all storage engines, it should work as follows:
    1. Create the new database.
    2. For every table, do a rename of the table to another database, as we do with the RENAME command.
    3. Drop the old database.
  • New internal file interface change. This will make all file handling much more general and make it easier to add extensions like RAID.

1.6.3 New Features Planned for the Near Future

New functionality
  • Oracle-like CONNECT BY PRIOR to search tree-like (hierarchical) structures.
  • Add all missing standard SQL and ODBC 3.0 types.
  • Add SUM(DISTINCT).
  • INSERT SQL_CONCURRENT and @command{mysqld --concurrent-insert} to do a concurrent insert at the end of a table if the table is read-locked.
  • Allow variables to be updated in UPDATE statements. For example: UPDATE foo SET @a:=a+b,a=@a, b=@a+c.
  • Change when user variables are updated so that you can use them with GROUP BY, as in the following statement: SELECT id, @a:=COUNT(*), SUM(sum_col)/@a FROM tbl_name GROUP BY id.
  • Add an IMAGE option to LOAD DATA INFILE to not update TIMESTAMP and AUTO_INCREMENT columns.
  • Add LOAD DATA INFILE ... UPDATE syntax that works like this:
    • For tables with primary keys, if an input record contains a primary key value, existing rows matching that primary key value are updated from the remainder of the input columns. However, columns corresponding to columns that are missing from the input record are not touched.
    • For tables with primary keys, if an input record does not contain the primary key value or is missing some part of the key, the record is treated as LOAD DATA INFILE ... REPLACE INTO.
  • Make LOAD DATA INFILE understand syntax like this: 
    LOAD DATA INFILE 'file_name.txt' INTO TABLE tbl_name
     TEXT_FIELDS (text_col1, text_col2, text_col3)
     SET table_col1=CONCAT(text_col1, text_col2),
         table_col3=23
     IGNORE text_col3
    This can be used to skip over extra columns in the text file, or update columns based on expressions of the read data.
  • New functions for working with SET type columns:
    • ADD_TO_SET(value,set)
    • REMOVE_FROM_SET(value,set)
  • If you abort @command{mysql} in the middle of a query, you should open another connection and kill the old running query. Alternatively, an attempt should be made to detect this in the server.
  • Add a storage engine interface for table information so that you can use it as a system table. This would be a bit slow if you requested information about all tables, but very flexible. SHOW INFO FROM tbl_name for basic table information should be implemented.
  • Allow SELECT a FROM tbl_name1 LEFT JOIN tbl_name2 USING (a); in this case a is assumed to come from tbl_name1.
  • DELETE and REPLACE options to the UPDATE statement (this will delete rows when a duplicate-key error occurs while updating).
  • Change the format of DATETIME to store fractions of seconds.
  • Make it possible to use the new GNU regexp library instead of the current one (the new library should be much faster than the current one).
Standards compliance, portability and migration
  • Add ANY(), EVERY(), and SOME() group functions. In standard SQL, these work only on boolean columns, but we can extend these to work on any columns or expressions by treating a value of zero as FALSE and non-zero values as TRUE.
  • Fix the type of MAX(column) to be the same as the column type: 
    mysql> CREATE TABLE t1 (a DATE);
mysql> INSERT INTO t1 VALUES (NOW());
mysql> CREATE TABLE t2 SELECT MAX(a) FROM t1;
mysql> SHOW COLUMNS FROM t2;
Speed enhancements
  • Don't allow more than a defined number of threads to run MyISAM recovery at the same time.
  • Change INSERT INTO ... SELECT to optionally use concurrent inserts.
  • Add an option to periodically flush key pages for tables with delayed keys if they haven't been used in a while.
  • Allow join on key parts (optimization issue).
  • Add a log file analyzer that can extract information about which tables are hit most often, how often multiple-table joins are executed, and so on. This should help users identify areas of table design that could be optimized to execute much more efficient queries.
Usability enhancements
  • Return the original column types when doing SELECT MIN(column) ... GROUP BY.
  • Make it possible to specify long_query_time with a granularity in microseconds.
  • Link the @command{myisampack} code into the server so that it can perform PACK or COMPRESS operations.
  • Add a temporary key buffer cache during INSERT/DELETE/UPDATE so that we can gracefully recover if the index file gets full.
  • If you perform an ALTER TABLE on a table that is symlinked to another disk, create temporary tables on that disk.
  • Implement a DATE/DATETIME type that handles time zone information properly, to make dealing with dates in different time zones easier.
  • Fix @command{configure} so that all libraries (like MyISAM) can be compiled without threads.
  • Allow user variables as LIMIT arguments; for example, LIMIT @a,@b.
  • Automatic output from @command{mysql} to a Web browser.
  • LOCK DATABASES (with various options).
  • Many more variables for SHOW STATUS. Record reads and updates. Selects on a single table and selects with joins. Mean number of tables in selects. Number of ORDER BY and GROUP BY queries.
  • @command{mysqladmin copy database new-database}; this requires a COPY operation to be added to @command{mysqld}.
  • Processlist output should indicate the number of queries/threads.
  • SHOW HOSTS for printing information about the hostname cache.
  • Change table names from empty strings to NULL for calculated columns.
  • Don't use Item_copy_string on numerical values to avoid number-to-string-to-number conversion in case of SELECT COUNT(*)*(id+0) FROM tbl_name GROUP BY id.
  • Change so that ALTER TABLE doesn't abort clients that execute INSERT DELAYED.
  • Fix so that when columns are referenced in an UPDATE clause, they contain the old values from before the update started.
New operating systems
  • Port the MySQL clients to LynxOS.

1.6.4 New Features Planned for the Mid-Term Future

1.6.5 New Features We Don't Plan to Implement

We aim toward full compliance with ANSI/ISO SQL. There are no features we plan not to implement.

1.7 MySQL Information Sources

1.7.1 MySQL Mailing Lists

This section introduces the MySQL mailing lists and provides guidelines as to how the lists should be used. When you subscribe to a mailing list, you will receive all postings to the list as email messages. You can also send your own questions and answers to the list.

1.7.1.1 The MySQL Mailing Lists

To subscribe to or unsubscribe from any of the mailing lists described in this section, visit http://lists.mysql.com/. For most of them, you can select the regular version of the list where you get individual messages, or a digest version where you get one large message per day.

Please do not send messages about subscribing or unsubscribing to any of the mailing lists, because such messages are distributed automatically to thousands of other users.

Your local site may have many subscribers to a MySQL mailing list. If so, the site may have a local mailing list, so that messages sent from lists.mysql.com to your site are propagated to the local list. In such cases, please contact your system administrator to be added to or dropped from the local MySQL list.

If you wish to have traffic for a mailing list go to a separate mailbox in your mail program, set up a filter based on the message headers. You can use either the List-ID: or Delivered-To: headers to identify list messages.

The MySQL mailing lists are as follows:

announce
This list is for announcements of new versions of MySQL and related programs. This is a low-volume list to which all MySQL users should subscribe.
mysql
This is the main list for general MySQL discussion. Please note that some topics are better discussed on the more-specialized lists. If you post to the wrong list, you may not get an answer.
bugs
This list will be of interest to you if you want to stay informed about issues reported since the last release of MySQL or if you want to be actively involved in the process of bug hunting and fixing. See section 1.7.1.3 How to Report Bugs or Problems.
internals
This list is for people who work on the MySQL code. This is also the forum for discussions on MySQL development and for posting patches.
mysqldoc
This list is for people who work on the MySQL documentation: people from MySQL AB, translators, and other community members.
benchmarks
This list is for anyone interested in performance issues. Discussions concentrate on database performance (not limited to MySQL), but also include broader categories such as performance of the kernel, filesystem, disk system, and so on.
packagers
This list is for discussions on packaging and distributing MySQL. This is the forum used by distribution maintainers to exchange ideas on packaging MySQL and on ensuring that MySQL looks and feels as similar as possible on all supported platforms and operating systems.
java
This list is for discussions about the MySQL server and Java. It is mostly used to discuss JDBC drivers, including MySQL Connector/J.
win32
This list is for all topics concerning the MySQL software on Microsoft operating systems, such as Windows 9x, Me, NT, 2000, and XP.
myodbc
This list is for all topics concerning connecting to the MySQL server with ODBC.
gui-tools
This list is for all topics concerning MySQL GUI tools, including MySQL Administrator and the MySQL Control Center graphical client.
cluster
This list is for discussion of MySQL Cluster.
dotnet
This list is for discussion of the MySQL server and the .NET platform. Mostly related to the MySQL Connector/Net provider.
plusplus
This list is for all topics concerning programming with the C++ API for MySQL.
perl
This list is for all topics concerning the Perl support for MySQL with DBD::mysql.

If you're unable to get an answer to your questions from a MySQL mailing list, one option is to purchase support from MySQL AB. This will put you in direct contact with MySQL developers. See section 1.4.1 Support Offered by MySQL AB.

The following table shows some MySQL mailing lists in languages other than English. These lists are not operated by MySQL AB.

[email protected]
A French mailing list.
[email protected]
A Korean mailing list. Email subscribe mysql [email protected] to this list.
[email protected]
A German mailing list. Email subscribe mysql-de [email protected] to this list. You can find information about this mailing list at http://www.4t2.com/mysql/.
[email protected]
A Portuguese mailing list. Email subscribe mysql-br [email protected] to this list.
[email protected]
A Spanish mailing list. Email subscribe mysql [email protected] to this list.

1.7.1.2 Asking Questions or Reporting Bugs

Before posting a bug report or question, please do the following:

If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, please follow the guidelines on sending mail to a MySQL mailing list, outlined in the next section, before contacting us.

1.7.1.3 How to Report Bugs or Problems

The normal place to report bugs is http://bugs.mysql.com/, which is the address for our bugs database. This database is public, and can be browsed and searched by anyone. If you log in to the system, you will also be able to enter new reports.

Writing a good bug report takes patience, but doing it right the first time saves time both for us and for yourself. A good bug report, containing a full test case for the bug, makes it very likely that we will fix the bug in the next release. This section will help you write your report correctly so that you don't waste your time doing things that may not help us much or at all.

We encourage everyone to use the @command{mysqlbug} script to generate a bug report (or a report about any problem). @command{mysqlbug} can be found in the `scripts' directory (source distribution) and in the `bin' directory under your MySQL installation directory (binary distribution). If you are unable to use @command{mysqlbug} (for example, if you are running on Windows), it is still vital that you include all the necessary information noted in this section (most importantly, a description of the operating system and the MySQL version).

The @command{mysqlbug} script helps you generate a report by determining much of the following information automatically, but if something important is missing, please include it with your message. Please read this section carefully and make sure that all the information described here is included in your report.

Preferably, you should test the problem using the latest production or development version of MySQL Server before posting. Anyone should be able to repeat the bug by just using mysql test < script_file on the included test case or by running the shell or Perl script that is included in the bug report.

All bugs posted in the bugs database at http://bugs.mysql.com/ will be corrected or documented in the next MySQL release. If only minor code changes are needed to correct a problem, we may also post a patch that fixes the problem.

If you have found a sensitive security bug in MySQL, you can send email to [email protected].

If you have a repeatable bug report, please report it to the bugs database at http://bugs.mysql.com/. Note that even in this case it's good to run the @command{mysqlbug} script first to find information about your system. Any bug that we are able to repeat has a high chance of being fixed in the next MySQL release.

To report other problems, you can use one of the MySQL mailing lists.

Remember that it is possible for us to respond to a message containing too much information, but not to one containing too little. People often omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is this: If you are in doubt about stating something, state it. It is faster and less troublesome to write a couple more lines in your report than to wait longer for the answer if we must ask you to provide information that was missing from the initial report.

The most common errors made in bug reports are (a) not including the version number of the MySQL distribution used, and (b) not fully describing the platform on which the MySQL server is installed (including the platform type and version number). This is highly relevant information, and in 99 cases out of 100, the bug report is useless without it. Very often we get questions like, ``Why doesn't this work for me?'' Then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has already been fixed in newer MySQL versions. Sometimes the error is platform-dependent; in such cases, it is next to impossible for us to fix anything without knowing the operating system and the version number of the platform.

If you compiled MySQL from source, remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether your problem depends on your compiler, we need to know what compiler you use. Note that every compiling problem should be regarded as a bug and reported accordingly.

It is most helpful when a good description of the problem is included in the bug report. That is, give a good example of everything you did that led to the problem and describe, in exact detail, the problem itself. The best reports are those that include a full example showing how to reproduce the bug or problem. See section D.1.6 Making a Test Case If You Experience Table Corruption.

If a program produces an error message, it is very important to include the message in your report. If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the lettercase should be observed.) You should never try to reproduce from memory what the error message was; instead, copy and paste the entire message into your report.

If you have a problem with Connector/ODBC (MyODBC), please try to generate a trace file and send it with your report. See section 22.1.1.9 How to Report MyODBC Problems or Bugs.

Please remember that many of the people who will read your report will do so using an 80-column display. When generating reports or examples using the @command{mysql} command-line tool, you should therefore use the --vertical option (or the \G statement terminator) for output that would exceed the available width for such a display (for example, with the EXPLAIN SELECT statement; see the example later in this section).

Please include the following information in your report:

If you are a support customer, please cross-post the bug report to [email protected] for higher-priority treatment, as well as to the appropriate mailing list to see whether someone else has experienced (and perhaps solved) the problem.

For information on reporting bugs in MyODBC, see section 22.1.1.9 How to Report MyODBC Problems or Bugs.

For solutions to some common problems, see section A Problems and Common Errors.

When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarize the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem.

1.7.1.4 Guidelines for Answering Questions on the Mailing List

If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.

Try to summarize the essential part of the question in your reply; don't feel obliged to quote the entire original message.

Please don't post mail messages from your browser with HTML mode turned on. Many users don't read mail with a browser.

1.7.2 MySQL Community Support on IRC (Internet Relay Chat)

In addition to the various MySQL mailing lists, you can find experienced community people on IRC (Internet Relay Chat). These are the best networks/channels currently known to us:

If you are looking for IRC client software to connect to an IRC network, take a look at X-Chat (http://www.xchat.org/). X-Chat (GPL licensed) is available for Unix as well as for Windows platforms.

1.8 MySQL Standards Compliance

This section describes how MySQL relates to the ANSI/ISO SQL standards. MySQL Server has many extensions to the SQL standard, and here you will find out what they are and how to use them. You will also find information about functionality missing from MySQL Server, and how to work around some differences.

The SQL standard has been evolving since 1986 and several versions exist. In this manual, ``SQL-92'' refers to the standard released in 1992, ``SQL:1999'' refers to the standard released in 1999, and ``SQL:2003'' refers to the current version of the standard. We use the phrase ``the SQL standard'' to mean the current version of the SQL Standard at any time.

Our goal is to not restrict MySQL Server usability for any usage without a very good reason for doing so. Even if we don't have the resources to perform development for every possible use, we are always willing to help and offer suggestions to people who are trying to use MySQL Server in new territories.

One of our main goals with the product is to continue to work toward compliance with the SQL standard, but without sacrificing speed or reliability. We are not afraid to add extensions to SQL or support for non-SQL features if this greatly increases the usability of MySQL Server for a large segment of our user base. The HANDLER interface in MySQL Server 4.0 is an example of this strategy. See section 14.1.3 HANDLER Syntax.

We will continue to support transactional and non-transactional databases to satisfy both mission-critical 24/7 usage and heavy Web or logging usage.

MySQL Server was originally designed to work with medium size databases (10-100 million rows, or about 100MB per table) on small computer systems. Today MySQL Server handles terabyte-size databases, but the code can also be compiled in a reduced version suitable for hand-held and embedded devices. The compact design of the MySQL server makes development in both directions possible without any conflicts in the source tree.

Currently, we are not targeting realtime support, although MySQL replication capabilities already offer significant functionality.

Database cluster support now exists through third-party clustering solutions as well as the integration of our acquired NDB Cluster technology into a new storage engine, available from version 4.1.2. See section 17 MySQL Cluster.

We are also looking at providing XML support in the database server.

1.8.1 What Standards MySQL Follows

We are aiming toward supporting the full ANSI/ISO SQL standard, but without making concessions to speed and quality of the code.

ODBC levels 0-3.51.

1.8.2 Selecting SQL Modes

The MySQL server can operate in different SQL modes, and can apply these modes differentially for different clients. This allows an application to tailor server operation to its own requirements.

Modes define what SQL syntax MySQL should support and what kind of validation checks it should perform on the data. This makes it easier to use MySQL in a lot of different environments and to use MySQL together with other database servers.

You can set the default SQL mode by starting @command{mysqld} with the --sql-mode="modes" option. Beginning with MySQL 4.1, you can also change the mode after startup time by setting the sql_mode variable with a SET [SESSION|GLOBAL] sql_mode='modes' statement.

For more information on setting the server mode, see section 5.2.2 The Server SQL Mode.

1.8.3 Running MySQL in ANSI Mode

You can tell @command{mysqld} to use the ANSI mode with the --ansi startup option. See section 5.2.1 @command{mysqld} Command-Line Options.

Running the server in ANSI mode is the same as starting it with these options (specify the --sql_mode value on a single line): 

--transaction-isolation=SERIALIZABLE
--sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
IGNORE_SPACE,ONLY_FULL_GROUP_BY

In MySQL 4.1, you can achieve the same effect with these two statements (specify the sql_mode value on a single line): 

SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET GLOBAL sql_mode = 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
IGNORE_SPACE,ONLY_FULL_GROUP_BY';

See section 1.8.2 Selecting SQL Modes.

In MySQL 4.1.1, the sql_mode options shown can be also be set with this statement: 

SET GLOBAL sql_mode='ansi';

In this case, the value of the sql_mode variable will be set to all options that are relevant for ANSI mode. You can check the result like this: 

mysql> SET GLOBAL sql_mode='ansi';
mysql> SELECT @@global.sql_mode;
        -> 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
            IGNORE_SPACE,ONLY_FULL_GROUP_BY,ANSI';

1.8.4 MySQL Extensions to Standard SQL

MySQL Server includes some extensions that you probably will not find in other SQL databases. Be warned that if you use them, your code will not be portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the form /*! ... */. In this case, MySQL Server will parse and execute the code within the comment as it would any other MySQL statement, but other SQL servers will ignore the extensions. For example: 

SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...

If you add a version number after the `!' character, the syntax within the comment will be executed only if the MySQL version is equal to or newer than the specified version number: 

CREATE /*!32302 TEMPORARY */ TABLE t (a INT);

This means that if you have Version 3.23.02 or newer, MySQL Server will use the TEMPORARY keyword.

The following descriptions list MySQL extensions, organized by category.

Organization of data on disk
MySQL Server maps each database to a directory under the MySQL data directory, and tables within a database to filenames in the database directory. This has a few implications:
  • Database names and table names are case sensitive in MySQL Server on operating systems that have case-sensitive filenames (such as most Unix systems). See section 10.2.2 Identifier Case Sensitivity.
  • You can use standard system commands to back up, rename, move, delete, and copy tables that are managed by the MyISAM or ISAM storage engines. For example, to rename a MyISAM table, rename the `.MYD', `.MYI', and `.frm' files to which the table corresponds.
Database, table, index, column, or alias names may begin with a digit (but may not consist solely of digits).
General language syntax
  • Strings may be enclosed by either `"' or `'', not just by `''.
  • Use of `\' as an escape character in strings.
  • In SQL statements, you can access tables from different databases with the db_name.tbl_name syntax. Some SQL servers provide the same functionality but call this User space. MySQL Server doesn't support tablespaces such as used in statements like this: CREATE TABLE ralph.my_table...IN my_tablespace.
SQL statement syntax
  • The ANALYZE TABLE, CHECK TABLE, OPTIMIZE TABLE, and REPAIR TABLE statements.
  • The CREATE DATABASE and DROP DATABASE statements. See section 14.2.4 CREATE DATABASE Syntax.
  • The DO statement.
  • EXPLAIN SELECT to get a description of how tables are joined.
  • The FLUSH and RESET statements.
  • The SET statement. See section 14.5.3.1 SET Syntax.
  • The SHOW statement. See section 14.5.3 SET and SHOW Syntax.
  • Use of LOAD DATA INFILE. In many cases, this syntax is compatible with Oracle's LOAD DATA INFILE. See section 14.1.5 LOAD DATA INFILE Syntax.
  • Use of RENAME TABLE. See section 14.2.12 RENAME TABLE Syntax.
  • Use of REPLACE instead of DELETE + INSERT. See section 14.1.6 REPLACE Syntax.
  • Use of CHANGE col_name, DROP col_name, or DROP INDEX, IGNORE or RENAME in an ALTER TABLE statement. Use of multiple ADD, ALTER, DROP, or CHANGE clauses in an ALTER TABLE statement. See section 14.2.2 ALTER TABLE Syntax.
  • Use of index names, indexes on a prefix of a field, and use of INDEX or KEY in a CREATE TABLE statement. See section 14.2.6 CREATE TABLE Syntax.
  • Use of TEMPORARY or IF NOT EXISTS with CREATE TABLE.
  • Use of IF EXISTS with DROP TABLE.
  • You can drop multiple tables with a single DROP TABLE statement.
  • The ORDER BY and LIMIT clauses of the UPDATE and DELETE statements.
  • INSERT INTO ... SET col_name = ... syntax.
  • The DELAYED clause of the INSERT and REPLACE statements.
  • The LOW_PRIORITY clause of the INSERT, REPLACE, DELETE, and UPDATE statements.
  • Use of INTO OUTFILE and STRAIGHT_JOIN in a SELECT statement. See section 14.1.7 SELECT Syntax.
  • The SQL_SMALL_RESULT option in a SELECT statement.
  • You don't need to name all selected columns in the GROUP BY part. This gives better performance for some very specific, but quite normal queries. See section 13.9 Functions and Modifiers for Use with GROUP BY Clauses.
  • You can specify ASC and DESC with GROUP BY.
  • The ability to set variables in a statement with the := assignment operator: 
    mysql> SELECT @a:=SUM(total),@b=COUNT(*),@a/@b AS avg
    -> FROM test_table;
mysql> SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
Column types
  • The column types MEDIUMINT, SET, ENUM, and the different BLOB and TEXT types.
  • The column attributes AUTO_INCREMENT, BINARY, NULL, UNSIGNED, and ZEROFILL.
Functions and operators
  • To make it easier for users who come from other SQL environments, MySQL Server supports aliases for many functions. For example, all string functions support both standard SQL syntax and ODBC syntax.
  • MySQL Server understands the || and && operators to mean logical OR and AND, as in the C programming language. In MySQL Server, || and OR are synonyms, as are && and AND. Because of this nice syntax, MySQL Server doesn't support the standard SQL || operator for string concatenation; use CONCAT() instead. Because CONCAT() takes any number of arguments, it's easy to convert use of the || operator to MySQL Server.
  • Use of COUNT(DISTINCT list) where list has more than one element.
  • All string comparisons are case-insensitive by default, with sort ordering determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, you should declare your columns with the BINARY attribute or use the BINARY cast, which causes comparisons to be done using the underlying character code values rather then a lexical ordering.
  • The % operator is a synonym for MOD(). That is, N % M is equivalent to MOD(N,M). % is supported for C programmers and for compatibility with PostgreSQL.
  • The =, <>, <= ,<, >=,>, <<, >>, <=>, AND, OR, or LIKE operators may be used in column comparisons to the left of the FROM in SELECT statements. For example: 
    mysql> SELECT col1=1 AND col2=2 FROM tbl_name;
  • The LAST_INSERT_ID() function that returns the most recent AUTO_INCREMENT value. See section 13.8.3 Information Functions.
  • LIKE is allowed on numeric columns.
  • The REGEXP and NOT REGEXP extended regular expression operators.
  • CONCAT() or CHAR() with one argument or more than two arguments. (In MySQL Server, these functions can take any number of arguments.)
  • The BIT_COUNT(), CASE, ELT(), FROM_DAYS(), FORMAT(), IF(), PASSWORD(), ENCRYPT(), MD5(), ENCODE(), DECODE(), PERIOD_ADD(), PERIOD_DIFF(), TO_DAYS(), and WEEKDAY() functions.
  • Use of TRIM() to trim substrings. Standard SQL supports removal of single characters only.
  • The GROUP BY functions STD(), BIT_OR(), BIT_AND(), BIT_XOR(), and GROUP_CONCAT(). See section 13.9 Functions and Modifiers for Use with GROUP BY Clauses.

For a prioritized list indicating when new extensions will be added to MySQL Server, you should consult the online MySQL TODO list at http://dev.mysql.com/doc/mysql/en/TODO.html. That is the latest version of the TODO list in this manual. See section 1.6 MySQL and the Future (the TODO).

1.8.5 MySQL Differences from Standard SQL

We try to make MySQL Server follow the ANSI SQL standard and the ODBC SQL standard, but MySQL Server performs operations differently in some cases:

1.8.5.1 Subqueries

MySQL 4.1 supports subqueries and derived tables. A ``subquery'' is a SELECT statement nested within another statement. A ``derived table'' (an unnamed view) is a subquery in the FROM clause of another statement. See section 14.1.8 Subquery Syntax.

For MySQL versions older than 4.1, most subqueries can be rewritten using joins or other methods. See section 14.1.8.11 Rewriting Subqueries as Joins for Earlier MySQL Versions for examples that show how to do this.

1.8.5.2 SELECT INTO TABLE

MySQL Server doesn't support the Sybase SQL extension: SELECT ... INTO TABLE .... Instead, MySQL Server supports the standard SQL syntax INSERT INTO ... SELECT ..., which is basically the same thing. See section 14.1.4.1 INSERT ... SELECT Syntax. 

INSERT INTO tbl_temp2 (fld_id)
    SELECT tbl_temp1.fld_order_id
    FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;

Alternatively, you can use SELECT INTO OUTFILE ... or CREATE TABLE ... SELECT.

From version 5.0, MySQL supports SELECT ... INTO with user variables. The same syntax may also be used inside stored procedures using cursors and local variables. See section 20.1.6.3 SELECT ... INTO Statement.

1.8.5.3 Transactions and Atomic Operations

MySQL Server (version 3.23-max and all versions 4.0 and above) supports transactions with the InnoDB and BDB transactional storage engines. InnoDB provides full ACID compliance. See section 15 MySQL Storage Engines and Table Types.

The other non-transactional storage engines in MySQL Server (such as MyISAM) follow a different paradigm for data integrity called ``atomic operations.'' In transactional terms, MyISAM tables effectively always operate in AUTOCOMMIT=1 mode. Atomic operations often offer comparable integrity with higher performance.

With MySQL Server supporting both paradigms, you can decide whether your applications are best served by the speed of atomic operations or the use of transactional features. This choice can be made on a per-table basis.

As noted, the trade-off for transactional versus non-transactional table types lies mostly in performance. Transactional tables have significantly higher memory and diskspace requirements, and more CPU overhead. On the other hand, transactional table types such as InnoDB also offer many significant features. MySQL Server's modular design allows the concurrent use of different storage engines to suit different requirements and deliver optimum performance in all situations.

But how do you use the features of MySQL Server to maintain rigorous integrity even with the non-transactional MyISAM tables, and how do these features compare with the transactional table types?

  1. If your applications are written in a way that is dependent on being able to call ROLLBACK rather than COMMIT in critical situations, transactions are more convenient. Transactions also ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved. If you use non-transactional tables, MySQL Server in almost all cases allows you to resolve potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such an inconsistency occurs. Note that just by using the MySQL log or even adding one extra log, you can normally fix tables perfectly with no data integrity loss.
  2. More often than not, critical transactional updates can be rewritten to be atomic. Generally speaking, all integrity problems that transactions solve can be done with LOCK TABLES or atomic updates, ensuring that you never will get an automatic abort from the server, which is a common problem with transactional database systems.
  3. Even a transactional system can lose data if the server goes down. The difference between different systems lies in just how small the time-lag is where they could lose data. No system is 100% secure, only ``secure enough.'' Even Oracle, reputed to be the safest of transactional database systems, is reported to sometimes lose data in such situations. To be safe with MySQL Server, whether or not using transactional tables, you only need to have backups and have binary logging turned on. With this you can recover from any situation that you could with any other transactional database system. It is always good to have backups, regardless of which database system you use.

The transactional paradigm has its benefits and its drawbacks. Many users and application developers depend on the ease with which they can code around problems where an abort appears to be, or is necessary. However, even if you are new to the atomic operations paradigm, or more familiar with transactions, do consider the speed benefit that non-transactional tables can offer on the order of three to five times the speed of the fastest and most optimally tuned transactional tables.

In situations where integrity is of highest importance, MySQL Server offers transaction-level reliability and integrity even for non-transactional tables. If you lock tables with LOCK TABLES, all updates will stall until any integrity checks are made. If you obtain a READ LOCAL lock (as opposed to a write lock) for a table that allows concurrent inserts at the end of the table, reads are allowed, as are inserts by other clients. The new inserted records will not be seen by the client that has the read lock until it releases the lock. With INSERT DELAYED, you can queue inserts into a local queue, until the locks are released, without having the client wait for the insert to complete. See section 14.1.4.2 INSERT DELAYED Syntax.

``Atomic,'' in the sense that we mean it, is nothing magical. It only means that you can be sure that while each specific update is running, no other user can interfere with it, and there will never be an automatic rollback (which can happen with transactional tables if you are not very careful). MySQL Server also guarantees that there will not be any dirty reads.

Following are some techniques for working with non-transactional tables:

1.8.5.4 Stored Procedures and Triggers

Stored procedures are implemented in MySQL version 5.0. See section 20 Stored Procedures and Functions.

Triggers are currently being implemented, with basic functionality in MySQL version 5.0, with further development planned for version 5.1. A ``trigger'' is effectively a type of stored procedure, one that is invoked when a particular event occurs. For example, you could set up a stored procedure that is triggered each time a record is deleted from a transactional table, and that stored procedure automatically deletes the corresponding customer from a customer table when all their transactions are deleted.

1.8.5.5 Foreign Keys

In MySQL Server 3.23.44 and up, the InnoDB storage engine supports checking of foreign key constraints, including CASCADE, ON DELETE, and ON UPDATE. See section 16.7.4 FOREIGN KEY Constraints.

For storage engines other than InnoDB, MySQL Server parses the FOREIGN KEY syntax in CREATE TABLE statements, but does not use or store it. In the future, the implementation will be extended to store this information in the table specification file so that it may be retrieved by @command{mysqldump} and ODBC. At a later stage, foreign key constraints will be implemented for MyISAM tables as well.

Foreign key enforcement offers several benefits to database developers:

Do keep in mind that these benefits come at the cost of additional overhead for the database server to perform the necessary checks. Additional checking by the server affects performance, which for some applications may be sufficiently undesirable as to be avoided if possible. (Some major commercial applications have coded the foreign-key logic at the application level for this reason.)

MySQL gives database developers the choice of which approach to use. If you don't need foreign keys and want to avoid the overhead associated with enforcing referential integrity, you can choose another table type instead, such as MyISAM. (For example, the MyISAM storage engine offers very fast performance for applications that perform only INSERT and SELECT operations, because the inserts can be performed concurrently with retrievals. See section 7.3.2 Table Locking Issues.)

If you choose not to take advantage of referential integrity checks, keep the following considerations in mind:

Be aware that the use of foreign keys can in some instances lead to problems:

Note that foreign keys in SQL are used to check and enforce referential integrity, not to join tables. If you want to get results from multiple tables from a SELECT statement, you do this by performing a join between them: 

SELECT * FROM t1, t2 WHERE t1.id = t2.id;

See section 14.1.7.1 JOIN Syntax. See section 3.6.6 Using Foreign Keys.

The FOREIGN KEY syntax without ON DELETE ... is often used by ODBC applications to produce automatic WHERE clauses.

1.8.5.6 Views

Views (updatable) are being implemented in the 5.0 version of MySQL Server. They are already available in binary releases from 5.0.1 and up. See section 14.2.7 CREATE VIEW Syntax.

Views are useful for allowing users to access a set of relations (tables) as if it were a single table, and limiting their access to just that. Views can also be used to restrict access to rows (a subset of a particular table). For access control to columns, you can also use the sophisticated privilege system in MySQL Server. See section 5.5 The MySQL Access Privilege System.

In designing an implementation of views, our ambitious goal, as much as is possible within the confines of SQL, has been full compliance with ``Codd's Rule #6'' for relational database systems: ``All views that are theoretically updatable, should in practice also be updatable.''

1.8.5.7 `--' as the Start of a Comment

Some other SQL databases use `--' to start comments. MySQL Server uses `#' as the start comment character. You can also use the C comment style /* this is a comment */ with MySQL Server. See section 10.5 Comment Syntax.

MySQL Server 3.23.3 and above support the `--' comment style, provided the comment is followed by a space (or by a control character such as a newline). The requirement for a space is to prevent problems with automatically generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for !payment!: 

UPDATE account SET credit=credit-!payment!

Think about what happens if the value of payment is a negative value such as -1: 

UPDATE account SET credit=credit--1

credit--1 is a legal expression in SQL, but if -- is interpreted as the start of a comment, part of the expression is discarded. The result is a statement that has a completely different meaning than intended: 

UPDATE account SET credit=credit

The statement produces no change in value at all! This illustrates that allowing comments to start with `--' can have serious consequences.

Using our implementation of this method of commenting in MySQL Server 3.23.3 and up, credit--1 is actually safe.

Another safe feature is that the @command{mysql} command-line client removes all lines that start with `--'.

The following information is relevant only if you are running a MySQL version earlier than 3.23.3:

If you have an SQL program in a text file that contains `--' comments, you should use the @command{replace} utility as follows to convert the comments to use `#' characters: 

shell> replace " --" " #" < text-file-with-funny-comments.sql \
         | mysql db_name

instead of the usual: 

shell> mysql db_name < text-file-with-funny-comments.sql

You can also edit the command file ``in place'' to change the `--' comments to `#' comments: 

shell> replace " --" " #" -- text-file-with-funny-comments.sql

Change them back with this command: 

shell> replace " #" " --" -- text-file-with-funny-comments.sql

1.8.6 How MySQL Deals with Constraints

MySQL allows you to work with both transactional tables that allow rollback and non-transactional tables that do not, so constraint handling is a bit different in MySQL than in other databases.

We have to handle the case when you have updated a lot of rows in a non-transactional table that cannot roll back when an error occurs.

The basic philosophy is to try to give an error for anything that we can detect at compile time but try to recover from any errors we get at runtime. We do this in most cases, but not yet for all. See section 1.6.3 New Features Planned for the Near Future.

The options MySQL has when an error occurs are to stop the statement in the middle or to recover as well as possible from the problem and continue.

The following sections describe what happens for the different types of constraints.

1.8.6.1 Constraint PRIMARY KEY / UNIQUE

Normally you will get an error when you try to INSERT or UPDATE a row that causes a primary key, unique key, or foreign key violation. If you are using a transactional storage engine such as InnoDB, MySQL will automatically roll back the transaction. If you are using a non-transactional storage engine, MySQL will stop at the incorrect row and leave any remaining rows unprocessed.

To make life easier, MySQL supports an IGNORE keyword for most commands that can cause a key violation (such as INSERT IGNORE and UPDATE IGNORE). In this case, MySQL will ignore any key violation and continue with processing the next row. You can get information about what MySQL did with the mysql_info() C API function. See section 21.2.3.126 mysql_info(). In MySQL 4.1 and up, you also can use the SHOW WARNINGS statement. See section 14.5.3.21 SHOW WARNINGS Syntax.

Note that, for the moment, only InnoDB tables support foreign keys. See section 16.7.4 FOREIGN KEY Constraints. Foreign key support in MyISAM tables is scheduled for implementation in MySQL 5.1.

1.8.6.2 Constraint NOT NULL and DEFAULT Values

To be able to support easy handling of non-transactional tables, all columns in MySQL have implicit/explicit default values.

The following holds true when you are not using the STRICT_ALL_TABLES or if you are using STRICT_TRANS_TABLES and the first update to a non-transactional table didn't produce an error. See section 5.2.2 The Server SQL Mode. It's also true if you are using INSERT IGNORE or UPDATE IGNORE.

If you insert an ``incorrect'' value into a column, such as a NULL into a NOT NULL column or a too-large numerical value into a numerical column, MySQL sets the column to the ``best possible value'' instead of producing an error:

The reason for the preceding rules is that we can't check these conditions until the query has begun executing. We can't just roll back if we encounter a problem after updating a few rows, because the storage engine may not support rollback. The option of terminating the statement is not that good; in this case, the update would be ``half done,'' which is probably the worst possible scenario. In this case, it's better to ``do the best you can'' and then continue as if nothing happened.

In other words, if you are using STRICT_ALL_TABLES, you risk getting half updates. A safer setting is to use STRICT_TRANS_TABLES. In this case, a wrong value causes MySQL to roll back, if it can, all updates done so far (that is, if we have updated only transactional tables).

If you are using non-transactional tables, you should not use MySQL to check column content. In general, the safest (and often fastest) way is to let the application ensure that it passes only legal values to the database.

1.8.6.3 Constraint ENUM and SET

In MySQL 4.x and earlier, ENUM is not a real constraint, but is a more efficient way to define columns that can contain only a given set of values. This is for the same reasons that NOT NULL is not honored. See section 1.8.6.2 Constraint NOT NULL and DEFAULT Values.

ENUM columns always have a default value. If you don't specify a default value, then it will be NULL for columns that can have NULL, otherwise the first enumeration value is used as the default value.

If you insert an incorrect value into an ENUM column or if you force a value into an ENUM column with IGNORE, it is set to the reserved enumeration value of 0, which is displayed as an empty string in string context. See section 12.4.3 The ENUM Type.

If you insert an incorrect value into a SET column, the incorrect value is ignored. For example, if the column can contain the values 'a', 'b', and 'c', an attempt to assign 'a,x,b,y' results in a value of 'a,b'. See section 12.4.4 The SET Type.

As of MySQL 5.0.2, strict SQL mode is implemented. See section 5.2.2 The Server SQL Mode. If strict mode is not enabled, values entered into ENUM and SET columns are handled as just described for MySQL 4.x. If strict mode is enabled, the definition of a ENUM or SET column acts as a constraint on values entered into the column. An error occurs for values that do not satisify these conditions:

In strict mode, errors for invalid values can be suppressed if you use INSERT IGNORE or UPDATE IGNORE. In this case, a warning is generated rather than an error. For ENUM, the value is inserted as the error member (0). For SET, the value is inserted as given except that that any invalid substrings are deleted. For example, 'a,x,b,y' results in a value of 'a,b', as described earlier.

1.8.7 Known Errors and Design Deficiencies in MySQL

1.8.7.1 Errors in 3.23 Fixed in a Later MySQL Version

The following known errors or bugs are not fixed in MySQL 3.23 because fixing them would involve changing a lot of code that could introduce other even worse bugs. The bugs are also classified as ``not fatal'' or ``bearable.''

1.8.7.2 Errors in 4.0 Fixed in a Later MySQL Version

The following known errors or bugs are not fixed in MySQL 4.0 because fixing them would involve changing a lot of code that could introduce other even worse bugs. The bugs are also classified as ``not fatal'' or ``bearable.''

1.8.7.3 Open Bugs and Design Deficiencies in MySQL

The following problems are known and fixing them is a high priority:

The following problems are known and will be fixed in due time:

The following are known bugs in earlier versions of MySQL:

For information about platform-specific bugs, see the installation and porting instructions in section 2.6 Operating System-Specific Notes and section D Porting to Other Systems.

2 Installing MySQL

This chapter describes how to obtain and install MySQL:

  1. Determine whether your platform is supported. Please note that not all supported systems are equally good for running MySQL on them. On some it is much more robust and efficient than others. See section 2.1.1 Operating Systems Supported by MySQL for details.
  2. Choose which distribution to install. Several versions of MySQL are available, and most are available in several distribution formats. You can choose from pre-packaged distributions containing binary (precompiled) programs or source code. When in doubt, use a binary distribution. We also provide public access to our current source tree for those who want to see our most recent developments and help us test new code. To determine which version and type of distribution you should use, see section 2.1.2 Choosing Which MySQL Distribution to Install.
  3. Download the distribution that you want to install. For a list of sites from which you can obtain MySQL, see section 2.1.3 How to Get MySQL. You can verify the integrity of the distribution using the instructions in section 2.1.4 Verifying Package Integrity Using MD5 Checksums or GnuPG.
  4. Install the distribution. To install MySQL from a binary distribution, use the instructions in section 2.2 Standard MySQL Installation Using a Binary Distribution. To install MySQL from a source distribution or from the current development source tree, use the instructions in section 2.3 MySQL Installation Using a Source Distribution. Note: If you plan to upgrade an existing version of MySQL to a newer version rather than installing MySQL for the first time, see section 2.5 Upgrading/Downgrading MySQL for information about upgrade procedures and about issues that you should consider before upgrading. If you encounter installation difficulties, see section 2.6 Operating System-Specific Notes for information on solving problems for particular platforms.
  5. Perform any necessary post-installation setup. After installing MySQL, read section 2.4 Post-Installation Setup and Testing. This section contains important information about making sure the MySQL server is working properly. It also describes how to secure the initial MySQL user accounts, which have no passwords until you assign passwords. The section applies whether you install MySQL using a binary or source distribution.
  6. If you want to run the MySQL benchmark scripts, Perl support for MySQL must be available. See section 2.7 Perl Installation Notes.

2.1 General Installation Issues

Before installing MySQL, you should do the following:

  1. Determine whether or not MySQL runs on your platform.
  2. Choose a distribution to install.
  3. Download the distribution and verify its integrity.

This section contains the information necessary to carry out these steps. After doing so, you can use the instructions in later sections of the chapter to install the distribution that you choose.

2.1.1 Operating Systems Supported by MySQL

This section lists the operating systems on which you can expect to be able to run MySQL.

We use GNU Autoconf, so it is possible to port MySQL to all modern systems that have a C++ compiler and a working implementation of POSIX threads. (Thread support is needed for the server. To compile only the client code, the only requirement is a C++ compiler.) We use and develop the software ourselves primarily on Linux (SuSE and Red Hat), FreeBSD, and Sun Solaris (Versions 8 and 9).

MySQL has been reported to compile successfully on the following combinations of operating system and thread package. Note that for many operating systems, native thread support works only in the latest versions.

Not all platforms are equally well-suited for running MySQL. How well a certain platform is suited for a high-load mission-critical MySQL server is determined by the following factors:

Based on the preceding criteria, the best platforms for running MySQL at this point are x86 with SuSE Linux using a 2.4 kernel, and ReiserFS (or any similar Linux distribution) and SPARC with Solaris (2.7-9). FreeBSD comes third, but we really hope it will join the top club once the thread library is improved. We also hope that at some point we will be able to include into the top category all other platforms on which MySQL currently compiles and runs okay, but not quite with the same level of stability and performance. This will require some effort on our part in cooperation with the developers of the operating system and library components that MySQL depends on. If you are interested in improving one of those components, are in a position to influence its development, and need more detailed instructions on what MySQL needs to run better, send an email message to the MySQL internals mailing list. See section 1.7.1.1 The MySQL Mailing Lists.

Please note that the purpose of the preceding comparison is not to say that one operating system is better or worse than another in general. We are talking only about choosing an OS for the specific purpose of running MySQL. With this in mind, the result of this comparison would be different if we considered more factors. In some cases, the reason one OS is better than the other could simply be that we have been able to put more effort into testing and optimizing for a particular platform. We are just stating our observations to help you decide which platform to use for running MySQL.

2.1.2 Choosing Which MySQL Distribution to Install

When preparing to install MySQL, you should decide which version to use. MySQL development occurs in several release series, and you can pick the one that best fits your needs. After deciding which version to install, you can choose a distribution format. Releases are available in binary or source format.

2.1.2.1 Choosing Which Version of MySQL to Install

The first decision to make is whether you want to use a production (stable) release or a development release. In the MySQL development process, multiple release series co-exist, each at a different stage of maturity:

We don't believe in a complete freeze, as this also leaves out bugfixes and things that ``must be done.'' ``Somewhat frozen'' means that we may add small things that ``almost surely will not affect anything that's already working.'' Naturally, relevant bugfixes from an earlier series propagate to later series.

Normally, if you are beginning to use MySQL for the first time or trying to port it to some system for which there is no binary distribution, we recommend going with the production release series. Currently this is MySQL 4.0. All MySQL releases, even those from development series, are checked with the MySQL benchmarks and an extensive test suite before being issued.

If you are running an old system and want to upgrade, but don't want to take the chance of having a non-seamless upgrade, you should upgrade to the latest version in the same release series you are using (where only the last part of the version number is newer than yours). We have tried to fix only fatal bugs and make small, relatively safe changes to that version.

If you want to use new features not present in the production release series, you can use a version from a development series. Note that development releases are not as stable as production releases.

If you want to use the very latest sources containing all current patches and bugfixes, you can use one of our BitKeeper repositories. These are not ``releases'' as such, but are available as previews of the code on which future releases will be based.

The MySQL naming scheme uses release names that consist of three numbers and a suffix; for example, mysql-4.1.2-alpha. The numbers within the release name are interpreted like this:

For each minor update, the last number in the version string is incremented. When there are major new features or minor incompatibilities with previous versions, the second number in the version string is incremented. When the file format changes, the first number is increased.

Release names also include a suffix to indicates the stability level of the release. Releases within a series progress through a set of suffixes to indicate how the stability level improves. The possible suffixes are:

MySQL uses a naming scheme that is slightly different from most other products. In general, it's relatively safe to use any version that has been out for a couple of weeks without being replaced with a new version within the release series.

All releases of MySQL are run through our standard tests and benchmarks to ensure that they are relatively safe to use. Because the standard tests are extended over time to check for all previously found bugs, the test suite keeps getting better.

All releases have been tested at least with:

An internal test suite
The `mysql-test' directory contains an extensive set of test cases. We run these tests for virtually every server binary. See section 24.1.2 MySQL Test Suite for more information about this test suite.
The MySQL benchmark suite
This suite runs a range of common queries. It is also a test to see whether the latest batch of optimizations actually made the code faster. See section 7.1.4 The MySQL Benchmark Suite.
The crash-me test
This test tries to determine what features the database supports and what its capabilities and limitations are. See section 7.1.4 The MySQL Benchmark Suite.

Another test is that we use the newest MySQL version in our internal production environment, on at least one machine. We have more than 100GB of data to work with.

2.1.2.2 Choosing a Distribution Format

After choosing which version of MySQL to install, you should decide whether to use a binary distribution or a source distribution. In most cases, you should probably use a binary distribution, if one exists for your platform. Binary distributions are available in native format for many platforms, such as RPM files for Linux or DMG package installers for Mac OS X. Distributions also are available as Zip archives or compressed @command{tar} files.

Reasons to choose a binary distribution include the following:

Under some circumstances, you probably will be better off installing MySQL from a source distribution:

2.1.2.3 How and When Updates Are Released

MySQL is evolving quite rapidly here at MySQL AB and we want to share new developments with other MySQL users. We try to make a release when we have very useful features that others seem to have a need for.

We also try to help out users who request features that are easy to implement. We take note of what our licensed users want to have, and we especially take note of what our extended email-supported customers want and try to help them out.

No one has to download a new release. The News section will tell you if the new release has something you really want. See section C MySQL Change History.

We use the following policy when updating MySQL:

2.1.2.4 Release Philosophy--No Known Bugs in Releases

We put a lot of time and effort into making our releases bug-free. To our knowledge, we have not released a single MySQL version with any known ``fatal'' repeatable bugs. (A ``fatal'' bug is something that crashes MySQL under normal usage, produces incorrect answers for normal queries, or has a security problem.)

We have documented all open problems, bugs, and issues that are dependent on design decisions. See section 1.8.7 Known Errors and Design Deficiencies in MySQL.

Our aim is to fix everything that is fixable without risk of making a stable MySQL version less stable. In certain cases, this means we can fix an issue in the development versions, but not in the stable (production) version. Naturally, we document such issues so that users are aware of them.

Here is a description of how our build process works:

2.1.2.5 MySQL Binaries Compiled by MySQL AB

As a service of MySQL AB, we provide a set of binary distributions of MySQL that are compiled on systems at our site or on systems where supporters of MySQL kindly have given us access to their machines.

In addition to the binaries provided in platform-specific package formats, we offer binary distributions for a number of platforms in the form of compressed @command{tar} files (.tar.gz files). See section 2.2 Standard MySQL Installation Using a Binary Distribution.

For Windows distributions, see section 2.2.1 Installing MySQL on Windows.

These distributions are generated using the script Build-tools/Do-compile, which compiles the source code and creates the binary tar.gz archive using @command{scripts/make_binary_distribution}.

These binaries are configured and built with the following compilers and options. This information can also be obtained by looking at the variables COMP_ENV_INFO and CONFIGURE_LINE inside the script @command{bin/mysqlbug} of every binary @command{tar} file distribution.

The following binaries are built on MySQL AB development systems:

Linux 2.4.xx x86 with @command{gcc 2.95.3:}
CFLAGS="-O2 -mcpu=pentiumpro" CXX=gcc CXXFLAGS="-O2 -mcpu=pentiumpro -felide-constructors" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --enable-assembler --disable-shared --with-client-ldflags=-all-static --with-mysqld-ldflags=-all-static
Linux 2.4.x x86 with @command{icc (Intel C++ Compiler 8.0):}
CC=icc CXX=icc CFLAGS="-O3 -unroll2 -ip -mp -no-gcc -restrict" CXXFLAGS="-O3 -unroll2 -ip -mp -no-gcc -restrict" ./configure --prefix=/usr/local/mysql --localstatedir=/usr/local/mysql/data --libexecdir=/usr/local/mysql/bin --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --enable-assembler --disable-shared --with-client-ldflags=-all-static --with-mysqld-ldflags=-all-static --with-embedded-server --with-innodb
Linux 2.4.xx Intel Itanium 2 with @command{ecc (Intel C++ Itanium Compiler 7.0):}
CC=ecc CFLAGS="-O2 -tpp2 -ip -nolib_inline" CXX=ecc CXXFLAGS="-O2 -tpp2 -ip -nolib_inline" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile
Linux 2.4.xx Intel Itanium with @command{ecc (Intel C++ Itanium Compiler 7.0):}
CC=ecc CFLAGS=-tpp1 CXX=ecc CXXFLAGS=-tpp1 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile
Linux 2.4.xx alpha with ccc (Compaq C V6.2-505 / Compaq C++ V6.3-006):
CC=ccc CFLAGS="-fast -arch generic" CXX=cxx CXXFLAGS="-fast -arch generic -noexceptions -nortti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --with-mysqld-ldflags=-non_shared --with-client-ldflags=-non_shared --disable-shared
Linux 2.x.xx ppc with @command{gcc 2.95.4:}
CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --localstatedir=/usr/local/mysql/data --libexecdir=/usr/local/mysql/bin --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared --with-embedded-server --with-innodb
Linux 2.4.xx s390 with @command{gcc 2.95.3:}
CFLAGS="-O2" CXX=gcc CXXFLAGS="-O2 -felide-constructors" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared --with-client-ldflags=-all-static --with-mysqld-ldflags=-all-static
Linux 2.4.xx x86_64 (AMD64) with @command{gcc 3.2.1:}
CXX=gcc ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared
Sun Solaris 8 x86 with @command{gcc 3.2.3:}
CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --localstatedir=/usr/local/mysql/data --libexecdir=/usr/local/mysql/bin --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared --with-innodb
Sun Solaris 8 SPARC with @command{gcc 3.2:}
CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --enable-assembler --with-named-z-libs=no --with-named-curses-libs=-lcurses --disable-shared
Sun Solaris 8 SPARC 64-bit with @command{gcc 3.2:}
CC=gcc CFLAGS="-O3 -m64 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -m64 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --with-named-z-libs=no --with-named-curses-libs=-lcurses --disable-shared
Sun Solaris 9 SPARC with @command{gcc 2.95.3:}
CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --enable-assembler --with-named-curses-libs=-lcurses --disable-shared
Sun Solaris 9 SPARC with cc-5.0 (Sun Forte 5.0):
CC=cc-5.0 CXX=CC ASFLAGS="-xarch=v9" CFLAGS="-Xa -xstrconst -mt -D_FORTEC_ -xarch=v9" CXXFLAGS="-noex -mt -D_FORTEC_ -xarch=v9" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --enable-assembler --with-named-z-libs=no --enable-thread-safe-client --disable-shared
IBM AIX 4.3.2 ppc with @command{gcc 3.2.3:}
CFLAGS="-O2 -mcpu=powerpc -Wa,-many " CXX=gcc CXXFLAGS="-O2 -mcpu=powerpc -Wa,-many -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --with-named-z-libs=no --disable-shared
IBM AIX 4.3.3 ppc with xlC_r (IBM Visual Age C/C++ 6.0):
CC=xlc_r CFLAGS="-ma -O2 -qstrict -qoptimize=2 -qmaxmem=8192" CXX=xlC_r CXXFLAGS ="-ma -O2 -qstrict -qoptimize=2 -qmaxmem=8192" ./configure --prefix=/usr/local/mysql --localstatedir=/usr/local/mysql/data --libexecdir=/usr/local/mysql/bin --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --with-named-z-libs=no --disable-shared --with-innodb
IBM AIX 5.1.0 ppc with @command{gcc 3.3:}
CFLAGS="-O2 -mcpu=powerpc -Wa,-many" CXX=gcc CXXFLAGS="-O2 -mcpu=powerpc -Wa,-many -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --with-named-z-libs=no --disable-shared
IBM AIX 5.2.0 ppc with xlC_r (IBM Visual Age C/C++ 6.0):
CC=xlc_r CFLAGS="-ma -O2 -qstrict -qoptimize=2 -qmaxmem=8192" CXX=xlC_r CXXFLAGS="-ma -O2 -qstrict -qoptimize=2 -qmaxmem=8192" ./configure --prefix=/usr/local/mysql --localstatedir=/usr/local/mysql/data --libexecdir=/usr/local/mysql/bin --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --with-named-z-libs=no --disable-shared --with-embedded-server --with-innodb
HP-UX 10.20 pa-risc1.1 with @command{gcc 3.1:}
CFLAGS="-DHPUX -I/opt/dce/include -O3 -fPIC" CXX=gcc CXXFLAGS="-DHPUX -I/opt/dce /include -felide-constructors -fno-exceptions -fno-rtti -O3 -fPIC" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --with-pthread --with-named-thread-libs=-ldce --with-lib-ccflags=-fPIC --disable-shared
HP-UX 11.00 pa-risc with aCC (HP ANSI C++ B3910B A.03.50):
CC=cc CXX=aCC CFLAGS=+DAportable CXXFLAGS=+DAportable ./configure --prefix=/usr/local/mysql --localstatedir=/usr/local/mysql/data --libexecdir=/usr/local/mysql/bin --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared --with-embedded-server --with-innodb
HP-UX 11.11 pa-risc2.0 64bit with aCC (HP ANSI C++ B3910B A.03.33):
CC=cc CXX=aCC CFLAGS=+DD64 CXXFLAGS=+DD64 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared
HP-UX 11.11 pa-risc2.0 32bit with aCC (HP ANSI C++ B3910B A.03.33):
CC=cc CXX=aCC CFLAGS="+DAportable" CXXFLAGS="+DAportable" ./configure --prefix=/usr/local/mysql --localstatedir=/usr/local/mysql/data --libexecdir=/usr/local/mysql/bin --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared --with-innodb
HP-UX 11.22 ia64 64bit with aCC (HP aC++/ANSI C B3910B A.05.50):
CC=cc CXX=aCC CFLAGS="+DD64 +DSitanium2" CXXFLAGS="+DD64 +DSitanium2" ./configure --prefix=/usr/local/mysql --localstatedir=/usr/local/mysql/data --libexecdir=/usr/local/mysql/bin --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared --with-embedded-server --with-innodb
Apple Mac OS X 10.2 powerpc with @command{gcc 3.1:}
CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared
FreeBSD 4.7 i386 with @command{gcc 2.95.4:}
CFLAGS=-DHAVE_BROKEN_REALPATH ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --enable-assembler --with-named-z-libs=not-used --disable-shared
FreeBSD 4.7 i386 using LinuxThreads with @command{gcc 2.95.4:}
CFLAGS="-DHAVE_BROKEN_REALPATH -D__USE_UNIX98 -D_REENTRANT -D_THREAD_SAFE -I/usr/local/include/pthread/linuxthreads" CXXFLAGS="-DHAVE_BROKEN_REALPATH -D__USE_UNIX98 -D_REENTRANT -D_THREAD_SAFE -I/usr/local/include/pthread/linuxthreads" ./configure --prefix=/usr/local/mysql --localstatedir=/usr/local/mysql/data --libexecdir=/usr/local/mysql/bin --enable-thread-safe-client --enable-local-infile --enable-assembler --with-named-thread-libs="-DHAVE_GLIBC2_STYLE_GETHOSTBYNAME_R -D_THREAD_SAFE -I /usr/local/include/pthread/linuxthreads -L/usr/local/lib -llthread -llgcc_r" --disable-shared --with-embedded-server --with-innodb
QNX Neutrino 6.2.1 i386 with @command{gcc 2.95.3qnx-nto 20010315:}
CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared

The following binaries are built on third-party systems kindly provided to MySQL AB by other users. These are provided only as a courtesy; MySQL AB does not have full control over these systems, so we can provide only limited support for the binaries built on them.

SCO Unix 3.2v5.0.6 i386 with @command{gcc 2.95.3:}
CFLAGS="-O3 -mpentium" LDFLAGS=-static CXX=gcc CXXFLAGS="-O3 -mpentium -felide-constructors" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --with-named-z-libs=no --enable-thread-safe-client --disable-shared
SCO OpenUnix 8.0.0 i386 with @command{CC 3.2:}
CC=cc CFLAGS="-O" CXX=CC ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --with-named-z-libs=no --enable-thread-safe-client --disable-shared
Compaq Tru64 OSF/1 V5.1 732 alpha with cc/cxx (Compaq C V6.3-029i / DIGITAL C++ V6.1-027):
CC="cc -pthread" CFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all" CXX="cxx -pthread" CXXFLAGS="-O4 -ansi_alias -fast -inline speed -speculate all -noexceptions -nortti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --with-prefix=/usr/local/mysql --with-named-thread-libs="-lpthread -lmach -lexc -lc" --disable-shared --with-mysqld-ldflags=-all-static
SGI Irix 6.5 IP32 with @command{gcc 3.0.1:}
CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared
FreeBSD/sparc64 5.0 with @command{gcc 3.2.1:}
CFLAGS=-DHAVE_BROKEN_REALPATH ./configure --prefix=/usr/local/mysql --localstatedir=/usr/local/mysql/data --libexecdir=/usr/local/mysql/bin --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --disable-shared --with-innodb

The following compile options have been used for binary packages that MySQL AB provided in the past. These binaries no longer are being updated, but the compile options are listed here for reference purposes.

Linux 2.2.xx SPARC with @command{egcs 1.1.2:}
CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex --enable-thread-safe-client --enable-local-infile --enable-assembler --disable-shared
Linux 2.2.x with x686 with @command{gcc 2.95.2:}
CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-extra-charsets=complex
SunOS 4.1.4 2 sun4c with @command{gcc 2.7.2.1:}
CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors" ./configure --prefix=/usr/local/mysql --disable-shared --with-extra-charsets=complex --enable-assembler
SunOS 5.5.1 (and above) sun4u with @command{egcs 1.0.3a or 2.90.27 or}
@command{gcc} 2.95.2 and newer: CC=gcc CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex --enable-assembler
SunOS 5.6 i86pc with @command{gcc 2.8.1:}
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex
BSDI BSD/OS 3.1 i386 with @command{gcc 2.7.2.1:}
CC=gcc CXX=gcc CXXFLAGS=-O ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
BSDI BSD/OS 2.1 i386 with @command{gcc 2.7.2:}
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
AIX 4.2 with @command{gcc 2.7.2.2:}
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex

Anyone who has more optimal options for any of the preceding configurations listed can always mail them to the MySQL internals mailing list. See section 1.7.1.1 The MySQL Mailing Lists.

RPM distributions prior to MySQL 3.22 are user-contributed. Beginning with MySQL 3.22, RPM distributions are generated by MySQL AB.

If you want to compile a debug version of MySQL, you should add --with-debug or --with-debug=full to the preceding @command{configure} commands and remove any -fomit-frame-pointer options.

2.1.3 How to Get MySQL

Check the MySQL downloads page (http://dev.mysql.com/downloads/) for information about the current version and for downloading instructions. For a complete up-to-date list of MySQL download mirror sites, see http://dev.mysql.com/downloads/mirrors.html. There you will also find information about becoming a MySQL mirror site and how to report a bad or out-of-date mirror.

Our main mirror is located at http://mirrors.sunsite.dk/mysql/.

2.1.4 Verifying Package Integrity Using MD5 Checksums or GnuPG

After you have downloaded the MySQL package that suits your needs and before you attempt to install it, you should make sure that it is intact and has not been tampered with. MySQL AB offers three means of integrity checking:

The following sections describe how to use these methods.

If you notice that the MD5 checksum or GPG signatures do not match, first try to download the respective package one more time, perhaps from another mirror site. If you repeatedly cannot successfully verify the integrity of the package, please notify us about such incidents, including the full package name and the download site you have been using, at [email protected] or [email protected]. Do not report downloading problems using the bug-reporting system.

2.1.4.1 Verifying the MD5 Checksum

After you have downloaded a MySQL package, you should make sure that its MD5 checksum matches the one provided on the MySQL download pages. Each package has an individual checksum that you can verify with the following command, where package_name is the name of the package you downloaded: 

shell> md5sum package_name

Example: 

shell> md5sum mysql-standard-4.0.17-pc-linux-i686.tar.gz
60f5fe969d61c8f82e4f7f62657e1f06  mysql-standard-4.0.17-pc-linux-i686.tar.gz

You should verify that the resulting checksum (the string of hexadecimal digits) matches the one displayed on the download page immediately below the respective package.

Note that not all operating systems support the md5sum command. On some, it is simply called md5 and others do not ship it at all. On Linux, it is part of the GNU Text Utilities package, which is available for a wide range of platforms. You can download the source code from http://www.gnu.org/software/textutils/ as well. If you have OpenSSL installed, you can also use the command openssl md5 package_name instead. A DOS/Windows implementation of the md5 command is available from http://www.fourmilab.ch/md5/.

2.1.4.2 Signature Checking Using GnuPG

Another method of verifying the integrity and authenticity of a package is to use cryptographic signatures. This is more reliable than using MD5 checksums, but requires more work.

Beginning with MySQL 4.0.10 (February 2003), MySQL AB started signing downloadable packages with GnuPG (GNU Privacy Guard). GnuPG is an Open Source alternative to the very well-known Pretty Good Privacy (PGP) by Phil Zimmermann. See http://www.gnupg.org/ for more information about GnuPG and how to obtain and install it on your system. Most Linux distributions already ship with GnuPG installed by default. For more information about OpenPGP, see http://www.openpgp.org/.

To verify the signature for a specific package, you first need to obtain a copy of MySQL AB's public GPG build key. You can download the key from http://www.keyserver.net/. The key that you want to obtain is named [email protected]. Alternatively, you can cut and paste the key directly from the following text: 

Key ID:
pub  1024D/5072E1F5 2003-02-03
     MySQL Package signing key (www.mysql.com) <[email protected]>
Fingerprint: A4A9 4068 76FC BD3C 4567  70C8 8C71 8D3B 5072 E1F5

Public Key (ASCII-armored):

-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: GnuPG v1.0.6 (GNU/Linux)
Comment: For info see http://www.gnupg.org
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=YJkx
-----END PGP PUBLIC KEY BLOCK-----

You can import the build key into your personal public GPG keyring by using gpg --import. For example, if you save the key in a file named `mysql_pubkey.asc', the import command looks like this: 

shell> gpg --import mysql_pubkey.asc

See the GPG documentation for more information on how to work with public keys.

After you have downloaded and imported the public build key, download your desired MySQL package and the corresponding signature, which also is available from the download page. The signature file has the same name as the distribution file with an `.asc' extension. For example:

Distribution file mysql-standard-4.0.17-pc-linux-i686.tar.gz
Signature file mysql-standard-4.0.17-pc-linux-i686.tar.gz.asc

Make sure that both files are stored in the same directory and then run the following command to verify the signature for the distribution file: 

shell> gpg --verify package_name.asc

Example: 

shell> gpg --verify mysql-standard-4.0.17-pc-linux-i686.tar.gz.asc
gpg: Warning: using insecure memory!
gpg: Signature made Mon 03 Feb 2003 08:50:39 PM MET
using DSA key ID 5072E1F5
gpg: Good signature from
     "MySQL Package signing key (www.mysql.com) <[email protected]>"

The Good signature message indicates that everything is all right. You can ignore the insecure memory warning.

2.1.4.3 Signature Checking Using RPM

For RPM packages, there is no separate signature. RPM packages have a built-in GPG signature and MD5 checksum. You can verify a package by running the following command: 

shell> rpm --checksig package_name.rpm

Example: 

shell> rpm --checksig MySQL-server-4.0.10-0.i386.rpm
MySQL-server-4.0.10-0.i386.rpm: md5 gpg OK

Note: If you are using RPM 4.1 and it complains about (GPG) NOT OK (MISSING KEYS: GPG#5072e1f5), even though you have imported the MySQL public build key into your own GPG keyring, you need to import the key into the RPM keyring first. RPM 4.1 no longer uses your personal GPG keyring (or GPG itself). Rather, it maintains its own keyring because it is a system-wide application and a user's GPG public keyring is a user-specific file. To import the MySQL public key into the RPM keyring, first obtain the key as described in the previous section. Then use @command{rpm --import} to import the key. For example, if you have the public key stored in a file named `mysql_pubkey.asc', import it using this command: 

shell> rpm --import mysql_pubkey.asc

2.1.5 Installation Layouts

This section describes the default layout of the directories created by installing binary or source distributions provided by MySQL AB. If you install a distribution provided by another vendor, some other layout might be used.

On Windows, the default installation directory is `C:\mysql', which has the following subdirectories:

Directory Contents of Directory
`bin' Client programs and the @command{mysqld} server
`data' Log files, databases
`Docs' Documentation
`examples' Example programs and scripts
`include' Include (header) files
`lib' Libraries
`scripts' Utility scripts
`share' Error message files

Installations created from Linux RPM distributions result in files under the following system directories:

Directory Contents of Directory
`/usr/bin' Client programs and scripts
`/usr/sbin' The @command{mysqld} server
`/var/lib/mysql' Log files, databases
`/usr/share/doc/packages' Documentation
`/usr/include/mysql' Include (header) files
`/usr/lib/mysql' Libraries
`/usr/share/mysql' Error message and character set files
`/usr/share/sql-bench' Benchmarks

On Unix, a @command{tar} file binary distribution is installed by unpacking it at the installation location you choose (typically `/usr/local/mysql') and creates the following directories in that location:

Directory Contents of Directory
`bin' Client programs and the @command{mysqld} server
`data' Log files, databases
`docs' Documentation, ChangeLog
`include' Include (header) files
`lib' Libraries
`scripts' @command{mysql_install_db}
`share/mysql' Error message files
`sql-bench' Benchmarks

A source distribution is installed after you configure and compile it. By default, the installation step installs files under `/usr/local', in the following subdirectories:

Directory Contents of Directory
`bin' Client programs and scripts
`include/mysql' Include (header) files
`info' Documentation in Info format
`lib/mysql' Libraries
`libexec' The @command{mysqld} server
`share/mysql' Error message files
`sql-bench' Benchmarks and crash-me test
`var' Databases and log files

Within an installation directory, the layout of a source installation differs from that of a binary installation in the following ways:

You can create your own binary installation from a compiled source distribution by executing the `scripts/make_binary_distribution' script from the top directory of the source distribution.

2.2 Standard MySQL Installation Using a Binary Distribution

This section covers the installation of MySQL on platforms where we offer packages using the native packaging format of the respective platform. (This is also known as performing a ``binary install.'') However, binary distributions of MySQL are available for many other platforms as well. See section 2.2.5 Installing MySQL on Other Unix-Like Systems for generic installation instructions for these packages that apply to all platforms.

See section 2.1 General Installation Issues for more information on what other binary distributions are available and how to obtain them.

2.2.1 Installing MySQL on Windows

The installation process for MySQL on Windows has the following steps:

  1. Obtain and install the distribution.
  2. Set up an option file if necessary.
  3. Select the server you want to use.
  4. Start the server.
  5. Assign passwords to the initial MySQL accounts.

MySQL for Windows is available in two distribution formats:

Generally speaking, you should use the binary distribution. It's simpler, and you need no additional tools to get MySQL up and running.

This section describes how to install MySQL on Windows using a binary distribution. To install using a source distribution, see section 2.3.6 Installing MySQL from Source on Windows.

2.2.1.1 Windows System Requirements

To run MySQL on Windows, you need the following:

2.2.1.2 Installing a Windows Binary Distribution

To install MySQL on Windows using a binary distribution, follow this procedure:

  1. If you are working on a Windows NT, 2000, or XP machine, make sure that you have logged in as a user with administrator privileges.
  2. If you are doing an upgrade of an earlier MySQL installation, it is necessary to stop the current server. On Windows NT, 2000, or XP machines, if you are running the server as a Windows service, stop it as follows from the command prompt: 
    C:\> NET STOP MySQL
    If you plan to use a different server after the upgrade (for example, if you want to run @command{mysqld-max} rather than @command{mysqld}), remove the existing service: 
    C:\> C:\mysql\bin\mysqld --remove
    You can reinstall the service to use the proper server after upgrading. If you are not running the MySQL server as a service, stop it like this: 
    C:\> C:\mysql\bin\mysqladmin -u root shutdown
  3. Exit the WinMySQLAdmin program if it is running.
  4. Unzip the distribution file to a temporary directory.
  5. Run the setup.exe program to begin the installation process. If you want to install MySQL into a location other than the default directory (`C:\mysql'), use the Browse button to specify your preferred directory. If you do not install MySQL into the default location, you will need to specify the location whenever you start the server. The easiest way to do this is to use an option file, as described in section 2.2.1.3 Preparing the Windows MySQL Environment.
  6. Finish the install process.

Important note: Early alpha Windows distributions for MySQL 4.1 do not contain an installer program. A 4.1 distribution is a Zip file that you just unzip in the location where you want to install MySQL. For example, to install `mysql-4.1.1-alpha-win.zip' as `C:\mysql', unzip the distribution file on the C: drive, then rename the resulting `mysql-4.1.1-alpha' directory to `mysql'.

If you are upgrading to MySQL 4.1 from an earlier version, you will want to preserve your existing `data' directory that contains the grant tables in the mysql database and your own databases. Before installing 4.1, stop the server if it is running, and save your `data' directory to another location. Then either rename the existing `C:\mysql' directory or remove it. Install 4.1 as described in the preceding paragraph, and then replace its `data' directory with your old `data' directory. This will avoid the loss of your current databases. Start the new server and update the grant tables. See section 2.5.8 Upgrading the Grant Tables.

Please see section 2.2.1.8 Troubleshooting a MySQL Installation Under Windows if you encounter difficulties during installation.

2.2.1.3 Preparing the Windows MySQL Environment

If you need to specify startup options when you run the server, you can indicate them on the command line or place them in an option file. For options that will be used every time the server starts, you will find it most convenient to use an option file to specify your MySQL configuration. This is true particularly under the following circumstances:

On Windows, the MySQL installer places the data directory directly under the directory where you install MySQL. If you would like to use a data directory in a different location, you should copy the entire contents of the `data' directory to the new location. For example, by default, the installer places MySQL in `C:\mysql' and the data directory in `C:\mysql\data'. If you want to use a data directory of `E:\mydata', you must do two things:

When the MySQL server starts on Windows, it looks for options in two files: the `my.ini' file in the Windows directory, and the `C:\my.cnf' file. The Windows directory typically is named something like `C:\WINDOWS' or `C:\WinNT'. You can determine its exact location from the value of the WINDIR environment variable using the following command: 

C:\> echo %WINDIR%

MySQL looks for options first in the `my.ini' file, then in the `my.cnf' file. However, to avoid confusion, it's best if you use only one file. If your PC uses a boot loader where the C: drive isn't the boot drive, your only option is to use the `my.ini' file. Whichever option file you use, it must be a plain text file.

An option file can be created and modified with any text editor, such as the Notepad program. For example, if MySQL is installed at `E:\mysql' and the data directory is located at `E:\mydata\data', you can create the option file and set up a [mysqld] section to specify values for the basedir and datadir parameters: 

[mysqld]
# set basedir to your installation path
basedir=E:/mysql
# set datadir to the location of your data directory
datadir=E:/mydata/data

Note that Windows pathnames are specified in option files using forward slashes rather than backslashes. If you do use backslashes, you must double them.

Another way to manage an option file is to use the WinMySQLAdmin tool. You can find WinMySQLAdmin in the `bin' directory of your MySQL installation, as well as a help file containing instructions for using it. WinMySQLAdmin has the capability of editing your option file, but note these points:

Now you are ready to start the server.

2.2.1.4 Selecting a Windows Server

Starting with MySQL 3.23.38, the Windows distribution includes both the normal and the MySQL-Max server binaries.

Up through the early releases of MySQL 4.1, the servers included in Windows distributions are named like this:

Binary Description
@command{mysqld} Compiled with full debugging and automatic memory allocation checking, symbolic links, and InnoDB and BDB tables.
@command{mysqld-opt} Optimized binary. From version 4.0 on, InnoDB is enabled. Before 4.0, this server includes no transactional table support.
@command{mysqld-nt} Optimized binary for Windows NT, 2000, and XP with support for named pipes.
@command{mysqld-max} Optimized binary with support for symbolic links, and InnoDB and BDB tables.
@command{mysqld-max-nt} Like @command{mysqld-max}, but compiled with support for named pipes.

We have found that the server with the most generic name (@command{mysqld}) is the one that many users are likely to choose by default. However, that is also the server that results in the highest memory and CPU use due to the inclusion of full debugging support. The server named @command{mysqld-opt} is a better general-use server choice to make instead if you don't need debugging suport and don't want the maximal feature set offered by the -max servers or named pipe support offered by the -nt servers.

To make it less likely that the debugging server would be chosen inadvertantly, some name changes were made from MySQL 4.1.2 to 4.1.4: @command{mysqld} has been renamed to @command{mysqld-debug} and @command{mysqld-opt} has been renamed to @command{mysqld}. Thus, the server that includes debugging support indicates that in its name, and the server named @command{mysqld} is an efficient default choice. The other servers still have their same names. The resulting servers are named like this:

Binary Description
@command{mysqld-debug} Compiled with full debugging and automatic memory allocation checking, symbolic links, and InnoDB and BDB tables.
@command{mysqld} Optimized binary with InnoDB support.
@command{mysqld-nt} Optimized binary for Windows NT, 2000, and XP with support for named pipes.
@command{mysqld-max} Optimized binary with support for symbolic links, and InnoDB and BDB tables.
@command{mysqld-max-nt} Like @command{mysqld-max}, but compiled with support for named pipes.

The name changes were not both instituted at the same time. If you have MySQL 4.1.2 or 4.1.3, it might be that you have a server named @command{mysqld-debug} but not one named @command{mysqld}. In this case, you should have have a server @command{mysqld-opt}, which you should choose as your default server unless you need maximal features, named pipes, or debugging support.

All of the preceding binaries are optimized for modern Intel processors, but should work on any Intel i386-class or higher processor.

MySQL supports TCP/IP on all Windows platforms. The @command{mysqld-nt} and mysql-max-nt servers support named pipes on NT, 2000, and XP. However, the default is to use TCP/IP regardless of the platform. (Named pipes are slower than TCP/IP in many Windows configurations.) Named pipe use is subject to these conditions:

Note: Most of the examples in the following sections use @command{mysqld} as the server name. If you choose to use a different server, such as @command{mysqld-opt}, make the appropriate substitutions in the commands that are shown in the examples. One good reason to choose a different server is that because @command{mysqld} contains full debugging support, it uses more memory and runs slower than the other Windows servers.

2.2.1.5 Starting the Server for the First Time

On Windows 95, 98, or Me, MySQL clients always connect to the server using TCP/IP. (This will allow any machine on your network to connect to your MySQL server.) Because of this, you must make sure that TCP/IP support is installed on your machine before starting MySQL. You can find TCP/IP on your Windows CD-ROM.

Note that if you are using an old Windows 95 release (for example, OSR2), it's likely that you have an old Winsock package; MySQL requires Winsock 2! You can get the newest Winsock from http://www.microsoft.com/. Windows 98 has the new Winsock 2 library, so it is unnecessary to update the library.

On NT-based systems such as Windows NT, 2000, or XP, clients have two options. They can use TCP/IP, or they can use a named pipe if the server supports named pipe connections.

In MySQL 4.1 and up, Windows servers also support shared-memory connections if started with the --shared-memory option. Clients can connect this way by using the --protocol=memory option.

For information about which server binary to run, see section 2.2.1.4 Selecting a Windows Server.

This section gives a general overview of starting the MySQL server. The following sections provide more specific information for particular versions of Windows.

The examples in these sections assume that MySQL is installed under the default location of `C:\mysql'. Adjust the pathnames shown in the examples if you have MySQL installed in a different location.

Testing is best done from a command prompt in a console window (a ``DOS window''). This way you can have the server display status messages in the window where they are easy to see. If something is wrong with your configuration, these messages will make it easier for you to identify and fix any problems.

To start the server, enter this command: 

C:\> C:\mysql\bin\mysqld --console

For servers that include InnoDB support, you should see the following messages as the server starts: 

InnoDB: The first specified datafile c:\ibdata\ibdata1 did not exist:
InnoDB: a new database to be created!
InnoDB: Setting file c:\ibdata\ibdata1 size to 209715200
InnoDB: Database physically writes the file full: wait...
InnoDB: Log file c:\iblogs\ib_logfile0 did not exist: new to be created
InnoDB: Setting log file c:\iblogs\ib_logfile0 size to 31457280
InnoDB: Log file c:\iblogs\ib_logfile1 did not exist: new to be created
InnoDB: Setting log file c:\iblogs\ib_logfile1 size to 31457280
InnoDB: Log file c:\iblogs\ib_logfile2 did not exist: new to be created
InnoDB: Setting log file c:\iblogs\ib_logfile2 size to 31457280
InnoDB: Doublewrite buffer not found: creating new
InnoDB: Doublewrite buffer created
InnoDB: creating foreign key constraint system tables
InnoDB: foreign key constraint system tables created
011024 10:58:25  InnoDB: Started

When the server finishes its startup sequence, you should see something like this, which indicates that the server is ready to service client connections: 

mysqld: ready for connections
Version: '4.0.14-log'  socket: ''  port: 3306

The server will continue to write to the console any further diagnostic output it produces. You can open a new console window in which to run client programs.

If you omit the --console option, the server writes diagnostic output to the error log in the data directory (`C:\mysql\data' by default). The error log is the file with the `.err' extension.

Note: The accounts that are listed in the MySQL grant tables initially have no passwords. After starting the server, you should set up passwords for them using the instructions in section 2.4 Post-Installation Setup and Testing.

2.2.1.6 Starting MySQL from the Windows Command Line

The MySQL server can be started manually from the command line. This can be done on any version of Windows.

To start the @command{mysqld} server from the command line, you should start a console window (a ``DOS window'') and enter this command: 

C:\> C:\mysql\bin\mysqld

On non-NT versions of Windows, this will start @command{mysqld} in the background. That is, after the server starts, you should see another command prompt. If you start the server this way on Windows NT, 2000, or XP, the server will run in the foreground and no command prompt will appear until the server exits. Because of this, you should open another console window to run client programs while the server is running.

You can stop the MySQL server by executing this command: 

C:\> C:\mysql\bin\mysqladmin -u root shutdown

This invokes the MySQL administrative utility @command{mysqladmin} to connect to the server and tell it to shut down. The command connects as root, which is the default administrative account in the MySQL grant system. Note that users in the MySQL grant system are wholly independent from any login users under Windows.

If @command{mysqld} doesn't start, check the error log to see whether the server wrote any messages there to indicate the cause of the problem. The error log is located in the `C:\mysql\data' directory. It is the file with a suffix of `.err'. You can also try to start the server as @command{mysqld --console}; in this case, you may get some useful information on the screen that may help solve the problem.

The last option is to start @command{mysqld} with --standalone --debug. In this case, @command{mysqld} will write a log file `C:\mysqld.trace' that should contain the reason why @command{mysqld} doesn't start. See section D.1.2 Creating Trace Files.

Use @command{mysqld --help} to display all the options that @command{mysqld} understands!

2.2.1.7 Starting MySQL as a Windows Service

On the NT family (Windows NT, 2000, or XP), the recommended way to run MySQL is to install it as a Windows service. Then Windows starts and stops the MySQL server automatically when Windows starts and stops. A server installed as a service can also be controlled from the command line using @command{NET} commands, or with the graphical Services utility.

The Services utility (the Windows Service Control Manager) can be found in the Windows Control Panel (under Administrative Tools on Windows 2000 or XP). It is advisable to close the Services utility while performing server installation or removal operations from this command line. This prevents some odd errors.

To get MySQL to work with TCP/IP on Windows NT 4, you must install service pack 3 (or newer).

Before installing MySQL as a Windows service, you should first stop the current server if it is running by using the following command: 

C:\> C:\mysql\bin\mysqladmin -u root shutdown

This invokes the MySQL administrative utility @command{mysqladmin} to connect to the server and tell it to shut down. The command connects as root, which is the default administrative account in the MySQL grant system. Note that users in the MySQL grant system are wholly independent from any login users under Windows.

Now install the server as a service: 

C:\> mysqld --install

If you have problems installing @command{mysqld} as a service using just the server name, try installing it using its full pathname: 

C:\> C:\mysql\bin\mysqld --install

As of MySQL 4.0.2, you can specify a specific service name after the --install option. As of MySQL 4.0.3, you can in addition specify a --defaults-file option after the service name to indicate where the server should obtain options when it starts. The rules that determine the service name and option files the server uses are as follows:

Note: Prior to MySQL 4.0.17, a server installed as a Windows service has problems starting if its pathname or the service name contains spaces. For this reason, avoid installing MySQL in a directory such as `C:\Program Files' or using a service name containing spaces.

In the usual case that you install the server with --install but no service name, the server is installed with a service name of MySQL.

As a more complex example, consider the following command: 

C:\> C:\mysql\bin\mysqld --install mysql --defaults-file=C:\my-opts.cnf

Here, a service name is given after the --install option. If no --defaults-file option had been given, this command would have the effect of causing the server to read the [mysql] group from the standard option files. (This would be a bad idea, because that option group is for use by the @command{mysql} client program.) However, because the --defaults-file option is present, the server reads options only from the named file, and only from the [mysqld] option group.

You can also specify options as ``Start parameters'' in the Windows Services utility before you start the MySQL service.

Once a MySQL server is installed as a service, Windows will start the service automatically whenever Windows starts. The service also can be started immediately from the Services utility, or by using the command @command{NET START MySQL}. The @command{NET} command is not case sensitive.

When run as a service, @command{mysqld} has no access to a console window, so no messages can be seen there. If @command{mysqld} doesn't start, check the error log to see whether the server wrote any messages there to indicate the cause of the problem. The error log is located in the `C:\mysql\data' directory. It is the file with a suffix of `.err'.

When @command{mysqld} is running as a service, it can be stopped by using the Services utility, the command @command{NET STOP MySQL}, or the command @command{mysqladmin shutdown}. If the service is running when Windows shuts down, Windows will stop the server automatically.

From MySQL 3.23.44 on, you have the choice of installing the server as a Manual service if you don't wish the service to be started automatically during the boot process. To do this, use the --install-manual option rather than the --install option: 

C:\> C:\mysql\bin\mysqld --install-manual

To remove a server that is installed as a service, first stop it if it is running. Then use the --remove option to remove it: 

C:\> C:\mysql\bin\mysqld --remove

For MySQL versions older than 3.23.49, one problem with automatic MySQL service shutdown is that Windows waited only for a few seconds for the shutdown to complete, then killed the database server process if the time limit was exceeded. This had the potential to cause problems. (For example, the InnoDB storage engine had to perform crash recovery at the next startup.) Starting from MySQL 3.23.49, Windows waits longer for the MySQL server shutdown to complete. If you notice this still is not enough for your installation, it is safest not to run the MySQL server as a service. Instead, start it from the command-line prompt, and stop it with @command{mysqladmin shutdown}.

This change to tell Windows to wait longer when stopping the MySQL server works for Windows 2000 and XP. It does not work for Windows NT, where Windows waits only 20 seconds for a service to shut down, and after that kills the service process. You can increase this default by opening the Registry Editor `\winnt\system32\regedt32.exe' and editing the value of WaitToKillServiceTimeout at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control in the Registry tree. Specify the new larger value in milliseconds. For example, the value 120000 tells Windows NT to wait up to 120 seconds.

If you don't want to start @command{mysqld} as a service, you can start it from the command line. For instructions, see section 2.2.1.6 Starting MySQL from the Windows Command Line.

Please see section 2.2.1.8 Troubleshooting a MySQL Installation Under Windows if you encounter difficulties during installation.

2.2.1.8 Troubleshooting a MySQL Installation Under Windows

When installing and running MySQL for the first time, you may encounter certain errors that prevent the MySQL server from starting. The purpose of this section is to help you diagnose and correct some of these errors.

Your first resource when troubleshooting server issues is the error log. The MySQL server uses the error log to record information relevant to the error that is preventing the server from starting. The error log is located in the data directory specified in your `my.ini' file. The default data directory location is `C:\mysql\data'. See section 5.9.1 The Error Log.

Another source of information regarding possible errors is the console messages displayed when the MySQL service is starting. Use the @command{NET START mysql} command from the command line after installing @command{mysqld} as a service to see any error messages regarding the starting of the MySQL server as a service. See section 2.2.1.7 Starting MySQL as a Windows Service.

The following are examples of some of the more common error messages you may encounter when installing MySQL and starting the server for the first time: 

System error 1067 has occurred.
Fatal error: Can't open privilege tables: Table 'mysql.host' doesn't exist

These messages occur when the MySQL server cannot find the mysql privileges database or other critical files. This error is often encountered when the MySQL base or data directories are installed in different locations than the default locations (`C:\mysql' and `C:\mysql\data', respectively).

If you have installed MySQL to a directory other than `C:\mysql' you will need to ensure that the MySQL server is aware of this through the use of a configuration (my.ini) file. The my.ini file needs to be located in your Windows directory, typically located at `C:\WinNT' or `C:\WINDOWS'. You can determine its exact location from the value of the WINDIR environment variable by issuing the following command from the command prompt: 

C:\> echo %WINDIR%

An option file can be created and modified with any text editor, such as the Notepad program. For example, if MySQL is installed at `E:\mysql' and the data directory is located at `D:\MySQLdata', you can create the option file and set up a [mysqld] section to specify values for the basedir and datadir parameters: 

[mysqld]
# set basedir to your installation path
basedir=E:/mysql
# set datadir to the location of your data directory
datadir=D:/MySQLdata

Note that Windows pathnames are specified in option files using forward slashes rather than backslashes. If you do use backslashes, you must double them: 

[mysqld]
# set basedir to your installation path
basedir=C:\\Program Files\\mysql
# set datadir to the location of your data directory
datadir=D:\\MySQLdata

See section 2.2.1.3 Preparing the Windows MySQL Environment.

2.2.1.9 Running MySQL Client Programs on Windows

You can test whether the MySQL server is working by executing any of the following commands: 

C:\> C:\mysql\bin\mysqlshow
C:\> C:\mysql\bin\mysqlshow -u root mysql
C:\> C:\mysql\bin\mysqladmin version status proc
C:\> C:\mysql\bin\mysql test

If @command{mysqld} is slow to respond to TCP/IP connections from client programs on Windows 9x/Me, there is probably a problem with your DNS. In this case, start @command{mysqld} with the --skip-name-resolve option and use only localhost and IP numbers in the Host column of the MySQL grant tables.

You can force a MySQL client to use a named pipe connection rather than TCP/IP by specifying the --pipe option or by specifying . (period) as the host name. Use the --socket option to specify the name of the pipe. As of MySQL 4.1, you should use the --protocol=PIPE option.

There are two versions of the MySQL command-line tool:
Binary Description
@command{mysql} Compiled on native Windows, offering limited text editing capabilities.
@command{mysqlc} Compiled with the Cygnus GNU compiler and libraries, which offers readline editing.

If you want to use @command{mysqlc}, you must have a copy of the `cygwinb19.dll' library installed somewhere that @command{mysqlc} can find it. Current distributions of MySQL include this library in the same directory as @command{mysqlc} (the `bin' directory under the base directory of your MySQL installation). If your distribution does not have the cygwinb19.dll library in the `bin' directory, look for it in the lib directory and copy it to your Windows system directory (`\Windows\system' or a similar place).

2.2.1.10 MySQL on Windows Compared to MySQL on Unix

MySQL for Windows has by now proven itself to be very stable. The Windows version of MySQL has the same features as the corresponding Unix version, with the following exceptions:

Windows 95 and threads
Windows 95 leaks about 200 bytes of main memory for each thread creation. Each connection in MySQL creates a new thread, so you shouldn't run @command{mysqld} for an extended time on Windows 95 if your server handles many connections! Other versions of Windows don't suffer from this bug.
Limited number of ports
Windows systems have about 4,000 ports available for client connections, and after a connection on a port closes, it takes two to four minutes before the port can be reused. In situations where clients connect to and disconnect from the server at a high rate, it is possible for all available ports to be used up before closed ports become available again. If this happens, the MySQL server will appear to have become unresponsive even though it is running. Note that ports may be used by other applications running on the machine as well, in which case the number of ports available to MySQL is lower. For more information, see http://support.microsoft.com/default.aspx?scid=kb;en-us;196271.
Concurrent reads
MySQL depends on the pread() and pwrite() calls to be able to mix INSERT and SELECT. Currently we use mutexes to emulate pread()/pwrite(). We will, in the long run, replace the file level interface with a virtual interface so that we can use the readfile()/writefile() interface on NT, 2000, and XP to get more speed. The current implementation limits the number of open files MySQL can use to 2,048 (1,024 before MySQL 4.0.19), which means that you will not be able to run as many concurrent threads on NT, 2000, and XP as on Unix.
Blocking read
MySQL uses a blocking read for each connection, which has the following implications if named pipe connections are enabled:
  • A connection will not be disconnected automatically after eight hours, as happens with the Unix version of MySQL.
  • If a connection hangs, it's impossible to break it without killing MySQL.
  • @command{mysqladmin kill} will not work on a sleeping connection.
  • @command{mysqladmin shutdown} can't abort as long as there are sleeping connections.
We plan to fix this problem when our Windows developers have figured out a nice workaround.
ALTER TABLE
While you are executing an ALTER TABLE statement, the table is locked from being used by other threads. This has to do with the fact that on Windows, you can't delete a file that is in use by another thread. In the future, we may find some way to work around this problem.
DROP TABLE
DROP TABLE on a table that is in use by a MERGE table will not work on Windows because the MERGE handler does the table mapping hidden from the upper layer of MySQL. Because Windows doesn't allow you to drop files that are open, you first must flush all MERGE tables (with FLUSH TABLES) or drop the MERGE table before dropping the table. We will fix this at the same time we introduce views.
DATA DIRECTORY and INDEX DIRECTORY
The DATA DIRECTORY and INDEX DIRECTORY options for CREATE TABLE are ignored on Windows, because Windows doesn't support symbolic links. These options also are ignored on systems that have a non-functional realpath() call.
DROP DATABASE
You cannot drop a database that is in use by some thread.
Killing MySQL from the Task Manager
You cannot kill MySQL from the Task Manager or with the shutdown utility in Windows 95. You must take it down with @command{mysqladmin shutdown}.
Case-insensitive names
Filenames are not case sensitive on Windows, so MySQL database and table names are also not case sensitive on Windows. The only restriction is that database and table names must be specified using the same case throughout a given statement. See section 10.2.2 Identifier Case Sensitivity.
The `\' pathname separator character
Pathname components in Windows 95 are separated by the `\' character, which is also the escape character in MySQL. If you are using LOAD DATA INFILE or SELECT ... INTO OUTFILE, use Unix-style filenames with `/' characters: 
mysql> LOAD DATA INFILE 'C:/tmp/skr.txt' INTO TABLE skr;
mysql> SELECT * INTO OUTFILE 'C:/tmp/skr.txt' FROM skr;
Alternatively, you must double the `\' character: 
mysql> LOAD DATA INFILE 'C:\\tmp\\skr.txt' INTO TABLE skr;
mysql> SELECT * INTO OUTFILE 'C:\\tmp\\skr.txt' FROM skr;
Problems with pipes.
Pipes do not work reliably from the Windows command-line prompt. If the pipe includes the character ^Z / CHAR(24), Windows will think it has encountered end-of-file and abort the program. This is mainly a problem when you try to apply a binary log as follows: 
C:\> mysqlbinlog binary-log-name | mysql --user=root
If you have a problem applying the log and suspect that it is because of a ^Z / CHAR(24) character, you can use the following workaround: 
C:\> mysqlbinlog binary-log-file --result-file=/tmp/bin.sql
C:\> mysql --user=root --execute "source /tmp/bin.sql"
The latter command also can be used to reliably read in any SQL file that may contain binary data.
Can't open named pipe error
If you use a MySQL 3.22 server on Windows NT with the newest MySQL client programs, you will get the following error: 
error 2017: can't open named pipe to host: . pipe...
This happens because the release version of MySQL uses named pipes on NT by default. You can avoid this error by using the --host=localhost option to the new MySQL clients or by creating an option file `C:\my.cnf' that contains the following information: 
[client]
host = localhost
Starting from 3.23.50, named pipes are enabled only if @command{mysqld-nt} or @command{mysqld-max-nt} is started with --enable-named-pipe.
Access denied for user error
If you attempt to run a MySQL client program to connect to a server running on the same machine, but get the error Access denied for user 'some-user'@'unknown' to database 'mysql', this means that MySQL cannot resolve your hostname properly. To fix this, you should create a file named `\windows\hosts' containing the following information: 
127.0.0.1       localhost

Here are some open issues for anyone who might want to help us improve MySQL on Windows:

2.2.2 Installing MySQL on Linux

The recommended way to install MySQL on Linux is by using the RPM packages. The MySQL RPMs are currently built on a SuSE Linux 7.3 system, but should work on most versions of Linux that support @command{rpm} and use glibc. To obtain RPM packages, see section 2.1.3 How to Get MySQL.

Note: RPM distributions of MySQL often are provided by other vendors. Be aware that they may differ in features and capabilities from those built by MySQL AB, and that the instructions in this manual do not necessarily apply to installing them. The vendor's instructions should be consulted instead.

If you have problems with an RPM file (for example, if you receive the error ``Sorry, the host 'xxxx' could not be looked up''), see section 2.6.1.2 Linux Binary Distribution Notes.

In most cases, you only need to install the MySQL-server and MySQL-client packages to get a functional MySQL installation. The other packages are not required for a standard installation. If you want to run a MySQL-Max server that has additional capabilities, you should also install the MySQL-Max RPM. However, you should do so only after installing the MySQL-server RPM. @xref{mysqld-max, , @command{mysqld-max}}.

If you get a dependency failure when trying to install the MySQL 4.0 packages (for example, ``error: removing these packages would break dependencies: libmysqlclient.so.10 is needed by ...''), you should also install the package MySQL-shared-compat, which includes both the shared libraries for backward compatibility (libmysqlclient.so.12 for MySQL 4.0 and libmysqlclient.so.10 for MySQL 3.23).

Many Linux distributions still ship with MySQL 3.23 and they usually link applications dynamically to save disk space. If these shared libraries are in a separate package (for example, MySQL-shared), it is sufficient to simply leave this package installed and just upgrade the MySQL server and client packages (which are statically linked and do not depend on the shared libraries). For distributions that include the shared libraries in the same package as the MySQL server (for example, Red Hat Linux), you could either install our 3.23 MySQL-shared RPM, or use the MySQL-shared-compat package instead.

The following RPM packages are available:

To see all files in an RPM package (for example, a MySQL-server RPM), run: 

shell> rpm -qpl MySQL-server-VERSION.i386.rpm

To perform a standard minimal installation, run: 

shell> rpm -i MySQL-server-VERSION.i386.rpm
shell> rpm -i MySQL-client-VERSION.i386.rpm

To install just the client package, run: 

shell> rpm -i MySQL-client-VERSION.i386.rpm

RPM provides a feature to verify the integrity and authenticity of packages before installing them. If you would like to learn more about this feature, see section 2.1.4 Verifying Package Integrity Using MD5 Checksums or GnuPG.

The server RPM places data under the `/var/lib/mysql' directory. The RPM also creates a login account for a user named mysql (if one does not already exist) to use for running the MySQL server, and creates the appropriate entries in `/etc/init.d/' to start the server automatically at boot time. (This means that if you have performed a previous installation and have made changes to its startup script, you may want to make a copy of the script so that you don't lose it when you install a newer RPM.) See section 2.4.2.2 Starting and Stopping MySQL Automatically for more information on how MySQL can be started automatically on system startup.

If you want to install the MySQL RPM on older Linux distributions that do not support initialization scripts in `/etc/init.d' (directly or via a symlink), you should create a symbolic link that points to the location where your initialization scripts actually are installed. For example, if that location is `/etc/rc.d/init.d', use these commands before installing the RPM to create `/etc/init.d' as a symbolic link that points there: 

shell> cd /etc
shell> ln -s rc.d/init.d .

However, all current major Linux distributions should already support the new directory layout that uses `/etc/init.d', because it is required for LSB (Linux Standard Base) compliance.

If the RPM files that you install include MySQL-server, the @command{mysqld} server should be up and running after installation. You should now be able to start using MySQL.

If something goes wrong, you can find more information in the binary installation section. See section 2.2.5 Installing MySQL on Other Unix-Like Systems.

Note: The accounts that are listed in the MySQL grant tables initially have no passwords. After starting the server, you should set up passwords for them using the instructions in section 2.4 Post-Installation Setup and Testing.

2.2.3 Installing MySQL on Mac OS X

Beginning with MySQL 4.0.11, you can install MySQL on Mac OS X 10.2.x (``Jaguar'') and up using a Mac OS X binary package in PKG format instead of the binary tarball distribution. Please note that older versions of Mac OS X (for example, 10.1.x) are not supported by this package.

The package is located inside a disk image (.dmg) file that you first need to mount by double-clicking its icon in the Finder. It should then mount the image and display its contents.

To obtain MySQL, see section 2.1.3 How to Get MySQL.

Note: Before proceeding with the installation, be sure to shut down all running MySQL server instances by either using the MySQL Manager Application (on Mac OS X Server) or via @command{mysqladmin shutdown} on the command line.

To actually install the MySQL PKG file, double-click on the package icon. This launches the Mac OS X Package Installer, which will guide you through the installation of MySQL.

Due to a bug in the Mac OS X package installer, you may see this error message in the destination disk selection dialog: 

You cannot install this software on this disk. (null)

If this error occurs, simply click the Go Back button once to return to the previous screen. Then click Continue to advance to the destination disk selection again, and you should be able to choose the destination disk correctly. We have reported this bug to Apple and it is investigating this problem.

The Mac OS X PKG of MySQL will install itself into `/usr/local/mysql-VERSION' and will also install a symbolic link, `/usr/local/mysql', pointing to the new location. If a directory named `/usr/local/mysql' already exists, it will be renamed to `/usr/local/mysql.bak' first. Additionally, the installer will create the grant tables in the mysql database by executing @command{mysql_install_db} after the installation.

The installation layout is similar to that of a @command{tar} file binary distribution; all MySQL binaries are located in the directory `/usr/local/mysql/bin'. The MySQL socket file is created as `/tmp/mysql.sock' by default. See section 2.1.5 Installation Layouts.

MySQL installation requires a Mac OS X user account named mysql. A user account with this name should exist by default on Mac OS X 10.2 and up.

If you are running Mac OS X Server, you already have a version of MySQL installed. The versions of MySQL that ship with Mac OS X Server versions are shown in the following table:

Mac OS X Server Version MySQL Version
10.2-10.2.2 3.23.51
10.2.3-10.2.6 3.23.53
10.3 4.0.14
10.3.2 4.0.16

This manual section covers the installation of the official MySQL Mac OS X PKG only. Make sure to read Apple's help information about installing MySQL: Run the ``Help View'' application, select ``Mac OS X Server'' help, do a search for ``MySQL,'' and read the item entitled ``Installing MySQL.''

For pre-installed versions of MySQL on Mac OS X Server, note especially that you should start @command{mysqld} with @command{safe_mysqld} instead of @command{mysqld_safe} if MySQL is older than version 4.0.

If you previously used Marc Liyanage's MySQL packages for Mac OS X from http://www.entropy.ch, you can simply follow the update instructions for packages using the binary installation layout as given on his pages.

If you are upgrading from Marc's 3.23.xx versions or from the Mac OS X Server version of MySQL to the official MySQL PKG, you also need to convert the existing MySQL privilege tables to the current format, because some new security privileges have been added. See section 2.5.8 Upgrading the Grant Tables.

If you would like to automatically start up MySQL during system startup, you also need to install the MySQL Startup Item. Starting with MySQL 4.0.15, it is part of the Mac OS X installation disk images as a separate installation package. Simply double-click the MySQLStartupItem.pkg icon and follow the instructions to install it.

Note that the Startup Item need be installed only once! There is no need to install it each time you upgrade the MySQL package later.

The Startup Item will be installed into `/Library/StartupItems/MySQLCOM'. (Before MySQL 4.1.2, the location was `/Library/StartupItems/MySQL', but that collided with the MySQL Startup Item installed by Mac OS X Server.) Startup Item installation adds a variable MYSQLCOM=-YES- to the system configuration file `/etc/hostconfig'. If you would like to disable the automatic startup of MySQL, simply change this variable to MYSQLCOM=-NO-.

On Mac OS X Server, the default MySQL installation uses the variable MYSQL in the `/etc/hostconfig' file. The MySQL AB Startup Item installer disables this variable by setting it to MYSQL=-NO-. This avoids boot time conflicts with the MYSQLCOM variable used by the MySQL AB Startup Item. However, it does not shut down an already running MySQL server. You should do that yourself.

After the installation, you can start up MySQL by running the following commands in a terminal window. You must have administrator privileges to perform this task.

If you have installed the Startup Item: 

shell> sudo /Library/StartupItems/MySQLCOM/MySQLCOM start
(Enter your password, if necessary)
(Press Control-D or enter "exit" to exit the shell)

For versions of MySQL older than 4.1.3, substitute /Library/StartupItems/MySQLCOM/MySQLCOM with /Library/StartupItems/MySQL/MySQL above.

If you don't use the Startup Item, enter the following command sequence: 

shell> cd /usr/local/mysql
shell> sudo ./bin/mysqld_safe
(Enter your password, if necessary)
(Press Control-Z)
shell> bg
(Press Control-D or enter "exit" to exit the shell)

You should now be able to connect to the MySQL server, for example, by running `/usr/local/mysql/bin/mysql'.

Note: The accounts that are listed in the MySQL grant tables initially have no passwords. After starting the server, you should set up passwords for them using the instructions in section 2.4 Post-Installation Setup and Testing.

You might want to add aliases to your shell's resource file to make it easier to access commonly used programs such as @command{mysql} and @command{mysqladmin} from the command line. The syntax for @command{tcsh} is: 

alias mysql /usr/local/mysql/bin/mysql
alias mysqladmin /usr/local/mysql/bin/mysqladmin

For @command{bash}, use: 

alias mysql=/usr/local/mysql/bin/mysql
alias mysqladmin=/usr/local/mysql/bin/mysqladmin

Even better, add /usr/local/mysql/bin to your PATH environment variable. For example, add the following line to your `$HOME/.tcshrc' file if your shell is @command{tcsh}: 

setenv PATH ${PATH}:/usr/local/mysql/bin

If no `.tcshrc' file exists in your home directory, create it with a text editor.

If you are upgrading an existing installation, please note that installing a new MySQL PKG does not remove the directory of an older installation. Unfortunately, the Mac OS X Installer does not yet offer the functionality required to properly upgrade previously installed packages.

To use your existing databases with the new installation, you'll need to copy the contents of the old data directory to the new data directory. Make sure that neither the old server nor the new one is running when you do this. After you have copied over the MySQL database files from the previous installation and have successfully started the new server, you should consider removing the old installation files to save disk space. Additionally, you should also remove older versions of the Package Receipt directories located in `/Library/Receipts/mysql-VERSION.pkg'.

2.2.4 Installing MySQL on NetWare

Porting MySQL to NetWare was an effort spearheaded by Novell. Novell customers will be pleased to note that NetWare 6.5 ships with bundled MySQL binaries, complete with an automatic commercial use license for all servers running that version of NetWare.

MySQL for NetWare is compiled using a combination of Metrowerks CodeWarrior for NetWare and special cross-compilation versions of the GNU autotools.

The latest binary packages for NetWare can be obtained at http://dev.mysql.com/downloads/. See section 2.1.3 How to Get MySQL.

In order to host MySQL, the NetWare server must meet these requirements:

To install MySQL for NetWare, use the following procedure:

  1. If you are upgrading from a prior installation, stop the MySQL server. This is done from the server console, using the following command: 
    SERVER:  mysqladmin -u root shutdown
  2. Log on to the target server from a client machine with access to the location where you will install MySQL.
  3. Extract the binary package Zip file onto the server. Be sure to allow the paths in the Zip file to be used. It is safe to simply extract the file to `SYS:\'. If you are upgrading from a prior installation, you may need to copy the data directory (for example, `SYS:MYSQL\DATA') now, as well as `my.cnf', if you have customized it. You can then delete the old copy of MySQL.
  4. You might want to rename the directory to something more consistent and easy to use. We recommend using `SYS:MYSQL'; examples in this manual use this name to refer to the installation directory in general.
  5. At the server console, add a search path for the directory containing the MySQL NLMs. For example: 
    SERVER:  SEARCH ADD SYS:MYSQL\BIN
  6. Initialize the data directory and the grant tables, if needed, by executing @command{mysql_install_db} at the server console.
  7. Start the MySQL server using @command{mysqld_safe} at the server console.
  8. To finish the installation, you should also add the following commands to autoexec.ncf. For example, if your MySQL installation is in `SYS:MYSQL' and you want MySQL to start automatically, you could add these lines: 
    #Starts the MySQL 4.0.x database server
SEARCH ADD SYS:MYSQL\BIN
MYSQLD_SAFE
    If you are running MySQL on NetWare 6.0, we strongly suggest that you use the --skip-external-locking option on the command line: 
    #Starts the MySQL 4.0.x database server
SEARCH ADD SYS:MYSQL\BIN
MYSQLD_SAFE --skip-external-locking
    It will also be necessary to use CHECK TABLE and REPAIR TABLE instead of @command{myisamchk}, because @command{myisamchk} makes use of external locking. External locking is known to have problems on NetWare 6.0; the problem has been eliminated in NetWare 6.5. @command{mysqld_safe} on NetWare provides a screen presence. When you unload (shut down) the @command{mysqld_safe} NLM, the screen does not by default go away. Instead, it prompts for user input: 
    *<NLM has terminated; Press any key to close the screen>*
    If you want NetWare to close the screen automatically instead, use the --autoclose option to @command{mysqld_safe}. For example: 
    #Starts the MySQL 4.0.x database server
SEARCH ADD SYS:MYSQL\BIN
MYSQLD_SAFE --autoclose
  9. The latest Netware Perl and PHP modules for MySQL can be downloaded from http://developer.novell.com/ndk/perl5.htm and http://developer.novell.com/ndk/php2.htm respectively.

The behavior of @command{mysqld_safe} on NetWare is described further in @ref{mysqld_safe, , @command{mysqld_safe}}.

If there was an existing installation of MySQL on the server, be sure to check for existing MySQL startup commands in autoexec.ncf, and edit or delete them as necessary.

Note: The accounts that are listed in the MySQL grant tables initially have no passwords. After starting the server, you should set up passwords for them using the instructions in section 2.4 Post-Installation Setup and Testing.

2.2.5 Installing MySQL on Other Unix-Like Systems

This section covers the installation of MySQL binary distributions that are provided for various platforms in the form of compressed @command{tar} files (files with a .tar.gz extension). See section 2.1.2.5 MySQL Binaries Compiled by MySQL AB for a detailed list.

To obtain MySQL, see section 2.1.3 How to Get MySQL.

MySQL @command{tar} file binary distributions have names of the form `mysql-VERSION-OS.tar.gz', where VERSION is a number (for example, 4.0.17), and OS indicates the type of operating system for which the distribution is intended (for example, pc-linux-i686).

In addition to these generic packages, we also offer binaries in platform-specific package formats for selected platforms. See section 2.2 Standard MySQL Installation Using a Binary Distribution for more information on how to install these.

You need the following tools to install a MySQL @command{tar} file binary distribution:

If you run into problems, please always use @command{mysqlbug} when posting questions to a MySQL mailing list. Even if the problem isn't a bug, @command{mysqlbug} gathers system information that will help others solve your problem. By not using @command{mysqlbug}, you lessen the likelihood of getting a solution to your problem. You will find @command{mysqlbug} in the `bin' directory after you unpack the distribution. See section 1.7.1.3 How to Report Bugs or Problems.

The basic commands you must execute to install and use a MySQL binary distribution are: 

shell> groupadd mysql
shell> useradd -g mysql mysql
shell> cd /usr/local
shell> gunzip < /path/to/mysql-VERSION-OS.tar.gz | tar xvf -
shell> ln -s full-path-to-mysql-VERSION-OS mysql
shell> cd mysql
shell> scripts/mysql_install_db --user=mysql
shell> chown -R root  .
shell> chown -R mysql data
shell> chgrp -R mysql .
shell> bin/mysqld_safe --user=mysql &

For versions of MySQL older than 4.0, substitute @command{bin/safe_mysqld} for @command{bin/mysqld_safe} in the final command.

Note: This procedure does not set up any passwords for MySQL accounts. After following the procedure, proceed to section 2.4 Post-Installation Setup and Testing.

A more detailed version of the preceding description for installing a binary distribution follows:

  1. Add a login user and group for @command{mysqld} to run as: 
    shell> groupadd mysql
shell> useradd -g mysql mysql
    These commands add the mysql group and the mysql user. The syntax for @command{useradd} and @command{groupadd} may differ slightly on different versions of Unix. They may also be called @command{adduser} and @command{addgroup}. You might want to call the user and group something else instead of mysql. If so, substitute the appropriate name in the following steps.
  2. Pick the directory under which you want to unpack the distribution, and change location into it. In the following example, we unpack the distribution under `/usr/local'. (The instructions, therefore, assume that you have permission to create files and directories in `/usr/local'. If that directory is protected, you will need to perform the installation as root.) 
    shell> cd /usr/local
  3. Obtain a distribution file from one of the sites listed in section 2.1.3 How to Get MySQL. For a given release, binary distributions for all platforms are built from the same MySQL source distribution.
  4. Unpack the distribution, which will create the installation directory. Then create a symbolic link to that directory: 
    shell> gunzip < /path/to/mysql-VERSION-OS.tar.gz | tar xvf -
shell> ln -s full-path-to-mysql-VERSION-OS mysql
    The @command{tar} command creates a directory named `mysql-VERSION-OS'. The ln command makes a symbolic link to that directory. This lets you refer more easily to the installation directory as `/usr/local/mysql'. With GNU @command{tar}, no separate invocation of gunzip is necessary. You can replace the first line with the following alternative command to uncompress and extract the distribution: 
    shell> tar zxvf /path/to/mysql-VERSION-OS.tar.gz
  5. Change location into the installation directory: 
    shell> cd mysql
    You will find several files and subdirectories in the mysql directory. The most important for installation purposes are the `bin' and `scripts' subdirectories.
    `bin'
    This directory contains client programs and the server. You should add the full pathname of this directory to your PATH environment variable so that your shell finds the MySQL programs properly. See section E Environment Variables.
    `scripts'
    This directory contains the @command{mysql_install_db} script used to initialize the mysql database containing the grant tables that store the server access permissions.
  6. If you haven't installed MySQL before, you must create the MySQL grant tables: 
    shell> scripts/mysql_install_db --user=mysql
    If you run the command as root, you should use the --user option as shown. The value of the option should be the name of the login account that you created in the first step to use for running the server. If you run the command while logged in as that user, you can omit the --user option. Note that for MySQL versions older than 3.22.10, @command{mysql_install_db} left the server running after creating the grant tables. This is no longer true; you will need to restart the server after performing the remaining steps in this procedure.
  7. Change the ownership of program binaries to root and ownership of the data directory to the user that you will run @command{mysqld} as. Assuming that you are located in the installation directory (`/usr/local/mysql'), the commands look like this: 
    shell> chown -R root  .
shell> chown -R mysql data
shell> chgrp -R mysql .
    The first command changes the owner attribute of the files to the root user. The second changes the owner attribute of the data directory to the mysql user. The third changes the group attribute to the mysql group.
  8. If you would like MySQL to start automatically when you boot your machine, you can copy support-files/mysql.server to the location where your system has its startup files. More information can be found in the support-files/mysql.server script itself and in section 2.4.2.2 Starting and Stopping MySQL Automatically.
  9. You can set up new accounts using the @command{bin/mysql_setpermission} script if you install the DBI and DBD::mysql Perl modules. For instructions, see section 2.7 Perl Installation Notes.
  10. If you would like to use @command{mysqlaccess} and have the MySQL distribution in some non-standard place, you must change the location where @command{mysqlaccess} expects to find the @command{mysql} client. Edit the `bin/mysqlaccess' script at approximately line 18. Search for a line that looks like this: 
    $MYSQL     = '/usr/local/bin/mysql';    # path to mysql executable
    Change the path to reflect the location where mysql actually is stored on your system. If you do not do this, you will get a Broken pipe error when you run @command{mysqlaccess}.

After everything has been unpacked and installed, you should test your distribution.

You can start the MySQL server with the following command: 

shell> bin/mysqld_safe --user=mysql &

For versions of MySQL older than 4.0, substitute @command{bin/safe_mysqld} for @command{bin/mysqld_safe} in the command.

More information about @command{mysqld_safe} is given in @ref{mysqld_safe, , @command{mysqld_safe}}.

Note: The accounts that are listed in the MySQL grant tables initially have no passwords. After starting the server, you should set up passwords for them using the instructions in section 2.4 Post-Installation Setup and Testing.

2.3 MySQL Installation Using a Source Distribution

Before you proceed with the source installation, check first to see whether our binary is available for your platform and whether it will work for you. We put a lot of effort into making sure that our binaries are built with the best possible options.

To obtain a source distribution for MySQL, section 2.1.3 How to Get MySQL.

MySQL source distributions are provided as compressed @command{tar} archives and have names of the form `mysql-VERSION.tar.gz', where VERSION is a number like 4.1.7.

You need the following tools to build and install MySQL from source:

If you are using a version of @command{gcc} recent enough to understand the -fno-exceptions option, it is very important that you use this option. Otherwise, you may compile a binary that crashes randomly. We also recommend that you use -felide-constructors and -fno-rtti along with -fno-exceptions. When in doubt, do the following: 

CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors \
       -fno-exceptions -fno-rtti" ./configure \
       --prefix=/usr/local/mysql --enable-assembler \
       --with-mysqld-ldflags=-all-static

On most systems, this will give you a fast and stable binary.

If you run into problems, please always use @command{mysqlbug} when posting questions to a MySQL mailing list. Even if the problem isn't a bug, @command{mysqlbug} gathers system information that will help others solve your problem. By not using @command{mysqlbug}, you lessen the likelihood of getting a solution to your problem. You will find @command{mysqlbug} in the `scripts' directory after you unpack the distribution. See section 1.7.1.3 How to Report Bugs or Problems.

2.3.1 Source Installation Overview

The basic commands you must execute to install a MySQL source distribution are: 

shell> groupadd mysql
shell> useradd -g mysql mysql
shell> gunzip < mysql-VERSION.tar.gz | tar -xvf -
shell> cd mysql-VERSION
shell> ./configure --prefix=/usr/local/mysql
shell> make
shell> make install
shell> cp support-files/my-medium.cnf /etc/my.cnf
shell> cd /usr/local/mysql
shell> bin/mysql_install_db --user=mysql
shell> chown -R root  .
shell> chown -R mysql var
shell> chgrp -R mysql .
shell> bin/mysqld_safe --user=mysql &

For versions of MySQL older than 4.0, substitute @command{bin/safe_mysqld} for @command{bin/mysqld_safe} in the final command.

If you start from a source RPM, do the following: 

shell> rpmbuild --rebuild --clean MySQL-VERSION.src.rpm

This will make a binary RPM that you can install. For older versions of RPM, you may have to replace the command @command{rpmbuild} with @command{rpm} instead.

Note: This procedure does not set up any passwords for MySQL accounts. After following the procedure, proceed to section 2.4 Post-Installation Setup and Testing, for post-installation setup and testing.

A more detailed version of the preceding description for installing MySQL from a source distribution follows:

  1. Add a login user and group for @command{mysqld} to run as: 
    shell> groupadd mysql
shell> useradd -g mysql mysql
    These commands add the mysql group and the mysql user. The syntax for @command{useradd} and @command{groupadd} may differ slightly on different versions of Unix. They may also be called @command{adduser} and @command{addgroup}. You might want to call the user and group something else instead of mysql. If so, substitute the appropriate name in the following steps.
  2. Pick the directory under which you want to unpack the distribution, and change location into it.
  3. Obtain a distribution file from one of the sites listed in section 2.1.3 How to Get MySQL.
  4. Unpack the distribution into the current directory: 
    shell> gunzip < /path/to/mysql-VERSION.tar.gz | tar xvf -
    This command creates a directory named `mysql-VERSION'. With GNU @command{tar}, no separate invocation of gunzip is necessary. You can use the following alternative command to uncompress and extract the distribution: 
    shell> tar zxvf /path/to/mysql-VERSION-OS.tar.gz
  5. Change location into the top-level directory of the unpacked distribution: 
    shell> cd mysql-VERSION
    Note that currently you must configure and build MySQL from this top-level directory. You cannot build it in a different directory.
  6. Configure the release and compile everything: 
    shell> ./configure --prefix=/usr/local/mysql
shell> make
    When you run @command{configure}, you might want to specify some options. Run @command{./configure --help} for a list of options. @ref{configure options, , @command{configure} options}, discusses some of the more useful options. If @command{configure} fails and you are going to send mail to a MySQL mailing list to ask for assistance, please include any lines from `config.log' that you think can help solve the problem. Also include the last couple of lines of output from @command{configure}. Post the bug report using the @command{mysqlbug} script. See section 1.7.1.3 How to Report Bugs or Problems. If the compile fails, see section 2.3.4 Dealing with Problems Compiling MySQL for help.
  7. Install the distribution: 
    shell> make install
    If you want to set up an option file, use one of those present in the `support-files' directory as a template. For example: 
    shell> cp support-files/my-medium.cnf /etc/my.cnf
    You might need to run these commands as root. If you want to configure support for InnoDB tables, you should edit the /etc/my.cnf file, remove the # character before the option lines that start with innodb_..., and modify the option values to be what you want. See section 4.3.2 Using Option Files and section 16.4 InnoDB Configuration.
  8. Change location into the installation directory: 
    shell> cd /usr/local/mysql
  9. If you haven't installed MySQL before, you must create the MySQL grant tables: 
    shell> bin/mysql_install_db --user=mysql
    If you run the command as root, you should use the --user option as shown. The value of the option should be the name of the login account that you created in the first step to use for running the server. If you run the command while logged in as that user, you can omit the --user option. Note that for MySQL versions older than 3.22.10, @command{mysql_install_db} left the server running after creating the grant tables. This is no longer true; you will need to restart the server after performing the remaining steps in this procedure.
  10. Change the ownership of program binaries to root and ownership of the data directory to the user that you will run @command{mysqld} as. Assuming that you are located in the installation directory (`/usr/local/mysql'), the commands look like this: 
    shell> chown -R root  .
shell> chown -R mysql var
shell> chgrp -R mysql .
    The first command changes the owner attribute of the files to the root user. The second changes the owner attribute of the data directory to the mysql user. The third changes the group attribute to the mysql group.
  11. If you would like MySQL to start automatically when you boot your machine, you can copy support-files/mysql.server to the location where your system has its startup files. More information can be found in the support-files/mysql.server script itself and in section 2.4.2.2 Starting and Stopping MySQL Automatically.
  12. You can set up new accounts using the @command{bin/mysql_setpermission} script if you install the DBI and DBD::mysql Perl modules. For instructions, see section 2.7 Perl Installation Notes.

After everything has been installed, you should initialize and test your distribution using this command: 

shell> /usr/local/mysql/bin/mysqld_safe --user=mysql &

For versions of MySQL older than 4.0, substitute @command{safe_mysqld} for @command{mysqld_safe} in the command.

If that command fails immediately and prints mysqld ended, you can find some information in the `host_name.err' file in the data directory.

More information about @command{mysqld_safe} is given in @ref{mysqld_safe, , @command{mysqld_safe}}.

Note: The accounts that are listed in the MySQL grant tables initially have no passwords. After starting the server, you should set up passwords for them using the instructions in section 2.4 Post-Installation Setup and Testing.

2.3.2 Typical @command{configure} Options

The @command{configure} script gives you a great deal of control over how you configure a MySQL source distribution. Typically you do this using options on the @command{configure} command line. You can also affect @command{configure} using certain environment variables. See section E Environment Variables. For a list of options supported by @command{configure}, run this command: 

shell> ./configure --help

Some of the more commonly used @command{configure} options are described here:

2.3.3 Installing from the Development Source Tree

Caution: You should read this section only if you are interested in helping us test our new code. If you just want to get MySQL up and running on your system, you should use a standard release distribution (either a binary or source distribution will do).

To obtain our most recent development source tree, use these instructions:

  1. Download BitKeeper from http://www.bitmover.com/cgi-bin/download.cgi. You will need Bitkeeper 3.0 or newer to access our repository.
  2. Follow the instructions to install it.
  3. After BitKeeper has been installed, first go to the directory you want to work from, and then use one of the following commands to clone the MySQL version branch of your choice: To clone the old 3.23 branch, use this command: 
    shell> bk clone bk://mysql.bkbits.net/mysql-3.23 mysql-3.23
    To clone the 4.0 stable (production) branch, use this command: 
    shell> bk clone bk://mysql.bkbits.net/mysql-4.0 mysql-4.0
    To clone the 4.1 gamma branch, use this command: 
    shell> bk clone bk://mysql.bkbits.net/mysql-4.1 mysql-4.1
    To clone the 5.0 development branch, use this command: 
    shell> bk clone bk://mysql.bkbits.net/mysql-5.0 mysql-5.0
    In the preceding examples, the source tree will be set up in the `mysql-3.23/', `mysql-4.0/', `mysql-4.1/', or `mysql-5.0/' subdirectory of your current directory. If you are behind a firewall and can only initiate HTTP connections, you can also use BitKeeper via HTTP. If you are required to use a proxy server, set the environment variable http_proxy to point to your proxy: 
    shell> export http_proxy="http://your.proxy.server:8080/"
    Now, simply replace the bk:// with http:// when doing a clone. Example: 
    shell> bk clone http://mysql.bkbits.net/mysql-4.1 mysql-4.1
    The initial download of the source tree may take a while, depending on the speed of your connection. Please be patient.
  4. You will need GNU @command{make}, @command{autoconf} 2.53 (or newer), @command{automake} 1.5, @command{libtool} 1.5, and @command{m4} to run the next set of commands. Even though many operating systems already come with their own implementation of @command{make}, chances are high that the compilation will fail with strange error messages. Therefore, it is highly recommended that you use GNU @command{make} (sometimes named @command{gmake}) instead. Fortunately, a large number of operating systems already ship with the GNU toolchain preinstalled or supply installable packages of these. In any case, they can also be downloaded from the following locations: If you are trying to configure MySQL 4.1 or later, you will also need GNU @command{bison} 1.75 or later. Older versions of @command{bison} may report this error: 
    sql_yacc.yy:#####: fatal error: maximum table size (32767) exceeded
    Note: The maximum table size is not actually exceeded; the error is caused by bugs in older versions of @command{bison}. Versions of MySQL before version 4.1 may also compile with other @command{yacc} implementations (for example, BSD @command{yacc} 91.7.30). For later versions, GNU @command{bison} is required. The following example shows the typical commands required to configure a source tree. The first cd command changes location into the top-level directory of the tree; replace `mysql-4.0' with the appropriate directory name. 
    shell> cd mysql-4.0
shell> bk -r edit
shell> aclocal; autoheader; autoconf; automake
shell> (cd innobase; aclocal; autoheader; autoconf; automake)
shell> (cd bdb/dist; sh s_all)
shell> ./configure  # Add your favorite options here
make
    The command lines that change directory into the `innobase' and `bdb/dist' directories are used to configure the InnoDB and Berkeley DB (BDB) storage engines. You can omit these command lines if you to not require InnoDB or BDB support. If you get some strange errors during this stage, verify that you really have @command{libtool} installed. A collection of our standard configuration scripts is located in the `BUILD/' subdirectory. You may find it more convenient to use the `BUILD/compile-pentium-debug' script than the preceding set of shell commands. To compile on a different architecture, modify the script by removing flags that are Pentium-specific.
  5. When the build is done, run @command{make install}. Be careful with this on a production machine; the command may overwrite your live release installation. If you have another installation of MySQL, we recommend that you run @command{./configure} with different values for the --prefix, --with-tcp-port, and --unix-socket-path options than those used for your production server.
  6. Play hard with your new installation and try to make the new features crash. Start by running @command{make test}. See section 24.1.2 MySQL Test Suite.
  7. If you have gotten to the @command{make} stage and the distribution does not compile, please report it in our bugs database at http://bugs.mysql.com/. If you have installed the latest versions of the required GNU tools, and they crash trying to process our configuration files, please report that also. However, if you execute aclocal and get a command not found error or a similar problem, do not report it. Instead, make sure that all the necessary tools are installed and that your PATH variable is set correctly so that your shell can find them.
  8. After the initial bk clone operation to obtain the source tree, you should run bk pull periodically to get updates.
  9. You can examine the change history for the tree with all the diffs by using bk revtool. If you see some funny diffs or code that you have a question about, do not hesitate to send email to the MySQL internals mailing list. See section 1.7.1.1 The MySQL Mailing Lists. Also, if you think you have a better idea on how to do something, send an email message to the same address with a patch. bk diffs will produce a patch for you after you have made changes to the source. If you do not have the time to code your idea, just send a description.
  10. BitKeeper has a nice help utility that you can access via bk helptool.
  11. Please note that any commits (made via bk ci or bk citool) will trigger the posting of a message with the changeset to our internals mailing list, as well as the usual openlogging.org submission with just the changeset comments. Generally, you wouldn't need to use commit (since the public tree will not allow bk push), but rather use the bk diffs method described previously.

You can also browse changesets, comments, and source code online. For example, to browse this information for MySQL 4.1, go to http://mysql.bkbits.net:8080/mysql-4.1.

The manual is in a separate tree that can be cloned with: 

shell> bk clone bk://mysql.bkbits.net/mysqldoc mysqldoc

There are also public BitKeeper trees for MySQL Control Center and MyODBC. They can be cloned respectively as follows.

To clone MySQL Control center, use this command: 

shell> bk clone http://mysql.bkbits.net/mysqlcc mysqlcc

To clone MyODBC, use this command: 

shell> bk clone http://mysql.bkbits.net/myodbc3 myodbc3

To clone Connector/NET, use this command: 

shell> bk clone http://mysql.bkbits.net/connector-net connector-net

2.3.4 Dealing with Problems Compiling MySQL

All MySQL programs compile cleanly for us with no warnings on Solaris or Linux using @command{gcc}. On other systems, warnings may occur due to differences in system include files. See section 2.3.5 MIT-pthreads Notes for warnings that may occur when using MIT-pthreads. For other problems, check the following list.

The solution to many problems involves reconfiguring. If you do need to reconfigure, take note of the following:

To prevent old configuration information or object files from being used, run these commands before re-running @command{configure}: 

shell> rm config.cache
shell> make clean

Alternatively, you can run @command{make distclean}.

The following list describes some of the problems when compiling MySQL that have been found to occur most often:

2.3.5 MIT-pthreads Notes

This section describes some of the issues involved in using MIT-pthreads.

On Linux, you should not use MIT-pthreads. Use the installed LinuxThreads implementation instead. See section 2.6.1 Linux Notes.

If your system does not provide native thread support, you will need to build MySQL using the MIT-pthreads package. This includes older FreeBSD systems, SunOS 4.x, Solaris 2.4 and earlier, and some others. See section 2.1.1 Operating Systems Supported by MySQL.

Beginning with MySQL 4.0.2, MIT-pthreads is no longer part of the source distribution. If you require this package, you need to download it separately from http://www.mysql.com/Downloads/Contrib/pthreads-1_60_beta6-mysql.tar.gz

After downloading, extract this source archive into the top level of the MySQL source directory. It will create a new subdirectory named mit-pthreads.

2.3.6 Installing MySQL from Source on Windows

These instructions describe how to build MySQL binaries from source for versions 4.1 and above on Windows. Instructions are provided for building binaries from a standard source distribution or from the BitKeeper tree that contains the latest development source.

Note: The instructions in this document are strictly for users who want to test MySQL on Windows from the latest source distribution or from the BitKeeper tree. For production use, MySQL AB does not advise using a MySQL server built by yourself from source. Normally, it is best to use precompiled binary distributions of MySQL that are built specifically for optimal performance on Windows by MySQL AB. Instructions for installing a binary distributions are available at section 2.2.1 Installing MySQL on Windows.

To build MySQL on Windows from source, you need the following compiler and resources available on your Windows system:

You'll also need a MySQL source distribution for Windows. There are two ways you can get a source distribution for MySQL version 4.1 and above:

  1. Obtain a source distribution packaged by MySQL AB for the particular version of MySQL in which you are interested. Prepackaged source distributions are available for released versions of MySQL and can be obtained from http://dev.mysql.com/downloads/.
  2. You can package a source distribution yourself from the latest BitKeeper developer source tree. If you plan to do this, you must create the package on a Unix system and then transfer it to your Windows system. (The reason for this is that some of the configuration and build steps require tools that work only on Unix.) The BitKeeper approach thus requires:
    • A system running Unix, or a Unix-like system such as Linux.
    • BitKeeper 3.0 installed on that system. You can obtain BitKeeper from http://www.bitkeeper.com/.

If you are using a Windows source distribution, you can go directly to section 2.3.6.1 Building MySQL Using VC++. To build from the BitKeeper tree, proceed to section 2.3.6.2 Creating a Windows Source Package from the Latest Development Source.

If you find something not working as expected, or you have suggestions about ways to improve the current build process on Windows, please send a message to the win32 mailing list. See section 1.7.1.1 The MySQL Mailing Lists.

2.3.6.1 Building MySQL Using VC++

Note: VC++ workspace files for MySQL 4.1 and above are compatible with Microsoft Visual Studio 6.0 and above (7.0/.NET) editions and tested by MySQL AB staff before each release.

Follow this procedure to build MySQL:

  1. Create a work directory (for example, `C:\workdir').
  2. Unpack the source distribution in the aforementioned directory using WinZip or other Windows tool that can read `.zip' files.
  3. Start the VC++ 6.0 compiler.
  4. In the File menu, select Open Workspace.
  5. Open the `mysql.dsw' workspace you find in the work directory.
  6. From the Build menu, select the Set Active Configuration menu.
  7. Click over the screen selecting mysqld - Win32 Debug and click OK.
  8. Press F7 to begin the build of the debug server, libraries, and some client applications.
  9. Compile the release versions that you want in the same way.
  10. Debug versions of the programs and libraries are placed in the `client_debug' and `lib_debug' directories. Release versions of the programs and libraries are placed in the `client_release' and `lib_release' directories. Note that if you want to build both debug and release versions, you can select the Build All option from the Build menu.
  11. Test the server. The server built using the preceding instructions will expect that the MySQL base directory and data directory are `C:\mysql' and `C:\mysql\data' by default. If you want to test your server using the source tree root directory and its data directory as the base directory and data directory, you will need to tell the server their pathnames. You can either do this on the command line with the --basedir and --datadir options, or place appropriate options in an option file (the `my.ini' file in your Windows directory or `C:\my.cnf'). If you have an existing data directory elsewhere that you want to use, you can specify its pathname instead.
  12. Start your server from the `client_release' or `client_debug' directory, depending on which server you want to use. The general server startup instructions are at section 2.2.1 Installing MySQL on Windows. You'll need to adapt the instructions appropriately if you want to use a different base directory or data directory.
  13. When the server is running in standalone fashion or as a service based on your configuration, try to connect to it from the @command{mysql} interactive command-line utility that exists in your `client_release' or `client_debug' directory.

When you are satisfied that the programs you have built are working correctly, stop the server. Then install MySQL as follows:

  1. Create the directories where you want to install MySQL. For example, to install into `C:\mysql', use these commands: 
    C:\> mkdir C:\mysql
C:\> mkdir C:\mysql\bin
C:\> mkdir C:\mysql\data
C:\> mkdir C:\mysql\share
C:\> mkdir C:\mysql\scripts
    If you want to compile other clients and link them to MySQL, you should also create several additional directories: 
    C:\> mkdir C:\mysql\include
C:\> mkdir C:\mysql\lib
C:\> mkdir C:\mysql\lib\debug
C:\> mkdir C:\mysql\lib\opt
    If you want to benchmark MySQL, create this directory: 
    C:\> mkdir C:\mysql\sql-bench
    Benchmarking requires Perl support.
  2. From the `workdir' directory, copy into the C:\mysql directory the following directories: 
    C:\> cd \workdir
C:\workdir> copy client_release\*.exe C:\mysql\bin
C:\workdir> copy client_debug\mysqld.exe C:\mysql\bin\mysqld-debug.exe
C:\workdir> xcopy scripts\*.* C:\mysql\scripts /E
C:\workdir> xcopy share\*.* C:\mysql\share /E
    If you want to compile other clients and link them to MySQL, you should also copy several libraries and header files: 
    C:\workdir> copy lib_debug\mysqlclient.lib C:\mysql\lib\debug
C:\workdir> copy lib_debug\libmysql.* C:\mysql\lib\debug
C:\workdir> copy lib_debug\zlib.* C:\mysql\lib\debug
C:\workdir> copy lib_release\mysqlclient.lib C:\mysql\lib\opt
C:\workdir> copy lib_release\libmysql.* C:\mysql\lib\opt
C:\workdir> copy lib_release\zlib.* C:\mysql\lib\opt
C:\workdir> copy include\*.h C:\mysql\include
C:\workdir> copy libmysql\libmysql.def C:\mysql\include
    If you want to benchmark MySQL, you should also do this: 
    C:\workdir> xcopy sql-bench\*.* C:\mysql\bench /E

Set up and start the server in the same way as for the binary Windows distribution. See section 2.2.1 Installing MySQL on Windows.

2.3.6.2 Creating a Windows Source Package from the Latest Development Source

To create a Windows source package from the current BitKeeper source tree, use the following instructions. Please note that this procedure must be performed on a system running a Unix or Unix-like operating system. For example, the procedure is known to work well on Linux.

  1. Clone the BitKeeper source tree for MySQL (version 4.1 or above, as desired). For more information on how to clone the source tree, see the instructions at section 2.3.3 Installing from the Development Source Tree.
  2. Configure and build the distribution so that you have a server binary to work with. One way to do this is to run the following command in the top-level directory of your source tree: 
    shell> ./BUILD/compile-pentium-max
  3. After making sure that the build process completed successfully, run the following utility script from top-level directory of your source tree: 
    shell> ./scripts/make_win_src_distribution
    This script creates a Windows source package to be used on your Windows system. You can supply different options to the script based on your needs. It accepts the following options:
    --help
    Display a help message.
    --debug
    Print information about script operations, do not create package.
    --tmp
    Specify the temporary location.
    --suffix
    Suffix name for the package.
    --dirname
    Directory name to copy files (intermediate).
    --silent
    Do not print verbose list of files processed.
    --tar
    Create `tar.gz' package instead of `.zip' package.
    By default, @command{make_win_src_distribution} creates a Zip-format archive with the name `mysql-VERSION-win-src.zip', where VERSION represents the version of your MySQL source tree.
  4. Copy or upload to your Windows machine the Windows source package that you have just created. To compile it, use the instructions in section 2.3.6.1 Building MySQL Using VC++.

2.3.7 Compiling MySQL Clients on Windows

In your source files, you should include `my_global.h' before `mysql.h': 

#include <my_global.h>
#include <mysql.h>

`my_global.h' includes any other files needed for Windows compatibility (such as `windows.h') if you compile your program on Windows.

You can either link your code with the dynamic `libmysql.lib' library, which is just a wrapper to load in `libmysql.dll' on demand, or link with the static `mysqlclient.lib' library.

The MySQL client libraries are compiled as threaded libraries, so you should also compile your code to be multi-threaded.

2.4 Post-Installation Setup and Testing

After installing MySQL, there are some issues you should address. For example, on Unix, you should initialize the data directory and create the MySQL grant tables. On all platforms, an important security concern is that the initial accounts in the grant tables have no passwords. You should assign passwords to prevent unauthorized access to the MySQL server. For MySQL 4.1.3 and up, you can create time zone tables to enable recognition of named time zones. (Currently, these tables can be populated only on Unix. This problem will be addressed soon for Windows.)

The following sections include post-installation procedures that are specific to Windows systems and to Unix systems. Another section, section 2.4.2.3 Starting and Troubleshooting the MySQL Server, applies to all platforms; it describes what to do if you have trouble getting the server to start. section 2.4.3 Securing the Initial MySQL Accounts also applies to all platforms. You should follow its instructions to make sure that you have properly protected your MySQL accounts by assigning passwords to them.

When you are ready to create additional user accounts, you can find information on the MySQL access control system and account management in section 5.5 The MySQL Access Privilege System and section 5.6 MySQL User Account Management.

2.4.1 Windows Post-Installation Procedures

On Windows, the data directory and the grant tables do not have to be created. MySQL Windows distributions include the grant tables already set up with a set of preinitialized accounts in the mysql database under the data directory. You do not run the @command{mysql_install_db} script that is used on Unix. However, you should assign passwords to the accounts. The procedure for this is given in section 2.4.3 Securing the Initial MySQL Accounts.

Before setting up passwords, you might want to try running some client programs to make sure that you can connect to the server and that it is operating properly. Make sure the server is running (see section 2.2.1.5 Starting the Server for the First Time), then issue the following commands to verify that you can retrieve information from the server. The output should be similar to what is shown here: 

C:\> C:\mysql\bin\mysqlshow
+-----------+
| Databases |
+-----------+
| mysql     |
| test      |
+-----------+

C:\> C:\mysql\bin\mysqlshow mysql
Database: mysql
+--------------+
|    Tables    |
+--------------+
| columns_priv |
| db           |
| func         |
| host         |
| tables_priv  |
| user         |
+--------------+

C:\> C:\mysql\bin\mysql -e "SELECT Host,Db,User FROM db" mysql
+------+-------+------+
| host | db    | user |
+------+-------+------+
| %    | test% |      |
+------+-------+------+

If you are running a version of Windows that supports services and you want the MySQL server to run automatically when Windows starts, see section 2.2.1.7 Starting MySQL as a Windows Service.

2.4.2 Unix Post-Installation Procedures

After installing MySQL on Unix, you need to initialize the grant tables, start the server, and make sure that the server works okay. You may also wish to arrange for the server to be started and stopped automatically when your system starts and stops. You should also assign passwords to the accounts in the grant tables.

On Unix, the grant tables are set up by the @command{mysql_install_db} program. For some installation methods, this program is run for you automatically:

Otherwise, you'll need to run @command{mysql_install_db} yourself.

The following procedure describes how to initialize the grant tables (if that has not already been done) and then start the server. It also suggests some commands that you can use to test whether the server is accessible and working properly. For information about starting and stopping the server automatically, see section 2.4.2.2 Starting and Stopping MySQL Automatically.

After you complete the procedure and have the server running, you should assign passwords to the accounts created by @command{mysql_install_db}. Instructions for doing so are given in section 2.4.3 Securing the Initial MySQL Accounts.

In the examples shown here, the server runs under the user ID of the mysql login account. This assumes that such an account exists. Either create the account if it does not exist, or substitute the name of a different existing login account that you plan to use for running the server.

  1. Change location into the top-level directory of your MySQL installation, represented here by BASEDIR: 
    shell> cd BASEDIR
    BASEDIR is likely to be something like `/usr/local/mysql' or `/usr/local'. The following steps assume that you are located in this directory.
  2. If necessary, run the @command{mysql_install_db} program to set up the initial MySQL grant tables containing the privileges that determine how users are allowed to connect to the server. You'll need to do this if you used a distribution type that doesn't run the program for you. Typically, @command{mysql_install_db} needs to be run only the first time you install MySQL, so you can skip this step if you are upgrading an existing installation, However, @command{mysql_install_db} does not overwrite any existing privilege tables, so it should be safe to run in any circumstances. To initialize the grant tables, use one of the following commands, depending on whether @command{mysql_install_db} is located in the bin or scripts directory: 
    shell> bin/mysql_install_db --user=mysql
shell> scripts/mysql_install_db --user=mysql
    The @command{mysql_install_db} script creates the data directory, the mysql database that holds all database privileges, and the test database that you can use to test MySQL. The script also creates privilege table entries for root accounts and anonymous-user accounts. The accounts have no passwords initially. A description of their initial privileges is given in section 2.4.3 Securing the Initial MySQL Accounts. Briefly, these privileges allow the MySQL root user to do anything, and allow anybody to create or use databases with a name of test or starting with test_. It is important to make sure that the database directories and files are owned by the mysql login account so that the server has read and write access to them when you run it later. To ensure this, the --user option should be used as shown if you run @command{mysql_install_db} as root. Otherwise, you should execute the script while logged in as mysql, in which case you can omit the --user option from the command. @command{mysql_install_db} creates several tables in the mysql database: user, db, host, tables_priv, columns_priv, func, and possibly others depending on your version of MySQL. If you don't want to have the test database, you can remove it with @command{mysqladmin -u root drop test} after starting the server. If you have problems with @command{mysql_install_db}, see @ref{mysql_install_db, , @command{mysql_install_db}}. There are some alternatives to running the @command{mysql_install_db} script as it is provided in the MySQL distribution:
    • If you want the initial privileges to be different from the standard defaults, you can modify @command{mysql_install_db} before you run it. However, a preferable technique is to use GRANT and REVOKE to change the privileges after the grant tables have been set up. In other words, you can run @command{mysql_install_db}, and then use @code{mysql -u root mysql} to connect to the server as the MySQL root user so that you can issue the GRANT and REVOKE statements. If you want to install MySQL on a lot of machines with the same privileges, you can put the GRANT and REVOKE statements in a file and execute the file as a script using mysql after running @command{mysql_install_db}. For example: 
      shell> bin/mysql_install_db --user=mysql
shell> bin/mysql -u root < your_script_file
      By doing this, you can avoid having to issue the statements manually on each machine.
    • It is possible to re-create the grant tables completely after they have already been created. You might want to do this if you're just learning how to use GRANT and REVOKE and have made so many modifications after running @command{mysql_install_db} that you want to wipe out the tables and start over. To re-create the grant tables, remove all the `.frm', `.MYI', and `.MYD' files in the directory containing the mysql database. (This is the directory named `mysql' under the data directory, which is listed as the datadir value when you run @command{mysqld --help}.) Then run the @command{mysql_install_db} script again. Note: For MySQL versions older than 3.22.10, you should not delete the `.frm' files. If you accidentally do this, you should copy them back into the `mysql' directory from your MySQL distribution before running @command{mysql_install_db}.
    • You can start @command{mysqld} manually using the --skip-grant-tables option and add the privilege information yourself using @command{mysql}: 
      shell> bin/mysqld_safe --user=mysql --skip-grant-tables &
shell> bin/mysql mysql
      From @command{mysql}, manually execute the SQL commands contained in @command{mysql_install_db}. Make sure that you run @command{mysqladmin flush-privileges} or @command{mysqladmin reload} afterward to tell the server to reload the grant tables. Note that by not using @command{mysql_install_db}, you not only have to populate the grant tables manually, you also have to create them first.
  3. Start the MySQL server: 
    shell> bin/mysqld_safe --user=mysql &
    For versions of MySQL older than 4.0, substitute @command{bin/safe_mysqld} for @command{bin/mysqld_safe} in this command. It is important that the MySQL server be run using an unprivileged (non-root) login account. To ensure this, the --user option should be used as shown if you run mysql_safe as root. Otherwise, you should execute the script while logged in as mysql, in which case you can omit the --user option from the command. Further instructions for running MySQL as an unprivileged user are given in section A.3.2 How to Run MySQL as a Normal User. If you neglected to create the grant tables before proceeding to this step, the following message will appear in the error log file when you start the server: 
    mysqld: Can't find file: 'host.frm'
    If you have other problems starting the server, see section 2.4.2.3 Starting and Troubleshooting the MySQL Server.
  4. Use @command{mysqladmin} to verify that the server is running. The following commands provide simple tests to check whether the server is up and responding to connections: 
    shell> bin/mysqladmin version
shell> bin/mysqladmin variables
    The output from @command{mysqladmin version} varies slightly depending on your platform and version of MySQL, but should be similar to that shown here: 
    shell> bin/mysqladmin version
mysqladmin  Ver 8.40 Distrib 4.0.18, for linux on i586
Copyright (C) 2000 MySQL AB & MySQL Finland AB & TCX DataKonsult AB
This software comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to modify and redistribute it under the GPL license

Server version          4.0.18-log
Protocol version        10
Connection              Localhost via Unix socket
TCP port                3306
UNIX socket             /tmp/mysql.sock
Uptime:                 16 sec

Threads: 1  Questions: 9  Slow queries: 0
Opens: 7  Flush tables: 2  Open tables: 0
Queries per second avg: 0.000
Memory in use: 132K  Max memory used: 16773K
    To see what else you can do with @command{mysqladmin}, invoke it with the --help option.
  5. Verify that you can shut down the server: 
    shell> bin/mysqladmin -u root shutdown
  6. Verify that you can restart the server. Do this by using @command{mysqld_safe} or by invoking @command{mysqld} directly. For example: 
    shell> bin/mysqld_safe --user=mysql --log &
    If @command{mysqld_safe} fails, see section 2.4.2.3 Starting and Troubleshooting the MySQL Server.
  7. Run some simple tests to verify that you can retrieve information from the server. The output should be similar to what is shown here: 
    shell> bin/mysqlshow
+-----------+
| Databases |
+-----------+
| mysql     |
| test      |
+-----------+

shell> bin/mysqlshow mysql
Database: mysql
+--------------+
|    Tables    |
+--------------+
| columns_priv |
| db           |
| func         |
| host         |
| tables_priv  |
| user         |
+--------------+

shell> bin/mysql -e "SELECT Host,Db,User FROM db" mysql
+------+--------+------+
| host | db     | user |
+------+--------+------+
| %    | test   |      |
| %    | test_% |      |
+------+--------+------+
  8. There is a benchmark suite in the `sql-bench' directory (under the MySQL installation directory) that you can use to compare how MySQL performs on different platforms. The benchmark suite is written in Perl. It uses the Perl DBI module to provide a database-independent interface to the various databases, and some other additional Perl modules are required to run the benchmark suite. You must have the following modules installed: 
    DBI
DBD::mysql
Data::Dumper
Data::ShowTable
    These modules can be obtained from CPAN (http://www.cpan.org/). See section 2.7.1 Installing Perl on Unix. The `sql-bench/Results' directory contains the results from many runs against different databases and platforms. To run all tests, execute these commands: 
    shell> cd sql-bench
shell> perl run-all-tests
    If you don't have the `sql-bench' directory, you probably installed MySQL using RPM files other than the source RPM. (The source RPM includes the `sql-bench' benchmark directory.) In this case, you must first install the benchmark suite before you can use it. Beginning with MySQL 3.22, there are separate benchmark RPM files named `mysql-bench-VERSION-i386.rpm' that contain benchmark code and data. If you have a source distribution, there are also tests in its `tests' subdirectory that you can run. For example, to run `auto_increment.tst', execute this command from the top-level directory of your source distribution: 
    shell> mysql -vvf test < ./tests/auto_increment.tst
    The expected result of the test can be found in the `./tests/auto_increment.res' file.
  9. At this point, you should have the server running. However, none of the initial MySQL accounts have a password, so you should assign passwords using the instructions in section 2.4.3 Securing the Initial MySQL Accounts.

As of MySQL 4.1.3, the installation procedure creates time zone tables in the mysql database. However, you must populate the tables manually. Instructions to do this are given in section 5.8.8 MySQL Server Time Zone Support.

2.4.2.1 Problems Running @command{mysql_install_db}

The purpose of the @command{mysql_install_db} script is to generate new MySQL privilege tables. It will not overwrite existing MySQL privilege tables, and it will not affect any other data.

If you want to re-create your privilege tables, first stop the @command{mysqld} server if it's running. Then rename the `mysql' directory under the data directory to save it, and then run @command{mysql_install_db}. For example: 

shell> mv mysql-data-directory/mysql mysql-data-directory/mysql-old
shell> mysql_install_db --user=mysql

This section lists problems you might encounter when you run @command{mysql_install_db}:

@command{mysql_install_db doesn't install the grant tables}
You may find that @command{mysql_install_db} fails to install the grant tables and terminates after displaying the following messages: 
Starting mysqld daemon with databases from XXXXXX
mysqld ended
In this case, you should examine the error log file very carefully. The log should be located in the directory `XXXXXX' named by the error message, and should indicate why @command{mysqld} didn't start. If you don't understand what happened, include the log when you post a bug report. See section 1.7.1.3 How to Report Bugs or Problems.
There is already a @command{mysqld process running}
This indicates that the server is already running, in which case the grant tables probably have already been created. If so, you don't have to run @command{mysql_install_db} at all because it need be run only once (when you install MySQL the first time).
Installing a second @command{mysqld server doesn't work when one server is running}
This can happen when you already have an existing MySQL installation, but want to put a new installation in a different location. For example, you might have a production installation already, but you want to create a second installation for testing purposes. Generally the problem that occurs when you try to run a second server is that it tries to use a network interface that is already in use by the first server. In this case, you will see one of the following error messages: 
Can't start server: Bind on TCP/IP port:
Address already in use
Can't start server: Bind on unix socket...
For instructions on setting up multiple servers, see section 5.10 Running Multiple MySQL Servers on the Same Machine.
You don't have write access to `/tmp'
If you don't have write access to create temporary files or a Unix socket file in the default location (the `/tmp' directory), an error will occur when you run @command{mysql_install_db} or the @command{mysqld} server. You can specify different temporary directory and Unix socket file locations by executing these commands prior to starting @command{mysql_install_db} or @command{mysqld}: 
shell> TMPDIR=/some_tmp_dir/
shell> MYSQL_UNIX_PORT=/some_tmp_dir/mysql.sock
shell> export TMPDIR MYSQL_UNIX_PORT
`some_tmp_dir' should be the full pathname to some directory for which you have write permission. After this, you should be able to run @command{mysql_install_db} and start the server with these commands: 
shell> bin/mysql_install_db --user=mysql
shell> bin/mysqld_safe --user=mysql &
If @command{mysql_install_db} is located in the `scripts' directory, modify the first command to use scripts/mysql_install_db. See section A.4.5 How to Protect or Change the MySQL Socket File `/tmp/mysql.sock'. See section E Environment Variables.

2.4.2.2 Starting and Stopping MySQL Automatically

Generally, you start the @command{mysqld} server in one of these ways:

The @command{mysql.server} and @command{mysqld_safe} scripts and the Mac OS X Startup Item can be used to start the server manually, or automatically at system startup time. @command{mysql.server} and the Startup Item also can be used to stop the server.

To start or stop the server manually using the @command{mysql.server} script, invoke it with start or stop arguments: 

shell> mysql.server start
shell> mysql.server stop

Before @command{mysql.server} starts the server, it changes location to the MySQL installation directory, and then invokes @command{mysqld_safe}. If you want the server to run as some specific user, add an appropriate user option to the [mysqld] group of the `/etc/my.cnf' option file, as shown later in this section. (It is possible that you'll need to edit @command{mysql.server} if you've installed a binary distribution of MySQL in a non-standard location. Modify it to cd into the proper directory before it runs @command{mysqld_safe}. If you do this, your modified version of @command{mysql.server} may be overwritten if you upgrade MySQL in the future, so you should make a copy of your edited version that you can reinstall.)

@command{mysql.server stop} brings down the server by sending a signal to it. You can also stop the server manually by executing @command{mysqladmin shutdown}.

To start and stop MySQL automatically on your server, you need to add start and stop commands to the appropriate places in your `/etc/rc*' files.

If you use the Linux server RPM package (MySQL-server-VERSION.rpm), the @command{mysql.server} script will already have been installed in the `/etc/init.d' directory with the name `mysql'. You need not install it manually. See section 2.2.2 Installing MySQL on Linux for more information on the Linux RPM packages.

Some vendors provide RPM packages that install a startup script under a different name such as @command{mysqld}.

If you install MySQL from a source distribution or using a binary distribution format that does not install @command{mysql.server} automatically, you can install it manually. The script can be found in the `support-files' directory under the MySQL installation directory or in a MySQL source tree.

To install @command{mysql.server} manually, copy it to the `/etc/init.d' directory with the name @command{mysql}, and then make it executable. Do this by changing location into the appropriate directory where @command{mysql.server} is located and executing these commands: 

shell> cp mysql.server /etc/init.d/mysql
shell> chmod +x /etc/init.d/mysql

Older Red Hat systems use the `/etc/rc.d/init.d' directory rather than `/etc/init.d'. Adjust the preceding commands accordingly. Alternatively, first create `/etc/init.d' as a symbolic link that points to `/etc/rc.d/init.d': 

shell> cd /etc
shell> ln -s rc.d/init.d .

After installing the script, the commands needed to activate it to run at system startup depend on your operating system. On Linux, you can use chkconfig: 

shell> chkconfig --add mysql

On some Linux systems, the following command also seems to be necessary to fully enable the @command{mysql} script: 

shell> chkconfig --level 345 mysql on

On FreeBSD, startup scripts generally should go in `/usr/local/etc/rc.d/'. The rc(8) manual page states that scripts in this directory are executed only if their basename matches the *.sh shell filename pattern. Any other files or directories present within the directory are silently ignored. In other words, on FreeBSD, you should install the `mysql.server' script as `/usr/local/etc/rc.d/mysql.server.sh' to enable automatic startup.

As an alternative to the preceding setup, some operating systems also use `/etc/rc.local' or `/etc/init.d/boot.local' to start additional services on startup. To start up MySQL using this method, you could append a command like the one following to the appropriate startup file: 

/bin/sh -c 'cd /usr/local/mysql; ./bin/mysqld_safe --user=mysql &'

For other systems, consult your operating system documentation to see how to install startup scripts.

You can add options for @command{mysql.server} in a global `/etc/my.cnf' file. A typical `/etc/my.cnf' file might look like this: 

[mysqld]
datadir=/usr/local/mysql/var
socket=/var/tmp/mysql.sock
port=3306
user=mysql

[mysql.server]
basedir=/usr/local/mysql

The @command{mysql.server} script understands the following options: basedir, datadir, and pid-file. If specified, they must be placed in an option file, not on the command line. @command{mysql.server} understands only start and stop as command-line arguments.

The following table shows which option groups the server and each startup script read from option files:

Script Option Groups
@command{mysqld} [mysqld], [server], [mysqld-major-version]
@command{mysql.server} [mysqld], [mysql.server]
@command{mysqld_safe} [mysqld], [server], [mysqld_safe]

[mysqld-major-version] means that groups with names like [mysqld-4.0], [mysqld-4.1], and [mysqld-5.0] will be read by servers having versions 4.0.x, 4.1.x, 5.0.x, and so forth. This feature was added in MySQL 4.0.14. It can be used to specify options that will be read only by servers within a given release series.

For backward compatibility, @command{mysql.server} also reads the [mysql_server] group and @command{mysqld_safe} also reads the [safe_mysqld] group. However, you should update your option files to use the [mysql.server] and [mysqld_safe] groups instead when you begin using MySQL 4.0 or later.

See section 4.3.2 Using Option Files.

2.4.2.3 Starting and Troubleshooting the MySQL Server

If you have problems starting the server, here are some things you can try:

Some storage engines have options that control their behavior. You can create a `my.cnf' file and set startup options for the engines you plan to use. If you are going to use storage engines that support transactional tables (InnoDB, BDB), be sure that you have them configured the way you want before starting the server:

When the @command{mysqld} server starts, it changes location to the data directory. This is where it expects to find databases and where it expects to write log files. On Unix, the server also writes the pid (process ID) file in the data directory.

The data directory location is hardwired in when the server is compiled. This is where the server looks for the data directory by default. If the data directory is located somewhere else on your system, the server will not work properly. You can find out what the default path settings are by invoking @command{mysqld} with the --verbose and --help options. (Prior to MySQL 4.1, omit the --verbose option.)

If the defaults don't match the MySQL installation layout on your system, you can override them by specifying options on the command line to @command{mysqld} or @command{mysqld_safe}. You can also list the options in an option file.

To specify the location of the data directory explicitly, use the --datadir option. However, normally you can tell @command{mysqld} the location of the base directory under which MySQL is installed and it will look for the data directory there. You can do this with the --basedir option.

To check the effect of specifying path options, invoke @command{mysqld} with those options followed by the --verbose and --help options. For example, if you change location into the directory where @command{mysqld} is installed, and then run the following command, it will show the effect of starting the server with a base directory of `/usr/local': 

shell> ./mysqld --basedir=/usr/local --verbose --help

You can specify other options such as --datadir as well, but note that --verbose and --help must be the last options. (Prior to MySQL 4.1, omit the --verbose option.)

Once you determine the path settings you want, start the server without --verbose and --help.

If @command{mysqld} is currently running, you can find out what path settings it is using by executing this command: 

shell> mysqladmin variables

Or: 

shell> mysqladmin -h host_name variables

host_name is the name of the MySQL server host.

If you get Errcode 13 (which means Permission denied) when starting @command{mysqld}, this means that the access privileges of the data directory or its contents do not allow the server access. In this case, you change the permissions for the involved files and directories so that the server has the right to use them. You can also start the server as root, but this can raise security issues and should be avoided.

On Unix, change location into the data directory and check the ownership of the data directory and its contents to make sure the server has access. For example, if the data directory is `/usr/local/mysql/var', use this command: 

shell> ls -la /usr/local/mysql/var

If the data directory or its files or subdirectories are not owned by the account that you use for running the server, change their ownership to that account: 

shell> chown -R mysql /usr/local/mysql/var
shell> chgrp -R mysql /usr/local/mysql/var

If the server fails to start up correctly, check the error log file to see if you can find out why. Log files are located in the data directory (typically `C:\mysql\data' on Windows, `/usr/local/mysql/data' for a Unix binary distribution, and `/usr/local/var' for a Unix source distribution). Look in the data directory for files with names of the form `host_name.err' and `host_name.log', where host_name is the name of your server host. (Older servers on Windows use `mysql.err' as the error log name.) Then check the last few lines of these files. On Unix, you can use tail to display the last few lines: 

shell> tail host_name.err
shell> tail host_name.log

The error log contains information that indicates why the server couldn't start. For example, you might see something like this in the log: 

000729 14:50:10  bdb:  Recovery function for LSN 1 27595 failed
000729 14:50:10  bdb:  warning: ./test/t1.db: No such file or directory
000729 14:50:10  Can't init databases

This means that you didn't start @command{mysqld} with the --bdb-no-recover option and Berkeley DB found something wrong with its own log files when it tried to recover your databases. To be able to continue, you should move away the old Berkeley DB log files from the database directory to some other place, where you can later examine them. The BDB log files are named in sequence beginning with `log.0000000001', where the number increases over time.

If you are running @command{mysqld} with BDB table support and @command{mysqld} dumps core at startup, this could be due to problems with the BDB recovery log. In this case, you can try starting @command{mysqld} with --bdb-no-recover. If that helps, then you should remove all BDB log files from the data directory and try starting @command{mysqld} again without the --bdb-no-recover option.

If either of the following errors occur, it means that some other program (perhaps another @command{mysqld} server) is already using the TCP/IP port or Unix socket file that @command{mysqld} is trying to use: 

Can't start server: Bind on TCP/IP port: Address already in use
Can't start server: Bind on unix socket...

Use @command{ps} to determine whether you have another @command{mysqld} server running. If so, shut down the server before starting @command{mysqld} again. (If another server is running, and you really want to run multiple servers, you can find information about how to do so in section 5.10 Running Multiple MySQL Servers on the Same Machine.)

If no other server is running, try to execute the command telnet your-host-name tcp-ip-port-number. (The default MySQL port number is 3306.) Then press Enter a couple of times. If you don't get an error message like telnet: Unable to connect to remote host: Connection refused, some other program is using the TCP/IP port that @command{mysqld} is trying to use. You'll need to track down what program this is and disable it, or else tell @command{mysqld} to listen to a different port with the --port option. In this case, you'll also need to specify the port number for client programs when connecting to the server via TCP/IP.

Another reason the port might be inaccessible is that you have a firewall running that blocks connections to it. If so, modify the firewall settings to allow access to the port.

If the server starts but you can't connect to it, you should make sure that you have an entry in `/etc/hosts' that looks like this: 

127.0.0.1       localhost

This problem occurs only on systems that don't have a working thread library and for which MySQL must be configured to use MIT-pthreads.

If you can't get @command{mysqld} to start, you can try to make a trace file to find the problem by using the --debug option. See section D.1.2 Creating Trace Files.

2.4.3 Securing the Initial MySQL Accounts

Part of the MySQL installation process is to set up the mysql database containing the grant tables:

The grant tables define the initial MySQL user accounts and their access privileges. These accounts are set up as follows:

As noted, none of the initial accounts have passwords. This means that your MySQL installation is unprotected until you do something about it:

The following instructions describe how to set up passwords for the initial MySQL accounts, first for the anonymous accounts and then for the root accounts. Replace ``newpwd'' in the examples with the actual password that you want to use. The instructions also cover how to remove the anonymous accounts, should you prefer not to allow anonymous access at all.

You might want to defer setting the passwords until later, so that you don't need to specify them while you perform additional setup or testing. However, be sure to set them before using your installation for any real production work.

To assign passwords to the anonymous accounts, you can use either SET PASSWORD or UPDATE. In both cases, be sure to encrypt the password using the PASSWORD() function.

To use SET PASSWORD on Windows, do this: 

shell> mysql -u root
mysql> SET PASSWORD FOR ''@'localhost' = PASSWORD('newpwd');
mysql> SET PASSWORD FOR ''@'%' = PASSWORD('newpwd');

To use SET PASSWORD on Unix, do this: 

shell> mysql -u root
mysql> SET PASSWORD FOR ''@'localhost' = PASSWORD('newpwd');
mysql> SET PASSWORD FOR ''@'host_name' = PASSWORD('newpwd');

In the second SET PASSWORD statement, replace host_name with the name of the server host. This is the name that is specified in the Host column of the non-localhost record for root in the user table. If you don't know what hostname this is, issue the following statement before using SET PASSWORD: 

mysql> SELECT Host, User FROM mysql.user;

Look for the record that has root in the User column and something other than localhost in the Host column. Then use that Host value in the second SET PASSWORD statement.

The other way to assign passwords to the anonymous accounts is by using UPDATE to modify the user table directly. Connect to the server as root and issue an UPDATE statement that assigns a value to the Password column of the appropriate user table records. The procedure is the same for Windows and Unix. The following UPDATE statement assigns a password to both anonymous accounts at once: 

shell> mysql -u root
mysql> UPDATE mysql.user SET Password = PASSWORD('newpwd')
    ->     WHERE User = '';
mysql> FLUSH PRIVILEGES;

After you update the passwords in the user table directly using UPDATE, you must tell the server to re-read the grant tables with FLUSH PRIVILEGES. Otherwise, the change will go unnoticed until you restart the server.

If you prefer to remove the anonymous accounts instead, do so as follows: 

shell> mysql -u root
mysql> DELETE FROM mysql.user WHERE User = '';
mysql> FLUSH PRIVILEGES;

The DELETE statement applies both to Windows and to Unix. On Windows, if you want to remove only the anonymous account that has the same privileges as root, do this instead: 

shell> mysql -u root
mysql> DELETE FROM mysql.user WHERE Host='localhost' AND User='';
mysql> FLUSH PRIVILEGES;

This account allows anonymous access but has full privileges, so removing it improves security.

You can assign passwords to the root accounts in several ways. The following discussion demonstrates three methods:

To assign passwords using SET PASSWORD, connect to the server as root and issue two SET PASSWORD statements. Be sure to encrypt the password using the PASSWORD() function.

For Windows, do this: 

shell> mysql -u root
mysql> SET PASSWORD FOR 'root'@'localhost' = PASSWORD('newpwd');
mysql> SET PASSWORD FOR 'root'@'%' = PASSWORD('newpwd');

For Unix, do this: 

shell> mysql -u root
mysql> SET PASSWORD FOR 'root'@'localhost' = PASSWORD('newpwd');
mysql> SET PASSWORD FOR 'root'@'host_name' = PASSWORD('newpwd');

In the second SET PASSWORD statement, replace host_name with the name of the server host. This is the same hostname that you used when you assigned the anonymous account passwords.

To assign passwords to the root accounts using @command{mysqladmin}, execute the following commands: 

shell> mysqladmin -u root password "newpwd"
shell> mysqladmin -u root -h host_name password "newpwd"

These commands apply both to Windows and to Unix. In the second command, replace host_name with the name of the server host. The double quotes around the password are not always necessary, but you should use them if the password contains spaces or other characters that are special to your command interpreter.

If you are using a server from a very old version of MySQL, the @command{mysqladmin} commands to set the password will fail with the message parse error near 'SET password'. The solution to this problem is to upgrade the server to a newer version of MySQL.

You can also use UPDATE to modify the user table directly. The following UPDATE statement assigns a password to both root accounts at once: 

shell> mysql -u root
mysql> UPDATE mysql.user SET Password = PASSWORD('newpwd')
    ->     WHERE User = 'root';
mysql> FLUSH PRIVILEGES;

The UPDATE statement applies both to Windows and to Unix.

After the passwords have been set, you must supply the appropriate password whenever you connect to the server. For example, if you want to use @command{mysqladmin} to shut down the server, you can do so using this command: 

shell> mysqladmin -u root -p shutdown
Enter password: (enter root password here)

Note: If you forget your root password after setting it up, the procedure for resetting it is covered in section A.4.1 How to Reset the Root Password.

To set up new accounts, you can use the GRANT statement. For instructions, see section 5.6.2 Adding New User Accounts to MySQL.

2.5 Upgrading/Downgrading MySQL

As a general rule, we recommend that when upgrading from one release series to another, you should go to the next series rather than skipping a series. For example, if you currently are running MySQL 3.23 and wish to upgrade to a newer series, upgrade to MySQL 4.0 rather than to 4.1 or 5.0.

The following items form a checklist of things you should do whenever you perform an upgrade:

You can always move the MySQL format files and data files between different versions on the same architecture as long as you stay within versions for the same release series of MySQL. The current production release series is 4.0. If you change the character set when running MySQL, you must run @command{myisamchk -r -q --set-character-set=charset} on all MyISAM tables. Otherwise, your indexes may not be ordered correctly, because changing the character set may also change the sort order.

If you upgrade or downgrade from one release series to another, there may be incompatibilities in table storage formats. In this case, you can use @command{mysqldump} to dump your tables before upgrading. After upgrading, reload the dump file using @command{mysql} to re-create your tables.

If you are cautious about using new versions, you can always rename your old @command{mysqld} before installing a newer one. For example, if you are using MySQL 4.0.18 and want to upgrade to 4.1.1, rename your current server from @command{mysqld} to @command{mysqld-4.0.18}. If your new @command{mysqld} then does something unexpected, you can simply shut it down and restart with your old @command{mysqld}.

If, after an upgrade, you experience problems with recompiled client programs, such as Commands out of sync or unexpected core dumps, you probably have used old header or library files when compiling your programs. In this case, you should check the date for your `mysql.h' file and `libmysqlclient.a' library to verify that they are from the new MySQL distribution. If not, recompile your programs with the new headers and libraries.

If problems occur, such as that the new @command{mysqld} server doesn't want to start or that you can't connect without a password, verify that you don't have some old `my.cnf' file from your previous installation. You can check this with the --print-defaults option (for example, @command{mysqld --print-defaults}). If this displays anything other than the program name, you have an active `my.cnf' file that affects server or client operation.

It is a good idea to rebuild and reinstall the Perl DBD::mysql module whenever you install a new release of MySQL. The same applies to other MySQL interfaces as well, such as the PHP mysql extension and the Python MySQLdb module.

2.5.1 Upgrading from Version 4.1 to 5.0

In general, you should do the following when upgrading to MySQL 5.0 from 4.1:

The following list describes changes that may affect applications and that you should watch out for when upgrading to version 5.0:

2.5.2 Upgrading from Version 4.0 to 4.1

In general, you should do the following when upgrading to MySQL 4.1 from 4.0:

Several visible behaviors have changed between MySQL 4.0 and MySQL 4.1 to fix some critical bugs and make MySQL more compatible with standard SQL. These changes may affect your applications.

Some of the 4.1 behaviors can be tested in 4.0 before performing a full upgrade to 4.1. We have added to later MySQL 4.0 releases (from 4.0.12 on) a --new startup option for @command{mysqld}. See section 5.2.1 @command{mysqld} Command-Line Options.

This option gives you the 4.1 behavior for the most critical changes. You can also enable these behaviors for a given client connection with the SET @@new=1 command, or turn them off if they are on with SET @@new=0.

If you believe that some of the 4.1 changes will affect you, we recommend that before upgrading to 4.1, you download the latest MySQL 4.0 version and run it with the --new option by adding the following to your config file: 

[mysqld-4.0]
new

That way you can test the new behaviors in 4.0 to make sure that your applications work with them. This will help you have a smooth, painless transition when you perform a full upgrade to 4.1 later. Putting the --new option in the [mysqld-4.0] option group ensures that you don't accidentally later run the 4.1 version with the --new option.

The following lists describe changes that may affect applications and that you should watch out for when upgrading to version 4.1:

Server Changes:

Client Changes:

SQL Changes:

C API Changes:

Password-Handling Changes:

The password hashing mechanism has changed in 4.1 to provide better security, but this may cause compatibility problems if you still have clients that use the client library from 4.0 or earlier. (It is very likely that you will have 4.0 clients in situations where clients connect from remote hosts that have not yet upgraded to 4.1.) The following list indicates some possible upgrade strategies. They represent various tradeoffs between the goal of compatibility with old clients and the goal of security.

Further background on password hashing with respect to client authentication and password-changing operations may be found in section 5.5.9 Password Hashing in MySQL 4.1 and section A.2.3 Client does not support authentication protocol.

2.5.3 Upgrading from Version 3.23 to 4.0

In general, you should do the following when upgrading to MySQL 4.0 from 3.23:

MySQL 4.0 will work even if you don't perform the preceding actions, but you will not be able to use the new security privileges in MySQL 4.0 and you may run into problems when upgrading later to MySQL 4.1 or newer. The ISAM file format still works in MySQL 4.0, but is deprecated and is not compiled in by default as of MySQL 4.1. MyISAM tables should be used instead.

Old clients should work with a MySQL 4.0 server without any problems.

Even if you perform the preceding actions, you can still downgrade to MySQL 3.23.52 or newer if you run into problems with the MySQL 4.0 series. In this case, you must use @command{mysqldump} to dump any tables that use full-text indexes and reload the dump file into the 3.23 server. This is necessary because 4.0 uses a new format for full-text indexing.

The following lists describe changes that may affect applications and that you should watch out for when upgrading to version 4.0:

Server Changes:

SQL Changes:

C API Changes:

Other Changes:

2.5.4 Upgrading from Version 3.22 to 3.23

MySQL 3.22 and 3.21 clients will work without any problems with a MySQL 3.23 server.

When upgrading to MySQL 3.23 from an earlier version, note the following changes:

Table Changes:

Client Program Changes:

SQL Changes:

C API Changes:

2.5.5 Upgrading from Version 3.21 to 3.22

Nothing that affects compatibility has changed between versions 3.21 and 3.22. The only pitfall is that new tables that are created with DATE type columns will use the new way to store the date. You can't access these new columns from an old version of @command{mysqld}.

When upgrading to MySQL 3.23 from an earlier version, note the following changes:

2.5.6 Upgrading from Version 3.20 to 3.21

If you are running a version older than Version 3.20.28 and want to switch to Version 3.21, you need to do the following:

You can start the @command{mysqld} Version 3.21 server with the --old-protocol option to use it with clients from a Version 3.20 distribution. In this case, the server uses the old pre-3.21 password() checking rather than the new method. Also, the new client function mysql_errno() will not return any server error, only CR_UNKNOWN_ERROR. The function does work for client errors.

If you are not using the --old-protocol option to @command{mysqld}, you will need to make the following changes:

MySQL 3.20.28 and above can handle the new user table format without affecting clients. If you have a MySQL version earlier than 3.20.28, passwords will no longer work with it if you convert the user table. So to be safe, you should first upgrade to at least Version 3.20.28 and then upgrade to Version 3.21.

The new client code works with a 3.20.x @command{mysqld} server, so if you experience problems with 3.21.x, you can use the old 3.20.x server without having to recompile the clients again.

If you are not using the --old-protocol option to @command{mysqld}, old clients will be unable to connect and will issue the following error message: 

ERROR: Protocol mismatch. Server Version = 10 Client Version = 9

The Perl DBI interface also supports the old mysqlperl interface. The only change you have to make if you use mysqlperl is to change the arguments to the connect() function. The new arguments are: host, database, user, and password (note that the user and password arguments have changed places).

The following changes may affect queries in old applications:

2.5.7 Upgrading MySQL Under Windows

When upgrading MySQL under Windows, please follow these steps:

  1. Download the latest Windows distribution of MySQL.
  2. Choose a time of day with low usage, where a maintenance break is acceptable.
  3. Alert the users who still are active about the maintenance break.
  4. Stop the running MySQL Server (for example, with @command{NET STOP MySQL} or with the Services utility if you are running MySQL as a service, or with @command{mysqladmin shutdown} otherwise).
  5. Exit the WinMySQLAdmin program if it is running.
  6. Run the installation script of the Windows distribution by clicking the Install button in WinZip and following the installation steps of the script. Important note: Early alpha Windows distributions for MySQL 4.1 do not contain an installer program. See section 2.2.1.2 Installing a Windows Binary Distribution for instructions on how to install such a distribution.
  7. You may either overwrite your old MySQL installation (usually located at `C:\mysql'), or install it into a different directory, such as C:\mysql4. Overwriting the old installation is recommended.
  8. Restart the server. For example, use @command{NET START MySQL} if you run MySQL as a service, or invoke @command{mysqld} directly otherwise.
  9. Update the grant tables. The procedure is described in section 2.5.8 Upgrading the Grant Tables.

Possible error situations: 

A system error has occurred.
System error 1067 has occurred.
The process terminated unexpectedly.

These errors mean that your option file (by default `C:\my.cnf') contains an option that cannot be recognized by MySQL. You can verify that this is the case by trying to restart MySQL with the option file renamed to prevent the server from using it. (For example, rename `C:\my.cnf' to `C:\my_cnf.old'.) Once you have verified it, you need to identify which option is the culprit. Create a new `my.cnf' file and move parts of the old file to it (restarting the server after you move each part) until you determine which option causes server startup to fail.

2.5.8 Upgrading the Grant Tables

Some releases introduce changes to the structure of the grant tables (the tables in the mysql database) to add new privileges or features. To make sure that your grant tables are current when you update to a new version of MySQL, you should update your grant tables as well.

On Unix or Unix-like systems, update the grant tables by running the @command{mysql_fix_privilege_tables} script: 

shell> mysql_fix_privilege_tables

You must run this script while the server is running. It attempts to connect to the server running on the local host as root. If your root account requires a password, indicate the password on the command line. For MySQL 4.1 and up, specify the password like this: 

shell> mysql_fix_privilege_tables --password=root_password

Prior to MySQL 4.1, specify the password like this: 

shell> mysql_fix_privilege_tables root_password

The @command{mysql_fix_privilege_tables} script performs any actions necessary to convert your grant tables to the current format. You might see some Duplicate column name warnings as it runs; you can ignore them.

After running the script, stop the server and restart it.

On Windows systems, there isn't an easy way to update the grant tables until MySQL 4.0.15. From version 4.0.15 on, MySQL distributions include a `mysql_fix_privilege_tables.sql' SQL script that you can run using the @command{mysql} client. If your MySQL installation is located at `C:\mysql', the commands look like this: 

C:\> C:\mysql\bin\mysql -u root -p mysql
mysql> SOURCE C:\mysql\scripts\mysql_fix_privilege_tables.sql

If your installation is located in some other directory, adjust the pathnames appropriately.

The @command{mysql} command will prompt you for the root password; enter it when prompted.

As with the Unix procedure, you might see some Duplicate column name warnings as @command{mysql} processes the statements in the `mysql_fix_privilege_tables.sql' script; you can ignore them.

After running the script, stop the server and restart it.

If you are upgrading to MySQL 5.0.1 or later, the grant table upgrade procedure just described will add view-related columns for the CREATE VIEW and SHOW VIEW privileges. These privileges exist at the global and database levels. Their initial values are assigned as follows:

2.5.9 Copying MySQL Databases to Another Machine

If you are using MySQL 3.23 or later, you can copy the `.frm', `.MYI', and `.MYD' files for MyISAM tables between different architectures that support the same floating-point format. (MySQL takes care of any byte-swapping issues.) See section 15.1 The MyISAM Storage Engine.

The MySQL ISAM data and index files (`.ISD' and `*.ISM', respectively) are architecture dependent and in some cases operating system dependent. If you want to move your applications to another machine that has a different architecture or operating system than your current machine, you should not try to move a database by simply copying the files to the other machine. Use @command{mysqldump} instead.

By default, @command{mysqldump} will create a file containing SQL statements. You can then transfer the file to the other machine and feed it as input to the @command{mysql} client.

Try @command{mysqldump --help} to see what options are available. If you are moving the data to a newer version of MySQL, you should use @command{mysqldump --opt} to take advantage of any optimizations that result in a dump file that is smaller and can be processed faster.

The easiest (although not the fastest) way to move a database between two machines is to run the following commands on the machine on which the database is located: 

shell> mysqladmin -h 'other_hostname' create db_name
shell> mysqldump --opt db_name | mysql -h 'other_hostname' db_name

If you want to copy a database from a remote machine over a slow network, you can use: 

shell> mysqladmin create db_name
shell> mysqldump -h 'other_hostname' --opt --compress db_name | mysql db_name

You can also store the result in a file, then transfer the file to the target machine and load the file into the database there. For example, you can dump a database to a file on the source machine like this: 

shell> mysqldump --quick db_name | gzip > db_name.contents.gz

(The file created in this example is compressed.) Transfer the file containing the database contents to the target machine and run these commands there: 

shell> mysqladmin create db_name
shell> gunzip < db_name.contents.gz | mysql db_name

You can also use @command{mysqldump} and @command{mysqlimport} to transfer the database. For big tables, this is much faster than simply using @command{mysqldump}. In the following commands, DUMPDIR represents the full pathname of the directory you use to store the output from @command{mysqldump}.

First, create the directory for the output files and dump the database: 

shell> mkdir DUMPDIR
shell> mysqldump --tab=DUMPDIR db_name

Then transfer the files in the DUMPDIR directory to some corresponding directory on the target machine and load the files into MySQL there: 

shell> mysqladmin create db_name           # create database
shell> cat DUMPDIR/*.sql | mysql db_name   # create tables in database
shell> mysqlimport db_name DUMPDIR/*.txt   # load data into tables

Also, don't forget to copy the mysql database because that is where the user, db, and host grant tables are stored. You might have to run commands as the MySQL root user on the new machine until you have the mysql database in place.

After you import the mysql database on the new machine, execute @command{mysqladmin flush-privileges} so that the server reloads the grant table information.

2.6 Operating System-Specific Notes

2.6.1 Linux Notes

This section discusses issues that have been found to occur on Linux. The first few subsections describe general operating system-related issues, problems that can occur when using binary or source distributions, and post-installation issues. The remaining subsections discuss problems that occur with Linux on specific platforms.

Note that most of these problems occur on older versions of Linux. If you are running a recent version, you likely will see none of them.

2.6.1.1 Linux Operating System Notes

MySQL needs at least Linux Version 2.0.

Warning: We have seen some strange problems with Linux 2.2.14 and MySQL on SMP systems. We also have reports from some MySQL users that they have encountered serious stability problems using MySQL with kernel 2.2.14. If you are using this kernel, you should upgrade to 2.2.19 (or newer) or to a 2.4 kernel. If you have a multiple-CPU box, then you should seriously consider using 2.4 because it will give you a significant speed boost. Your system also will be more stable.

When using LinuxThreads, you will see a minimum of three @command{mysqld} processes running. These are in fact threads. There will be one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals.

2.6.1.2 Linux Binary Distribution Notes

The Linux-Intel binary and RPM releases of MySQL are configured for the highest possible speed. We are always trying to use the fastest stable compiler available.

The binary release is linked with -static, which means you do not normally need to worry about which version of the system libraries you have. You need not install LinuxThreads, either. A program linked with -static is slightly larger than a dynamically linked program, but also slightly faster (3-5%). However, one problem with a statically linked program is that you can't use user-defined functions (UDFs). If you are going to write or use UDFs (this is something for C or C++ programmers only), you must compile MySQL yourself using dynamic linking.

A known issue with binary distributions is that on older Linux systems that use libc (such as Red Hat 4.x or Slackware), you will get some non-fatal problems with hostname resolution. If your system uses libc rather than glibc2, you probably will encounter some difficulties with hostname resolution and getpwnam(). This happens because glibc unfortunately depends on some external libraries to implement hostname resolution and getpwent(), even when compiled with -static. These problems manifest themselves in two ways:

Another solution, which solves both problems, is to not use a binary distribution. Get a MySQL source distribution (in RPM or tar.gz format) and install that instead.

On some Linux 2.2 versions, you may get the error Resource temporarily unavailable when clients make a lot of new connections to a @command{mysqld} server over TCP/IP. The problem is that Linux has a delay between the time that you close a TCP/IP socket and the time that the system actually frees it. There is room for only a finite number of TCP/IP slots, so you will encounter the resource-unavailable error if clients attempt too many new TCP/IP connections during a short time. For example, you may see the error when you run the MySQL `test-connect' benchmark over TCP/IP.

We have inquired about this problem a few times on different Linux mailing lists but have never been able to find a suitable resolution. The only known ``fix'' is for the clients to use persistent connections, or, if you are running the database server and clients on the same machine, to use Unix socket file connections rather than TCP/IP connections.

2.6.1.3 Linux Source Distribution Notes

The following notes regarding glibc apply only to the situation when you build MySQL yourself. If you are running Linux on an x86 machine, in most cases it is much better for you to just use our binary. We link our binaries against the best patched version of glibc we can come up with and with the best compiler options, in an attempt to make it suitable for a high-load server. For a typical user, even for setups with a lot of concurrent connections or tables exceeding the 2GB limit, our binary is the best choice in most cases. After reading the following text, if you are in doubt about what to do, try our binary first to see whether it meets your needs. If you discover that it is not good enough, then you may want to try your own build. In that case, we would appreciate a note about it so that we can build a better binary next time.

MySQL uses LinuxThreads on Linux. If you are using an old Linux version that doesn't have glibc2, you must install LinuxThreads before trying to compile MySQL. You can get LinuxThreads at http://dev.mysql.com/downloads/os-linux.html.

Note that glibc versions before and including Version 2.1.1 have a fatal bug in pthread_mutex_timedwait() handling, which is used when you issue INSERT DELAYED statements. We recommend that you not use INSERT DELAYED before upgrading glibc.

Note that Linux kernel and the LinuxThread library can by default only have 1,024 threads. If you plan to have more than 1,000 concurrent connections, you will need to make some changes to LinuxThreads:

The page http://www.volano.com/linuxnotes.html contains additional information about circumventing thread limits in LinuxThreads.

There is another issue that greatly hurts MySQL performance, especially on SMP systems. The mutex implementation in LinuxThreads in glibc 2.1 is very bad for programs with many threads that hold the mutex only for a short time. This produces a paradoxical result: If you link MySQL against an unmodified LinuxThreads, removing processors from an SMP actually improves MySQL performance in many cases. We have made a patch available for glibc 2.1.3 to correct this behavior (http://www.mysql.com/Downloads/Linux/linuxthreads-2.1-patch).

With glibc 2.2.2, MySQL 3.23.36 will use the adaptive mutex, which is much better than even the patched one in glibc 2.1.3. Be warned, however, that under some conditions, the current mutex code in glibc 2.2.2 overspins, which hurts MySQL performance. The likelihood that this condition will occur can be reduced by renicing the @command{mysqld} process to the highest priority. We have also been able to correct the overspin behavior with a patch, available at http://www.mysql.com/Downloads/Linux/linuxthreads-2.2.2.patch. It combines the correction of overspin, maximum number of threads, and stack spacing all in one. You will need to apply it in the linuxthreads directory with patch -p0 </tmp/linuxthreads-2.2.2.patch. We hope it will be included in some form in future releases of glibc 2.2. In any case, if you link against glibc 2.2.2, you still need to correct STACK_SIZE and PTHREAD_THREADS_MAX. We hope that the defaults will be corrected to some more acceptable values for high-load MySQL setup in the future, so that the commands needed to produce your own build can be reduced to @command{./configure; make; make install}.

We recommend that you use these patches to build a special static version of libpthread.a and use it only for statically linking against MySQL. We know that the patches are safe for MySQL and significantly improve its performance, but we cannot say anything about other applications. If you link other applications that require LinuxThreads against the patched static version of the library, or build a patched shared version and install it on your system, you do so at your own risk.

If you experience any strange problems during the installation of MySQL, or with some common utilities hanging, it is very likely that they are either library or compiler related. If this is the case, using our binary will resolve them.

If you link your own MySQL client programs, you may see the following error at runtime: 

ld.so.1: fatal: libmysqlclient.so.#:
open failed: No such file or directory

This problem can be avoided by one of the following methods:

If you are using the Fujitsu compiler (fcc/FCC), you will have some problems compiling MySQL because the Linux header files are very @command{gcc} oriented. The following @command{configure} line should work with @command{fcc/FCC}: 

CC=fcc CFLAGS="-O -K fast -K lib -K omitfp -Kpreex -D_GNU_SOURCE \
    -DCONST=const -DNO_STRTOLL_PROTO" \
CXX=FCC CXXFLAGS="-O -K fast -K lib \
    -K omitfp -K preex --no_exceptions --no_rtti -D_GNU_SOURCE \
    -DCONST=const -Dalloca=__builtin_alloca -DNO_STRTOLL_PROTO \
    '-D_EXTERN_INLINE=static __inline'" \
./configure \
    --prefix=/usr/local/mysql --enable-assembler \
    --with-mysqld-ldflags=-all-static --disable-shared \
    --with-low-memory

2.6.1.4 Linux Post-Installation Notes

@command{mysql.server} can be found in the `support-files' directory under the MySQL installation directory or in a MySQL source tree. You can install it as `/etc/init.d/mysql' for automatic MySQL startup and shutdown. See section 2.4.2.2 Starting and Stopping MySQL Automatically.

If MySQL can't open enough files or connections, it may be that you haven't configured Linux to handle enough files.

In Linux 2.2 and onward, you can check the number of allocated file handles as follows: 

shell> cat /proc/sys/fs/file-max
shell> cat /proc/sys/fs/dquot-max
shell> cat /proc/sys/fs/super-max

If you have more than 16MB of memory, you should add something like the following to your init scripts (for example, `/etc/init.d/boot.local' on SuSE Linux): 

echo 65536 > /proc/sys/fs/file-max
echo 8192 > /proc/sys/fs/dquot-max
echo 1024 > /proc/sys/fs/super-max

You can also run the echo commands from the command line as root, but these settings will be lost the next time your computer restarts.

Alternatively, you can set these parameters on startup by using the sysctl tool, which is used by many Linux distributions (SuSE has added it as well, beginning with SuSE Linux 8.0). Just put the following values into a file named `/etc/sysctl.conf': 

# Increase some values for MySQL
fs.file-max = 65536
fs.dquot-max = 8192
fs.super-max = 1024

You should also add the following to `/etc/my.cnf': 

[mysqld_safe]
open-files-limit=8192

This should allow the server a limit of 8,192 for the combined number of connections and open files.

The STACK_SIZE constant in LinuxThreads controls the spacing of thread stacks in the address space. It needs to be large enough so that there will be plenty of room for each individual thread stack, but small enough to keep the stack of some threads from running into the global @command{mysqld} data. Unfortunately, as we have experimentally discovered, the Linux implementation of mmap() will successfully unmap an already mapped region if you ask it to map out an address already in use, zeroing out the data on the entire page instead of returning an error. So, the safety of @command{mysqld} or any other threaded application depends on ``gentlemanly'' behavior of the code that creates threads. The user must take measures to make sure that the number of running threads at any time is sufficiently low for thread stacks to stay away from the global heap. With @command{mysqld}, you should enforce this behavior by setting a reasonable value for the max_connections variable.

If you build MySQL yourself, you can patch LinuxThreads for better stack use. See section 2.6.1.3 Linux Source Distribution Notes. If you do not want to patch LinuxThreads, you should set max_connections to a value no higher than 500. It should be even less if you have a large key buffer, large heap tables, or some other things that make @command{mysqld} allocate a lot of memory, or if you are running a 2.2 kernel with a 2GB patch. If you are using our binary or RPM version 3.23.25 or later, you can safely set max_connections at 1500, assuming no large key buffer or heap tables with lots of data. The more you reduce STACK_SIZE in LinuxThreads the more threads you can safely create. We recommend values between 128KB and 256KB.

If you use a lot of concurrent connections, you may suffer from a ``feature'' in the 2.2 kernel that attempts to prevent fork bomb attacks by penalizing a process for forking or cloning a child. This causes MySQL not to scale well as you increase the number of concurrent clients. On single-CPU systems, we have seen this manifested as very slow thread creation: It may take a long time to connect to MySQL (as long as one minute), and it may take just as long to shut it down. On multiple-CPU systems, we have observed a gradual drop in query speed as the number of clients increases. In the process of trying to find a solution, we have received a kernel patch from one of our users who claimed it made a lot of difference for his site. The patch is available at http://www.mysql.com/Downloads/Patches/linux-fork.patch. We have now done rather extensive testing of this patch on both development and production systems. It has significantly improved MySQL performance without causing any problems and we now recommend it to our users who still run high-load servers on 2.2 kernels.

This issue has been fixed in the 2.4 kernel, so if you are not satisfied with the current performance of your system, rather than patching your 2.2 kernel, it might be easier to upgrade to 2.4. On SMP systems, upgrading also will give you a nice SMP boost in addition to fixing the fairness bug.

We have tested MySQL on the 2.4 kernel on a two-CPU machine and found MySQL scales much better. There was virtually no slowdown on query throughput all the way up to 1,000 clients, and the MySQL scaling factor (computed as the ratio of maximum throughput to the throughput for one client) was 180%. We have observed similar results on a four-CPU system: Virtually no slowdown as the number of clients was increased up to 1,000, and a 300% scaling factor. Based on these results, for a high-load SMP server using a 2.2 kernel, we definitely recommend upgrading to the 2.4 kernel at this point.

We have discovered that it is essential to run the @command{mysqld} process with the highest possible priority on the 2.4 kernel to achieve maximum performance. This can be done by adding a renice -20 $$ command to @command{mysqld_safe}. In our testing on a four-CPU machine, increasing the priority resulted in a 60% throughput increase with 400 clients.

We are currently also trying to collect more information on how well MySQL performs with a 2.4 kernel on four-way and eight-way systems. If you have access such a system and have done some benchmarks, please send an email message to [email protected] with the results. We will review them for inclusion in the manual.

If you see a dead @command{mysqld} server process with @command{ps}, this usually means that you have found a bug in MySQL or you have a corrupted table. See section A.4.2 What to Do If MySQL Keeps Crashing.

To get a core dump on Linux if @command{mysqld} dies with a SIGSEGV signal, you can start @command{mysqld} with the --core-file option. Note that you also probably need to raise the core file size by adding @command{ulimit -c 1000000} to @command{mysqld_safe} or starting @command{mysqld_safe} with --core-file-size=1000000. @xref{mysqld_safe, , @command{mysqld_safe}}.

2.6.1.5 Linux x86 Notes

MySQL requires libc Version 5.4.12 or newer. It's known to work with libc 5.4.46. glibc Version 2.0.6 and later should also work. There have been some problems with the glibc RPMs from Red Hat, so if you have problems, check whether there are any updates. The glibc 2.0.7-19 and 2.0.7-29 RPMs are known to work.

If you are using Red Hat 8.0 or a new glibc 2.2.x library, you may see @command{mysqld} die in gethostbyaddr(). This happens because the new glibc library requires a stack size greater than 128KB for this call. To fix the problem, start @command{mysqld} with the --thread-stack=192K option. (Use -O thread_stack=192K before MySQL 4.) This stack size is now the default on MySQL 4.0.10 and above, so you should not see the problem.

If you are using @command{gcc} 3.0 and above to compile MySQL, you must install the libstdc++v3 library before compiling MySQL; if you don't do this, you will get an error about a missing __cxa_pure_virtual symbol during linking.

On some older Linux distributions, @command{configure} may produce an error like this: 

Syntax error in sched.h. Change _P to __P in the
/usr/include/sched.h file.
See the Installation chapter in the Reference Manual.

Just do what the error message says. Add an extra underscore to the _P macro name that has only one underscore, then try again.

You may get some warnings when compiling. Those shown here can be ignored: 

mysqld.cc -o objs-thread/mysqld.o
mysqld.cc: In function `void init_signals()':
mysqld.cc:315: warning: assignment of negative value `-1' to
`long unsigned int'
mysqld.cc: In function `void * signal_hand(void *)':
mysqld.cc:346: warning: assignment of negative value `-1' to
`long unsigned int'

If @command{mysqld} always dumps core when it starts, the problem may be that you have an old `/lib/libc.a'. Try renaming it, then remove `sql/mysqld' and do a new @command{make install} and try again. This problem has been reported on some Slackware installations.

If you get the following error when linking @command{mysqld}, it means that your `libg++.a' is not installed correctly: 

/usr/lib/libc.a(putc.o): In function `_IO_putc':
putc.o(.text+0x0): multiple definition of `_IO_putc'

You can avoid using `libg++.a' by running @command{configure} like this: 

shell> CXX=gcc ./configure

If @command{mysqld} crashes immediately and you are running Red Hat Version 5.0 with a version of glibc older than 2.0.7-5, you should make sure that you have installed all glibc patches. There is a lot of information about this in the MySQL mail archives, available online at http://lists.mysql.com/.

2.6.1.6 Linux SPARC Notes

In some implementations, readdir_r() is broken. The symptom is that the SHOW DATABASES statement always returns an empty set. This can be fixed by removing HAVE_READDIR_R from `config.h' after configuring and before compiling.

2.6.1.7 Linux Alpha Notes

MySQL 3.23.12 is the first MySQL version that is tested on Linux-Alpha. If you plan to use MySQL on Linux-Alpha, you should ensure that you have this version or newer.

We have tested MySQL on Alpha with our benchmarks and test suite, and it appears to work nicely.

We currently build the MySQL binary packages on SuSE Linux 7.0 for AXP, kernel 2.4.4-SMP, Compaq C compiler (V6.2-505) and Compaq C++ compiler (V6.3-006) on a Compaq DS20 machine with an Alpha EV6 processor.

You can find the preceding compilers at http://www.support.compaq.com/alpha-tools/. By using these compilers rather than @command{gcc}, we get about 9-14% better MySQL performance.

Note that until MySQL version 3.23.52 and 4.0.2, we optimized the binary for the current CPU only (by using the -fast compile option). This means that for older versions, you can use our Alpha binaries only if you have an Alpha EV6 processor.

For all following releases, we added the -arch generic flag to our compile options, which makes sure that the binary runs on all Alpha processors. We also compile statically to avoid library problems. The @command{configure} command looks like this: 

CC=ccc CFLAGS="-fast -arch generic" CXX=cxx \
CXXFLAGS="-fast -arch generic -noexceptions -nortti" \
./configure --prefix=/usr/local/mysql --disable-shared \
    --with-extra-charsets=complex --enable-thread-safe-client \
    --with-mysqld-ldflags=-non_shared --with-client-ldflags=-non_shared

If you want to use @command{egcs}, the following @command{configure} line worked for us: 

CFLAGS="-O3 -fomit-frame-pointer" CXX=gcc \
CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors \
    -fno-exceptions -fno-rtti" \
./configure --prefix=/usr/local/mysql --disable-shared

Some known problems when running MySQL on Linux-Alpha:

2.6.1.8 Linux PowerPC Notes

MySQL should work on MkLinux with the newest glibc package (tested with glibc 2.0.7).

2.6.1.9 Linux MIPS Notes

To get MySQL to work on Qube2 (Linux Mips), you need the newest glibc libraries. glibc-2.0.7-29C2 is known to work. You must also use the @command{egcs} C++ compiler (@command{egcs} 1.0.2-9, @command{gcc} 2.95.2 or newer).

2.6.1.10 Linux IA-64 Notes

To get MySQL to compile on Linux IA-64, we use the following @command{configure} command for building with @command{gcc} 2.96: 

CC=gcc \
CFLAGS="-O3 -fno-omit-frame-pointer" \
CXX=gcc \
CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors \
    -fno-exceptions -fno-rtti" \
    ./configure --prefix=/usr/local/mysql \
    "--with-comment=Official MySQL binary" \
    --with-extra-charsets=complex

On IA-64, the MySQL client binaries use shared libraries. This means that if you install our binary distribution at a location other than `/usr/local/mysql', you need to add the path of the directory where you have `libmysqlclient.so' installed either to the `/etc/ld.so.conf' file or to the value of your LD_LIBRARY_PATH environment variable.

See section A.3.1 Problems Linking to the MySQL Client Library.

2.6.2 Mac OS X Notes

On Mac OS X, @command{tar} cannot handle long filenames. If you need to unpack a `.tar.gz' distribution, use @command{gnutar} instead.

2.6.2.1 Mac OS X 10.x (Darwin)

MySQL should work without any problems on Mac OS X 10.x (Darwin).

Our binary for Mac OS X is compiled on Darwin 6.3 with the following @command{configure} line: 

CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc \
CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors \
    -fno-exceptions -fno-rtti" \
    ./configure --prefix=/usr/local/mysql \
    --with-extra-charsets=complex --enable-thread-safe-client \
    --enable-local-infile --disable-shared

See section 2.2.3 Installing MySQL on Mac OS X.

2.6.2.2 Mac OS X Server 1.2 (Rhapsody)

For current versions of Mac OS X Server, no operating system changes are necessary before compiling MySQL. Compiling for the Server platform is the same as for the client version of Mac OS X. (However, note that MySQL comes preinstalled on Mac OS X Server, so you need not build it yourself.)

For older versions (Mac OS X Server 1.2, a.k.a. Rhapsody), you must first install a pthread package before trying to configure MySQL.

See section 2.2.3 Installing MySQL on Mac OS X.

2.6.3 Solaris Notes

On Solaris, you may run into trouble even before you get the MySQL distribution unpacked! Solaris @command{tar} can't handle long filenames, so you may see an error like this when you unpack MySQL: 

x mysql-3.22.12-beta/bench/Results/ATIS-mysql_odbc-NT_4.0-cmp-db2,
informix,ms-sql,mysql,oracle,solid,sybase, 0 bytes, 0 tape blocks
tar: directory checksum error

In this case, you must use GNU @command{tar} (@command{gtar}) to unpack the distribution. You can find a precompiled copy for Solaris at http://dev.mysql.com/downloads/os-solaris.html.

Sun native threads work only on Solaris 2.5 and higher. For Version 2.4 and earlier, MySQL automatically uses MIT-pthreads. See section 2.3.5 MIT-pthreads Notes.

If you get the following error from @command{configure}, it means that you have something wrong with your compiler installation: 

checking for restartable system calls... configure: error can not
run test programs while cross compiling

In this case, you should upgrade your compiler to a newer version. You may also be able to solve this problem by inserting the following row into the `config.cache' file: 

ac_cv_sys_restartable_syscalls=${ac_cv_sys_restartable_syscalls='no'}

If you are using Solaris on a SPARC, the recommended compiler is @command{gcc} 2.95.2 or 3.2. You can find this at http://gcc.gnu.org/. Note that @command{egcs} 1.1.1 and @command{gcc} 2.8.1 don't work reliably on SPARC!

The recommended @command{configure} line when using @command{gcc} 2.95.2 is: 

CC=gcc CFLAGS="-O3" \
CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" \
./configure --prefix=/usr/local/mysql --with-low-memory \
    --enable-assembler

If you have an UltraSPARC system, you can get 4% better performance by adding -mcpu=v8 -Wa,-xarch=v8plusa to the CFLAGS and CXXFLAGS environment variables.

If you have Sun's Forte 5.0 (or newer) compiler, you can run @command{configure} like this: 

CC=cc CFLAGS="-Xa -fast -native -xstrconst -mt" \
CXX=CC CXXFLAGS="-noex -mt" \
./configure --prefix=/usr/local/mysql --enable-assembler

To create a 64-bit binary with Sun's Forte compiler, use the following configuration options: 

CC=cc CFLAGS="-Xa -fast -native -xstrconst -mt -xarch=v9" \
CXX=CC CXXFLAGS="-noex -mt -xarch=v9" ASFLAGS="-xarch=v9" \
./configure --prefix=/usr/local/mysql --enable-assembler

To create a 64-bit Solaris binary using @command{gcc}, add -m64 to CFLAGS and CXXFLAGS and remove --enable-assembler from the @command{configure} line. This works only with MySQL 4.0 and up; MySQL 3.23 does not include the required modifications to support this.

In the MySQL benchmarks, we got a 4% speedup on an UltraSPARC when using Forte 5.0 in 32-bit mode compared to using @command{gcc} 3.2 with the -mcpu flag.

If you create a 64-bit @command{mysqld} binary, it is 4% slower than the 32-bit binary, but can handle more threads and memory.

If you get a problem with fdatasync or sched_yield, you can fix this by adding LIBS=-lrt to the @command{configure} line

For compilers older than WorkShop 5.3, you might have to edit the @command{configure} script. Change this line: 

#if !defined(__STDC__) || __STDC__ != 1

To this: 

#if !defined(__STDC__)

If you turn on __STDC__ with the -Xc option, the Sun compiler can't compile with the Solaris `pthread.h' header file. This is a Sun bug (broken compiler or broken include file).

If @command{mysqld} issues the following error message when you run it, you have tried to compile MySQL with the Sun compiler without enabling the -mt multi-thread option: 

libc internal error: _rmutex_unlock: rmutex not held

Add -mt to CFLAGS and CXXFLAGS and recompile.

If you are using the SFW version of @command{gcc} (which comes with Solaris 8), you must add `/opt/sfw/lib' to the environment variable LD_LIBRARY_PATH before running @command{configure}.

If you are using the @command{gcc} available from sunfreeware.com, you may have many problems. To avoid this, you should recompile @command{gcc} and GNU binutils on the machine where you will be running them.

If you get the following error when compiling MySQL with @command{gcc}, it means that your @command{gcc} is not configured for your version of Solaris: 

shell> gcc -O3 -g -O2 -DDBUG_OFF  -o thr_alarm ...
./thr_alarm.c: In function `signal_hand':
./thr_alarm.c:556: too many arguments to function `sigwait'

The proper thing to do in this case is to get the newest version of @command{gcc} and compile it with your current @command{gcc} compiler. At least for Solaris 2.5, almost all binary versions of @command{gcc} have old, unusable include files that will break all programs that use threads, and possibly other programs!

Solaris doesn't provide static versions of all system libraries (libpthreads and libdl), so you can't compile MySQL with --static. If you try to do so, you will get one of the following errors: 

ld: fatal: library -ldl: not found
undefined reference to `dlopen'
cannot find -lrt

If you link your own MySQL client programs, you may see the following error at runtime: 

ld.so.1: fatal: libmysqlclient.so.#:
open failed: No such file or directory

This problem can be avoided by one of the following methods:

If you have problems with @command{configure} trying to link with -lz when you don't have zlib installed, you have two options:

If you are using @command{gcc} and have problems with loading user-defined functions (UDFs) into MySQL, try adding -lgcc to the link line for the UDF.

If you would like MySQL to start automatically, you can copy `support-files/mysql.server' to `/etc/init.d' and create a symbolic link to it named `/etc/rc3.d/S99mysql.server'.

If too many processes try to connect very rapidly to @command{mysqld}, you will see this error in the MySQL log: 

Error in accept: Protocol error

You might try starting the server with the --back_log=50 option as a workaround for this. (Use -O back_log=50 before MySQL 4.)

Solaris doesn't support core files for setuid() applications, so you can't get a core file from @command{mysqld} if you are using the --user option.

2.6.3.1 Solaris 2.7/2.8 Notes

Normally, you can use a Solaris 2.6 binary on Solaris 2.7 and 2.8. Most of the Solaris 2.6 issues also apply for Solaris 2.7 and 2.8.

MySQL 3.23.4 and above should be able to detect new versions of Solaris automatically and enable workarounds for the following problems.

Solaris 2.7 / 2.8 has some bugs in the include files. You may see the following error when you use @command{gcc}: 

/usr/include/widec.h:42: warning: `getwc' redefined
/usr/include/wchar.h:326: warning: this is the location of the previous
definition

If this occurs, you can fix the problem by copying /usr/include/widec.h to .../lib/gcc-lib/os/gcc-version/include and changing line 41 from this: 

#if     !defined(lint) && !defined(__lint)

To this: 

#if     !defined(lint) && !defined(__lint) && !defined(getwc)

Alternatively, you can edit `/usr/include/widec.h' directly. Either way, after you make the fix, you should remove `config.cache' and run @command{configure} again.

If you get the following errors when you run @command{make}, it's because @command{configure} didn't detect the `curses.h' file (probably because of the error in `/usr/include/widec.h'): 

In file included from mysql.cc:50:
/usr/include/term.h:1060: syntax error before `,'
/usr/include/term.h:1081: syntax error before `;'

The solution to this problem is to do one of the following:

If your linker can't find -lz when linking client programs, the problem is probably that your `libz.so' file is installed in `/usr/local/lib'. You can fix this problem by one of the following methods:

2.6.3.2 Solaris x86 Notes

On Solaris 8 on x86, @command{mysqld} will dump core if you remove the debug symbols using strip.

If you are using @command{gcc} or @command{egcs} on Solaris x86 and you experience problems with core dumps under load, you should use the following @command{configure} command: 

CC=gcc CFLAGS="-O3 -fomit-frame-pointer -DHAVE_CURSES_H" \
CXX=gcc \
CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors \
    -fno-exceptions -fno-rtti -DHAVE_CURSES_H" \
./configure --prefix=/usr/local/mysql

This will avoid problems with the libstdc++ library and with C++ exceptions.

If this doesn't help, you should compile a debug version and run it with a trace file or under @command{gdb}. @xref{Using gdb on mysqld, , Using @command{gdb} on @command{mysqld}}.

2.6.4 BSD Notes

This section provides information about using MySQL on variants of BSD Unix.

2.6.4.1 FreeBSD Notes

FreeBSD 4.x or newer is recommended for running MySQL, because the thread package is much more integrated. To get a secure and stable system, you should use only FreeBSD kernels that are marked -RELEASE.

The easiest (and preferred) way to install MySQL is to use the @command{mysql-server} and mysql-client ports available at http://www.freebsd.org/. Using these ports gives you the following benefits:

It is recommended you use MIT-pthreads on FreeBSD 2.x, and native threads on Versions 3 and up. It is possible to run with native threads on some late 2.2.x versions, but you may encounter problems shutting down @command{mysqld}.

Unfortunately, certain function calls on FreeBSD are not yet fully thread-safe. Most notably, this includes the gethostbyname() function, which is used by MySQL to convert hostnames into IP addresses. Under certain circumstances, the @command{mysqld} process will suddenly cause 100% CPU load and will be unresponsive. If you encounter this problem, try to start MySQL using the --skip-name-resolve option.

Alternatively, you can link MySQL on FreeBSD 4.x against the LinuxThreads library, which avoids a few of the problems that the native FreeBSD thread implementation has. For a very good comparison of LinuxThreads versus native threads, see Jeremy Zawodny's article FreeBSD or Linux for your MySQL Server? at http://jeremy.zawodny.com/blog/archives/000697.html.

A known problem when using LinuxThreads on FreeBSD is that the wait_timeout value is not honored (probably a signal handling problem in FreeBSD/LinuxThreads). This is supposed to be fixed in FreeBSD 5.0. The symptom is that persistent connections can hang for a very long time without getting closed down.

The MySQL build process requires GNU make (@command{gmake}) to work. If GNU @command{make} is not available, you must install it first before compiling MySQL.

The recommended way to compile and install MySQL on FreeBSD with @command{gcc} (2.95.2 and up) is: 

CC=gcc CFLAGS="-O2 -fno-strength-reduce" \
    CXX=gcc CXXFLAGS="-O2 -fno-rtti -fno-exceptions \
    -felide-constructors -fno-strength-reduce" \
    ./configure --prefix=/usr/local/mysql --enable-assembler
gmake
gmake install
cd /usr/local/mysql
bin/mysql_install_db --user=mysql
bin/mysqld_safe &

If you notice that @command{configure} will use MIT-pthreads, you should read the MIT-pthreads notes. See section 2.3.5 MIT-pthreads Notes.

If you get an error from @command{make install} that it can't find `/usr/include/pthreads', @command{configure} didn't detect that you need MIT-pthreads. To fix this problem, remove `config.cache', then re-run @command{configure} with the --with-mit-threads option.

Be sure that your name resolver setup is correct. Otherwise, you may experience resolver delays or failures when connecting to @command{mysqld}. Also make sure that the localhost entry in the `/etc/hosts' file is correct. The file should start with a line similar to this: 

127.0.0.1       localhost localhost.your.domain

FreeBSD is known to have a very low default file handle limit. See section A.2.17 File Not Found. Start the server by using the --open-files-limit option for @command{mysqld_safe}, or raise the limits for the @command{mysqld} user in `/etc/login.conf' and rebuild it with cap_mkdb /etc/login.conf. Also be sure that you set the appropriate class for this user in the password file if you are not using the default (use chpass mysqld-user-name). @xref{mysqld_safe, , @command{mysqld_safe}}.

If you have a lot of memory, you should consider rebuilding the kernel to allow MySQL to use more than 512MB of RAM. Take a look at option MAXDSIZ in the LINT config file for more information.

If you get problems with the current date in MySQL, setting the TZ variable will probably help. See section E Environment Variables.

2.6.4.2 NetBSD Notes

To compile on NetBSD, you need GNU @command{make}. Otherwise, the build process will fail when @command{make} tries to run lint on C++ files.

2.6.4.3 OpenBSD 2.5 Notes

On OpenBSD Version 2.5, you can compile MySQL with native threads with the following options: 

CFLAGS=-pthread CXXFLAGS=-pthread ./configure --with-mit-threads=no

2.6.4.4 OpenBSD 2.8 Notes

Our users have reported that OpenBSD 2.8 has a threading bug that causes problems with MySQL. The OpenBSD Developers have fixed the problem, but as of January 25, 2001, it's only available in the ``-current'' branch. The symptoms of this threading bug are slow response, high load, high CPU usage, and crashes.

If you get an error like Error in accept:: Bad file descriptor or error 9 when trying to open tables or directories, the problem is probably that you have not allocated enough file descriptors for MySQL.

In this case, try starting @command{mysqld_safe} as root with the following options: 

mysqld_safe --user=mysql --open-files-limit=2048 &

2.6.4.5 BSD/OS Version 2.x Notes

If you get the following error when compiling MySQL, your @command{ulimit} value for virtual memory is too low: 

item_func.h: In method
`Item_func_ge::Item_func_ge(const Item_func_ge &)':
item_func.h:28: virtual memory exhausted
make[2]: *** [item_func.o] Error 1

Try using @command{ulimit -v 80000} and run @command{make} again. If this doesn't work and you are using @command{bash}, try switching to @command{csh} or @command{sh}; some BSDI users have reported problems with @command{bash} and @command{ulimit}.

If you are using @command{gcc}, you may also use have to use the --with-low-memory flag for @command{configure} to be able to compile `sql_yacc.cc'.

If you get problems with the current date in MySQL, setting the TZ variable will probably help. See section E Environment Variables.

2.6.4.6 BSD/OS Version 3.x Notes

Upgrade to BSD/OS Version 3.1. If that is not possible, install BSDIpatch M300-038.

Use the following command when configuring MySQL: 

env CXX=shlicc++ CC=shlicc2 \
./configure \
    --prefix=/usr/local/mysql \
    --localstatedir=/var/mysql \
    --without-perl \
    --with-unix-socket-path=/var/mysql/mysql.sock

The following is also known to work: 

env CC=gcc CXX=gcc CXXFLAGS=-O3 \
./configure \
    --prefix=/usr/local/mysql \
    --with-unix-socket-path=/var/mysql/mysql.sock

You can change the directory locations if you wish, or just use the defaults by not specifying any locations.

If you have problems with performance under heavy load, try using the --skip-thread-priority option to @command{mysqld}! This will run all threads with the same priority. On BSDI Version 3.1, this gives better performance, at least until BSDI fixes its thread scheduler.

If you get the error virtual memory exhausted while compiling, you should try using @command{ulimit -v 80000} and running @command{make} again. If this doesn't work and you are using @command{bash}, try switching to @command{csh} or @command{sh}; some BSDI users have reported problems with @command{bash} and @command{ulimit}.

2.6.4.7 BSD/OS Version 4.x Notes

BSDI Version 4.x has some thread-related bugs. If you want to use MySQL on this, you should install all thread-related patches. At least M400-023 should be installed.

On some BSDI Version 4.x systems, you may get problems with shared libraries. The symptom is that you can't execute any client programs, for example, @command{mysqladmin}. In this case, you need to reconfigure not to use shared libraries with the --disable-shared option to configure.

Some customers have had problems on BSDI 4.0.1 that the @command{mysqld} binary after a while can't open tables. This is because some library/system-related bug causes @command{mysqld} to change current directory without having asked for that to happen.

The fix is to either upgrade MySQL to at least version 3.23.34 or, after running @command{configure}, remove the line #define HAVE_REALPATH from config.h before running @command{make}.

Note that this means that you can't symbolically link a database directories to another database directory or symbolic link a table to another database on BSDI. (Making a symbolic link to another disk is okay).

2.6.5 Other Unix Notes

2.6.5.1 HP-UX Version 10.20 Notes

There are a couple of small problems when compiling MySQL on HP-UX. We recommend that you use @command{gcc} instead of the HP-UX native compiler, because @command{gcc} produces better code.

We recommend using @command{gcc} 2.95 on HP-UX. Don't use high optimization flags (such as -O6) because they may not be safe on HP-UX.

The following @command{configure} line should work with @command{gcc} 2.95: 

CFLAGS="-I/opt/dce/include -fpic" \
CXXFLAGS="-I/opt/dce/include -felide-constructors -fno-exceptions \
-fno-rtti" \
CXX=gcc \
./configure --with-pthread \
    --with-named-thread-libs='-ldce' \
    --prefix=/usr/local/mysql --disable-shared

The following @command{configure} line should work with @command{gcc} 3.1: 

CFLAGS="-DHPUX -I/opt/dce/include -O3 -fPIC" CXX=gcc \
CXXFLAGS="-DHPUX -I/opt/dce/include -felide-constructors \
    -fno-exceptions -fno-rtti -O3 -fPIC" \
./configure --prefix=/usr/local/mysql \
    --with-extra-charsets=complex --enable-thread-safe-client \
    --enable-local-infile  --with-pthread \
    --with-named-thread-libs=-ldce --with-lib-ccflags=-fPIC
    --disable-shared

2.6.5.2 HP-UX Version 11.x Notes

For HP-UX Version 11.x, we recommend MySQL 3.23.15 or later.

Because of some critical bugs in the standard HP-UX libraries, you should install the following patches before trying to run MySQL on HP-UX 11.0: 

PHKL_22840 Streams cumulative
PHNE_22397 ARPA cumulative

This will solve the problem of getting EWOULDBLOCK from recv() and EBADF from accept() in threaded applications.

If you are using @command{gcc} 2.95.1 on an unpatched HP-UX 11.x system, you will get the error: 

In file included from /usr/include/unistd.h:11,
                 from ../include/global.h:125,
                 from mysql_priv.h:15,
                 from item.cc:19:
/usr/include/sys/unistd.h:184: declaration of C function ...
/usr/include/sys/pthread.h:440: previous declaration ...
In file included from item.h:306,
                 from mysql_priv.h:158,
                 from item.cc:19:

The problem is that HP-UX doesn't define pthreads_atfork() consistently. It has conflicting prototypes in `/usr/include/sys/unistd.h':184 and `/usr/include/sys/pthread.h':440.

One solution is to copy `/usr/include/sys/unistd.h' into `mysql/include' and edit `unistd.h' and change it to match the definition in `pthread.h'. Look for this line: 

extern int pthread_atfork(void (*prepare)(), void (*parent)(),
                                          void (*child)());

Change it to look like this: 

extern int pthread_atfork(void (*prepare)(void), void (*parent)(void),
                                          void (*child)(void));

After making the change, the following @command{configure} line should work: 

CFLAGS="-fomit-frame-pointer -O3 -fpic" CXX=gcc \
CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -O3" \
./configure --prefix=/usr/local/mysql --disable-shared

If you are using MySQL 4.0.5 with the HP-UX compiler, you can use the following command (which has been tested with @command{cc} B.11.11.04): 

CC=cc CXX=aCC CFLAGS=+DD64 CXXFLAGS=+DD64 ./configure \
    --with-extra-character-set=complex

You can ignore any errors of the following type: 

aCC: warning 901: unknown option: `-3': use +help for online
documentation

If you get the following error from @command{configure}, verify that you don't have the path to the K&R compiler before the path to the HP-UX C and C++ compiler: 

checking for cc option to accept ANSI C... no
configure: error: MySQL requires an ANSI C compiler (and a C++ compiler).
Try gcc. See the Installation chapter in the Reference Manual.

Another reason for not being able to compile is that you didn't define the +DD64 flags as just described.

Another possibility for HP-UX 11 is to use MySQL binaries for HP-UX 10.20. We have received reports from some users that these binaries work fine on HP-UX 11.00. If you encounter problems, be sure to check your HP-UX patch level.

2.6.5.3 IBM-AIX notes

Automatic detection of xlC is missing from Autoconf, so a number of variables need to be set before running @command{configure}. The following example uses the IBM compiler: 

export CC="xlc_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192 "
export CXX="xlC_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192"
export CFLAGS="-I /usr/local/include"
export LDFLAGS="-L /usr/local/lib"
export CPPFLAGS=$CFLAGS
export CXXFLAGS=$CFLAGS

./configure --prefix=/usr/local \
                --localstatedir=/var/mysql \
                --sbindir='/usr/local/bin' \
                --libexecdir='/usr/local/bin' \
                --enable-thread-safe-client \
                --enable-large-files

The preceding options are used to compile the MySQL distribution that can be found at http://www-frec.bull.com/.

If you change the -O3 to -O2 in the preceding @command{configure} line, you must also remove the -qstrict option. This is a limitation in the IBM C compiler.

If you are using @command{gcc} or @command{egcs} to compile MySQL, you must use the -fno-exceptions flag, because the exception handling in @command{gcc}/@command{egcs} is not thread-safe! (This is tested with @command{egcs} 1.1.) There are also some known problems with IBM's assembler that may cause it to generate bad code when used with @command{gcc}.

We recommend the following @command{configure} line with @command{egcs} and @command{gcc} 2.95 on AIX: 

CC="gcc -pipe -mcpu=power -Wa,-many" \
CXX="gcc -pipe -mcpu=power -Wa,-many" \
CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti" \
./configure --prefix=/usr/local/mysql --with-low-memory

The -Wa,-many option is necessary for the compile to be successful. IBM is aware of this problem but is in no hurry to fix it because of the workaround that is available. We don't know if the -fno-exceptions is required with @command{gcc} 2.95, but because MySQL doesn't use exceptions and the option generates faster code, we recommend that you should always use it with @command{egcs} / @command{gcc}.

If you get a problem with assembler code, try changing the -mcpu=xxx option to match your CPU. Typically power2, power, or powerpc may need to be used. Alternatively, you might need to use 604 or 604e. We are not positive but suspect that power would likely be safe most of the time, even on a power2 machine.

If you don't know what your CPU is, execute a uname -m command. It will produce a string that looks like 000514676700, with a format of xxyyyyyymmss where xx and ss are always 00, yyyyyy is a unique system ID and mm is the ID of the CPU Planar. A chart of these values can be found at http://publib.boulder.ibm.com/doc_link/en_US/a_doc_lib/cmds/aixcmds5/uname.htm. This will give you a machine type and a machine model you can use to determine what type of CPU you have.

If you have problems with signals (MySQL dies unexpectedly under high load), you may have found an OS bug with threads and signals. In this case, you can tell MySQL not to use signals by configuring as follows: 

CFLAGS=-DDONT_USE_THR_ALARM CXX=gcc \
CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti \
-DDONT_USE_THR_ALARM" \
./configure --prefix=/usr/local/mysql --with-debug \
    --with-low-memory

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are ``sleeping'' on a connection with @command{mysqladmin kill} or @command{mysqladmin shutdown}. Instead, the client will die when it issues its next command.

On some versions of AIX, linking with libbind.a makes getservbyname() dump core. This is an AIX bug and should be reported to IBM.

For AIX 4.2.1 and @command{gcc}, you have to make the following changes.

After configuring, edit `config.h' and `include/my_config.h' and change the line that says this: 

#define HAVE_SNPRINTF 1

to this: 

#undef HAVE_SNPRINTF

And finally, in `mysqld.cc', you need to add a prototype for initgroups(). 

#ifdef _AIX41
extern "C" int initgroups(const char *,int);
#endif

If you need to allocate a lot of memory to the @command{mysqld} process, it's not enough to just use @command{ulimit -d unlimited}. You may also have to modify @command{mysqld_safe} to add a line something like this: 

export LDR_CNTRL='MAXDATA=0x80000000'

You can find more information about using a lot of memory at http://publib16.boulder.ibm.com/pseries/en_US/aixprggd/genprogc/lrg_prg_support.htm.

2.6.5.4 SunOS 4 Notes

On SunOS 4, MIT-pthreads is needed to compile MySQL. This in turn means you will need GNU @command{make}.

Some SunOS 4 systems have problems with dynamic libraries and @command{libtool}. You can use the following @command{configure} line to avoid this problem: 

./configure --disable-shared --with-mysqld-ldflags=-all-static

When compiling readline, you may get warnings about duplicate defines. These can be ignored.

When compiling @command{mysqld}, there will be some @code{implicit declaration of function} warnings. These can be ignored.

2.6.5.5 Alpha-DEC-UNIX Notes (Tru64)

If you are using @command{egcs} 1.1.2 on Digital Unix, you should upgrade to @command{gcc} 2.95.2, because @command{egcs} on DEC has some serious bugs!

When compiling threaded programs under Digital Unix, the documentation recommends using the -pthread option for @command{cc} and @command{cxx} and the -lmach -lexc libraries (in addition to -lpthread). You should run @command{configure} something like this: 

CC="cc -pthread" CXX="cxx -pthread -O" \
./configure --with-named-thread-libs="-lpthread -lmach -lexc -lc"

When compiling @command{mysqld}, you may see a couple of warnings like this: 

mysqld.cc: In function void handle_connections()':
mysqld.cc:626: passing long unsigned int *' as argument 3 of
accept(int,sockadddr *, int *)'

You can safely ignore these warnings. They occur because @command{configure} can detect only errors, not warnings.

If you start the server directly from the command line, you may have problems with it dying when you log out. (When you log out, your outstanding processes receive a SIGHUP signal.) If so, try starting the server like this: 

nohup mysqld [options] &

nohup causes the command following it to ignore any SIGHUP signal sent from the terminal. Alternatively, start the server by running @command{mysqld_safe}, which invokes @command{mysqld} using @command{nohup} for you. @xref{mysqld_safe, , @command{mysqld_safe}}.

If you get a problem when compiling `mysys/get_opt.c', just remove the #define _NO_PROTO line from the start of that file.

If you are using Compaq's CC compiler, the following @command{configure} line should work: 

CC="cc -pthread"
CFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed all -arch host"
CXX="cxx -pthread"
CXXFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed all \
    -arch host -noexceptions -nortti"
export CC CFLAGS CXX CXXFLAGS
./configure \
    --prefix=/usr/local/mysql \
    --with-low-memory \
    --enable-large-files \
    --enable-shared=yes \
    --with-named-thread-libs="-lpthread -lmach -lexc -lc"
gnumake

If you get a problem with @command{libtool} when compiling with shared libraries as just shown, when linking @command{mysql}, you should be able to get around this by issuing these commands: 

cd mysql
/bin/sh ../libtool --mode=link cxx -pthread  -O3 -DDBUG_OFF \
    -O4 -ansi_alias -ansi_args -fast -inline speed \
    -speculate all \ -arch host  -DUNDEF_HAVE_GETHOSTBYNAME_R \
    -o mysql  mysql.o readline.o sql_string.o completion_hash.o \
    ../readline/libreadline.a -lcurses \
    ../libmysql/.libs/libmysqlclient.so  -lm
cd ..
gnumake
gnumake install
scripts/mysql_install_db

2.6.5.6 Alpha-DEC-OSF/1 Notes

If you have problems compiling and have DEC @command{CC} and @command{gcc} installed, try running @command{configure} like this: 

CC=cc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 \
./configure --prefix=/usr/local/mysql

If you get problems with the `c_asm.h' file, you can create and use a 'dummy' `c_asm.h' file with: 

touch include/c_asm.h
CC=gcc CFLAGS=-I./include \
CXX=gcc CXXFLAGS=-O3 \
./configure --prefix=/usr/local/mysql

Note that the following problems with the ld program can be fixed by downloading the latest DEC (Compaq) patch kit from: http://ftp.support.compaq.com/public/unix/.

On OSF/1 V4.0D and compiler "DEC C V5.6-071 on Digital Unix V4.0 (Rev. 878)," the compiler had some strange behavior (undefined asm symbols). /bin/ld also appears to be broken (problems with _exit undefined errors occurring while linking @command{mysqld}). On this system, we have managed to compile MySQL with the following @command{configure} line, after replacing /bin/ld with the version from OSF 4.0C: 

CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql

With the Digital compiler "C++ V6.1-029," the following should work: 

CC=cc -pthread
CFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed \
       -speculate all -arch host
CXX=cxx -pthread
CXXFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed \
         -speculate all -arch host -noexceptions -nortti
export CC CFLAGS CXX CXXFLAGS
./configure --prefix=/usr/mysql/mysql \
            --with-mysqld-ldflags=-all-static --disable-shared \
            --with-named-thread-libs="-lmach -lexc -lc"

In some versions of OSF/1, the alloca() function is broken. Fix this by removing the line in `config.h' that defines 'HAVE_ALLOCA'.

The alloca() function also may have an incorrect prototype in /usr/include/alloca.h. This warning resulting from this can be ignored.

@command{configure} will use the following thread libraries automatically: --with-named-thread-libs="-lpthread -lmach -lexc -lc".

When using @command{gcc}, you can also try running @command{configure} like this: 

CFLAGS=-D_PTHREAD_USE_D4 CXX=gcc CXXFLAGS=-O3 ./configure ...

If you have problems with signals (MySQL dies unexpectedly under high load), you may have found an OS bug with threads and signals. In this case, you can tell MySQL not to use signals by configuring with: 

CFLAGS=-DDONT_USE_THR_ALARM \
CXXFLAGS=-DDONT_USE_THR_ALARM \
./configure ...

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are ``sleeping'' on a connection with @command{mysqladmin kill} or @command{mysqladmin shutdown}. Instead, the client will die when it issues its next command.

With @command{gcc} 2.95.2, you will probably run into the following compile error: 

sql_acl.cc:1456: Internal compiler error in `scan_region',
at except.c:2566
Please submit a full bug report.

To fix this, you should change to the sql directory and do a cut-and-paste of the last @command{gcc} line, but change -O3 to -O0 (or add -O0 immediately after @command{gcc} if you don't have any -O option on your compile line). After this is done, you can just change back to the top-level directory and run @command{make} again.

2.6.5.7 SGI Irix Notes

If you are using Irix Version 6.5.3 or newer, @command{mysqld} will be able to create threads only if you run it as a user that has CAP_SCHED_MGT privileges (such as root) or give the @command{mysqld} server this privilege with the following shell command: 

chcap "CAP_SCHED_MGT+epi" /opt/mysql/libexec/mysqld

You may have to undefine some symbols in `config.h' after running @command{configure} and before compiling.

In some Irix implementations, the alloca() function is broken. If the @command{mysqld} server dies on some SELECT statements, remove the lines from `config.h' that define HAVE_ALLOC and HAVE_ALLOCA_H. If @command{mysqladmin create} doesn't work, remove the line from `config.h' that defines HAVE_READDIR_R. You may have to remove the HAVE_TERM_H line as well.

SGI recommends that you install all the patches on this page as a set: http://support.sgi.com/surfzone/patches/patchset/6.2_indigo.rps.html

At the very minimum, you should install the latest kernel rollup, the latest rld rollup, and the latest libc rollup.

You definitely need all the POSIX patches on this page, for pthreads support:

http://support.sgi.com/surfzone/patches/patchset/6.2_posix.rps.html

If you get the something like the following error when compiling `mysql.cc': 

"/usr/include/curses.h", line 82: error(1084):
invalid combination of type

Type the following in the top-level directory of your MySQL source tree: 

extra/replace bool curses_bool < /usr/include/curses.h > include/curses.h
make

There have also been reports of scheduling problems. If only one thread is running, performance is slow. Avoid this by starting another client. This may lead to a two-to-tenfold increase in execution speed thereafter for the other thread. This is a poorly understood problem with Irix threads; you may have to improvise to find solutions until this can be fixed.

If you are compiling with @command{gcc}, you can use the following @command{configure} command: 

CC=gcc CXX=gcc CXXFLAGS=-O3 \
./configure --prefix=/usr/local/mysql --enable-thread-safe-client \
    --with-named-thread-libs=-lpthread

On Irix 6.5.11 with native Irix C and C++ compilers ver. 7.3.1.2, the following is reported to work 

CC=cc CXX=CC CFLAGS='-O3 -n32 -TARG:platform=IP22 -I/usr/local/include \
-L/usr/local/lib' CXXFLAGS='-O3 -n32 -TARG:platform=IP22 \
-I/usr/local/include -L/usr/local/lib' \
./configure --prefix=/usr/local/mysql --with-innodb --with-berkeley-db \
    --with-libwrap=/usr/local \
    --with-named-curses-libs=/usr/local/lib/libncurses.a

2.6.5.8 SCO Notes

The current port is tested only on ``sco3.2v5.0.5,'' ``sco3.2v5.0.6,'' and ``sco3.2v5.0.7'' systems. There has also been a lot of progress on a port to ``sco 3.2v4.2.'' Open Server 5.0.8(Legend) will have native threads and allow files greater than 2GB. The current maximum file size is 2GB.

We have been able to compile MySQL with the following @command{configure} command on on OpenServer with @command{gcc} 2.95.3. 

CC=gcc CXX=gcc ./configure --prefix=/usr/local/mysql \
    --enable-thread-safe-client --with-innodb \
    --with-openssl --with-vio --with-extra-charsets=complex

@command{gcc} is available at ftp://ftp.sco.com/pub/openserver5/opensrc/gnutools-5.0.7Kj.

This development system requires the OpenServer Execution Enviroment Supplement oss646B on OpenServer 5.0.6 and oss656B and The OpenSource libraries found in gwxlibs. All OpenSource tools are in the `opensrc' directory. They are available at ftp://ftp.sco.com/pub/openserver5/opensrc/.

We recommend using the latest production release of MySQL. Currently MySQL-4.0.x is the latest production release. There were some problems with MySQL 4.0.17 and MySQL 4.0.18, but they have now been fixed.

SCO provides operating system patches at ftp://ftp.sco.com/pub/openserver5 for OpenServer 5.0.[0-6] and ftp://ftp.sco.com/pub/openserverv5/507 for OpenServer 5.0.7.

SCO provides information about security fixes at ftp://ftp.sco.com/pub/security/OpenServer for OpenServer 5.0.x.

The maximum file size on an OpenSever 5.0.x system is 2GB.

The total memory which could be allocated for streams buffers, clists and lock records cannot exceed 60MB on OpenServer 5.0.x.

Streams buffers are allocated in units of 4096 byte pages, clists are 70 bytes each, and lock records are 64 bytes each, so: 

(NSTRPAGES * 4096) + (NCLIST * 70) + (MAX_FLCKREC * 64) <= 62914560

Follow this procedure to configure the Database Services option. If you are unsure whether an application requires this, see the documentation provided with the application.

  1. Log in as root.
  2. Enable the SUDS driver by editing the `/etc/conf/sdevice.d/suds' file. Change the N in the second field to a Y.
  3. Use mkdev aio or the Hardware/Kernel Manager to enable support for asynchronous I/O and relink the kernel. To allow users to lock down memory for use with this type of I/O, update the aiomemlock(F) file. This file should be updated to include the names of users that can use AIO and the maximum amounts of memory they can lock down.
  4. Many applications use setuid binaries so that you need to specify only a single user. See the documentation provided with the application to see if this is the case for your application.

After you complete this process, reboot the system to create a new kernel incorporating these changes.

By default, the entries in `/etc/conf/cf.d/mtune' are set as follows: 

Value           Default         Min             Max
-----           -------         --             ---
NBUF            0               24              450000
NHBUF           0               32              524288
NMPBUF          0               12              512
MAX_INODE       0               100             64000
MAX_FILE        0               100             64000
CTBUFSIZE       128             0               256
MAX_PROC        0               50              16000
MAX_REGION      0               500             160000
NCLIST          170             120             16640
MAXUP           100             15              16000
NOFILES         110             60              11000
NHINODE         128             64              8192
NAUTOUP         10              0               60
NGROUPS         8               0               128
BDFLUSHR        30              1               300
MAX_FLCKREC     0               50              16000
PUTBUFSZ        8000            2000            20000
MAXSLICE        100             25              100
ULIMIT          4194303         2048            4194303
* Streams Parameters
NSTREAM         64              1               32768
NSTRPUSH        9               9               9
NMUXLINK        192             1               4096
STRMSGSZ        16384           4096            524288
STRCTLSZ        1024            1024            1024
STRMAXBLK       524288          4096            524288
NSTRPAGES       500             0               8000
STRSPLITFRAC    80              50              100
NLOG            3               3               3
NUMSP           64              1               256
NUMTIM          16              1               8192
NUMTRW          16              1               8192
* Semaphore Parameters
SEMMAP          10              10              8192
SEMMNI          10              10              8192
SEMMNS          60              60              8192
SEMMNU          30              10              8192
SEMMSL          25              25              150
SEMOPM          10              10              1024
SEMUME          10              10              25
SEMVMX          32767           32767           32767
SEMAEM          16384           16384           16384
* Shared Memory Parameters
SHMMAX          524288          131072          2147483647
SHMMIN          1               1               1
SHMMNI          100             100             2000
FILE            0               100             64000
NMOUNT          0               4               256
NPROC           0               50              16000
NREGION         0               500             160000

We recommend setting these values as follows:

NOFILES should be 4096 or 2048.

MAXUP should be 2048.

To make changes to the kernel, cd to `/etc/conf/bin' and use @command{./idtune} name parameter to make the changes. For example, to change SEMMS to 200, execute these commands as root: 

# cd /etc/conf/bin
# ./idtune SEMMNS 200

We recommend tuning the system, but the proper parameter values to use depend on the number of users accessing the application or database and size the of the database (that is, the used buffer pool). The following will affect the following kernel parameters defined in `/etc/conf/cf.d/stune':

SHMMAX (recommended setting: 128MB) and SHMSEG (recommended setting: 15). These parameters have influence on the MySQL database engine to create user buffer pools.

NOFILES and MAXUP should be at to at least 2048.

MAXPROC should be set to at least 3000/4000 (depends on number of users) or more.

Also is recommended to use following formula to count value for SEMMSL, SEMMNS and SEMMNU: 

SEMMSL = 13

The 13 is what has been found to be the best for both Progress and MySQL.

SEMMNS = SEMMSL * number of db servers to be run on the system.

Set SEMMNS to the value of SEMMSL multiplied by the number of db servers (maximum) that you will be running on the system at one time. 

SEMMNU = SEMMNS

Set the value of SEMMNU to equal the value of SEMMNS. You could probably set this to 75% of SEMMNS, but this is a conservative estimate.

You need to at least install the "SCO OpenServer Linker and Application Development Libraries" or the OpenServer Development System to use @command{gcc}. You cannot just use the GCC Dev system without installing one of these.

You should get the FSU Pthreads package and install it first. This can be found at http://moss.csc.ncsu.edu/~mueller/ftp/pub/PART/pthreads.tar.gz. You can also get a precompiled package from ftp://ftp.zenez.com/pub/zenez/prgms/FSU-threads-3.14.tar.gz.

FSU Pthreads can be compiled with SCO Unix 4.2 with tcpip, or using OpenServer 3.0 or Open Desktop 3.0 (OS 3.0 ODT 3.0) with the SCO Development System installed using a good port of GCC 2.5.x. For ODT or OS 3.0, you will need a good port of GCC 2.5.x. There are a lot of problems without a good port. The port for this product requires the SCO Unix Development system. Without it, you are missing the libraries and the linker that is needed. You will also need `SCO-3.2v4.2-includes.tar.gz'. This file contains the changes to the SCO Development include files that are needed to get MySQL to build. You need to replace the existing system include files with these modified header files. They can be obtained from ftp://ftp.zenez.com/pub/zenez/prgms/SCO-3.2v4.2-includes.tar.gz.

To build FSU Pthreads on your system, all you should need to do is run GNU @command{make}. The `Makefile' in FSU-threads-3.14.tar.gz is already set up to make FSU-threads.

You can run @command{./configure} in the `threads/src' directory and select the SCO OpenServer option. This command copies `Makefile.SCO5' to `Makefile'. Then run @command{make}.

To install in the default `/usr/include' directory, log in as root, then cd to the `thread/src' directory and run @command{make install}.

Remember that you must use GNU @command{make} when making MySQL.

Note: If you don't start @command{mysqld_safe} as root, you probably will get only the default 110 open files per process. @command{mysqld} will write a note about this in the log file.

With SCO 3.2V4.2, you should use FSU Pthreads version 3.14 or newer. The following @command{configure} command should work: 

CFLAGS="-D_XOPEN_XPG4" CXX=gcc CXXFLAGS="-D_XOPEN_XPG4" \
./configure \
    --prefix=/usr/local/mysql \
    --with-named-thread-libs="-lgthreads -lsocket -lgen -lgthreads" \
    --with-named-curses-libs="-lcurses"

You may get some problems with some include files. In this case, you can find new SCO-specific include files at ftp://ftp.zenez.com/pub/zenez/prgms/SCO-3.2v4.2-includes.tar.gz.

You should unpack this file in the `include' directory of your MySQL source tree.

SCO development notes:

Begining with Legend, OpenServer will have native threads and no 2GB file size limit.

2.6.5.9 SCO UnixWare Version 7.1.x Notes

We recommend using the latest production release of MySQL. Currently this is MySQL 4.0.x. Should you choose to use an older release of MySQL on UnixWare 7.1.x, you must use a version of MySQL at least as recent as 3.22.13 to get fixes for some portability and OS problems.

We have been able to compile MySQL with the following @command{configure} command on UnixWare Version 7.1.x: 

CC="cc" CFLAGS="-I/usr/local/include" \
CXX="CC" CXXFLAGS="-I/usr/local/include" \
./configure --prefix=/usr/local/mysql \
    --enable-thread-safe-client --with-berkeley-db=./bdb \
    --with-innodb --with-openssl --with-extra-charsets=complex

If you want to use @command{gcc}, you must use @command{gcc} 2.95.3 or newer. 

CC=gcc CXX=g++ ./configure --prefix=/usr/local/mysql

SCO provides operating system patches at ftp://ftp.sco.com/pub/unixware7 for UnixWare 7.1.1, ftp://ftp.sco.com/pub/unixware7/713/ for UnixWare 7.1.3, ftp://ftp.sco.com/pub/unixware7/714/ for UnixWare 7.1.4, and ftp://ftp.sco.com/pub/openunix8 for OpenUNIX 8.0.0.

SCO provides information about security fixes at ftp://ftp.sco.com/pub/security/OpenUNIX for OpenUNIX and ftp://ftp.sco.com/pub/security/UnixWare for UnixWare.

By default, the maximum file size on a UnixWare 7 system is 1GB. Many OS utilities have a limitation of 2GB. The maximum possible file size on UnixWare 7 is 1TB with VXFS.

To enable large file support on UnixWare 7.1.x, run fsadm. 

# fsadm -Fvxfs -o largefiles /
# fsadm /					* Note
# ulimit unlimited
# cd /etc/conf/bin
# ./idtune SFSZLIM 0x7FFFFFFF			** Note
# ./idtune HFSZLIM 0x7FFFFFFF			** Note
# ./idbuild -B

* This should report "largefiles".
** 0x7FFFFFFF represents infinity for these values.

Reboot the system using shutdown.

By default, the entries in `/etc/conf/cf.d/mtune' are set to: 

Value           Default         Min             Max
-----           -------         --             ---
SVMMLIM         0x9000000       0x1000000       0x7FFFFFFF
HVMMLIM         0x9000000       0x1000000       0x7FFFFFFF
SSTKLIM         0x1000000       0x2000          0x7FFFFFFF
HSTKLIM         0x1000000       0x2000          0x7FFFFFFF

We recommend setting these values as follows: 

SDATLIM 0x7FFFFFFF
HDATLIM 0x7FFFFFFF
SSTKLIM 0x7FFFFFFF
HSTKLIM 0x7FFFFFFF
SVMMLIM 0x7FFFFFFF
HVMMLIM 0x7FFFFFFF
SFNOLIM 2048
HFNOLIM 2048

We recommend tuning the system, but the proper parameter values to use depend on the number of users accessing the application or database and size the of the database (that is, the used buffer pool). The following will affect the following kernel parameters defined in `/etc/conf/cf.d/stune':

SHMMAX (recommended setting: 128MB) and SHMSEG (recommended setting: 15). These parameters have influence on the MySQL database engine to create user buffer pools.

SFNOLIM and HFNOLIM should be at maximum 2048.

NPROC should be set to at least 3000/4000 (depends on number of users).

Also is recommended to use following formula to count value for SEMMSL, SEMMNS, and SEMMNU: 

SEMMSL = 13

13 is what has been found to be the best for both Progress and MySQL.

SEMMNS = SEMMSL * number of db servers to be run on the system.

Set SEMMNS to the value of SEMMSL multiplied by the number of db servers (maximum) that you will be running on the system at one time.

SEMMNU = SEMMNS

Set the value of SEMMNU to equal the value of SEMMNS. You could probably set this to 75% of SEMMNS, but this is a conservative estimate.

2.6.6 OS/2 Notes

MySQL uses quite a few open files. Because of this, you should add something like the following to your `CONFIG.SYS' file: 

SET EMXOPT=-c -n -h1024

If you don't do this, you will probably run into the following error: 

File 'xxxx' not found (Errcode: 24)

When using MySQL with OS/2 Warp 3, FixPack 29 or above is required. With OS/2 Warp 4, FixPack 4 or above is required. This is a requirement of the Pthreads library. MySQL must be installed on a partition with a type that supports long filenames, such as HPFS, FAT32, and so on.

The `INSTALL.CMD' script must be run from OS/2's own `CMD.EXE' and may not work with replacement shells such as `4OS2.EXE'.

The `scripts/mysql-install-db' script has been renamed. It is now called `install.cmd' and is a REXX script, which will set up the default MySQL security settings and create the WorkPlace Shell icons for MySQL.

Dynamic module support is compiled in but not fully tested. Dynamic modules should be compiled using the Pthreads runtime library. 

gcc -Zdll -Zmt -Zcrtdll=pthrdrtl -I../include -I../regex -I.. \
    -o example udf_example.cc -L../lib -lmysqlclient udf_example.def
mv example.dll example.udf

Note: Due to limitations in OS/2, UDF module name stems must not exceed eight characters. Modules are stored in the `/mysql2/udf' directory; the safe-mysqld.cmd script will put this directory in the BEGINLIBPATH environment variable. When using UDF modules, specified extensions are ignored--it is assumed to be `.udf'. For example, in Unix, the shared module might be named `example.so' and you would load a function from it like this: 

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME 'example.so';

In OS/2, the module would be named `example.udf', but you would not specify the module extension: 

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME 'example';

2.6.7 BeOS Notes

We have in the past talked with some BeOS developers who have said that MySQL is 80% ported to BeOS, but we haven't heard from them in a while.

2.7 Perl Installation Notes

Perl support for MySQL is provided by means of the DBI/DBD client interface. The interface requires Perl Version 5.6.0 or later. It will not work if you have an older version of Perl.

If you want to use transactions with Perl DBI, you need to have DBD::mysql version 1.2216 or newer. Version 2.9003 or newer is recommended.

If you are using the MySQL 4.1 client library, you must use DBD::mysql 2.9003 or newer.

As of MySQL 3.22.8, Perl support is no longer included with MySQL distributions. You can obtain the necessary modules from http://search.cpan.org for Unix, or by using the ActiveState ppm program on Windows. The following sections describe how to do this.

Perl support for MySQL must be installed if you want to run the MySQL benchmark scripts. See section 7.1.4 The MySQL Benchmark Suite.

2.7.1 Installing Perl on Unix

MySQL Perl support requires that you've installed MySQL client programming support (libraries and header files). Most installation methods install the necessary files. However, if you installed MySQL from RPM files on Linux, be sure that you've installed the developer RPM. The client programs are in the client RPM, but client programming support is in the developer RPM.

If you want to install Perl support, the files you will need can be obtained from the CPAN (Comprehensive Perl Archive Network) at http://search.cpan.org.

The easiest way to install Perl modules on Unix is to use the CPAN module. For example: 

shell> perl -MCPAN -e shell
cpan> install DBI
cpan> install DBD::mysql

The DBD::mysql installation runs a number of tests. These tests require being able to connect to the local MySQL server as the anonymous user with no password. If you have removed anonymous accounts or assigned them passwords, the tests fail. You can use force install DBD::mysql to ignore the failed tests.

DBI requires the Data::Dumper module. It may already be installed; if not, you should install it before installing DBI.

It is also possible to download the module distributions in the form of compressed @command{tar} archives and build the modules manually. For example, to unpack and build a DBI distribution, use a procedure such as this:

  1. Unpack the distribution into the current directory: 
    shell> gunzip < DBI-VERSION.tar.gz | tar xvf -
    This command creates a directory named `DBI-VERSION'.
  2. Change location into the top-level directory of the unpacked distribution: 
    shell> cd DBI-VERSION
  3. Build the distribution and compile everything: 
    shell> perl Makefile.PL
shell> make
shell> make test
shell> make install

The @command{make test} command is important because it verifies that the module is working. Note that when you run that command during the DBD::mysql installation to exercise the interface code, the MySQL server must be running or the test will fail.

It is a good idea to rebuild and reinstall the DBD::mysql distribution whenever you install a new release of MySQL, particularly if you notice symptoms such as that all your DBI scripts fail after you upgrade MySQL.

If you don't have access rights to install Perl modules in the system directory or if you want to install local Perl modules, the following reference may be useful: http://servers.digitaldaze.com/extensions/perl/modules.html#modules

Look under the heading ``Installing New Modules that Require Locally Installed Modules.''

2.7.2 Installing ActiveState Perl on Windows

On Windows, you should do the following to install the MySQL DBD module with ActiveState Perl:

This procedure should work at least with ActiveState Perl Version 5.6.

If you can't get the procedure to work, you should instead install the MyODBC driver and connect to the MySQL server through ODBC: 

use DBI;
$dbh= DBI->connect("DBI:ODBC:$dsn",$user,$password) ||
  die "Got error $DBI::errstr when connecting to $dsn\n";

2.7.3 Problems Using the Perl DBI/DBD Interface

If Perl reports that it can't find the `../mysql/mysql.so' module, then the problem is probably that Perl can't locate the shared library `libmysqlclient.so'.

You should be able to fix this by one of the following methods:

Note that you may also need to modify the -L options if there are other libraries that the linker fails to find. For example, if the linker cannot find libc because it is in `/lib' and the link command specifies -L/usr/lib, change the -L option to -L/lib or add -L/lib to the existing link command.

If you get the following errors from DBD::mysql, you are probably using @command{gcc} (or using an old binary compiled with @command{gcc}): 

/usr/bin/perl: can't resolve symbol '__moddi3'
/usr/bin/perl: can't resolve symbol '__divdi3'

Add -L/usr/lib/gcc-lib/... -lgcc to the link command when the `mysql.so' library gets built (check the output from @command{make} for `mysql.so' when you compile the Perl client). The -L option should specify the pathname of the directory where `libgcc.a' is located on your system.

Another cause of this problem may be that Perl and MySQL aren't both compiled with @command{gcc}. In this case, you can solve the mismatch by compiling both with @command{gcc}.

You may see the following error from DBD::mysql when you run the tests: 

t/00base............install_driver(mysql) failed:
Can't load '../blib/arch/auto/DBD/mysql/mysql.so' for module DBD::mysql:
../blib/arch/auto/DBD/mysql/mysql.so: undefined symbol:
uncompress at /usr/lib/perl5/5.00503/i586-linux/DynaLoader.pm line 169.

This means that you need to include the -lz compression library on the link line. That can be done by changing the following line in the file `lib/DBD/mysql/Install.pm': 

$sysliblist .= " -lm";

Change that line to: 

$sysliblist .= " -lm -lz";

After this, you must run @command{make realclean} and then proceed with the installation from the beginning.

If you want to install DBI on SCO, you have to edit the `Makefile' in DBI-xxx and each subdirectory. Note that the following assumes @command{gcc} 2.95.2 or newer: 

OLD:                                  NEW:
CC = cc                               CC = gcc
CCCDLFLAGS = -KPIC -W1,-Bexport       CCCDLFLAGS = -fpic
CCDLFLAGS = -wl,-Bexport              CCDLFLAGS =

LD = ld                               LD = gcc -G -fpic
LDDLFLAGS = -G -L/usr/local/lib       LDDLFLAGS = -L/usr/local/lib
LDFLAGS = -belf -L/usr/local/lib      LDFLAGS = -L/usr/local/lib

LD = ld                               LD = gcc -G -fpic
OPTIMISE = -Od                        OPTIMISE = -O1

OLD:
CCCFLAGS = -belf -dy -w0 -U M_XENIX -DPERL_SCO5 -I/usr/local/include

NEW:
CCFLAGS = -U M_XENIX -DPERL_SCO5 -I/usr/local/include

These changes are necessary because the Perl dynaloader will not load the DBI modules if they were compiled with @command{icc} or @command{cc}.

If you want to use the Perl module on a system that doesn't support dynamic linking (such as SCO), you can generate a static version of Perl that includes DBI and DBD::mysql. The way this works is that you generate a version of Perl with the DBI code linked in and install it on top of your current Perl. Then you use that to build a version of Perl that additionally has the DBD code linked in, and install that.

On SCO, you must have the following environment variables set: 

LD_LIBRARY_PATH=/lib:/usr/lib:/usr/local/lib:/usr/progressive/lib

Or: 

LD_LIBRARY_PATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:\
    /usr/progressive/lib:/usr/skunk/lib
LIBPATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:\
    /usr/progressive/lib:/usr/skunk/lib
MANPATH=scohelp:/usr/man:/usr/local1/man:/usr/local/man:\
    /usr/skunk/man:

First, create a Perl that includes a statically linked DBI module by running these commands in the directory where your DBI distribution is located: 

shell> perl Makefile.PL -static -config
shell> make
shell> make install
shell> make perl

Then you must install the new Perl. The output of @command{make perl} will indicate the exact @command{make} command you will need to execute to perform the installation. On SCO, this is @command{make -f Makefile.aperl inst_perl MAP_TARGET=perl}.

Next, use the just-created Perl to create another Perl that also includes a statically linked DBD::mysql by running these commands in the directory where your DBD::mysql distribution is located: 

shell> perl Makefile.PL -static -config
shell> make
shell> make install
shell> make perl

Finally, you should install this new Perl. Again, the output of @command{make perl} indicates the command to use.

3 MySQL Tutorial

This chapter provides a tutorial introduction to MySQL by showing how to use the @command{mysql} client program to create and use a simple database. @command{mysql} (sometimes referred to as the ``terminal monitor'' or just ``monitor'') is an interactive program that allows you to connect to a MySQL server, run queries, and view the results. @command{mysql} may also be used in batch mode: you place your queries in a file beforehand, then tell @command{mysql} to execute the contents of the file. Both ways of using @command{mysql} are covered here.

To see a list of options provided by @command{mysql}, invoke it with the --help option: 

shell> mysql --help

This chapter assumes that @command{mysql} is installed on your machine and that a MySQL server is available to which you can connect. If this is not true, contact your MySQL administrator. (If you are the administrator, you will need to consult other sections of this manual.)

This chapter describes the entire process of setting up and using a database. If you are interested only in accessing an already-existing database, you may want to skip over the sections that describe how to create the database and the tables it contains.

Because this chapter is tutorial in nature, many details are necessarily omitted. Consult the relevant sections of the manual for more information on the topics covered here.

3.1 Connecting to and Disconnecting from the Server

To connect to the server, you'll usually need to provide a MySQL username when you invoke @command{mysql} and, most likely, a password. If the server runs on a machine other than the one where you log in, you'll also need to specify a hostname. Contact your administrator to find out what connection parameters you should use to connect (that is, what host, username, and password to use). Once you know the proper parameters, you should be able to connect like this: 

shell> mysql -h host -u user -p
Enter password: ********

host and user represent the hostname where your MySQL server is running and the username of your MySQL account. Substitute appropriate values for your setup. The ******** represents your password; enter it when @command{mysql} displays the Enter password: prompt.

If that works, you should see some introductory information followed by a mysql> prompt: 

shell> mysql -h host -u user -p
Enter password: ********
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 25338 to server version: 4.0.14-log

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql>

The prompt tells you that @command{mysql} is ready for you to enter commands.

Some MySQL installations allow users to connect as the anonymous (unnamed) user to the server running on the local host. If this is the case on your machine, you should be able to connect to that server by invoking @command{mysql} without any options: 

shell> mysql

After you have connected successfully, you can disconnect any time by typing QUIT (or \q) at the mysql> prompt: 

mysql> QUIT
Bye

On Unix, you can also disconnect by pressing Control-D.

Most examples in the following sections assume that you are connected to the server. They indicate this by the mysql> prompt.

3.2 Entering Queries

Make sure that you are connected to the server, as discussed in the previous section. Doing so will not in itself select any database to work with, but that's okay. At this point, it's more important to find out a little about how to issue queries than to jump right in creating tables, loading data into them, and retrieving data from them. This section describes the basic principles of entering commands, using several queries you can try out to familiarize yourself with how @command{mysql} works.

Here's a simple command that asks the server to tell you its version number and the current date. Type it in as shown here following the mysql> prompt and press Enter: 

mysql> SELECT VERSION(), CURRENT_DATE;
+--------------+--------------+
| VERSION()    | CURRENT_DATE |
+--------------+--------------+
| 3.22.20a-log | 1999-03-19   |
+--------------+--------------+
1 row in set (0.01 sec)
mysql>

This query illustrates several things about @command{mysql}:

Keywords may be entered in any lettercase. The following queries are equivalent: 

mysql> SELECT VERSION(), CURRENT_DATE;
mysql> select version(), current_date;
mysql> SeLeCt vErSiOn(), current_DATE;

Here's another query. It demonstrates that you can use @command{mysql} as a simple calculator: 

mysql> SELECT SIN(PI()/4), (4+1)*5;
+-------------+---------+
| SIN(PI()/4) | (4+1)*5 |
+-------------+---------+
|    0.707107 |      25 |
+-------------+---------+

The queries shown thus far have been relatively short, single-line statements. You can even enter multiple statements on a single line. Just end each one with a semicolon: 

mysql> SELECT VERSION(); SELECT NOW();
+--------------+
| VERSION()    |
+--------------+
| 3.22.20a-log |
+--------------+

+---------------------+
| NOW()               |
+---------------------+
| 1999-03-19 00:15:33 |
+---------------------+

A command need not be given all on a single line, so lengthy commands that require several lines are not a problem. @command{mysql} determines where your statement ends by looking for the terminating semicolon, not by looking for the end of the input line. (In other words, @command{mysql} accepts free-format input: it collects input lines but does not execute them until it sees the semicolon.)

Here's a simple multiple-line statement: 

mysql> SELECT
    -> USER()
    -> ,
    -> CURRENT_DATE;
+--------------------+--------------+
| USER()             | CURRENT_DATE |
+--------------------+--------------+
| joesmith@localhost | 1999-03-18   |
+--------------------+--------------+

In this example, notice how the prompt changes from mysql> to -> after you enter the first line of a multiple-line query. This is how @command{mysql} indicates that it hasn't seen a complete statement and is waiting for the rest. The prompt is your friend, because it provides valuable feedback. If you use that feedback, you will always be aware of what @command{mysql} is waiting for.

If you decide you don't want to execute a command that you are in the process of entering, cancel it by typing \c: 

mysql> SELECT
    -> USER()
    -> \c
mysql>

Here, too, notice the prompt. It switches back to mysql> after you type \c, providing feedback to indicate that @command{mysql} is ready for a new command.

The following table shows each of the prompts you may see and summarizes what they mean about the state that @command{mysql} is in:

Prompt Meaning
mysql> Ready for new command.
-> Waiting for next line of multiple-line command.
'> Waiting for next line, collecting a string that begins with a single quote (`'').
"> Waiting for next line, collecting a string that begins with a double quote (`"').
`> Waiting for next line, collecting an identifier that begins with a backtick (``').

Multiple-line statements commonly occur by accident when you intend to issue a command on a single line, but forget the terminating semicolon. In this case, @command{mysql} waits for more input: 

mysql> SELECT USER()
    ->

If this happens to you (you think you've entered a statement but the only response is a -> prompt), most likely @command{mysql} is waiting for the semicolon. If you don't notice what the prompt is telling you, you might sit there for a while before realising what you need to do. Enter a semicolon to complete the statement, and @command{mysql} will execute it: 

mysql> SELECT USER()
    -> ;
+--------------------+
| USER()             |
+--------------------+
| joesmith@localhost |
+--------------------+

The '> and "> prompts occur during string collection. In MySQL, you can write strings surrounded by either `'' or `"' characters (for example, 'hello' or "goodbye"), and @command{mysql} lets you enter strings that span multiple lines. When you see a '> or "> prompt, it means that you've entered a line containing a string that begins with a `'' or `"' quote character, but have not yet entered the matching quote that terminates the string. That's fine if you really are entering a multiple-line string, but how likely is that? Not very. More often, the '> and "> prompts indicate that you've inadvertantly left out a quote character. For example: 

mysql> SELECT * FROM my_table WHERE name = 'Smith AND age < 30;
    '>

If you enter this SELECT statement, then press Enter and wait for the result, nothing will happen. Instead of wondering why this query takes so long, notice the clue provided by the '> prompt. It tells you that @command{mysql} expects to see the rest of an unterminated string. (Do you see the error in the statement? The string 'Smith is missing the second quote.)

At this point, what do you do? The simplest thing is to cancel the command. However, you cannot just type \c in this case, because @command{mysql} interprets it as part of the string that it is collecting! Instead, enter the closing quote character (so @command{mysql} knows you've finished the string), then type \c: 

mysql> SELECT * FROM my_table WHERE name = 'Smith AND age < 30;
    '> '\c
mysql>

The prompt changes back to mysql>, indicating that @command{mysql} is ready for a new command.

The `> prompt is similar to th '> and "> prompts, but indicates that you have begun but not completed a backtick-quoted identifier.

It's important to know what the '>, ">, and `> prompts signify, because if you mistakenly enter an unterminated string, any further lines you type will appear to be ignored by @command{mysql}---including a line containing QUIT! This can be quite confusing, especially if you don't know that you need to supply the terminating quote before you can cancel the current command.

3.3 Creating and Using a Database

Now that you know how to enter commands, it's time to access a database.

Suppose that you have several pets in your home (your menagerie) and you'd like to keep track of various types of information about them. You can do so by creating tables to hold your data and loading them with the desired information. Then you can answer different sorts of questions about your animals by retrieving data from the tables. This section shows you how to:

The menagerie database will be simple (deliberately), but it is not difficult to think of real-world situations in which a similar type of database might be used. For example, a database like this could be used by a farmer to keep track of livestock, or by a veterinarian to keep track of patient records. A menagerie distribution containing some of the queries and sample data used in the following sections can be obtained from the MySQL Web site. It's available in either compressed @command{tar} format (http://www.mysql.com/Downloads/Contrib/Examples/menagerie.tar.gz) or Zip format (http://www.mysql.com/Downloads/Contrib/Examples/menagerie.zip).

Use the SHOW statement to find out what databases currently exist on the server: 

mysql> SHOW DATABASES;
+----------+
| Database |
+----------+
| mysql    |
| test     |
| tmp      |
+----------+

The list of databases is probably different on your machine, but the mysql and test databases are likely to be among them. The mysql database is required because it describes user access privileges. The test database is often provided as a workspace for users to try things out.

Note that you may not see all databases if you don't have the SHOW DATABASES privilege. See section 14.5.1.2 GRANT and REVOKE Syntax.

If the test database exists, try to access it: 

mysql> USE test
Database changed

Note that USE, like QUIT, does not require a semicolon. (You can terminate such statements with a semicolon if you like; it does no harm.) The USE statement is special in another way, too: it must be given on a single line.

You can use the test database (if you have access to it) for the examples that follow, but anything you create in that database can be removed by anyone else with access to it. For this reason, you should probably ask your MySQL administrator for permission to use a database of your own. Suppose that you want to call yours menagerie. The administrator needs to execute a command like this: 

mysql> GRANT ALL ON menagerie.* TO 'your_mysql_name'@'your_client_host';

where your_mysql_name is the MySQL username assigned to you and your_client_host is the host from which you connect to the server.

3.3.1 Creating and Selecting a Database

If the administrator creates your database for you when setting up your permissions, you can begin using it. Otherwise, you need to create it yourself: 

mysql> CREATE DATABASE menagerie;

Under Unix, database names are case sensitive (unlike SQL keywords), so you must always refer to your database as menagerie, not as Menagerie, MENAGERIE, or some other variant. This is also true for table names. (Under Windows, this restriction does not apply, although you must refer to databases and tables using the same lettercase throughout a given query.)

Creating a database does not select it for use; you must do that explicitly. To make menagerie the current database, use this command: 

mysql> USE menagerie
Database changed

Your database needs to be created only once, but you must select it for use each time you begin a @command{mysql} session. You can do this by issuing a USE statement as shown in the example. Alternatively, you can select the database on the command line when you invoke @command{mysql}. Just specify its name after any connection parameters that you might need to provide. For example: 

shell> mysql -h host -u user -p menagerie
Enter password: ********

Note that menagerie is not your password on the command just shown. If you want to supply your password on the command line after the -p option, you must do so with no intervening space (for example, as -pmypassword, not as -p mypassword). However, putting your password on the command line is not recommended, because doing so exposes it to snooping by other users logged in on your machine.

3.3.2 Creating a Table

Creating the database is the easy part, but at this point it's empty, as SHOW TABLES will tell you: 

mysql> SHOW TABLES;
Empty set (0.00 sec)

The harder part is deciding what the structure of your database should be: what tables you will need and what columns will be in each of them.

You'll want a table that contains a record for each of your pets. This can be called the pet table, and it should contain, as a bare minimum, each animal's name. Because the name by itself is not very interesting, the table should contain other information. For example, if more than one person in your family keeps pets, you might want to list each animal's owner. You might also want to record some basic descriptive information such as species and sex.

How about age? That might be of interest, but it's not a good thing to store in a database. Age changes as time passes, which means you'd have to update your records often. Instead, it's better to store a fixed value such as date of birth. Then, whenever you need age, you can calculate it as the difference between the current date and the birth date. MySQL provides functions for doing date arithmetic, so this is not difficult. Storing birth date rather than age has other advantages, too:

You can probably think of other types of information that would be useful in the pet table, but the ones identified so far are sufficient for now: name, owner, species, sex, birth, and death.

Use a CREATE TABLE statement to specify the layout of your table: 

mysql> CREATE TABLE pet (name VARCHAR(20), owner VARCHAR(20),
    -> species VARCHAR(20), sex CHAR(1), birth DATE, death DATE);

VARCHAR is a good choice for the name, owner, and species columns because the column values will vary in length. The lengths of those columns need not all be the same, and need not be 20. You can pick any length from 1 to 255, whatever seems most reasonable to you. (If you make a poor choice and it turns out later that you need a longer field, MySQL provides an ALTER TABLE statement.)

Several types of values can be chosen to represent sex in animal records, such as 'm' and 'f', or perhaps 'male' and 'female'. It's simplest to use the single characters 'm' and 'f'.

The use of the DATE data type for the birth and death columns is a fairly obvious choice.

Now that you have created a table, SHOW TABLES should produce some output: 

mysql> SHOW TABLES;
+---------------------+
| Tables in menagerie |
+---------------------+
| pet                 |
+---------------------+

To verify that your table was created the way you expected, use a DESCRIBE statement: 

mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(20) | YES  |     | NULL    |       |
| owner   | varchar(20) | YES  |     | NULL    |       |
| species | varchar(20) | YES  |     | NULL    |       |
| sex     | char(1)     | YES  |     | NULL    |       |
| birth   | date        | YES  |     | NULL    |       |
| death   | date        | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

You can use DESCRIBE any time, for example, if you forget the names of the columns in your table or what types they have.

3.3.3 Loading Data into a Table

After creating your table, you need to populate it. The LOAD DATA and INSERT statements are useful for this.

Suppose that your pet records can be described as shown here. (Observe that MySQL expects dates in 'YYYY-MM-DD' format; this may be different from what you are used to.)

name owner species sex birth death
Fluffy Harold cat f 1993-02-04
Claws Gwen cat m 1994-03-17
Buffy Harold dog f 1989-05-13
Fang Benny dog m 1990-08-27
Bowser Diane dog m 1979-08-31 1995-07-29
Chirpy Gwen bird f 1998-09-11
Whistler Gwen bird 1997-12-09
Slim Benny snake m 1996-04-29

Because you are beginning with an empty table, an easy way to populate it is to create a text file containing a row for each of your animals, then load the contents of the file into the table with a single statement.

You could create a text file `pet.txt' containing one record per line, with values separated by tabs, and given in the order in which the columns were listed in the CREATE TABLE statement. For missing values (such as unknown sexes or death dates for animals that are still living), you can use NULL values. To represent these in your text file, use \N (backslash, capital-N). For example, the record for Whistler the bird would look like this (where the whitespace between values is a single tab character):

name owner species sex birth death
Whistler Gwen bird \N 1997-12-09 \N

To load the text file `pet.txt' into the pet table, use this command: 

mysql> LOAD DATA LOCAL INFILE '/path/pet.txt' INTO TABLE pet;

Note that if you created the file on Windows with an editor that uses \r\n as a line terminator, you should use: 

mysql> LOAD DATA LOCAL INFILE '/path/pet.txt' INTO TABLE pet
    -> LINES TERMINATED BY '\r\n';

You can specify the column value separator and end of line marker explicitly in the LOAD DATA statement if you wish, but the defaults are tab and linefeed. These are sufficient for the statement to read the file `pet.txt' properly.

If the statement fails, it is likely that your MySQL installation does not have local file capability enabled by default. See section 5.4.4 Security Issues with LOAD DATA LOCAL for information on how to change this.

When you want to add new records one at a time, the INSERT statement is useful. In its simplest form, you supply values for each column, in the order in which the columns were listed in the CREATE TABLE statement. Suppose that Diane gets a new hamster named Puffball. You could add a new record using an INSERT statement like this: 

mysql> INSERT INTO pet
    -> VALUES ('Puffball','Diane','hamster','f','1999-03-30',NULL);

Note that string and date values are specified as quoted strings here. Also, with INSERT, you can insert NULL directly to represent a missing value. You do not use \N like you do with LOAD DATA.

From this example, you should be able to see that there would be a lot more typing involved to load your records initially using several INSERT statements rather than a single LOAD DATA statement.

3.3.4 Retrieving Information from a Table

The SELECT statement is used to pull information from a table. The general form of the statement is: 

SELECT what_to_select
FROM which_table
WHERE conditions_to_satisfy;

what_to_select indicates what you want to see. This can be a list of columns, or * to indicate ``all columns.'' which_table indicates the table from which you want to retrieve data. The WHERE clause is optional. If it's present, conditions_to_satisfy specifies conditions that rows must satisfy to qualify for retrieval.

3.3.4.1 Selecting All Data

The simplest form of SELECT retrieves everything from a table: 

mysql> SELECT * FROM pet;
+----------+--------+---------+------+------------+------------+
| name     | owner  | species | sex  | birth      | death      |
+----------+--------+---------+------+------------+------------+
| Fluffy   | Harold | cat     | f    | 1993-02-04 | NULL       |
| Claws    | Gwen   | cat     | m    | 1994-03-17 | NULL       |
| Buffy    | Harold | dog     | f    | 1989-05-13 | NULL       |
| Fang     | Benny  | dog     | m    | 1990-08-27 | NULL       |
| Bowser   | Diane  | dog     | m    | 1979-08-31 | 1995-07-29 |
| Chirpy   | Gwen   | bird    | f    | 1998-09-11 | NULL       |
| Whistler | Gwen   | bird    | NULL | 1997-12-09 | NULL       |
| Slim     | Benny  | snake   | m    | 1996-04-29 | NULL       |
| Puffball | Diane  | hamster | f    | 1999-03-30 | NULL       |
+----------+--------+---------+------+------------+------------+

This form of SELECT is useful if you want to review your entire table, for example, after you've just loaded it with your initial dataset. For example, you may happen to think that the birth date for Bowser doesn't seem quite right. Consulting your original pedigree papers, you find that the correct birth year should be 1989, not 1979.

There are least a couple of ways to fix this:

3.3.4.2 Selecting Particular Rows

As shown in the preceding section, it is easy to retrieve an entire table. Just omit the WHERE clause from the SELECT statement. But typically you don't want to see the entire table, particularly when it becomes large. Instead, you're usually more interested in answering a particular question, in which case you specify some constraints on the information you want. Let's look at some selection queries in terms of questions about your pets that they answer.

You can select only particular rows from your table. For example, if you want to verify the change that you made to Bowser's birth date, select Bowser's record like this: 

mysql> SELECT * FROM pet WHERE name = 'Bowser';
+--------+-------+---------+------+------------+------------+
| name   | owner | species | sex  | birth      | death      |
+--------+-------+---------+------+------------+------------+
| Bowser | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+-------+---------+------+------------+------------+

The output confirms that the year is correctly recorded now as 1989, not 1979.

String comparisons normally are case-insensitive, so you can specify the name as 'bowser', 'BOWSER', etc. The query result will be the same.

You can specify conditions on any column, not just name. For example, if you want to know which animals were born after 1998, test the birth column: 

mysql> SELECT * FROM pet WHERE birth >= '1998-1-1';
+----------+-------+---------+------+------------+-------+
| name     | owner | species | sex  | birth      | death |
+----------+-------+---------+------+------------+-------+
| Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
| Puffball | Diane | hamster | f    | 1999-03-30 | NULL  |
+----------+-------+---------+------+------------+-------+

You can combine conditions, for example, to locate female dogs: 

mysql> SELECT * FROM pet WHERE species = 'dog' AND sex = 'f';
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

The preceding query uses the AND logical operator. There is also an OR operator: 

mysql> SELECT * FROM pet WHERE species = 'snake' OR species = 'bird';
+----------+-------+---------+------+------------+-------+
| name     | owner | species | sex  | birth      | death |
+----------+-------+---------+------+------------+-------+
| Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL  |
| Slim     | Benny | snake   | m    | 1996-04-29 | NULL  |
+----------+-------+---------+------+------------+-------+

AND and OR may be intermixed, although AND has higher precedence than OR. If you use both operators, it's a good idea to use parentheses to indicate explicitly how conditions should be grouped: 

mysql> SELECT * FROM pet WHERE (species = 'cat' AND sex = 'm')
    -> OR (species = 'dog' AND sex = 'f');
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

3.3.4.3 Selecting Particular Columns

If you don't want to see entire rows from your table, just name the columns in which you're interested, separated by commas. For example, if you want to know when your animals were born, select the name and birth columns: 

mysql> SELECT name, birth FROM pet;
+----------+------------+
| name     | birth      |
+----------+------------+
| Fluffy   | 1993-02-04 |
| Claws    | 1994-03-17 |
| Buffy    | 1989-05-13 |
| Fang     | 1990-08-27 |
| Bowser   | 1989-08-31 |
| Chirpy   | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim     | 1996-04-29 |
| Puffball | 1999-03-30 |
+----------+------------+

To find out who owns pets, use this query: 

mysql> SELECT owner FROM pet;
+--------+
| owner  |
+--------+
| Harold |
| Gwen   |
| Harold |
| Benny  |
| Diane  |
| Gwen   |
| Gwen   |
| Benny  |
| Diane  |
+--------+

However, notice that the query simply retrieves the owner field from each record, and some of them appear more than once. To minimize the output, retrieve each unique output record just once by adding the keyword DISTINCT: 

mysql> SELECT DISTINCT owner FROM pet;
+--------+
| owner  |
+--------+
| Benny  |
| Diane  |
| Gwen   |
| Harold |
+--------+

You can use a WHERE clause to combine row selection with column selection. For example, to get birth dates for dogs and cats only, use this query: 

mysql> SELECT name, species, birth FROM pet
    -> WHERE species = 'dog' OR species = 'cat';
+--------+---------+------------+
| name   | species | birth      |
+--------+---------+------------+
| Fluffy | cat     | 1993-02-04 |
| Claws  | cat     | 1994-03-17 |
| Buffy  | dog     | 1989-05-13 |
| Fang   | dog     | 1990-08-27 |
| Bowser | dog     | 1989-08-31 |
+--------+---------+------------+

3.3.4.4 Sorting Rows

You may have noticed in the preceding examples that the result rows are displayed in no particular order. It's often easier to examine query output when the rows are sorted in some meaningful way. To sort a result, use an ORDER BY clause.

Here are animal birthdays, sorted by date: 

mysql> SELECT name, birth FROM pet ORDER BY birth;
+----------+------------+
| name     | birth      |
+----------+------------+
| Buffy    | 1989-05-13 |
| Bowser   | 1989-08-31 |
| Fang     | 1990-08-27 |
| Fluffy   | 1993-02-04 |
| Claws    | 1994-03-17 |
| Slim     | 1996-04-29 |
| Whistler | 1997-12-09 |
| Chirpy   | 1998-09-11 |
| Puffball | 1999-03-30 |
+----------+------------+

On character type columns, sorting--like all other comparison operations--is normally performed in a case-insensitive fashion. This means that the order will be undefined for columns that are identical except for their case. You can force a case-sensitive sort for a column by using the BINARY cast: ORDER BY BINARY col_name.

The default sort order is ascending, with smallest values first. To sort in reverse (descending) order, add the DESC keyword to the name of the column you are sorting by: 

mysql> SELECT name, birth FROM pet ORDER BY birth DESC;
+----------+------------+
| name     | birth      |
+----------+------------+
| Puffball | 1999-03-30 |
| Chirpy   | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim     | 1996-04-29 |
| Claws    | 1994-03-17 |
| Fluffy   | 1993-02-04 |
| Fang     | 1990-08-27 |
| Bowser   | 1989-08-31 |
| Buffy    | 1989-05-13 |
+----------+------------+

You can sort on multiple columns, and you can sort columns in different directions. For example, to sort by type of animal in ascending order, then by birth date within animal type in descending order (youngest animals first), use the following query: 

mysql> SELECT name, species, birth FROM pet
    -> ORDER BY species, birth DESC;
+----------+---------+------------+
| name     | species | birth      |
+----------+---------+------------+
| Chirpy   | bird    | 1998-09-11 |
| Whistler | bird    | 1997-12-09 |
| Claws    | cat     | 1994-03-17 |
| Fluffy   | cat     | 1993-02-04 |
| Fang     | dog     | 1990-08-27 |
| Bowser   | dog     | 1989-08-31 |
| Buffy    | dog     | 1989-05-13 |
| Puffball | hamster | 1999-03-30 |
| Slim     | snake   | 1996-04-29 |
+----------+---------+------------+

Note that the DESC keyword applies only to the column name immediately preceding it (birth); it does not affect the species column sort order.

3.3.4.5 Date Calculations

MySQL provides several functions that you can use to perform calculations on dates, for example, to calculate ages or extract parts of dates.

To determine how many years old each of your pets is, compute the difference in the year part of the current date and the birth date, then subtract one if the current date occurs earlier in the calendar year than the birth date. The following query shows, for each pet, the birth date, the current date, and the age in years. 

mysql> SELECT name, birth, CURDATE(),
    -> (YEAR(CURDATE())-YEAR(birth))
    -> - (RIGHT(CURDATE(),5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet;
+----------+------------+------------+------+
| name     | birth      | CURDATE()  | age  |
+----------+------------+------------+------+
| Fluffy   | 1993-02-04 | 2003-08-19 |   10 |
| Claws    | 1994-03-17 | 2003-08-19 |    9 |
| Buffy    | 1989-05-13 | 2003-08-19 |   14 |
| Fang     | 1990-08-27 | 2003-08-19 |   12 |
| Bowser   | 1989-08-31 | 2003-08-19 |   13 |
| Chirpy   | 1998-09-11 | 2003-08-19 |    4 |
| Whistler | 1997-12-09 | 2003-08-19 |    5 |
| Slim     | 1996-04-29 | 2003-08-19 |    7 |
| Puffball | 1999-03-30 | 2003-08-19 |    4 |
+----------+------------+------------+------+

Here, YEAR() pulls out the year part of a date and RIGHT() pulls off the rightmost five characters that represent the MM-DD (calendar year) part of the date. The part of the expression that compares the MM-DD values evaluates to 1 or 0, which adjusts the year difference down a year if CURDATE() occurs earlier in the year than birth. The full expression is somewhat ungainly, so an alias (age) is used to make the output column label more meaningful.

The query works, but the result could be scanned more easily if the rows were presented in some order. This can be done by adding an ORDER BY name clause to sort the output by name: 

mysql> SELECT name, birth, CURDATE(),
    -> (YEAR(CURDATE())-YEAR(birth))
    -> - (RIGHT(CURDATE(),5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet ORDER BY name;
+----------+------------+------------+------+
| name     | birth      | CURDATE()  | age  |
+----------+------------+------------+------+
| Bowser   | 1989-08-31 | 2003-08-19 |   13 |
| Buffy    | 1989-05-13 | 2003-08-19 |   14 |
| Chirpy   | 1998-09-11 | 2003-08-19 |    4 |
| Claws    | 1994-03-17 | 2003-08-19 |    9 |
| Fang     | 1990-08-27 | 2003-08-19 |   12 |
| Fluffy   | 1993-02-04 | 2003-08-19 |   10 |
| Puffball | 1999-03-30 | 2003-08-19 |    4 |
| Slim     | 1996-04-29 | 2003-08-19 |    7 |
| Whistler | 1997-12-09 | 2003-08-19 |    5 |
+----------+------------+------------+------+

To sort the output by age rather than name, just use a different ORDER BY clause: 

mysql> SELECT name, birth, CURDATE(),
    -> (YEAR(CURDATE())-YEAR(birth))
    -> - (RIGHT(CURDATE(),5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet ORDER BY age;
+----------+------------+------------+------+
| name     | birth      | CURDATE()  | age  |
+----------+------------+------------+------+
| Chirpy   | 1998-09-11 | 2003-08-19 |    4 |
| Puffball | 1999-03-30 | 2003-08-19 |    4 |
| Whistler | 1997-12-09 | 2003-08-19 |    5 |
| Slim     | 1996-04-29 | 2003-08-19 |    7 |
| Claws    | 1994-03-17 | 2003-08-19 |    9 |
| Fluffy   | 1993-02-04 | 2003-08-19 |   10 |
| Fang     | 1990-08-27 | 2003-08-19 |   12 |
| Bowser   | 1989-08-31 | 2003-08-19 |   13 |
| Buffy    | 1989-05-13 | 2003-08-19 |   14 |
+----------+------------+------------+------+

A similar query can be used to determine age at death for animals that have died. You determine which animals these are by checking whether the death value is NULL. Then, for those with non-NULL values, compute the difference between the death and birth values: 

mysql> SELECT name, birth, death,
    -> (YEAR(death)-YEAR(birth)) - (RIGHT(death,5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet WHERE death IS NOT NULL ORDER BY age;
+--------+------------+------------+------+
| name   | birth      | death      | age  |
+--------+------------+------------+------+
| Bowser | 1989-08-31 | 1995-07-29 |    5 |
+--------+------------+------------+------+

The query uses death IS NOT NULL rather than death <> NULL because NULL is a special value that cannot be compared using the usual comparison operators. This is discussed later. See section 3.3.4.6 Working with NULL Values.

What if you want to know which animals have birthdays next month? For this type of calculation, year and day are irrelevant; you simply want to extract the month part of the birth column. MySQL provides several date-part extraction functions, such as YEAR(), MONTH(), and DAYOFMONTH(). MONTH() is the appropriate function here. To see how it works, run a simple query that displays the value of both birth and MONTH(birth): 

mysql> SELECT name, birth, MONTH(birth) FROM pet;
+----------+------------+--------------+
| name     | birth      | MONTH(birth) |
+----------+------------+--------------+
| Fluffy   | 1993-02-04 |            2 |
| Claws    | 1994-03-17 |            3 |
| Buffy    | 1989-05-13 |            5 |
| Fang     | 1990-08-27 |            8 |
| Bowser   | 1989-08-31 |            8 |
| Chirpy   | 1998-09-11 |            9 |
| Whistler | 1997-12-09 |           12 |
| Slim     | 1996-04-29 |            4 |
| Puffball | 1999-03-30 |            3 |
+----------+------------+--------------+

Finding animals with birthdays in the upcoming month is easy, too. Suppose that the current month is April. Then the month value is 4 and you look for animals born in May (month 5) like this: 

mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5;
+-------+------------+
| name  | birth      |
+-------+------------+
| Buffy | 1989-05-13 |
+-------+------------+

There is a small complication if the current month is December. You don't just add one to the month number (12) and look for animals born in month 13, because there is no such month. Instead, you look for animals born in January (month 1).

You can even write the query so that it works no matter what the current month is. That way you don't have to use a particular month number in the query. DATE_ADD() allows you to add a time interval to a given date. If you add a month to the value of CURDATE(), then extract the month part with MONTH(), the result produces the month in which to look for birthdays: 

mysql> SELECT name, birth FROM pet
    -> WHERE MONTH(birth) = MONTH(DATE_ADD(CURDATE(),INTERVAL 1 MONTH));

A different way to accomplish the same task is to add 1 to get the next month after the current one (after using the modulo function (MOD) to wrap around the month value to 0 if it is currently 12): 

mysql> SELECT name, birth FROM pet
    -> WHERE MONTH(birth) = MOD(MONTH(CURDATE()), 12) + 1;

Note that MONTH returns a number between 1 and 12. And MOD(something,12) returns a number between 0 and 11. So the addition has to be after the MOD(), otherwise we would go from November (11) to January (1).

3.3.4.6 Working with NULL Values

The NULL value can be surprising until you get used to it. Conceptually, NULL means missing value or unknown value and it is treated somewhat differently than other values. To test for NULL, you cannot use the arithmetic comparison operators such as =, <, or <>. To demonstrate this for yourself, try the following query: 

mysql> SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;
+----------+-----------+----------+----------+
| 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL |
+----------+-----------+----------+----------+
|     NULL |      NULL |     NULL |     NULL |
+----------+-----------+----------+----------+

Clearly you get no meaningful results from these comparisons. Use the IS NULL and IS NOT NULL operators instead: 

mysql> SELECT 1 IS NULL, 1 IS NOT NULL;
+-----------+---------------+
| 1 IS NULL | 1 IS NOT NULL |
+-----------+---------------+
|         0 |             1 |
+-----------+---------------+

Note that in MySQL, 0 or NULL means false and anything else means true. The default truth value from a boolean operation is 1.

This special treatment of NULL is why, in the previous section, it was necessary to determine which animals are no longer alive using death IS NOT NULL instead of death <> NULL.

Two NULL values are regarded as equal in a GROUP BY.

When doing an ORDER BY, NULL values are presented first if you do ORDER BY ... ASC and last if you do ORDER BY ... DESC.

Note that MySQL 4.0.2 to 4.0.10 incorrectly always sorts NULL values first regardless of the sort direction.

3.3.4.7 Pattern Matching

MySQL provides standard SQL pattern matching as well as a form of pattern matching based on extended regular expressions similar to those used by Unix utilities such as vi, grep, and sed.

SQL pattern matching allows you to use `_' to match any single character and `%' to match an arbitrary number of characters (including zero characters). In MySQL, SQL patterns are case-insensitive by default. Some examples are shown here. Note that you do not use = or <> when you use SQL patterns; use the LIKE or NOT LIKE comparison operators instead.

To find names beginning with `b': 

mysql> SELECT * FROM pet WHERE name LIKE 'b%';
+--------+--------+---------+------+------------+------------+
| name   | owner  | species | sex  | birth      | death      |
+--------+--------+---------+------+------------+------------+
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL       |
| Bowser | Diane  | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+--------+---------+------+------------+------------+

To find names ending with `fy': 

mysql> SELECT * FROM pet WHERE name LIKE '%fy';
+--------+--------+---------+------+------------+-------+
| name   | owner  | species | sex  | birth      | death |
+--------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat     | f    | 1993-02-04 | NULL  |
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL  |
+--------+--------+---------+------+------------+-------+

To find names containing a `w': 

mysql> SELECT * FROM pet WHERE name LIKE '%w%';
+----------+-------+---------+------+------------+------------+
| name     | owner | species | sex  | birth      | death      |
+----------+-------+---------+------+------------+------------+
| Claws    | Gwen  | cat     | m    | 1994-03-17 | NULL       |
| Bowser   | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL       |
+----------+-------+---------+------+------------+------------+

To find names containing exactly five characters, use five instances of the `_' pattern character: 

mysql> SELECT * FROM pet WHERE name LIKE '_____';
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

The other type of pattern matching provided by MySQL uses extended regular expressions. When you test for a match for this type of pattern, use the REGEXP and NOT REGEXP operators (or RLIKE and NOT RLIKE, which are synonyms).

Some characteristics of extended regular expressions are:

To demonstrate how extended regular expressions work, the LIKE queries shown previously are rewritten here to use REGEXP.

To find names beginning with `b', use `^' to match the beginning of the name: 

mysql> SELECT * FROM pet WHERE name REGEXP '^b';
+--------+--------+---------+------+------------+------------+
| name   | owner  | species | sex  | birth      | death      |
+--------+--------+---------+------+------------+------------+
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL       |
| Bowser | Diane  | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+--------+---------+------+------------+------------+

Prior to MySQL Version 3.23.4, REGEXP is case sensitive, and the previous query will return no rows. In this case, to match either lowercase or uppercase `b', use this query instead: 

mysql> SELECT * FROM pet WHERE name REGEXP '^[bB]';

From MySQL 3.23.4 on, if you really want to force a REGEXP comparison to be case sensitive, use the BINARY keyword to make one of the strings a binary string. This query will match only lowercase `b' at the beginning of a name: 

mysql> SELECT * FROM pet WHERE name REGEXP BINARY '^b';

To find names ending with `fy', use `$' to match the end of the name: 

mysql> SELECT * FROM pet WHERE name REGEXP 'fy$';
+--------+--------+---------+------+------------+-------+
| name   | owner  | species | sex  | birth      | death |
+--------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat     | f    | 1993-02-04 | NULL  |
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL  |
+--------+--------+---------+------+------------+-------+

To find names containing a `w', use this query: 

mysql> SELECT * FROM pet WHERE name REGEXP 'w';
+----------+-------+---------+------+------------+------------+
| name     | owner | species | sex  | birth      | death      |
+----------+-------+---------+------+------------+------------+
| Claws    | Gwen  | cat     | m    | 1994-03-17 | NULL       |
| Bowser   | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL       |
+----------+-------+---------+------+------------+------------+

Because a regular expression pattern matches if it occurs anywhere in the value, it is not necessary in the previous query to put a wildcard on either side of the pattern to get it to match the entire value like it would be if you used an SQL pattern.

To find names containing exactly five characters, use `^' and `$' to match the beginning and end of the name, and five instances of `.' in between: 

mysql> SELECT * FROM pet WHERE name REGEXP '^.....$';
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

You could also write the previous query using the `{n}' ``repeat-n-times'' operator: 

mysql> SELECT * FROM pet WHERE name REGEXP '^.{5}$';
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

3.3.4.8 Counting Rows

Databases are often used to answer the question, ``How often does a certain type of data occur in a table?'' For example, you might want to know how many pets you have, or how many pets each owner has, or you might want to perform various kinds of census operations on your animals.

Counting the total number of animals you have is the same question as ``How many rows are in the pet table?'' because there is one record per pet. COUNT(*) counts the number of rows, so the query to count your animals looks like this: 

mysql> SELECT COUNT(*) FROM pet;
+----------+
| COUNT(*) |
+----------+
|        9 |
+----------+

Earlier, you retrieved the names of the people who owned pets. You can use COUNT() if you want to find out how many pets each owner has: 

mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner;
+--------+----------+
| owner  | COUNT(*) |
+--------+----------+
| Benny  |        2 |
| Diane  |        2 |
| Gwen   |        3 |
| Harold |        2 |
+--------+----------+

Note the use of GROUP BY to group together all records for each owner. Without it, all you get is an error message: 

mysql> SELECT owner, COUNT(*) FROM pet;
ERROR 1140: Mixing of GROUP columns (MIN(),MAX(),COUNT()...)
with no GROUP columns is illegal if there is no GROUP BY clause

COUNT() and GROUP BY are useful for characterizing your data in various ways. The following examples show different ways to perform animal census operations.

Number of animals per species: 

mysql> SELECT species, COUNT(*) FROM pet GROUP BY species;
+---------+----------+
| species | COUNT(*) |
+---------+----------+
| bird    |        2 |
| cat     |        2 |
| dog     |        3 |
| hamster |        1 |
| snake   |        1 |
+---------+----------+

Number of animals per sex: 

mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex;
+------+----------+
| sex  | COUNT(*) |
+------+----------+
| NULL |        1 |
| f    |        4 |
| m    |        4 |
+------+----------+

(In this output, NULL indicates that the sex is unknown.)

Number of animals per combination of species and sex: 

mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| bird    | NULL |        1 |
| bird    | f    |        1 |
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
| hamster | f    |        1 |
| snake   | m    |        1 |
+---------+------+----------+

You need not retrieve an entire table when you use COUNT(). For example, the previous query, when performed just on dogs and cats, looks like this: 

mysql> SELECT species, sex, COUNT(*) FROM pet
    -> WHERE species = 'dog' OR species = 'cat'
    -> GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
+---------+------+----------+

Or, if you wanted the number of animals per sex only for known-sex animals: 

mysql> SELECT species, sex, COUNT(*) FROM pet
    -> WHERE sex IS NOT NULL
    -> GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| bird    | f    |        1 |
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
| hamster | f    |        1 |
| snake   | m    |        1 |
+---------+------+----------+

3.3.4.9 Using More Than one Table

The pet table keeps track of which pets you have. If you want to record other information about them, such as events in their lives like visits to the vet or when litters are born, you need another table. What should this table look like? It needs:

Given these considerations, the CREATE TABLE statement for the event table might look like this: 

mysql> CREATE TABLE event (name VARCHAR(20), date DATE,
    -> type VARCHAR(15), remark VARCHAR(255));

As with the pet table, it's easiest to load the initial records by creating a tab-delimited text file containing the information:

name date type remark
Fluffy 1995-05-15 litter 4 kittens, 3 female, 1 male
Buffy 1993-06-23 litter 5 puppies, 2 female, 3 male
Buffy 1994-06-19 litter 3 puppies, 3 female
Chirpy 1999-03-21 vet needed beak straightened
Slim 1997-08-03 vet broken rib
Bowser 1991-10-12 kennel
Fang 1991-10-12 kennel
Fang 1998-08-28 birthday Gave him a new chew toy
Claws 1998-03-17 birthday Gave him a new flea collar
Whistler 1998-12-09 birthday First birthday

Load the records like this: 

mysql> LOAD DATA LOCAL INFILE 'event.txt' INTO TABLE event;

Based on what you've learned from the queries you've run on the pet table, you should be able to perform retrievals on the records in the event table; the principles are the same. But when is the event table by itself insufficient to answer questions you might ask?

Suppose that you want to find out the ages at which each pet had its litters. We saw earlier how to calculate ages from two dates. The litter date of the mother is in the event table, but to calculate her age on that date you need her birth date, which is stored in the pet table. This means the query requires both tables: 

mysql> SELECT pet.name,
    -> (YEAR(date)-YEAR(birth)) - (RIGHT(date,5)<RIGHT(birth,5)) AS age,
    -> remark
    -> FROM pet, event
    -> WHERE pet.name = event.name AND type = 'litter';
+--------+------+-----------------------------+
| name   | age  | remark                      |
+--------+------+-----------------------------+
| Fluffy |    2 | 4 kittens, 3 female, 1 male |
| Buffy  |    4 | 5 puppies, 2 female, 3 male |
| Buffy  |    5 | 3 puppies, 3 female         |
+--------+------+-----------------------------+

There are several things to note about this query:

You need not have two different tables to perform a join. Sometimes it is useful to join a table to itself, if you want to compare records in a table to other records in that same table. For example, to find breeding pairs among your pets, you can join the pet table with itself to produce candidate pairs of males and females of like species: 

mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species
    -> FROM pet AS p1, pet AS p2
    -> WHERE p1.species = p2.species AND p1.sex = 'f' AND p2.sex = 'm';
+--------+------+--------+------+---------+
| name   | sex  | name   | sex  | species |
+--------+------+--------+------+---------+
| Fluffy | f    | Claws  | m    | cat     |
| Buffy  | f    | Fang   | m    | dog     |
| Buffy  | f    | Bowser | m    | dog     |
+--------+------+--------+------+---------+

In this query, we specify aliases for the table name in order to refer to the columns and keep straight which instance of the table each column reference is associated with.

3.4 Getting Information About Databases and Tables

What if you forget the name of a database or table, or what the structure of a given table is (for example, what its columns are called)? MySQL addresses this problem through several statements that provide information about the databases and tables it supports.

You have already seen SHOW DATABASES, which lists the databases managed by the server. To find out which database is currently selected, use the DATABASE() function: 

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| menagerie  |
+------------+

If you haven't selected any database yet, the result is NULL (or the empty string before MySQL 4.1.1).

To find out what tables the current database contains (for example, when you're not sure about the name of a table), use this command: 

mysql> SHOW TABLES;
+---------------------+
| Tables in menagerie |
+---------------------+
| event               |
| pet                 |
+---------------------+

If you want to find out about the structure of a table, the DESCRIBE command is useful; it displays information about each of a table's columns: 

mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(20) | YES  |     | NULL    |       |
| owner   | varchar(20) | YES  |     | NULL    |       |
| species | varchar(20) | YES  |     | NULL    |       |
| sex     | char(1)     | YES  |     | NULL    |       |
| birth   | date        | YES  |     | NULL    |       |
| death   | date        | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

Field indicates the column name, Type is the data type for the column, NULL indicates whether the column can contain NULL values, Key indicates whether the column is indexed, and Default specifies the column's default value.

If you have indexes on a table, SHOW INDEX FROM tbl_name produces information about them.

3.5 Using @command{mysql} in Batch Mode

In the previous sections, you used @command{mysql} interactively to enter queries and view the results. You can also run @command{mysql} in batch mode. To do this, put the commands you want to run in a file, then tell @command{mysql} to read its input from the file: 

shell> mysql < batch-file

If you are running @command{mysql} under Windows and have some special characters in the file that cause problems, you can do this: 

C:\> mysql -e "source batch-file"

If you need to specify connection parameters on the command line, the command might look like this: 

shell> mysql -h host -u user -p < batch-file
Enter password: ********

When you use @command{mysql} this way, you are creating a script file, then executing the script.

If you want the script to continue even if some of the statements in it produce errors, you should use the --force command-line option.

Why use a script? Here are a few reasons:

The default output format is different (more concise) when you run @command{mysql} in batch mode than when you use it interactively. For example, the output of SELECT DISTINCT species FROM pet looks like this when @command{mysql} is run interactively: 

+---------+
| species |
+---------+
| bird    |
| cat     |
| dog     |
| hamster |
| snake   |
+---------+

In batch mode, the output looks like this instead: 

species
bird
cat
dog
hamster
snake

If you want to get the interactive output format in batch mode, use mysql -t. To echo to the output the commands that are executed, use mysql -vvv.

You can also use scripts from the @command{mysql} prompt by using the source or \. command: 

mysql> source filename;
mysql> \. filename

3.6 Examples of Common Queries

Here are examples of how to solve some common problems with MySQL.

Some of the examples use the table shop to hold the price of each article (item number) for certain traders (dealers). Supposing that each trader has a single fixed price per article, then (article, dealer) is a primary key for the records.

Start the command-line tool @command{mysql} and select a database: 

shell> mysql your-database-name

(In most MySQL installations, you can use the database name test).

You can create and populate the example table with these statements: 

mysql> CREATE TABLE shop (
    -> article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
    -> dealer  CHAR(20)                 DEFAULT ''     NOT NULL,
    -> price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL,
    -> PRIMARY KEY(article, dealer));
mysql> INSERT INTO shop VALUES
    -> (1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),
    -> (3,'C',1.69),(3,'D',1.25),(4,'D',19.95);

After issuing the statements, the table should have the following contents: 

mysql> SELECT * FROM shop;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0001 | A      |  3.45 |
|    0001 | B      |  3.99 |
|    0002 | A      | 10.99 |
|    0003 | B      |  1.45 |
|    0003 | C      |  1.69 |
|    0003 | D      |  1.25 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

3.6.1 The Maximum Value for a Column

``What's the highest item number?'' 

SELECT MAX(article) AS article FROM shop;

+---------+
| article |
+---------+
|       4 |
+---------+

3.6.2 The Row Holding the Maximum of a Certain Column

``Find number, dealer, and price of the most expensive article.''

In standard SQL (and as of MySQL 4.1), this is easily done with a subquery: 

SELECT article, dealer, price
FROM   shop
WHERE  price=(SELECT MAX(price) FROM shop);

In MySQL versions prior to 4.1, just do it in two steps:

  1. Get the maximum price value from the table with a SELECT statement. 
    mysql> SELECT MAX(price) FROM shop;
+------------+
| MAX(price) |
+------------+
|      19.95 |
+------------+
  2. Using the value 19.95 shown by the previous query to be the maximum article price, write a query to locate and display the corresponding record: 
    mysql> SELECT article, dealer, price
    -> FROM   shop
    -> WHERE  price=19.95;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0004 | D      | 19.95 |
+---------+--------+-------+

Another solution is to sort all rows descending by price and only get the first row using the MySQL-specific LIMIT clause: 

SELECT article, dealer, price
FROM   shop
ORDER BY price DESC
LIMIT 1;

Note: If there were several most expensive articles, each with a price of 19.95, the LIMIT solution would show only one of them!

3.6.3 Maximum of Column per Group

``What's the highest price per article?'' 

SELECT article, MAX(price) AS price
FROM   shop
GROUP BY article

+---------+-------+
| article | price |
+---------+-------+
|    0001 |  3.99 |
|    0002 | 10.99 |
|    0003 |  1.69 |
|    0004 | 19.95 |
+---------+-------+

3.6.4 The Rows Holding the Group-wise Maximum of a Certain Field

``For each article, find the dealer or dealers with the most expensive price.''

In standard SQL (and as of MySQL 4.1), the problem can be solved with a subquery like this: 

SELECT article, dealer, price
FROM   shop s1
WHERE  price=(SELECT MAX(s2.price)
              FROM shop s2
              WHERE s1.article = s2.article);

In MySQL versions prior to 4.1, it's best do it in several steps:

  1. Get the list of (article,maxprice) pairs.
  2. For each article, get the corresponding rows that have the stored maximum price.

This can easily be done with a temporary table and a join: 

CREATE TEMPORARY TABLE tmp (
        article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
        price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL);

LOCK TABLES shop READ;

INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article;

SELECT shop.article, dealer, shop.price FROM shop, tmp
WHERE shop.article=tmp.article AND shop.price=tmp.price;

UNLOCK TABLES;

DROP TABLE tmp;

If you don't use a TEMPORARY table, you must also lock the tmp table.

``Can it be done with a single query?''

Yes, but only by using a quite inefficient trick called the ``MAX-CONCAT trick'': 

SELECT article,
       SUBSTRING( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 7) AS dealer,
  0.00+LEFT(      MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 6) AS price
FROM   shop
GROUP BY article;

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0001 | B      |  3.99 |
|    0002 | A      | 10.99 |
|    0003 | C      |  1.69 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

The last example can be made a bit more efficient by doing the splitting of the concatenated column in the client.

3.6.5 Using User Variables

You can use MySQL user variables to remember results without having to store them in temporary variables in the client. See section 10.3 User Variables.

For example, to find the articles with the highest and lowest price you can do this: 

mysql> SELECT @min_price:=MIN(price),@max_price:=MAX(price) FROM shop;
mysql> SELECT * FROM shop WHERE price=@min_price OR price=@max_price;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0003 | D      |  1.25 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

3.6.6 Using Foreign Keys

In MySQL 3.23.44 and up, InnoDB tables support checking of foreign key constraints. See section 16 The InnoDB Storage Engine. See also section 1.8.5.5 Foreign Keys.

You don't actually need foreign keys to join two tables. For table types other than InnoDB, the only things MySQL currently doesn't do are 1) CHECK to make sure that the keys you use really exist in the table or tables you're referencing and 2) automatically delete rows from a table with a foreign key definition. Using your keys to join tables will work just fine: 

CREATE TABLE person (
    id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
    name CHAR(60) NOT NULL,
    PRIMARY KEY (id)
);

CREATE TABLE shirt (
    id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
    style ENUM('t-shirt', 'polo', 'dress') NOT NULL,
    color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,
    owner SMALLINT UNSIGNED NOT NULL REFERENCES person(id),
    PRIMARY KEY (id)
);

INSERT INTO person VALUES (NULL, 'Antonio Paz');

INSERT INTO shirt VALUES
(NULL, 'polo', 'blue', LAST_INSERT_ID()),
(NULL, 'dress', 'white', LAST_INSERT_ID()),
(NULL, 't-shirt', 'blue', LAST_INSERT_ID());

INSERT INTO person VALUES (NULL, 'Lilliana Angelovska');

INSERT INTO shirt VALUES
(NULL, 'dress', 'orange', LAST_INSERT_ID()),
(NULL, 'polo', 'red', LAST_INSERT_ID()),
(NULL, 'dress', 'blue', LAST_INSERT_ID()),
(NULL, 't-shirt', 'white', LAST_INSERT_ID());

SELECT * FROM person;
+----+---------------------+
| id | name                |
+----+---------------------+
|  1 | Antonio Paz         |
|  2 | Lilliana Angelovska |
+----+---------------------+

SELECT * FROM shirt;
+----+---------+--------+-------+
| id | style   | color  | owner |
+----+---------+--------+-------+
|  1 | polo    | blue   |     1 |
|  2 | dress   | white  |     1 |
|  3 | t-shirt | blue   |     1 |
|  4 | dress   | orange |     2 |
|  5 | polo    | red    |     2 |
|  6 | dress   | blue   |     2 |
|  7 | t-shirt | white  |     2 |
+----+---------+--------+-------+

SELECT s.* FROM person p, shirt s
 WHERE p.name LIKE 'Lilliana%'
   AND s.owner = p.id
   AND s.color <> 'white';

+----+-------+--------+-------+
| id | style | color  | owner |
+----+-------+--------+-------+
|  4 | dress | orange |     2 |
|  5 | polo  | red    |     2 |
|  6 | dress | blue   |     2 |
+----+-------+--------+-------+

3.6.7 Searching on Two Keys

An OR using a single key is well optimized, as is the handling of AND.

The one tricky case is that of searching on two different keys combined with OR: 

SELECT field1_index, field2_index FROM test_table
WHERE field1_index = '1' OR  field2_index = '1'

This case is optimized from MySQL 5.0.0. See section 7.2.6 Index Merge Optimization.

In MySQL 4.0 and up, you can also solve the problem efficiently by using a UNION that combines the output of two separate SELECT statements. See section 14.1.7.2 UNION Syntax.

Each SELECT searches only one key and can be optimized: 

SELECT field1_index, field2_index
    FROM test_table WHERE field1_index = '1'
UNION
SELECT field1_index, field2_index
    FROM test_table WHERE field2_index = '1';

Prior to MySQL 4.0, you can achieve the same effect by using a TEMPORARY table and separate SELECT statements. This type of optimization is also very good if you are using very complicated queries where the SQL server does the optimizations in the wrong order. 

CREATE TEMPORARY TABLE tmp
SELECT field1_index, field2_index
    FROM test_table WHERE field1_index = '1';
INSERT INTO tmp
SELECT field1_index, field2_index
    FROM test_table WHERE field2_index = '1';
SELECT * from tmp;
DROP TABLE tmp;

This method of solving the problem is in effect a UNION of two queries.

3.6.8 Calculating Visits Per Day

The following example shows how you can use the bit group functions to calculate the number of days per month a user has visited a Web page. 

CREATE TABLE t1 (year YEAR(4), month INT(2) UNSIGNED ZEROFILL,
             day INT(2) UNSIGNED ZEROFILL);
INSERT INTO t1 VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2),
            (2000,2,23),(2000,2,23);

The example table contains year-month-day values representing visits by users to the page. To determine how many different days in each month these visits occur, use this query: 

SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1
       GROUP BY year,month;

Which returns: 

+------+-------+------+
| year | month | days |
+------+-------+------+
| 2000 |    01 |    3 |
| 2000 |    02 |    2 |
+------+-------+------+

The query calculates how many different days appear in the table for each year/month combination, with automatic removal of duplicate entries.

3.6.9 Using AUTO_INCREMENT

The AUTO_INCREMENT attribute can be used to generate a unique identity for new rows: 

CREATE TABLE animals (
             id MEDIUMINT NOT NULL AUTO_INCREMENT,
             name CHAR(30) NOT NULL,
             PRIMARY KEY (id)
             );
INSERT INTO animals (name) VALUES ('dog'),('cat'),('penguin'),
                                  ('lax'),('whale'),('ostrich');
SELECT * FROM animals;

Which returns: 

+----+---------+
| id | name    |
+----+---------+
|  1 | dog     |
|  2 | cat     |
|  3 | penguin |
|  4 | lax     |
|  5 | whale   |
|  6 | ostrich |
+----+---------+

You can retrieve the most recent AUTO_INCREMENT value with the LAST_INSERT_ID() SQL function or the mysql_insert_id() C API function. These functions are connection-specific, so their return value is not affected by another connection also doing inserts.

Note: For a multiple-row insert, LAST_INSERT_ID()/mysql_insert_id() will actually return the AUTO_INCREMENT key from the first of the inserted rows. This allows multiple-row inserts to be reproduced correctly on other servers in a replication setup.

For MyISAM and BDB tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column)+1 WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups. 

CREATE TABLE animals (
             grp ENUM('fish','mammal','bird') NOT NULL,
             id MEDIUMINT NOT NULL AUTO_INCREMENT,
             name CHAR(30) NOT NULL,
             PRIMARY KEY (grp,id)
             );
INSERT INTO animals (grp,name) VALUES('mammal','dog'),('mammal','cat'),
                  ('bird','penguin'),('fish','lax'),('mammal','whale'),
                  ('bird','ostrich');
SELECT * FROM animals ORDER BY grp,id;

Which returns: 

+--------+----+---------+
| grp    | id | name    |
+--------+----+---------+
| fish   |  1 | lax     |
| mammal |  1 | dog     |
| mammal |  2 | cat     |
| mammal |  3 | whale   |
| bird   |  1 | penguin |
| bird   |  2 | ostrich |
+--------+----+---------+

Note that in this case (when the AUTO_INCREMENT column is part of a multiple-column index), AUTO_INCREMENT values will be reused if you delete the row with the biggest AUTO_INCREMENT value in any group. This happens even for MyISAM tables, for which AUTO_INCREMENT values normally are not reused.)

3.7 Queries from the Twin Project

At Analytikerna and Lentus, we have been doing the systems and field work for a big research project. This project is a collaboration between the Institute of Environmental Medicine at Karolinska Institutet Stockholm and the Section on Clinical Research in Aging and Psychology at the University of Southern California.

The project involves a screening part where all twins in Sweden older than 65 years are interviewed by telephone. Twins who meet certain criteria are passed on to the next stage. In this latter stage, twins who want to participate are visited by a doctor/nurse team. Some of the examinations include physical and neuropsychological examination, laboratory testing, neuroimaging, psychological status assessment, and family history collection. In addition, data are collected on medical and environmental risk factors.

More information about Twin studies can be found at: http://www.mep.ki.se/twinreg/index_en.html

The latter part of the project is administered with a Web interface written using Perl and MySQL.

Each night all data from the interviews is moved into a MySQL database.

3.7.1 Find All Non-distributed Twins

The following query is used to determine who goes into the second part of the project: 

SELECT
    CONCAT(p1.id, p1.tvab) + 0 AS tvid,
    CONCAT(p1.christian_name, ' ', p1.surname) AS Name,
    p1.postal_code AS Code,
    p1.city AS City,
    pg.abrev AS Area,
    IF(td.participation = 'Aborted', 'A', ' ') AS A,
    p1.dead AS dead1,
    l.event AS event1,
    td.suspect AS tsuspect1,
    id.suspect AS isuspect1,
    td.severe AS tsevere1,
    id.severe AS isevere1,
    p2.dead AS dead2,
    l2.event AS event2,
    h2.nurse AS nurse2,
    h2.doctor AS doctor2,
    td2.suspect AS tsuspect2,
    id2.suspect AS isuspect2,
    td2.severe AS tsevere2,
    id2.severe AS isevere2,
    l.finish_date
FROM
    twin_project AS tp
    /* For Twin 1 */
    LEFT JOIN twin_data AS td ON tp.id = td.id
              AND tp.tvab = td.tvab
    LEFT JOIN informant_data AS id ON tp.id = id.id
              AND tp.tvab = id.tvab
    LEFT JOIN harmony AS h ON tp.id = h.id
              AND tp.tvab = h.tvab
    LEFT JOIN lentus AS l ON tp.id = l.id
              AND tp.tvab = l.tvab
    /* For Twin 2 */
    LEFT JOIN twin_data AS td2 ON p2.id = td2.id
              AND p2.tvab = td2.tvab
    LEFT JOIN informant_data AS id2 ON p2.id = id2.id
              AND p2.tvab = id2.tvab
    LEFT JOIN harmony AS h2 ON p2.id = h2.id
              AND p2.tvab = h2.tvab
    LEFT JOIN lentus AS l2 ON p2.id = l2.id
              AND p2.tvab = l2.tvab,
    person_data AS p1,
    person_data AS p2,
    postal_groups AS pg
WHERE
    /* p1 gets main twin and p2 gets his/her twin. */
    /* ptvab is a field inverted from tvab */
    p1.id = tp.id AND p1.tvab = tp.tvab AND
    p2.id = p1.id AND p2.ptvab = p1.tvab AND
    /* Just the sceening survey */
    tp.survey_no = 5 AND
    /* Skip if partner died before 65 but allow emigration (dead=9) */
    (p2.dead = 0 OR p2.dead = 9 OR
     (p2.dead = 1 AND
      (p2.death_date = 0 OR
       (((TO_DAYS(p2.death_date) - TO_DAYS(p2.birthday)) / 365)
        >= 65))))
    AND
    (
    /* Twin is suspect */
    (td.future_contact = 'Yes' AND td.suspect = 2) OR
    /* Twin is suspect - Informant is Blessed */
    (td.future_contact = 'Yes' AND td.suspect = 1
                               AND id.suspect = 1) OR
    /* No twin - Informant is Blessed */
    (ISNULL(td.suspect) AND id.suspect = 1
                        AND id.future_contact = 'Yes') OR
    /* Twin broken off - Informant is Blessed */
    (td.participation = 'Aborted'
     AND id.suspect = 1 AND id.future_contact = 'Yes') OR
    /* Twin broken off - No inform - Have partner */
    (td.participation = 'Aborted' AND ISNULL(id.suspect)
                                  AND p2.dead = 0))
    AND
    l.event = 'Finished'
    /* Get at area code */
    AND SUBSTRING(p1.postal_code, 1, 2) = pg.code
    /* Not already distributed */
    AND (h.nurse IS NULL OR h.nurse=00 OR h.doctor=00)
    /* Has not refused or been aborted */
    AND NOT (h.status = 'Refused' OR h.status = 'Aborted'
    OR h.status = 'Died' OR h.status = 'Other')
ORDER BY
    tvid;

Some explanations:

CONCAT(p1.id, p1.tvab) + 0 AS tvid
We want to sort on the concatenated id and tvab in numerical order. Adding 0 to the result causes MySQL to treat the result as a number.
column id
This identifies a pair of twins. It is a key in all tables.
column tvab
This identifies a twin in a pair. It has a value of 1 or 2.
column ptvab
This is an inverse of tvab. When tvab is 1 this is 2, and vice versa. It exists to save typing and to make it easier for MySQL to optimize the query.

This query demonstrates, among other things, how to do lookups on a table from the same table with a join (p1 and p2). In the example, this is used to check whether a twin's partner died before the age of 65. If so, the row is not returned.

All of the above exist in all tables with twin-related information. We have a key on both id,tvab (all tables), and id,ptvab (person_data) to make queries faster.

On our production machine (A 200MHz UltraSPARC), this query returns about 150-200 rows and takes less than one second.

The current number of records in the tables used in the query:
Table Rows
person_data 71074
lentus 5291
twin_project 5286
twin_data 2012
informant_data 663
harmony 381
postal_groups 100

3.7.2 Show a Table of Twin Pair Status

Each interview ends with a status code called event. The query shown here is used to display a table over all twin pairs combined by event. This indicates in how many pairs both twins are finished, in how many pairs one twin is finished and the other refused, and so on. 

SELECT
        t1.event,
        t2.event,
        COUNT(*)
FROM
        lentus AS t1,
        lentus AS t2,
        twin_project AS tp
WHERE
        /* We are looking at one pair at a time */
        t1.id = tp.id
        AND t1.tvab=tp.tvab
        AND t1.id = t2.id
        /* Just the sceening survey */
        AND tp.survey_no = 5
        /* This makes each pair only appear once */
        AND t1.tvab='1' AND t2.tvab='2'
GROUP BY
        t1.event, t2.event;

3.8 Using MySQL with Apache

There are programs that let you authenticate your users from a MySQL database and also let you write your log files into a MySQL table.

You can change the Apache logging format to be easily readable by MySQL by putting the following into the Apache configuration file: 

LogFormat \
        "\"%h\",%{%Y%m%d%H%M%S}t,%>s,\"%b\",\"%{Content-Type}o\",  \
        \"%U\",\"%{Referer}i\",\"%{User-Agent}i\""

To load a log file in that format into MySQL, you can use a statement something like this: 

LOAD DATA INFILE '/local/access_log' INTO TABLE tbl_name
FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '\\'

The named table should be created to have columns that correspond to those that the LogFormat line writes to the log file.

4 Using MySQL Programs

This chapter provides a brief overview of the programs provided by MySQL AB and discusses how to specify options when you run these programs. Most programs have options that are specific to their own operation, but the syntax for specifying options is similar for all of them. Later chapters provide more detailed descriptions of individual programs, including which options they recognize.

4.1 Overview of MySQL Programs

MySQL AB provides several types of programs:

The MYSQL server and server startup scripts:
  • @command{mysqld} is the MySQL server
  • @command{mysqld_safe}, @command{mysql.server}, and @command{mysqld_multi} are server startup scripts
  • @command{mysql_install_db} initializes the data directory and the initial databases
These programs are discussed further in section 5 Database Administration.
Client programs that access the server:
  • @command{mysql} is a command-line client for executing SQL statements interactively or in batch mode
  • @command{mysqlcc} (MySQL Control Center) is an interactive graphical tool for executing SQL statements and administration
  • @command{mysqladmin} is an administrative client
  • @command{mysqlcheck} performs table maintenance operations
  • @command{mysqldump} and @command{mysqlhotcopy} make database backups
  • @command{mysqlimport} imports data files
  • @command{mysqlshow} displays information about databases and tables
These programs are discussed further in section 8 MySQL Client and Utility Programs.
Utility programs that operate independently of the server:
  • @command{myisamchk} performs table maintenance operations
  • @command{myisampack} produces compressed, read-only tables
  • @command{mysqlbinlog} is a tool for processing binary log files
  • @command{perror} displays error code meanings
@command{myisamchk} is discussed further in section 5 Database Administration. The other programs are further in section 8 MySQL Client and Utility Programs.

Most MySQL distributions include all of these programs, except for those programs that are platform-specific. (For example, the server startup scripts are not used on Windows.) The exception is that RPM distributions are more specialized. There is one RPM for the server, another for the client programs, and so forth. If you appear to be missing one or more programs, see section 2 Installing MySQL for information on types of distributions and what they contain. It may be that you need to install something else.

4.2 Invoking MySQL Programs

To invoke a MySQL program at the command line (that is, from your shell or command prompt), enter the program name followed by any options or other arguments needed to instruct the program what you want it to do. The following commands show some sample program invocations. ``shell>'' represents the prompt for your command interpreter; it is not part of what you type. The particular prompt you will see depends on your command interpreter. Typical prompts are $ for @command{sh} or @command{bash}, % for @command{csh} or @command{tcsh}, and C:\> for Windows @command{command.com} or @command{cmd.exe}. 

shell> mysql test
shell> mysqladmin extended-status variables
shell> mysqlshow --help
shell> mysqldump --user=root personnel

Arguments that begin with a dash are option arguments. They typically specify the type of connection a program should make to the server or affect its operational mode. Options have a syntax that is described in section 4.3 Specifying Program Options.

Non-option arguments (arguments with no leading dash) provide additional information to the program. For example, the @command{mysql} program interprets the first non-option argument as a database name, so the command mysql test indicates that you want to use the test database.

Later sections that describe individual programs indicate which options a program understands and describe the meaning of any additional non-option arguments.

Some options are common to a number of programs. The most common of these are the --host, --user, and --password options that specify connection parameters. They indicate the host where the MySQL server is running, and the username and password of your MySQL account. All MySQL client programs understand these options; they allow you to specify which server to connect to and the account to use on that server.

You may find it necessary to invoke MySQL programs using the pathname to the `bin' directory in which they are installed. This is likely to be the case if you get a ``program not found'' error whenever you attempt to run a MySQL program from any directory other than the `bin' directory. To make it more convenient to use MySQL, you can add the pathname of the `bin' directory to your PATH environment variable setting. Then to run a program you need only type its name, not its entire pathname.

Consult the documentation for your command interpreter for instructions on setting your PATH. The syntax for setting environment variables is interpreter-specific.

4.3 Specifying Program Options

You can provide options for MySQL programs in several ways:

MySQL programs determine which options are given first by examining environment variables, then option files, and then the command line. If an option is specified multiple times, the last occurrence takes precedence. This means that environment variables have the lowest precedence and command-line options the highest.

You can take advantage of the way that MySQL programs process options by specifying the default values for a program's options in an option file. Then you need not type them each time you run the program, but can override the defaults if necessary by using command-line options.

4.3.1 Using Options on the Command Line

Program options specified on the command line follow these rules:

MySQL 4.0 introduced some additional flexibility in the way you specify options. These changes were made in MySQL 4.0.2. Some of them relate to the way you specify options that have ``enabled'' and ``disabled'' states, and to the use of options that might be present in one version of MySQL but not another. Those capabilities are discussed in this section. Another change pertains to the way you use options to set program variables. section 4.3.4 Using Options to Set Program Variables discusses that topic further.

Some options control behavior that can be turned on or off. For example, the @command{mysql} client supports a --column-names option that determines whether or not to display a row of column names at the beginning of query results. By default, this option is enabled. However, you may want to disable it in some instances, such as when sending the output of @command{mysql} into another program that expects to see only data and not an initial header line.

To disable column names, you can specify the option using any of these forms: 

--disable-column-names
--skip-column-names
--column-names=0

The --disable and --skip prefixes and the =0 suffix all have the same effect: They turn the option off.

The ``enabled'' form of the option may be specified in any of these ways: 

--column-names
--enable-column-names
--column-names=1

Another change to option processing introduced in MySQL 4.0 is that you can use the --loose prefix for command-line options. If an option is prefixed by --loose, the program will not exit with an error if it does not recognize the option, but instead will issue only a warning: 

shell> mysql --loose-no-such-option
mysql: WARNING: unknown option '--no-such-option'

The --loose prefix can be useful when you run programs from multiple installations of MySQL on the same machine, at least if all the versions are as recent as 4.0.2. This prefix is particularly useful when you list options in an option file. An option that may not be recognized by all versions of a program can be given using the --loose prefix (or loose in an option file). Versions of the program that do not recognize the option will issue a warning and ignore it. This strategy requires that versions involved be 4.0.2 or later, because earlier versions know nothing of the --loose convention.

4.3.2 Using Option Files

MySQL programs can read startup options from option files (also sometimes called configuration files). Option files provide a convenient way to specify commonly used options so that they need not be entered on the command line each time you run a program. Option file capability is available from MySQL 3.22 on.

The following programs support option files: @command{myisamchk}, @command{myisampack}, @command{mysql}, @command{mysql.server}, @command{mysqladmin}, @command{mysqlbinlog}, @command{mysqlcc}, @command{mysqlcheck}, @command{mysqld_safe}, @command{mysqldump}, @command{mysqld}, @command{mysqlhotcopy}, @command{mysqlimport}, and @command{mysqlshow}.

On Windows, MySQL programs read startup options from the following files:

Filename Purpose
WINDIR\my.ini Global options
C:\my.cnf Global options

WINDIR represents the location of your Windows directory. This is commonly `C:\Windows' or `C:\WinNT'. You can determine its exact location from the value of the WINDIR environment variable using the following command: 

C:\> echo %WINDIR%

On Unix, MySQL programs read startup options from the following files:

Filename Purpose
/etc/my.cnf Global options
DATADIR/my.cnf Server-specific options
defaults-extra-file The file specified with --defaults-extra-file=path, if any
~/.my.cnf User-specific options

DATADIR represents the location of the MySQL data directory. Typically this is `/usr/local/mysql/data' for a binary installation or `/usr/local/var' for a source installation. Note that this is the data directory location that was specified at configuration time, not the one specified with --datadir when @command{mysqld} starts. Use of --datadir at runtime has no effect on where the server looks for option files, because it looks for them before processing any command-line arguments.

MySQL looks for option files in the order just described and reads any that exist. If an option file that you want to use does not exist, create it with a plain text editor. If multiple option files exist, an option specified in a file read later takes precedence over the same option specified in a file read earlier.

Any long option that may be given on the command line when running a MySQL program can be given in an option file as well. To get the list of available options for a program, run it with the --help option.

The syntax for specifying options in an option file is similar to command-line syntax, except that you omit the leading two dashes. For example, --quick or --host=localhost on the command line should be specified as quick or host=localhost in an option file. To specify an option of the form --loose-opt_name in an option file, write it as loose-opt_name.

Empty lines in option files are ignored. Non-empty lines can take any of the following forms:

#comment
;comment
Comment lines start with `#' or `;'. As of MySQL 4.0.14, a `#'-comment can start in the middle of a line as well.
[group]
group is the name of the program or group for which you want to set options. After a group line, any opt_name or set-variable lines apply to the named group until the end of the option file or another group line is given.
opt_name
This is equivalent to --opt_name on the command line.
opt_name=value
This is equivalent to --opt_name=value on the command line. In an option file, you can have spaces around the `=' character, something that is not true on the command line. As of MySQL 4.0.16, you can quote the value with double quotes or single quotes. This is useful if the value contains a `#' comment character or whitespace.
set-variable = var_name=value
Set the program variable var_name to the given value. This is equivalent to --set-variable=var_name=value on the command line. Spaces are allowed around the first `=' character but not around the second. This syntax is deprecated as of MySQL 4.0. See section 4.3.4 Using Options to Set Program Variables for more information on setting program variables.

Leading and trailing blanks are automatically deleted from option names and values. You may use the escape sequences `\b', `\t', `\n', `\r', `\\', and `\s' in option values to represent the backspace, tab, newline, carriage return, and space characters.

On Windows, if an option value represents a pathname, you should specify the value using `/' rather than `\' as the pathname separator. If you use `\', you must double it as `\\', because `\' is the escape character in MySQL.

If an option group name is the same as a program name, options in the group apply specifically to that program.

The [client] option group is read by all client programs (but not by @command{mysqld}). This allows you to specify options that apply to every client. For example, [client] is the perfect group to use to specify the password that you use to connect to the server. (But make sure that the option file is readable and writable only by yourself, so that other people cannot find out your password.) Be sure not to put an option in the [client] group unless it is recognized by all client programs that you use. Programs that do not understand the option will quit after displaying an error message if you try to run them.

As of MySQL 4.0.14, if you want to create option groups that should be read only by one specific @command{mysqld} server release series, you can do this by using groups with names of [mysqld-4.0], [mysqld-4.1], and so forth. The following group indicates that the --new option should be used only by MySQL servers with 4.0.x version numbers: 

[mysqld-4.0]
new

Here is a typical global option file: 

[client]
port=3306
socket=/tmp/mysql.sock

[mysqld]
port=3306
socket=/tmp/mysql.sock
key_buffer_size=16M
max_allowed_packet=8M

[mysqldump]
quick

The preceding option file uses var_name=value syntax for the lines that set the key_buffer_size and max_allowed_packet variables. Prior to MySQL 4.0.2, you would need to use set-variable syntax instead (described earlier in this section).

Here is a typical user option file: 

[client]
# The following password will be sent to all standard MySQL clients
password="my_password"

[mysql]
no-auto-rehash
set-variable = connect_timeout=2

[mysqlhotcopy]
interactive-timeout

This option file uses set-variable syntax to set the connect_timeout variable. For MySQL 4.0.2 and up, you can also set the variable using just connect_timeout=2.

If you have a source distribution, you will find sample option files named `my-xxxx.cnf' in the `support-files' directory. If you have a binary distribution, look in the `support-files' directory under your MySQL installation directory (typically `C:\mysql' on Windows or `/usr/local/mysql' on Unix). Currently there are sample option files for small, medium, large, and very large systems. To experiment with one of these files, copy it to `C:\my.cnf' on Windows or to `.my.cnf' in your home directory on Unix.

Note: On Windows, the `.cnf' option file extension might not be displayed.

All MySQL programs that support option files handle the following command-line options:

--no-defaults
Don't read any option files.
--print-defaults
Print the program name and all options that it will get from option files.
--defaults-file=path_name
Use only the given option file. path_name is the full pathname to the file.
--defaults-extra-file=path_name
Read this option file after the global option file but before the user option file. path_name is the full pathname to the file.

To work properly, each of these options must immediately follow the command name on the command line, with the exception that --print-defaults may be used immediately after --defaults-file or --defaults-extra-file.

In shell scripts, you can use the @command{my_print_defaults} program to parse option files. The following example shows the output that @command{my_print_defaults} might produce when asked to show the options found in the [client] and [mysql] groups: 

shell> my_print_defaults client mysql
--port=3306
--socket=/tmp/mysql.sock
--no-auto-rehash

Note for developers: Option file handling is implemented in the C client library simply by processing all matching options (that is, options in the appropriate group) before any command-line arguments. This works nicely for programs that use the last instance of an option that is specified multiple times. If you have a C or C++ program that handles multiply specified options this way but doesn't read option files, you need add only two lines to give it that capability. Check the source code of any of the standard MySQL clients to see how to do this.

Several other language interfaces to MySQL are based on the C client library, and some of them provide a way to access option file contents. These include Perl and Python. See the documentation for your preferred interface for details.

4.3.3 Using Environment Variables to Specify Options

To specify an option using an environment variable, set the variable using the syntax appropriate for your comment processor. For example, on Windows or NetWare, you can set the USER variable to specify your MySQL account name. To do so, use this syntax: 

SET USER=your_name

The syntax on Unix depends on your shell. Suppose that you want to specify the TCP/IP port number using the MYSQL_TCP_PORT variable. The syntax for Bourne shell and variants (@command{sh}, @command{bash}, @command{zsh}, etc.) is: 

MYSQL_TCP_PORT=3306

For @command{csh} and @command{tcsh}, use this syntax: 

setenv MYSQL_TCP_PORT 3306

The commands to set environment variables can be executed at your command prompt to take effect immediately. These settings persist until you log out. To have the settings take effect each time you log in, place the appropriate command or commands in a startup file that your command interpreter reads each time it starts. Typical startup files are `AUTOEXEC.BAT' for Windows, `.bash_profile' for @command{bash}, or `.tcshrc' for @command{tcsh}. Consult the documentation for your command interpreter for specific details.

section E Environment Variables lists all environment variables that affect MySQL program operation.

4.3.4 Using Options to Set Program Variables

Many MySQL programs have internal variables that can be set at runtime. As of MySQL 4.0.2, program variables are set the same way as any other long option that takes a value. For example, @command{mysql} has a max_allowed_packet variable that controls the maximum size of its communication buffer. To set the max_allowed_packet variable for @command{mysql} to a value of 16MB, use either of the following commands: 

shell> mysql --max_allowed_packet=16777216
shell> mysql --max_allowed_packet=16M

The first command specifies the value in bytes. The second specifies the value in megabytes. Variable values can have a suffix of K, M, or G (either uppercase or lowercase) to indicate units of kilobytes, megabytes, or gigabytes.

In an option file, the variable setting is given without the leading dashes: 

[mysql]
max_allowed_packet=16777216

Or: 

[mysql]
max_allowed_packet=16M

If you like, underscores in a variable name can be specified as dashes.

Prior to MySQL 4.0.2, program variable names are not recognized as option names. Instead, use the --set-variable option to assign a value to a variable: 

shell> mysql --set-variable=max_allowed_packet=16777216
shell> mysql --set-variable=max_allowed_packet=16M

In an option file, omit the leading dashes: 

[mysql]
set-variable = max_allowed_packet=16777216

Or: 

[mysql]
set-variable = max_allowed_packet=16M

With --set-variable, underscores in variable names cannot be given as dashes for versions of MySQL older than 4.0.2.

The --set-variable option is still recognized in MySQL 4.0.2 and up, but is deprecated.

Some server variables can be set at runtime. For details, see section 5.2.3.1 Dynamic System Variables.

5 Database Administration

This chapter covers topics that deal with administering a MySQL installation, such as configuring the server, managing user accounts, and performing backups.

5.1 The MySQL Server and Server Startup Scripts

The MySQL server, @command{mysqld}, is the main program that does most of the work in a MySQL installation. The server is accompanied by several related scripts that perform setup operations when you install MySQL or that are helper programs to assist you in starting and stopping the server.

This section provides an overview of the server and related programs, and information about server startup scripts. Information about configuring the server itself is given in section 5.2 Configuring the MySQL Server.

5.1.1 Overview of the Server-Side Scripts and Utilities

All MySQL programs take many different options. However, every MySQL program provides a --help option that you can use to get a description of the program's options. For example, try @command{mysqld --help}.

You can override default options for all standard programs by specifying options on the command line or in an option file. section 4.3 Specifying Program Options.

The following list briefly describes the MySQL server and server-related programs:

mysqld
The SQL daemon (that is, the MySQL server). To use client programs, this program must be running, because clients gain access to databases by connecting to the server. See section 5.2 Configuring the MySQL Server.
mysqld-max
A version of the server that includes additional features. @xref{mysqld-max, , @command{mysqld-max}}.
mysqld_safe
A server startup script. @command{mysqld_safe} attempts to start @command{mysqld-max} if it exists, and @command{mysqld} otherwise. @xref{mysqld_safe, , @command{mysqld_safe}}.
mysql.server
A server startup script. This script is used on systems that use run directories containing scripts that start system services for particular run levels. It invokes @command{mysqld_safe} to start the MySQL server. @xref{mysql.server, , @command{mysql.server}}.
mysqld_multi
A server startup script that can start or stop multiple servers installed on the system. @xref{mysqld_multi, , @command{mysqld_multi}}.
mysql_install_db
This script creates the MySQL grant tables with default privileges. It is usually executed only once, when first installing MySQL on a system.
mysql_fix_privilege_tables
This script is used after an upgrade install operation, to update the grant tables with any changes that have been made in newer versions of MySQL.

There are several other programs that also are run on the server host:

myisamchk
A utility to describe, check, optimize, and repair MyISAM tables. @command{myisamchk} is described in section 5.7.2 Table Maintenance and Crash Recovery.
make_binary_distribution
This program makes a binary release of a compiled MySQL. This could be sent by FTP to `/pub/mysql/upload/' on ftp.mysql.com for the convenience of other MySQL users.
mysqlbug
The MySQL bug reporting script. It can be used to send a bug report to the MySQL mailing list. (You can also visit http://bugs.mysql.com/ to file a bug report online.)

5.1.2 The @command{mysqld-max} Extended MySQL Server

A MySQL-Max server is a version of the @command{mysqld} MySQL server that has been built to include additional features.

The distribution to use depends on your platform:

You can find the MySQL-Max binaries on the MySQL AB Web site at http://dev.mysql.com/downloads/mysql-4.0.html.

MySQL AB builds the MySQL-Max servers by using the following @command{configure} options:

--with-server-suffix=-max
This option adds a -max suffix to the @command{mysqld} version string.
--with-innodb
This option enables support for the InnoDB storage engine. MySQL-Max servers always include InnoDB support, but this option actually is needed only for MySQL 3.23. From MySQL 4 on, InnoDB is included by default in binary distributions, so you do not need a MySQL-Max server to obtain InnoDB support.
--with-bdb
This option enables support for the Berkeley DB (BDB) storage engine.
CFLAGS=-DUSE_SYMDIR
This define enables symbolic link support for Windows.

MySQL-Max binary distributions are a convenience for those who wish to install precompiled programs. If you build MySQL using a source distribution, you can build your own Max-like server by enabling the same features at configuration time that the MySQL-Max binary distributions are built with.

MySQL-Max servers include the BerkeleyDB (BDB) storage engine whenever possible, but not all platforms support BDB. The following table shows which platforms allow MySQL-Max binaries to include BDB:

System BDB Support
AIX 4.3 N
HP-UX 11.0 N
Linux-Alpha N
Linux-IA-64 N
Linux-Intel Y
Mac OS X N
NetWare N
SCO OSR5 Y
Solaris-Intel N
Solaris-SPARC Y
UnixWare Y
Windows/NT Y

To find out which storage engines your server supports, issue the following statement: 

mysql> SHOW ENGINES;

Before MySQL 4.1.2, SHOW ENGINES is unavailable. Use the following statement instead and check the value of the variable for the storage engine in which you are interested: 

mysql> SHOW VARIABLES LIKE 'have_%';
+------------------+----------+
| Variable_name    | Value    |
+------------------+----------+
| have_bdb         | NO       |
| have_crypt       | YES      |
| have_innodb      | YES      |
| have_isam        | NO       |
| have_raid        | NO       |
| have_symlink     | DISABLED |
| have_openssl     | NO       |
| have_query_cache | YES      |
+------------------+----------+

The values in the second column indicate the server's level of support for each feature:

Value Meaning
YES The feature is supported and is active.
NO The feature is not supported.
DISABLED The feature is supported but has been disabled.

A value of NO means that the server was compiled without support for the feature, so it cannot be activated at runtime.

A value of DISABLED occurs either because the server was started with an option that disables the feature, or because not all options required to enable it were given. In the latter case, the host_name.err error log file should contain a reason indicating why the option is disabled.

One situation in which you might see DISABLED occurs with MySQL 3.23 when the InnoDB storage engine is compiled in. In MySQL 3.23, you must supply at least the innodb_data_file_path option at runtime to set up the InnoDB tablespace. Without this option, InnoDB disables itself. See section 16.3 InnoDB in MySQL 3.23. You can specify configuration options for the BDB storage engine, too, but BDB will not disable itself if you do not provide them. See section 15.4.3 BDB Startup Options.

You might also see DISABLED for the InnoDB, BDB, or ISAM storage engines if the server was compiled to support them, but was started with the --skip-innodb, --skip-bdb, or --skip-isam options at runtime.

As of Version 3.23, all MySQL servers support MyISAM tables, because MyISAM is the default storage engine.

5.1.3 The @command{mysqld_safe} Server Startup Script

@command{mysqld_safe} is the recommended way to start a @command{mysqld} server on Unix and NetWare. @command{mysqld_safe} adds some safety features such as restarting the server when an error occurs and logging runtime information to an error log file. NetWare-specific behaviors are listed later in this section.

Note: Before MySQL 4.0, @command{mysqld_safe} is named @command{safe_mysqld}. To preserve backward compatibility, MySQL binary distributions for some time will include @command{safe_mysqld} as a symbolic link to @command{mysqld_safe}.

By default, @command{mysqld_safe} tries to start an executable named @command{mysqld-max} if it exists, or @command{mysqld} otherwise. Be aware of the implications of this behavior:

To override the default behavior and specify explicitly which server you want to run, specify a --mysqld or --mysqld-version option to @command{mysqld_safe}.

Many of the options to @command{mysqld_safe} are the same as the options to @command{mysqld}. See section 5.2.1 @command{mysqld} Command-Line Options.

All options specified to @command{mysqld_safe} on the command line are passed to @command{mysqld}. If you want to use any options that are specific to @command{mysqld_safe} and that @command{mysqld} doesn't support, do not specify them on the command line. Instead, list them in the [mysqld_safe] group of an option file. See section 4.3.2 Using Option Files.

@command{mysqld_safe} reads all options from the [mysqld], [server], and [mysqld_safe] sections in option files. For backward compatibility, it also reads [safe_mysqld] sections, although you should rename such sections to [mysqld_safe] when you begin using MySQL 4.0 or later.

@command{mysqld_safe} supports the following options:

--basedir=path
The path to the MySQL installation directory.
--core-file-size=size
The size of the core file @command{mysqld} should be able to create. The option value is passed to @command{ulimit -c}.
--datadir=path
The path to the data directory.
--defaults-extra-file=path
The name of an option file to be read in addition to the usual option files.
--defaults-file=path
The name of an option file to be read instead of the usual option files.
--err-log=path
The old form of the --log-error option, to be used before MySQL 4.0.
--ledir=path
The path to the directory containing the @command{mysqld} program. Use this option to explicitly indicate the location of the server.
--log-error=path
Write the error log to the given file. See section 5.9.1 The Error Log.
--mysqld=prog_name
The name of the server program (in the ledir directory) that you want to start. This option is needed if you use the MySQL binary distribution but have the data directory outside of the binary distribution.
--mysqld-version=suffix
This option is similar to the --mysqld option, but you specify only the suffix for the server program name. The basename is assumed to be @command{mysqld}. For example, if you use --mysqld-version=max, @command{mysqld_safe} will start the @command{mysqld-max} program in the ledir directory. If the argument to --mysqld-version is empty, @command{mysqld_safe} uses @command{mysqld} in the ledir directory.
--nice=priority
Use the nice program to set the server's scheduling priority to the given value. This option was added in MySQL 4.0.14.
--no-defaults
Do not read any option files.
--open-files-limit=count
The number of files @command{mysqld} should be able to open. The option value is passed to @command{ulimit -n}. Note that you need to start @command{mysqld_safe} as root for this to work properly!
--pid-file=path
The path to the process ID file.
--port=port_num
The port number to use when listening for TCP/IP connections.
--socket=path
The Unix socket file to use for local connections.
--timezone=zone
Set the TZ time zone environment variable to the given option value. Consult your operating system documentation for legal time zone specification formats.
--user={user_name | user_id}
Run the @command{mysqld} server as the user having the name user_name or the numeric user ID user_id. (``User'' in this context refers to a system login account, not a MySQL user listed in the grant tables.)

The @command{mysqld_safe} script is written so that it normally can start a server that was installed from either a source or a binary distribution of MySQL, even though these types of distributions typically install the server in slightly different locations. (See section 2.1.5 Installation Layouts.) @command{mysqld_safe} expects one of the following conditions to be true:

Because @command{mysqld_safe} will try to find the server and databases relative to its own working directory, you can install a binary distribution of MySQL anywhere, as long as you run @command{mysqld_safe} from the MySQL installation directory: 

shell> cd mysql_installation_directory
shell> bin/mysqld_safe &

If @command{mysqld_safe} fails, even when invoked from the MySQL installation directory, you can specify the --ledir and --datadir options to indicate the directories in which the server and databases are located on your system.

Normally, you should not edit the @command{mysqld_safe} script. Instead, configure @command{mysqld_safe} by using command-line options or options in the [mysqld_safe] section of a `my.cnf' option file. In rare cases, it might be necessary to edit @command{mysqld_safe} to get it to start the server properly. However, if you do this, your modified version of @command{mysqld_safe} might be overwritten if you upgrade MySQL in the future, so you should make a copy of your edited version that you can reinstall.

On NetWare, @command{mysqld_safe} is a NetWare Loadable Module (NLM) that is ported from the original Unix shell script. It does the following:

  1. Runs a number of system and option checks.
  2. Runs a check on MyISAM and ISAM tables.
  3. Provides a screen presence for the MySQL server.
  4. Starts @command{mysqld}, monitors it, and restarts it if it terminates in error.
  5. Sends error messages from @command{mysqld} to the `host_name.err' file in the data directory.
  6. Sends @command{mysqld_safe} screen output to the `host_name.safe' file in the data directory.

5.1.4 The @command{mysql.server} Server Startup Script

MySQL distributions on Unix include a script named @command{mysql.server}. It can be used on systems such as Linux and Solaris that use System V-style run directories to start and stop system services. It is also used by the Mac OS X Startup Item for MySQL.

@command{mysql.server} can be found in the `support-files' directory under your MySQL installation directory or in a MySQL source tree.

If you use the Linux server RPM package (MySQL-server-VERSION.rpm), the @command{mysql.server} script will already have been installed in the `/etc/init.d' directory with the name `mysql'. You need not install it manually. See section 2.2.2 Installing MySQL on Linux for more information on the Linux RPM packages.

Some vendors provide RPM packages that install a startup script under a different name such as @command{mysqld}.

If you install MySQL from a source distribution or using a binary distribution format that does not install @command{mysql.server} automatically, you can install it manually. Instructions are provided in section 2.4.2.2 Starting and Stopping MySQL Automatically.

@command{mysql.server} reads options from the [mysql.server] and [mysqld] sections of option files. (For backward compatibility, it also reads [mysql_server] sections, although you should rename such sections to [mysql.server] when you begin using MySQL 4.0 or later.)

5.1.5 The @command{mysqld_multi} Program for Managing Multiple MySQL Servers

@command{mysqld_multi} is meant for managing several @command{mysqld} processes that listen for connections on different Unix socket files and TCP/IP ports. It can start or stop servers, or report their current status.

The program searches for groups named [mysqld#] in `my.cnf' (or in the file named by the --config-file option). # can be any positive integer. This number is referred to in the following discussion as the option group number, or GNR. Group numbers distinguish option groups from one another and are used as arguments to @command{mysqld_multi} to specify which servers you want to start, stop, or obtain a status report for. Options listed in these groups are the same that you would use in the [mysqld] group used for starting @command{mysqld}. (See, for example, section 2.4.2.2 Starting and Stopping MySQL Automatically.) However, when using multiple servers it is necessary that each one use its own value for options such as the Unix socket file and TCP/IP port number. For more information on which options must be unique per server in a multiple-server environment, see section 5.10 Running Multiple MySQL Servers on the Same Machine.

To invoke @command{mysqld_multi}, use the following syntax: 

shell> mysqld_multi [options] {start|stop|report} [GNR[,GNR]...]

start, stop, and report indicate which operation you want to perform. You can perform the designated operation on a single server or multiple servers, depending on the GNR list that follows the option name. If there is no list, @command{mysqld_multi} performs the operation for all servers in the option file.

Each GNR value represents an option group number or range of group numbers. The value should be the number at the end of the group name in the option file. For example, the GNR for a group named [mysqld17] is 17. To specify a range of numbers, separate the first and last numbers by a dash. The GNR value 10-13 represents groups [mysqld10] through [mysqld13]. Multiple groups or group ranges can be specified on the command line, separated by commas. There must be no whitespace characters (spaces or tabs) in the GNR list; anything after a whitespace character is ignored.

This command starts a single server using option group [mysqld17]: 

shell> mysqld_multi start 17

This command stops several servers, using option groups [mysql8] and [mysqld10] through [mysqld13]: 

shell> mysqld_multi stop 8,10-13

For an example of how you might set up an option file, use this command: 

shell> mysqld_multi --example

@command{mysqld_multi} supports the following options:

--config-file=name
Specify the name of an alternative option file. This affects where @command{mysqld_multi} looks for [mysqld#] option groups. Without this option, all options are read from the usual `my.cnf' file. The option does not affect where @command{mysqld_multi} reads its own options, which are always taken from the [mysqld_multi] group in the usual `my.cnf' file.
--example
Display a sample option file.
--help
Display a help message and exit.
--log=name
Specify the name of the log file. If the file exists, log output is appended to it.
--mysqladmin=prog_name
The @command{mysqladmin} binary to be used to stop servers.
--mysqld=prog_name
The @command{mysqld} binary to be used. Note that you can specify @command{mysqld_safe} as the value for this option also. The options are passed to @command{mysqld}. Just make sure that you have the directory where @command{mysqld} is located in your PATH environment variable setting or fix @command{mysqld_safe}.
--no-log
Print log information to stdout rather than to the log file. By default, output goes to the log file.
--password=password
The password of the MySQL account to use when invoking @command{mysqladmin}. Note that the password value is not optional for this option, unlike for other MySQL programs.
--tcp-ip
Connect to each MySQL server via the TCP/IP port instead of the Unix socket file. (If a socket file is missing, the server might still be running, but accessible only via the TCP/IP port.) By default, connections are made using the Unix socket file. This option affects stop and report operations.
--user=user_name
The username of the MySQL account to use when invoking @command{mysqladmin}.
--version
Display version information and exit.

Some notes about @command{mysqld_multi}:

The following example shows how you might set up an option file for use with @command{mysqld_multi}. The first and fifth [mysqld#] group were intentionally left out from the example to illustrate that you can have ``gaps'' in the option file. This gives you more flexibility. The order in which the @command{mysqld} programs are started or stopped depends on the order in which they appear in the option file. 

# This file should probably be in your home dir (~/.my.cnf)
# or /etc/my.cnf
# Version 2.1 by Jani Tolonen

[mysqld_multi]
mysqld     = /usr/local/bin/mysqld_safe
mysqladmin = /usr/local/bin/mysqladmin
user       = multi_admin
password   = multipass

[mysqld2]
socket     = /tmp/mysql.sock2
port       = 3307
pid-file   = /usr/local/mysql/var2/hostname.pid2
datadir    = /usr/local/mysql/var2
language   = /usr/local/share/mysql/english
user       = john

[mysqld3]
socket     = /tmp/mysql.sock3
port       = 3308
pid-file   = /usr/local/mysql/var3/hostname.pid3
datadir    = /usr/local/mysql/var3
language   = /usr/local/share/mysql/swedish
user       = monty

[mysqld4]
socket     = /tmp/mysql.sock4
port       = 3309
pid-file   = /usr/local/mysql/var4/hostname.pid4
datadir    = /usr/local/mysql/var4
language   = /usr/local/share/mysql/estonia
user       = tonu

[mysqld6]
socket     = /tmp/mysql.sock6
port       = 3311
pid-file   = /usr/local/mysql/var6/hostname.pid6
datadir    = /usr/local/mysql/var6
language   = /usr/local/share/mysql/japanese
user       = jani

See section 4.3.2 Using Option Files.

5.2 Configuring the MySQL Server

This section discusses MySQL server configuration topics:

5.2.1 @command{mysqld} Command-Line Options

When you start the @command{mysqld} server, you can specify program options using any of the methods described in section 4.3 Specifying Program Options. The most common methods are to provide options in an option file or on the command line. However, in most cases it is desirable to make sure that the server uses the same options each time it runs. The best way to ensure this is to list them in an option file. See section 4.3.2 Using Option Files.

@command{mysqld} reads options from the [mysqld] and [server] groups. @command{mysqld_safe} reads options from the [mysqld], [server], [mysqld_safe], and [safe_mysqld] groups. @command{mysql.server} reads options from the [mysqld] and [mysql.server] groups. An embedded MySQL server usually reads options from the [server], [embedded], and [xxxxx_SERVER] groups, where xxxxx is the name of the application into which the server is embedded.

@command{mysqld} accepts many command-line options. For a list, execute @command{mysqld --help}. Before MySQL 4.1.1, --help prints the full help message. As of 4.1.1, it prints a brief message; to see the full list, use @command{mysqld --verbose --help}.

The following list shows some of the most common server options. Additional options are described elsewhere:

You can also set the value of a server system variable by using the variable name as an option, as described later in this section.

--help, -?
Display a short help message and exit. Before MySQL 4.1.1, --help displays the full help message. As of 4.1.1, it displays an abbreviated message only. Use both the --verbose and --help options to see the full message.
--ansi
Use standard SQL syntax instead of MySQL syntax. See section 1.8.3 Running MySQL in ANSI Mode. For more precise control over the server SQL mode, use the --sql-mode option instead.
--basedir=path, -b path
The path to the MySQL installation directory. All paths are usually resolved relative to this.
--big-tables
Allow large result sets by saving all temporary sets in files. This option prevents most ``table full'' errors, but also slows down queries for which in-memory tables would suffice. Since MySQL 3.23.2, the server is able to handle large result sets automatically by using memory for small temporary tables and switching to disk tables where necessary.
--bind-address=IP
The IP address to bind to.
--console
Write the error log messages to stderr/stdout even if --log-error is specified. On Windows, @command{mysqld} will not close the console screen if this option is used.
--character-sets-dir=path
The directory where character sets are installed. See section 5.8.1 The Character Set Used for Data and Sorting.
--chroot=path
Put the @command{mysqld} server in a closed environment during startup by using the chroot() system call. This is a recommended security measure as of MySQL 4.0. (MySQL 3.23 is not able to provide a chroot() jail that is 100% closed.) Note that use of this option somewhat limits LOAD DATA INFILE and SELECT ... INTO OUTFILE.
--character-set-server=charset
Use charset as the default server character set. This option is available as of MySQL 4.1.3. See section 5.8.1 The Character Set Used for Data and Sorting.
--core-file
Write a core file if @command{mysqld} dies. For some systems, you must also specify the --core-file-size option to @command{mysqld_safe}. @xref{mysqld_safe, , @command{mysqld_safe}}. Note that on some systems, such as Solaris, you will not get a core file if you are also using the --user option.
--collation-server=collation
Use collation as the default server collation. This option is available as of MySQL 4.1.3. See section 5.8.1 The Character Set Used for Data and Sorting.
--datadir=path, -h path
The path to the data directory.
--debug[=debug_options], -# [debug_options]
If MySQL is configured with --with-debug, you can use this option to get a trace file of what @command{mysqld} is doing. The debug_options string often is 'd:t:o,file_name'. See section D.1.2 Creating Trace Files.
--default-character-set=charset
Use charset as the default character set. This option is deprecated in favor of --character-set-server as of MySQL 4.1.3. See section 5.8.1 The Character Set Used for Data and Sorting.
--default-collation=collation
Use collation as the default collation. This option is deprecated in favor of --collation-server as of MySQL 4.1.3. See section 5.8.1 The Character Set Used for Data and Sorting.
--default-storage-engine=type
This option is a synonym for --default-table-type. It is available as of MySQL 4.1.2.
--default-table-type=type
Set the default table type for tables. See section 15 MySQL Storage Engines and Table Types.
--default-time-zone=type
Set the default server time zone. This option sets the global time_zone system variable. If this option is not given, the default time zone will be the same as the system time zone (given by the value of the system_time_zone system variable. This option is available as of MySQL 4.1.3.
--delay-key-write[= OFF | ON | ALL]
How the DELAYED KEYS option should be used. Delayed key writing causes key buffers not to be flushed between writes for MyISAM tables. OFF disables delayed key writes. ON enables delayed key writes for those tables that were created with the DELAYED KEYS option. ALL delays key writes for all MyISAM tables. Available as of MySQL 4.0.3. See section 7.5.2 Tuning Server Parameters. See section 15.1.1 MyISAM Startup Options. Note: If you set this variable to ALL, you should not use MyISAM tables from within another program (such as from another MySQL server or with @command{myisamchk}) when the table is in use. Doing so will lead to index corruption.
--delay-key-write-for-all-tables
Old form of --delay-key-write=ALL for use prior to MySQL 4.0.3. As of 4.0.3, use --delay-key-write instead.
--des-key-file=file_name
Read the default keys used by DES_ENCRYPT() and DES_DECRYPT() from this file.
--enable-named-pipe
Enable support for named pipes. This option applies only on Windows NT, 2000, and XP systems, and can be used only with the @command{mysqld-nt} and @command{mysqld-max-nt} servers that support named pipe connections.
--exit-info[=flags], -T [flags]
This is a bit mask of different flags you can use for debugging the @command{mysqld} server. Do not use this option unless you know exactly what it does!
--external-locking
Enable system locking. Note that if you use this option on a system on which lockd does not fully work (as on Linux), you will easily get @command{mysqld} to deadlock. This option previously was named --enable-locking. Note: If you use this option to enable updates to MyISAM tables from many MySQL processes, you have to ensure that these conditions are satisfied:
  • You should not use the query cache for queries that use tables that are updated by another process.
  • You should not use --delay-key-write=ALL or DELAY_KEY_WRITE=1 on any shared tables.
The easiest way to ensure this is to always use --external-locking together with --delay-key-write=OFF --query-cache-size=0. (This is not done by default because in many setups it's useful to have a mixture of the above options.)
--flush
Flush all changes to disk after each SQL statement. Normally MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synching to disk. See section A.4.2 What to Do If MySQL Keeps Crashing.
--init-file=file
Read SQL statements from this file at startup. Each statement must be on a single line and should not include comments.
--innodb-safe-binlog
Adds consistency guarantees between the content of InnoDB tables and the binary log. See section 5.9.4 The Binary Log.
--language=lang_name, -L lang_name
Client error messages in given language. lang_name can be given as the language name or as the full pathname to the directory where the language files are installed. See section 5.8.2 Setting the Error Message Language.
--log[=file], -l [file]
Log connections and queries to this file. See section 5.9.2 The General Query Log. If you don't specify a filename, MySQL will use host_name.log as the filename.
--log-bin=[file]
The binary log file. Log all queries that change data to this file. Used for backup and replication. See section 5.9.4 The Binary Log. If you don't specify a filename, MySQL will use host_name-bin as the filename.
--log-bin-index[=file]
The index file for binary log filenames. See section 5.9.4 The Binary Log. If you don't specify a filename, MySQL will use host_name-bin.index as the filename.
--log-error[=file]
Log errors and startup messages to this file. See section 5.9.1 The Error Log. If you don't specify a filename, MySQL will use host_name.err as the filename.
--log-isam[=file]
Log all ISAM/MyISAM changes to this file (used only when debugging ISAM/MyISAM).
--log-long-format
Log some extra information to the log files (update log, binary update log, and slow queries log, whatever log has been activated). For example, username and timestamp are logged for queries. If you are using --log-slow-queries and --log-long-format, queries that are not using indexes also are logged to the slow query log. Note that --log-long-format is deprecated as of MySQL version 4.1, when --log-short-format was introduced (the long log format is the default setting since version 4.1). Also note that starting with MySQL 4.1, the --log-queries-not-using-indexes option is available for the purpose of logging queries that do not use indexes to the slow query log.
--log-queries-not-using-indexes
If you are using this option with --log-slow-queries, then queries that are not using indexes also are logged to the slow query log. This option is available as of MySQL 4.1. See section 5.9.5 The Slow Query Log.
--log-short-format
Log less information to the log files (update log, binary update log, and slow queries log, whatever log has been activated). For example, username and timestamp are not logged for queries. This option was introduced in MySQL 4.1.
--log-slow-queries[=file]
Log all queries that have taken more than long_query_time seconds to execute to this file. See section 5.9.5 The Slow Query Log. Note that the default for the amount of information logged has changed in MySQL 4.1. See the --log-long-format and --log-short-format options for details.
--log-update[=file]
Log updates to file# where # is a unique number if not given. See section 5.9.3 The Update Log. The update log is deprecated and is removed in MySQL 5.0.0; you should use the binary log instead (--log-bin). See section 5.9.4 The Binary Log. Starting from version 5.0.0, using --log-update will just turn on the binary log instead (see section C.1.3 Changes in release 5.0.0 (22 Dec 2003: Alpha)).
--log-warnings, -W
Print out warnings such as Aborted connection... to the error log. Enabling this option is recommended, for example, if you use replication (you will get more information about what is happening, such as messages about network failures and reconnections). This option is enabled by default as of MySQL 4.0.19 and 4.1.2; to disable it, use --skip-log-warnings. As of MySQL 4.0.21 and 4.1.3, aborted connections are not logged to the error log unless the value is greater than 1. See section A.2.10 Communication Errors and Aborted Connections. This option was named --warnings before MySQL 4.0.
--low-priority-updates
Table-modifying operations (INSERT, REPLACE, DELETE, UPDATE) will have lower priority than selects. This can also be done via {INSERT | REPLACE | DELETE | UPDATE} LOW_PRIORITY ... to lower the priority of only one query, or by SET LOW_PRIORITY_UPDATES=1 to change the priority in one thread. See section 7.3.2 Table Locking Issues.
--memlock
Lock the @command{mysqld} process in memory. This works on systems such as Solaris that support the mlockall() system call. This might help if you have a problem where the operating system is causing @command{mysqld} to swap on disk. Note that use of this option requires that you run the server as root, which is normally not a good idea for security reasons.
--myisam-recover [=option[,option...]]]
Set the MyISAM storage engine recovery mode. The option value is any combination of the values of DEFAULT, BACKUP, FORCE, or QUICK. If you specify multiple values, separate them by commas. You can also use a value of "" to disable this option. If this option is used, @command{mysqld} will, when it opens a MyISAM table, open check whether the table is marked as crashed or wasn't closed properly. (The last option works only if you are running with --skip-external-locking.) If this is the case, @command{mysqld} will run a check on the table. If the table was corrupted, @command{mysqld} will attempt to repair it. The following options affect how the repair works:
Option Description
DEFAULT The same as not giving any option to --myisam-recover.
BACKUP If the data file was changed during recovery, save a backup of the `tbl_name.MYD' file as `tbl_name-datetime.BAK'.
FORCE Run recovery even if we will lose more than one row from the `.MYD' file.
QUICK Don't check the rows in the table if there aren't any delete blocks.
Before a table is automatically repaired, MySQL will add a note about this in the error log. If you want to be able to recover from most problems without user intervention, you should use the options BACKUP,FORCE. This will force a repair of a table even if some rows would be deleted, but it will keep the old data file as a backup so that you can later examine what happened. This option is available as of MySQL 3.23.25.
--ndb-connectstring=connect_string
When using the NDB storage engine, it is possible to point out the management server that distributes the cluster configuration by setting the connect string option.
--ndbcluster
If the binary includes support for the NDB Cluster storage engine (from version 4.1.3, the MySQL-Max binaries are built with NDB Cluster enabled) the default disabling of support for the NDB Cluster storage engine can be overruled by using this option. Using the NDB Cluster storage engine is necessary for using MySQL Cluster. See section 17 MySQL Cluster.
--new
The --new option can be used to make the server behave as 4.1 in certain respects, easing a 4.0 to 4.1 upgrade:
  • Hexadecimal strings such as 0xFF are treated as strings by default rather than as numbers. (Works in 4.0.12 and up.)
  • TIMESTAMP is returned as a string with the format 'YYYY-MM-DD HH:MM:SS'. (Works in 4.0.13 and up.) See section 12 Column Types.
This option can be used to help you see how your applications will behave in MySQL 4.1, without actually upgrading to 4.1.
--pid-file=path
The path to the process ID file used by @command{mysqld_safe}.
--port=port_num, -P port_num
The port number to use when listening for TCP/IP connections.
--old-protocol, -o
Use the 3.20 protocol for compatibility with some very old clients. See section 2.5.6 Upgrading from Version 3.20 to 3.21.
--one-thread
Only use one thread (for debugging under Linux). This option is available only if the server is built with debugging enabled. See section D.1 Debugging a MySQL Server.
--open-files-limit=count
To change the number of file descriptors available to @command{mysqld}. If this is not set or set to 0, then @command{mysqld} will use this value to reserve file descriptors to use with setrlimit(). If this value is 0 then @command{mysqld} will reserve max_connections*5 or max_connections + table_cache*2 (whichever is larger) number of files. You should try increasing this if @command{mysqld} gives you the error "Too many open files."
--safe-mode
Skip some optimization stages.
--safe-show-database
With this option, the SHOW DATABASES statement displays only the names of those databases for which the user has some kind of privilege. As of MySQL 4.0.2, this option is deprecated and doesn't do anything (it is enabled by default), because there is now a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis. See section 5.5.3 Privileges Provided by MySQL.
--safe-user-create
If this is enabled, a user can't create new users with the GRANT statement, if the user doesn't have the INSERT privilege for the mysql.user table or any column in the table.
--secure-auth
Disallow authentication for accounts that have old (pre-4.1) passwords. This option is available as of MySQL 4.1.1.
--shared-memory
Enable shared-memory connections by local clients. This option is available only on Windows. It was added in MySQL 4.1.0.
--shared-memory-base-name=name
The name to use for shared-memory connections. This option is available only on Windows. It was added in MySQL 4.1.0.
--skip-bdb
Disable the BDB storage engine. This saves memory and might speed up some operations. Do not use this option if you require BDB tables.
--skip-concurrent-insert
Turn off the ability to select and insert at the same time on MyISAM tables. (This is to be used only if you think you have found a bug in this feature.)
--skip-delay-key-write
Ignore the DELAY_KEY_WRITE option for all tables. As of MySQL 4.0.3, you should use --delay-key-write=OFF instead. See section 7.5.2 Tuning Server Parameters.
--skip-external-locking
Don't use system locking. To use @command{isamchk} or @command{myisamchk}, you must shut down the server. See section 1.2.3 MySQL Stability. In MySQL 3.23, you can use CHECK TABLE and REPAIR TABLE to check and repair MyISAM tables. This option previously was named --skip-locking.
--skip-grant-tables
This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing a @command{mysqladmin flush-privileges} or @command{mysqladmin reload} command, or by issuing a FLUSH PRIVILEGES statement.)
--skip-host-cache
Do not use the internal hostname cache for faster name-to-IP resolution. Instead, query the DNS server every time a client connects. See section 7.5.6 How MySQL Uses DNS.
--skip-innodb
Disable the InnoDB storage engine. This saves memory and disk space and might speed up some operations. Do not use this option if you require InnoDB tables.
--skip-isam
Disable the ISAM storage engine. As of MySQL 4.1, ISAM is disabled by default, so this option applies only if the server was configured with support for ISAM. This option was added in MySQL 4.1.1.
--skip-name-resolve
Do not resolve hostnames when checking client connections. Use only IP numbers. If you use this option, all Host column values in the grant tables must be IP numbers or localhost. See section 7.5.6 How MySQL Uses DNS.
--skip-ndbcluster
Disable the NDB Cluster storage engine. This is the default for binaries that were built with NDB Cluster storage engine support, this means that the system will only allocate memory and other resources for this storage engine if it is explicitly enabled.
--skip-networking
Don't listen for TCP/IP connections at all. All interaction with @command{mysqld} must be made via named pipes (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are allowed. See section 7.5.6 How MySQL Uses DNS.
--skip-new
Don't use new, possibly wrong routines.
--skip-symlink
This is the old form of --skip-symbolic-links, for use before MySQL 4.0.13.
--symbolic-links, --skip-symbolic-links
Enable or disable symbolic link support. This option has different effects on Windows and Unix:
  • On Windows, enabling symbolic links allows you to establish a symbolic link to a database directory by creating a directory.sym file that contains the path to the real directory. See section 7.6.1.3 Using Symbolic Links for Databases on Windows.
  • On Unix, enabling symbolic links means that you can link a MyISAM index file or data file to another directory with the INDEX DIRECTORY or DATA DIRECTORY options of the CREATE TABLE statement. If you delete or rename the table, the files that its symbolic links point to also are deleted or renamed. See section 14.2.6 CREATE TABLE Syntax.
This option was added in MySQL 4.0.13.
--skip-safemalloc
If MySQL is configured with --with-debug=full, all MySQL programs check for memory overruns during each memory allocation and memory freeing operation. This checking is very slow, so for the server you can avoid it when you don't need it by using the --skip-safemalloc option.
--skip-show-database
With this option, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. Without this option, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database.
--skip-stack-trace
Don't write stack traces. This option is useful when you are running @command{mysqld} under a debugger. On some systems, you also must use this option to get a core file. See section D.1 Debugging a MySQL Server.
--skip-thread-priority
Disable using thread priorities for faster response time.
--socket=path
On Unix, this option specifies the Unix socket file to use for local connections. The default value is `/tmp/mysql.sock'. On Windows, the option specifies the pipe name to use for local connections that use a named pipe. The default value is MySQL.
--sql-mode=value[,value[,value...]]
Set the SQL mode for MySQL. See section 5.2.2 The Server SQL Mode. This option was added in 3.23.41.
--temp-pool
This option causes most temporary files created by the server to use a small set of names, rather than a unique name for each new file. This works around a problem in the Linux kernel dealing with creating many new files with different names. With the old behavior, Linux seems to ``leak'' memory, because it's being allocated to the directory entry cache rather than to the disk cache.
--transaction-isolation=level
Sets the default transaction isolation level, which can be READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. See section 14.4.6 SET TRANSACTION Syntax.
--tmpdir=path, -t path
The path of the directory to use for creating temporary files. It might be useful if your default /tmp directory resides on a partition that is too small to hold temporary tables. Starting from MySQL 4.1, this option accepts several paths that are used in round-robin fashion. Paths should be separated by colon characters (`:') on Unix and semicolon characters (`;') on Windows, NetWare, and OS/2. If the MySQL server is acting as a replication slave, you should not set --tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication will fail.
--user={user_name | user_id}, -u {user_name | user_id}
Run the @command{mysqld} server as the user having the name user_name or the numeric user ID user_id. (``User'' in this context refers to a system login account, not a MySQL user listed in the grant tables.) This option is mandatory when starting @command{mysqld} as root. The server will change its user ID during its startup sequence, causing it to run as that particular user rather than as root. See section 5.4.1 General Security Guidelines. Starting from MySQL 3.23.56 and 4.0.12: To avoid a possible security hole where a user adds a --user=root option to some `my.cnf' file (thus causing the server to run as root), @command{mysqld} uses only the first --user option specified and produces a warning if there are multiple --user options. Options in `/etc/my.cnf' and `datadir/my.cnf' are processed before command-line options, so it is recommended that you put a --user option in `/etc/my.cnf' and specify a value other than root. The option in `/etc/my.cnf' will be found before any other --user options, which ensures that the server runs as a user other than root, and that a warning results if any other --user option is found.
--version, -V
Display version information and exit.

As of MySQL 4.0, you can assign a value to a server system variable by using an option of the form --var_name=value. For example, --key_buffer_size=32M sets the key_buffer_size variable to a value of 32MB.

Note that when setting a variable to a value, MySQL might automatically correct it to stay within a given range, or adjust the value to the closest allowable value if only certain values are allowed.

It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. However, this syntax is deprecated as of MySQL 4.0.

You can find a full description for all variables in section 5.2.3 Server System Variables. The section on tuning server parameters includes information on how to optimize them. See section 7.5.2 Tuning Server Parameters.

You can change the values of most system variables for a running server with the SET statement. See section 14.5.3.1 SET Syntax.

If you want to restrict the maximum value that a startup option can be set to with SET, you can define this by using the --maximum-var_name command-line option.

5.2.2 The Server SQL Mode

The MySQL server can operate in different SQL modes, and (as of MySQL 4.1) can apply these modes differentially for different clients. This allows an application to tailor server operation to its own requirements.

Modes define what SQL syntax MySQL should support and what kind of data validation checks it should perform. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.

You can set the default SQL mode by starting @command{mysqld} with the --sql-mode="modes" option. The value also can be empty (--sql-mode="") if you want to reset it.

Beginning with MySQL 4.1, you can also change the SQL mode after startup time by setting the sql_mode variable with a SET [SESSION|GLOBAL] sql_mode='modes' statement. Setting the GLOBAL variable requires the SUPER privilege and affects the operation of all clients that connect from that time on. Setting the SESSION variable affects only the current client. Any client can change its session sql_mode value.

modes is a list of different modes separated by comma (`,') characters. You can retrieve the current mode by issuing a SELECT @@sql_mode statement. The default value is empty (no modes set).

The most important sql_mode values are probably:

ANSI
Change syntax and behavior to be more conformant to standard SQL. (New in MySQL 4.1.1)
STRICT_TRANS_TABLES
If a value could not be inserted as given into a transactional table, abort the statement. For a non-transactional table, abort the statement if the value occurs in a single-row statement or the first row of a multiple-row statement. More detail is given later in this section. (New in MySQL 5.0.2)
TRADITIONAL
Make MySQL behave like a ``traditional'' SQL database system. A simple description of this mode is ``give an error instead a warning'' when inserting an incorrect value in a column. Note: If you are using non-transactional storage engines, the INSERT/UPDATE will abort as soon as the error is noticed, which may not be what you want. (New in MySQL 5.0.2)

When this manual refers to strict mode, it means a mode where at least one of STRICT_TRANS_TABLES or STRICT_ALL_TABLES is enabled.

The following list describes all the supported modes:

ALLOW_INVALID_DATES
Don't do full checking of dates. Check only that the month is in the range from 1 to 12 and the day is in the range from 1 to 31. This is very convenient for Web applications where you obtain year, month, and day in three different fields and you want to store exactly what the user inserted (without date validation). This mode applies to DATE and DATETIME columns. It does not apply TIMESTAMP columns, which always require a valid date. This mode is new in MySQL 5.0.2. Before 5.0.2, this was the default MySQL date-handling mode. As of 5.0.2, enabling strict mode causes the server to require that month and day values be legal, not just in the range from 1 to 12 and 1 to 31. For example, '2004-04-31' is legal with strict mode disabled, but illegal with strict mode enabled. To allow such dates in strict mode, enable ALLOW_INVALID_DATES as well.
ANSI_QUOTES
Treat `"' as an identifier quote character (like the ``' quote character) and not as a string quote character. You can still use ``' to quote identifers in ANSI mode. With ANSI_QUOTES enabled, you cannot use double quotes to quote a literal string, because it will be interpreted as an identifier. (New in MySQL 4.0.0)
ERROR_FOR_DIVISION_BY_ZERO
Produce an error in strict mode (otherwise a warning) when we encounter a division by zero (or MOD(X,0)) during an INSERT/ UPDATE. If this mode is not given, MySQL instead returns NULL for divisions by zero. If used with IGNORE, MySQL generates a warning for divisions by zero, but the result of the operation is NULL. (New in MySQL 5.0.2)
IGNORE_SPACE
Allow spaces between a function name and the `(' character. This forces all function names to be treated as reserved words. As a result, if you want to access any database, table, or column name that is a reserved word, you must quote it. For example, because there is a USER() function, the name of the user table in the mysql database and the User column in that table become reserved, so you must quote them: 
SELECT "User" FROM mysql."user";
(New in MySQL 4.0.0)
NO_AUTO_VALUE_ON_ZERO
NO_AUTO_VALUE_ON_ZERO affects handling of AUTO_INCREMENT columns. Normally, you generate the next sequence number for the column by inserting either NULL or 0 into it. NO_AUTO_VALUE_ON_ZERO suppresses this behavior for 0 so that only NULL generates the next sequence number. (New in MySQL 4.1.1) This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (This is not a recommended practice, by the way.) For example, if you dump the table with @command{mysqldump} and then reload it, MySQL normally generates new sequence numbers when it encounters the 0 values, resulting in a table with different contents than the one that was dumped. Enabling NO_AUTO_VALUE_ON_ZERO before reloading the dump file solves this problem. As of MySQL 4.1.1, @command{mysqldump} automatically includes a statement in the dump output to enable NO_AUTO_VALUE_ON_ZERO.
NO_DIR_IN_CREATE
When creating a table, ignore all INDEX DIRECTORY and DATA DIRECTORY directives. This option is useful on slave replication servers. (New in MySQL 4.0.15)
NO_FIELD_OPTIONS
Don't print MySQL-specific column options in the output of SHOW CREATE TABLE. This mode is used by @command{mysqldump} in portability mode. (New in MySQL 4.1.1)
NO_KEY_OPTIONS
Don't print MySQL-specific index options in the output of SHOW CREATE TABLE. This mode is used by @command{mysqldump} in portability mode. (New in MySQL 4.1.1)
NO_TABLE_OPTIONS
Don't print MySQL-specific table options (such as ENGINE) in the output of SHOW CREATE TABLE. This mode is used by @command{mysqldump} in portability mode. (New in MySQL 4.1.1)
NO_ZERO_DATE
Don't allow '0000-00-00' as a valid date. You can still insert zero dates with the IGNORE option. (New in MySQL 5.0.2)
NO_ZERO_IN_DATE
Don't accept dates where the month or day part is 0. If used with the IGNORE option, we insert a '0000-00-00' date for any such date. (New in MySQL 5.0.2)
NO_UNSIGNED_SUBTRACTION
In subtraction operations, don't mark the result as UNSIGNED if one of the operands is unsigned. Note that this makes UNSIGNED BIGINT not 100% usable in all contexts. See section 13.7 Cast Functions and Operators. (New in MySQL 4.0.2)
ONLY_FULL_GROUP_BY
Don't allow queries that in the GROUP BY part refer to a not selected column. (New in MySQL 4.0.0)
PIPES_AS_CONCAT
Treat || as a string concatenation operator (same as CONCAT()) rather than as a synonym for OR. (New in MySQL 4.0.0)
REAL_AS_FLOAT
Treat REAL as a synonym for FLOAT rather than as a synonym for DOUBLE. (New in MySQL 4.0.0)
STRICT_ALL_TABLES
Enable strict mode for all storage engines. Invalid data values are rejected. Additional detail follows. (New in MySQL 5.0.2)
STRICT_TRANS_TABLES
Enable strict mode for transactional storage engines, and when possible for non-transactional storage engines. Additional detail follows. (New in MySQL 5.0.2)

When strict mode is enabled, MySQL returns an error if a value is invalid or missing (no value given for a column that does not have an explicit DEFAULT value in its definition). For transactional tables, this occurs when either of STRICT_ALL_TABLES or STRICT_TRANS_TABLES is enabled. The statement is aborted and rolled back. For non-transactional tables, MySQL handles invalid or missing values as follows:

If the statement inserts or modifies a single row, an error occurs if a value is invalid or missing. The statement is aborted and the table remains unchanged. If the statement inserts or modifies multiple rows, the effect depends on which strict option is enabled:

Strict mode disallows invalid date values such as '2004-04-31'. It does not disallow dates with zero parts such as 2004-04-00' or ``zero'' dates. To disallow these, enable the NO_ZERO_IN_DATE and NO_ZERO_DATE SQL modes.

If you are not using strict mode (that is, neither STRICT_TRANS_TABLES nor STRICT_ALL_TABLES is enabled), MySQL produces a warning for all values that are adjusted when inserted. See section 14.5.3.21 SHOW WARNINGS Syntax.

The following special modes are provided as shorthand for combinations of mode values from the preceding list. All are available as of MySQL 4.1.1, except TRADITIONAL (5.0.2).

ANSI
Equivalent to REAL_AS_FLOAT, PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, ONLY_FULL_GROUP_BY. See section 1.8.3 Running MySQL in ANSI Mode.
DB2
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
MAXDB
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
MSSQL
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
MYSQL323
Equivalent to NO_FIELD_OPTIONS.
MYSQL40
Equivalent to NO_FIELD_OPTIONS.
ORACLE
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
POSTGRESQL
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
TRADITIONAL
Equivalent to STRICT_TRANS_TABLES, STRICT_ALL_TABLES, NO_ZERO_IN_DATE, NO_ZERO_DATE, ERROR_FOR_DIVISION_BY_ZERO.

5.2.3 Server System Variables

The server maintains many system variables that indicate how it is configured. All of them have default values. They can be set at server startup using options on the command line or in option files. Most of them can be set at runtime using the SET statement.

Beginning with MySQL 4.0.3, the @command{mysqld} server maintains two kinds of variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections.

When the server starts, it initializes all global variables to their default values. These defaults can be changed by options specified in option files or on the command line. After the server starts, those global variables that are dynamic can be changed by connecting to the server and issuing a SET GLOBAL var_name statement. To change a global variable, you must have the SUPER privilege.

The server also maintains a set of session variables for each client that connects. The client's session variables are initialized at connect time using the current values of the corresponding global variables. For those session variables that are dynamic, the client can change them by issuing a SET SESSION var_name statement. Setting a session variable requires no special privilege, but a client can change only its own session variables, not those of any other client.

A change to a global variable is visible to any client that accesses that global variable. However, it affects the corresponding session variable that is initialized from the global variable only for clients that connect after the change. It does not affect the session variable for any client that is already connected (not even that of the client that issues the SET GLOBAL statement).

When setting a variable using a startup option, variable values can be given with a suffix of K, M, or G to indicate kilobytes, megabytes, or gigabytes, respectively. For example, the following command starts the server with a key buffer size of 16 megabytes: 

mysqld --key_buffer_size=16M

Before MySQL 4.0, use this syntax instead: 

mysqld --set-variable=key_buffer_size=16M

The lettercase of suffix letters does not matter; 16M and 16m are equivalent.

At runtime, use the SET statement to set system variables. In this context, suffix letters cannot be used, but the value can take the form of an expression: 

mysql> SET sort_buffer_size = 10 * 1024 * 1024;

To specify explicitly whether to set the global or session variable, use the GLOBAL or SESSION options: 

mysql> SET GLOBAL sort_buffer_size = 10 * 1024 * 1024;
mysql> SET SESSION sort_buffer_size = 10 * 1024 * 1024;

Without either option, the statement sets the session variable.

The variables that can be set at runtime are listed in section 5.2.3.1 Dynamic System Variables.

If you want to restrict the maximum value to which a system variable can be set with the SET statement, you can specify this maximum by using an option of the form --maximum-var_name at server startup. For example, to prevent the value of query_cache_size from being increased to more than 32MB at runtime, use the option --maximum-query_cache_size=32M. This feature is available as of MySQL 4.0.2.

You can view system variables and their values by using the SHOW VARIABLES statement. See section 10.4 System Variables for more information. 

mysql> SHOW VARIABLES;
+---------------------------------+------------------------------+
| Variable_name                   | Value                        |
+---------------------------------+------------------------------|
| back_log                        | 50                           |
| basedir                         | /usr/local/mysql             |
| bdb_cache_size                  | 8388572                      |
| bdb_home                        | /usr/local/mysql             |
| bdb_log_buffer_size             | 32768                        |
| bdb_logdir                      |                              |
| bdb_max_lock                    | 10000                        |
| bdb_shared_data                 | OFF                          |
| bdb_tmpdir                      | /tmp/                        |
| bdb_version                     | Sleepycat Software: ...      |
| binlog_cache_size               | 32768                        |
| bulk_insert_buffer_size         | 8388608                      |
| character_set                   | latin1                       |
| character_sets                  | latin1 big5 czech euc_kr     |
| concurrent_insert               | ON                           |
| connect_timeout                 | 5                            |
| convert_character_set           |                              |
| datadir                         | /usr/local/mysql/data/       |
| default_week_format             | 0                            |
| delay_key_write                 | ON                           |
| delayed_insert_limit            | 100                          |
| delayed_insert_timeout          | 300                          |
| delayed_queue_size              | 1000                         |
| flush                           | OFF                          |
| flush_time                      | 0                            |
| ft_boolean_syntax               | + -><()~*:""&|               |
| ft_max_word_len                 | 84                           |
| ft_min_word_len                 | 4                            |
| ft_query_expansion_limit        | 20                           |
| ft_stopword_file                | (built-in)                   |
| have_bdb                        | YES                          |
| have_innodb                     | YES                          |
| have_isam                       | YES                          |
| have_openssl                    | YES                          |
| have_query_cache                | YES                          |
| have_raid                       | NO                           |
| have_symlink                    | DISABLED                     |
| init_file                       |                              |
| innodb_additional_mem_pool_size | 1048576                      |
| innodb_buffer_pool_size         | 8388608                      |
| innodb_data_file_path           | ibdata1:10M:autoextend       |
| innodb_data_home_dir            |                              |
| innodb_fast_shutdown            | ON                           |
| innodb_file_io_threads          | 4                            |
| innodb_flush_log_at_trx_commit  | 1                            |
| innodb_flush_method             |                              |
| innodb_force_recovery           | 0                            |
| innodb_lock_wait_timeout        | 50                           |
| innodb_log_arch_dir             |                              |
| innodb_log_archive              | OFF                          |
| innodb_log_buffer_size          | 1048576                      |
| innodb_log_file_size            | 5242880                      |
| innodb_log_files_in_group       | 2                            |
| innodb_log_group_home_dir       | ./                           |
| innodb_mirrored_log_groups      | 1                            |
| innodb_thread_concurrency       | 8                            |
| interactive_timeout             | 28800                        |
| join_buffer_size                | 131072                       |
| key_buffer_size                 | 16773120                     |
| key_cache_age_threshold         | 300                          |
| key_cache_block_size            | 1024                         |
| key_cache_division_limit        | 100                          |
| language                        | /usr/local/mysql/share/...   |
| large_files_support             | ON                           |
| local_infile                    | ON                           |
| locked_in_memory                | OFF                          |
| log                             | OFF                          |
| log_bin                         | OFF                          |
| log_slave_updates               | OFF                          |
| log_slow_queries                | OFF                          |
| log_update                      | OFF                          |
| log_warnings                    | 1                            |
| long_query_time                 | 10                           |
| low_priority_updates            | OFF                          |
| lower_case_table_names          | 0                            |
| max_allowed_packet              | 1047552                      |
| max_binlog_cache_size           | 4294967295                   |
| max_binlog_size                 | 1073741824                   |
| max_connect_errors              | 10                           |
| max_connections                 | 100                          |
| max_delayed_threads             | 20                           |
| max_error_count                 | 64                           |
| max_heap_table_size             | 16777216                     |
| max_join_size                   | 4294967295                   |
| max_relay_log_size              | 0                            |
| max_sort_length                 | 1024                         |
| max_tmp_tables                  | 32                           |
| max_user_connections            | 0                            |
| max_write_lock_count            | 4294967295                   |
| myisam_max_extra_sort_file_size | 268435456                    |
| myisam_max_sort_file_size       | 2147483647                   |
| myisam_recover_options          | force                        |
| myisam_repair_threads           | 1                            |
| myisam_sort_buffer_size         | 8388608                      |
| net_buffer_length               | 16384                        |
| net_read_timeout                | 30                           |
| net_retry_count                 | 10                           |
| net_write_timeout               | 60                           |
| open_files_limit                | 1024                         |
| pid_file                        | /usr/local/mysql/name.pid    |
| port                            | 3306                         |
| protocol_version                | 10                           |
| query_cache_limit               | 1048576                      |
| query_cache_size                | 0                            |
| query_cache_type                | ON                           |
| read_buffer_size                | 131072                       |
| read_rnd_buffer_size            | 262144                       |
| rpl_recovery_rank               | 0                            |
| server_id                       | 0                            |
| skip_external_locking           | ON                           |
| skip_networking                 | OFF                          |
| skip_show_database              | OFF                          |
| slave_net_timeout               | 3600                         |
| slow_launch_time                | 2                            |
| socket                          | /tmp/mysql.sock              |
| sort_buffer_size                | 2097116                      |
| sql_mode                        |                              |
| table_cache                     | 64                           |
| table_type                      | MYISAM                       |
| thread_cache_size               | 3                            |
| thread_stack                    | 131072                       |
| timezone                        | EEST                         |
| tmp_table_size                  | 33554432                     |
| tmpdir                          | /tmp/:/mnt/hd2/tmp/          |
| tx_isolation                    | READ-COMMITTED               |
| version                         | 4.0.4-beta                   |
| wait_timeout                    | 28800                        |
+---------------------------------+------------------------------+

Most system variables are described here. Variables with no version indicated have been present since at least MySQL 3.22. InnoDB system variables are listed at section 16.5 InnoDB Startup Options.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

Information on tuning these variables can be found in section 7.5.2 Tuning Server Parameters.

ansi_mode
This is ON if @command{mysqld} was started with --ansi. See section 1.8.3 Running MySQL in ANSI Mode. This variable was added in MySQL 3.23.6 and removed in 3.23.41. See the description for sql_mode.
back_log
The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time. In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen() system call should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set back_log higher than your operating system limit will be ineffective.
basedir
The MySQL installation base directory. This variable can be set with the --basedir option.
bdb_cache_size
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you don't use BDB tables, you should start @command{mysqld} with --skip-bdb to not waste memory for this cache. This variable was added in MySQL 3.23.14.
bdb_home
The base directory for BDB tables. This should be assigned the same value as the datadir variable. This variable was added in MySQL 3.23.14.
bdb_log_buffer_size
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you don't use BDB tables, you should set this to 0 or start @command{mysqld} with --skip-bdb to not waste memory for this cache. This variable was added in MySQL 3.23.31.
bdb_logdir
The directory where the BDB storage engine writes its log files. This variable can be set with the --bdb-logdir option. This variable was added in MySQL 3.23.14.
bdb_max_lock
The maximum number of locks you can have active on a BDB table (10,000 by default). You should increase this if errors such as the following occur when you perform long transactions or when @command{mysqld} has to examine many rows to calculate a query: 
bdb: Lock table is out of available locks
Got error 12 from ...
This variable was added in MySQL 3.23.29.
bdb_shared_data
This is ON if you are using --bdb-shared-data. This variable was added in MySQL 3.23.29.
bdb_tmpdir
The value of the --bdb-tmpdir option. This variable was added in MySQL 3.23.14.
bdb_version
See the description for version_bdb.
binlog_cache_size
The size of the cache to hold the SQL statements for the binary log during a transaction. A binary log cache is allocated for each client if the server supports any transactional storage engines and, starting from MySQL 4.1.2, if the server has binary log enabled (--log-bin option). If you often use big, multiple-statement transactions, you can increase this to get more performance. The Binlog_cache_use and Binlog_cache_disk_use status variables can be useful for tuning the size of this variable. This variable was added in MySQL 3.23.29. See section 5.9.4 The Binary Log.
bulk_insert_buffer_size
MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. Note: This cache is used only when adding data to a non-empty table. The default value is 8MB. This variable was added in MySQL 4.0.3. This variable previously was named myisam_bulk_insert_tree_size.
character_set
The default character set. This variable was added in MySQL 3.23.3, then removed in MySQL 4.1.1 and replaced by the various character_set_xxx variables.
character_set_client
The character set for statements that arrive from the client. This variable was added in MySQL 4.1.1.
character_set_connection
The character set used for literals that do not have a character set introducer and for number-to-string conversion. This variable was added in MySQL 4.1.1.
character_set_database
The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server. This variable was added in MySQL 4.1.1.
character_set_results
The character set used for returning query results to the client. This variable was added in MySQL 4.1.1.
character_set_server
The server default character set. This variable was added in MySQL 4.1.1.
character_set_system
The character set used by the server for storing identifiers. The value is always utf8. This variable was added in MySQL 4.1.1.
character_sets
The supported character sets. This variable was added in MySQL 3.23.15 and removed in MySQL 4.1.1. (Use SHOW CHARACTER SET for a list of character sets.)
character_sets_dir
The directory where character sets are installed. This variable was added in MySQL 4.1.2.
collation_connection
The collation of the connection character set. This variable was added in MySQL 4.1.1.
collation_database
The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server. This variable was added in MySQL 4.1.1.
collation_server
The server default collation. This variable was added in MySQL 4.1.1.
concurrent_insert
If ON (the default), MySQL allows INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle. You can turn this option off by starting @command{mysqld} with --safe or --skip-new. This variable was added in MySQL 3.23.7.
connect_timeout
The number of seconds the @command{mysqld} server waits for a connect packet before responding with Bad handshake.
convert_character_set
The current character set mapping that was set by SET CHARACTER SET. This variable was removed in MySQL 4.1.
datadir
The MySQL data directory. This variable can be set with the --datadir option.
default_week_format
The default mode value to use for the WEEK() function. This variable is available as of MySQL 4.0.14.
delay_key_write
This option applies only to MyISAM tables. It can have one of the following values to affect handling of the DELAY_KEY_WRITE table option that can be used in CREATE TABLE statements.
Option Description
OFF DELAYED_KEY_WRITE is ignored.
ON MySQL honors the DELAY_KEY_WRITE option for CREATE TABLE. This is the default value.
ALL All new opened tables are treated as if they were created with the DELAY_KEY_WRITE option enabled.
If DELAY_KEY_WRITE is enabled, this means that the key buffer for tables with this option are not flushed on every index update, but only when a table is closed. This will speed up writes on keys a lot, but if you use this feature, you should add automatic checking of all MyISAM tables by starting the server with the --myisam-recover option (for example, --myisam-recover=BACKUP,FORCE). See section 5.2.1 @command{mysqld} Command-Line Options and section 15.1.1 @code{MyISAM} Startup Options. Note that --external-locking doesn't offer any protection against index corruption for tables that use delayed key writes. This variable was added in MySQL 3.23.8.
delayed_insert_limit
After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it allows them to execute before continuing to insert delayed rows.
delayed_insert_timeout
How long an INSERT DELAYED handler thread should wait for INSERT statements before terminating.
delayed_queue_size
This is a per-table limit on the number of rows to queue when handling INSERT DELAYED statements. If the queue becomes full, any client that issues an INSERT DELAYED statement will wait until there is room in the queue again.
expire_logs_days
The number of days for automatic binary log removal. The default is 0, which means ``no automatic removal.'' This variable was added in MySQL 4.1.0.
flush
This is ON if you have started @command{mysqld} with the --flush option. This variable was added in MySQL 3.22.9.
flush_time
If this is set to a non-zero value, all tables will be closed every flush_time seconds to free up resources and sync unflushed data to disk. We recommend this option only on Windows 9x or Me, or on systems with minimal resources available. This variable was added in MySQL 3.22.18.
ft_boolean_syntax
The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. This variable was added in MySQL 4.0.1. See section 13.6.1 Boolean Full-Text Searches. The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:
  • Operator function is determined by position within the string.
  • The replacement value must be 14 characters.
  • Each character must be an ASCII non-alphanumeric character.
  • Either the first or second character must be a space.
  • No duplicates are allowed except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.
  • Positions 10, 13, and 14 (which by default are set to `:', `&', and `|') are reserved for future extensions.
ft_max_word_len
The maximum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0. Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
ft_min_word_len
The minimum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0. Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
ft_query_expansion_limit
The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION. This variable was added in MySQL 4.1.1.
ft_stopword_file
The file from which to read the list of stopwords for full-text searches. All the words from the file are used; comments are not honored. By default, a built-in list of stopwords is used (as defined in the `myisam/ft_static.c' file). Setting this variable to the empty string ('') disables stopword filtering. This variable was added in MySQL 4.0.10. Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
group_concat_max_len
The maximum allowed result length for the GROUP_CONCAT() function. This variable was added in MySQL 4.1.0.
have_archive
YES if @command{mysqld} supports ARCHIVE tables. This variable was added in MySQL 4.1.3.
have_bdb
YES if @command{mysqld} supports BDB tables. DISABLED if --skip-bdb is used. This variable was added in MySQL 3.23.30.
have_compress
Whether the zlib compression library is available to the server. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used. This variable was added in MySQL 4.1.1.
have_crypt
Whether the crypt() system call is available to the server. If not, the CRYPT() function cannot be used. This variable was added in MySQL 4.0.10.
have_geometry
Whether the server supports spatial data types. This variable was added in MySQL 4.1.3.
have_innodb
YES if @command{mysqld} supports InnoDB tables. DISABLED if --skip-innodb is used. This variable was added in MySQL 3.23.37.
have_isam
YES if @command{mysqld} supports ISAM tables. DISABLED if --skip-isam is used. This variable was added in MySQL 3.23.30.
have_ndbcluster
YES if @command{mysqld} supports NDB Cluster tables. DISABLED if --skip-ndbcluster is used. This variable was added in MySQL 4.1.2.
have_openssl
YES if @command{mysqld} supports SSL (encryption) of the client/server protocol. This variable was added in MySQL 3.23.43.
have_query_cache
YES if @command{mysqld} supports the query cache. This variable was added in MySQL 4.0.2.
have_raid
YES if @command{mysqld} supports the RAID option. This variable was added in MySQL 3.23.30.
have_rtree_keys
Whether RTREE indexes are available. (These are used for spatial indexed in MyISAM tables.) This variable was added in MySQL 4.1.3.
have_symlink
Whether symbolic link support is enabled. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options. This variable was added in MySQL 4.0.0.
init_connect
A string to be executed by the server for each client that connects. The string consists of one or more SQL statements. To specify multiple statements, separate them by semicolon characters. For example, each client begins by default with autocommit mode enabled. There is no global server variable to specify that autocommit should be disabled by default, but init_connect can be used to achieve the same effect: 
SET GLOBAL init_connect='SET AUTOCOMMIT=0';
This variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines: 
[mysqld]
init_connect='SET AUTOCOMMIT=0'
Note that the content of init_connect is not executed for users having the SUPER privilege; this is in case that content has been wrongly set (contains a wrong query, for example with a syntax error), thus making all connections fail. Not executing it for SUPER users enables those to open a connection and fix init_connect. This variable was added in MySQL 4.1.2.
init_file
The name of the file specified with the --init-file option when you start the server. This is a file containing SQL statements that you want the server to execute when it starts. Each statement must be on a single line and should not include comments. This variable was added in MySQL 3.23.2.
init_slave
This variable is similar to init_connect, but is a string to be executed by a slave server each time the SQL thread starts. The format of the string is the same as for the init_connect variable. This variable was added in MySQL 4.1.2.
innodb_xxx
The InnoDB system variables are listed at section 16.5 InnoDB Startup Options.
interactive_timeout
The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.
join_buffer_size
The size of the buffer that is used for full joins (joins that do not use indexes). Normally the best way to get fast joins is to add indexes. Increase the value of join_buffer_size to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.
key_buffer_size
Index blocks for MyISAM and ISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache. The maximum allowable setting for key_buffer_size is 4GB. The effective maximum size might be less, depending on your available physical RAM and per-process RAM limits imposed by your operating system or hardware platform. Increase the value to get better index handling (for all reads and multiple writes) to as much as you can afford. Using a value that is 25% of total memory on a machine that mainly runs MySQL is quite common. However, if you make the value too large (for example, more than 50% of your total memory) your system might start to page and become extremely slow. MySQL relies on the operating system to perform filesystem caching for data reads, so you must leave some room for the filesystem cache. For even more speed when writing many rows at the same time, use LOCK TABLES. See section 14.4.5 LOCK TABLES and UNLOCK TABLES Syntax. You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. See section 14.5.3 SET and SHOW Syntax. The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the DELAY_KEY_WRITE table option. The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_used status variable and the buffer block size. From MySQL 4.1.1 on, the buffer block size is available from the key_cache_block_size server variable. The fraction of the buffer in use is: 
(Key_blocks_used * key_cache_block_size) / key_buffer_size
Before MySQL 4.1.1, key cache blocks are 1024 bytes, so the fraction of the key buffer in use is: 
(Key_blocks_used * 1024) / key_buffer_size
See section 7.4.6 The MyISAM Key Cache.
key_cache_age_threshold
This value controls the demotion of buffers from the hot sub-chain of a key cache to the warm sub-chain. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. This variable was added in MySQL 4.1.1. See section 7.4.6 The MyISAM Key Cache.
key_cache_block_size
The size in bytes of blocks in the key cache. The default value is 1024. This variable was added in MySQL 4.1.1. See section 7.4.6 The MyISAM Key Cache.
key_cache_division_limit
The division point between the hot and warm sub-chains of the key cache buffer chain. The value is the percentage of the buffer chain to use for the warm sub-chain. Allowable values range from 1 to 100. The default value is 100. This variable was added in MySQL 4.1.1. See section 7.4.6 The MyISAM Key Cache.
language
The language used for error messages.
large_file_support
Whether @command{mysqld} was compiled with options for large file support. This variable was added in MySQL 3.23.28.
license
The type of license the server has. This variable was added in MySQL 4.0.19.
local_infile
Whether LOCAL is supported for LOAD DATA INFILE statements. This variable was added in MySQL 4.0.3.
locked_in_memory
Whether @command{mysqld} was locked in memory with --memlock. This variable was added in MySQL 3.23.25.
log
Whether logging of all queries to the general query log is enabled. See section 5.9.2 The General Query Log.
log_bin
Whether the binary log is enabled. This variable was added in MySQL 3.23.14. See section 5.9.4 The Binary Log.
log_error
The location of the error log. This variable was added in MySQL 4.0.10.
log_slave_updates
Whether updates received by a slave server from a master server should be logged to the slave's own binary log. Binary logging must be enabled on the slave for this to have any effect. This variable was added in MySQL 3.23.17. See section 6.8 Replication Startup Options.
log_slow_queries
Whether slow queries should be logged. ``Slow'' is determined by the value of the long_query_time variable. This variable was added in MySQL 4.0.2. See section 5.9.5 The Slow Query Log.
log_update
Whether the update log is enabled. This variable was added in MySQL 3.22.18. Note that the binary log is preferable to the update log, which is unavailable as of MySQL 5.0. See section 5.9.3 The Update Log.
log_warnings
Whether to produce additional warning messages. This variable was added in MySQL 4.0.3. It is enabled by default as of MySQL 4.0.19 and 4.1.2. As of MySQL 4.0.21 and 4.1.3, aborted connections are not logged to the error log unless the value is greater than 1.
long_query_time
If a query takes longer than this many seconds, the Slow_queries status variable is incremented. If you are using the --log-slow-queries option, the query is logged to the slow query log file. This value is measured in real time, not CPU time, so a query that is under the threshold on a lightly loaded system might be above the threshold on a heavily loaded one. See section 5.9.5 The Slow Query Log.
low_priority_updates
If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This variable previously was named sql_low_priority_updates. It was added in MySQL 3.22.5.
lower_case_file_system
This variable indicates whether the filesystem where the data directory is located has case insensitive filenames. ON means filenames are case insensitive, OFF means they are case sensitive. This variable was added in MySQL 4.0.19.
lower_case_table_names
If set to 1, table names are stored in lowercase on disk and table name comparisons are not case sensitive. This variable was added in MySQL 3.23.6. If set to 2 (new in 4.0.18), table names are stored as given but compared in lowercase. From MySQL 4.0.2, this option also applies to database names. From 4.1.1, it also applies to table aliases. See section 10.2.2 Identifier Case Sensitivity. You should not set this variable to 0 if you are running MySQL on a system that does not have case-sensitive filenames (such as Windows or Mac OS X). New in 4.0.18: If this variable is not set at startup and the filesystem on which the data directory is located does not have case-sensitive filenames, MySQL automatically sets lower_case_table_names to 2.
max_allowed_packet
The maximum size of one packet or any generated/intermediate string. The packet message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch big (possibly wrong) packets. You must increase this value if you are using big BLOB columns or long strings. It should be as big as the biggest BLOB you want to use. The protocol limit for max_allowed_packet is 16MB before MySQL 4.0 and 1GB thereafter.
max_binlog_cache_size
If a multiple-statement transaction requires more than this amount of memory, you will get the error Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage. This variable was added in MySQL 3.23.29.
max_binlog_size
If a write to the binary log exceeds the given value, rotate the binary logs. You cannot set this variable to more than 1GB or to less than 4096 bytes. (The minimum before MYSQL 4.0.14 is 1024 bytes.) The default value is 1GB. This variable was added in MySQL 3.23.33. Note if you are using transactions: A transaction is written in one chunk to the binary log, hence it is never split between several binary logs. Therefore, if you have big transactions, you might see binary logs bigger than max_binlog_size. If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well. max_relay_log_size was added in MySQL 4.0.14.
max_connect_errors
If there are more than this number of interrupted connections from a host, that host is blocked from further connections. You can unblock blocked hosts with the FLUSH HOSTS statement.
max_connections
The number of simultaneous client connections allowed. Increasing this value increases the number of file descriptors that @command{mysqld} requires. See section 7.4.8 How MySQL Opens and Closes Tables for comments on file descriptor limits. Also see section A.2.6 Too many connections.
max_delayed_threads
Don't start more than this number of threads to handle INSERT DELAYED statements. If you try to insert data into a new table after all INSERT DELAYED threads are in use, the row will be inserted as if the DELAYED attribute wasn't specified. If you set this to 0, MySQL never creates a thread to handle DELAYED rows; in effect, this disables DELAYED entirely. This variable was added in MySQL 3.23.0.
max_error_count
The maximum number of error, warning, and note messages to be stored for display by SHOW ERRORS or SHOW WARNINGS. This variable was added in MySQL 4.1.0.
max_heap_table_size
This variable sets the maximum size to which MEMORY (HEAP) tables are allowed to grow. The value of the variable is used to calculate MEMORY table MAX_ROWS values. Setting this variable has no effect on any existing MEMORY table, unless the table is re-created with a statement such as CREATE TABLE or TRUNCATE TABLE, or altered with ALTER TABLE. This variable was added in MySQL 3.23.0.
max_insert_delayed_threads
This variable is a synonym for max_delayed_threads. It was added in MySQL 4.0.19.
max_join_size
Don't allow SELECT statements that probably will need to examine more than max_join_size row combinations or are likely to do more than max_join_size disk seeks. By setting this value, you can catch SELECT statements where keys are not used properly and that would probably take a long time. Set it if your users tend to perform joins that lack a WHERE clause, that take a long time, or that return millions of rows. Setting this variable to a value other than DEFAULT resets the SQL_BIG_SELECTS value to 0. If you set the SQL_BIG_SELECTS value again, the max_join_size variable is ignored. If a query result already is in the query cache, no result size check is performed, because the result has already been computed and it does not burden the server to send it to the client. This variable previously was named sql_max_join_size.
max_length_for_sort_data
The cutoff on the size of index values that determines which filesort algorithm to use. See section 7.2.10 How MySQL Optimizes ORDER BY. This variable was added in MySQL 4.1.1
max_relay_log_size
If a write by a replication slave to its relay log exceeds the given value, rotate the relay log. This variable enables you to put different size constraints on relay logs and binary logs. However, setting the variable to 0 makes MySQL use max_binlog_size for both binary logs and relay logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. This variable was added in MySQL 4.0.14. See section 6.3 Replication Implementation Details.
max_seeks_for_key
Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer will assume that no more than this number of key seeks will be required when searching for matching rows in a table by scanning a key, regardless of the actual cardinality of the key (see section 14.5.3.12 SHOW INDEX Syntax). By setting this to a low value (100?), you can force MySQL to prefer keys instead of table scans. This variable was added in MySQL 4.0.14.
max_sort_length
The number of bytes to use when sorting BLOB or TEXT values. Only the first max_sort_length bytes of each value are used; the rest are ignored.
max_tmp_tables
The maximum number of temporary tables a client can keep open at the same time. (This option doesn't yet do anything.)
max_user_connections
The maximum number of simultaneous connections allowed to any given MySQL account. A value of 0 means ``no limit.'' This variable was added in MySQL 3.23.34.
max_write_lock_count
After this many write locks, allow some read locks to run in between. This variable was added in MySQL 3.23.7.
myisam_data_pointer_size
The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 8. The default value is 4. This variable was added in MySQL 4.1.2. See section A.2.11 The table is full.
myisam_max_extra_sort_file_size
If the temporary file used for fast MyISAM index creation would be larger than using the key cache by the amount specified here, prefer the key cache method. This is mainly used to force long character keys in large tables to use the slower key cache method to create the index. This variable was added in MySQL 3.23.37. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.
myisam_max_sort_file_size
The maximum size of the temporary file MySQL is allowed to use while re-creating a MyISAM index (during REPAIR TABLE, ALTER TABLE, or LOAD DATA INFILE). If the file size would be bigger than this value, the index will be created using the key cache instead, which is slower. This variable was added in MySQL 3.23.37. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.
myisam_recover_options
The value of the --myisam-recover option. This variable was added in MySQL 3.23.36.
myisam_repair_threads
If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process. The default value is 1. Note: Multi-threaded repair is still alpha quality code. This variable was added in MySQL 4.0.13.
myisam_sort_buffer_size
The buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE. This variable was added in MySQL 3.23.16.
named_pipe
On Windows, indicates whether the server supports connections over named pipes. This variable was added in MySQL 3.23.50.
net_buffer_length
The communication buffer is reset to this size between queries. This should not normally be changed, but if you have very little memory, you can set it to the expected length of SQL statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to max_allowed_packet bytes.
net_read_timeout
The number of seconds to wait for more data from a connection before aborting the read. When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout. This variable was added in MySQL 3.23.20.
net_retry_count
If a read on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads. This variable was added in MySQL 3.23.7.
net_write_timeout
The number of seconds to wait for a block to be written to a connection before aborting the write. See also net_read_timeout. This variable was added in MySQL 3.23.20.
new
This variable is used in MySQL 4.0 to turn on some 4.1 behaviors. This variable was added in MySQL 4.0.12.
old_passwords
Whether the server should use pre-4.1-style passwords for MySQL user accounts. This variable was added in MySQL 4.1.1.
open_files_limit
The number of files that the operating system allows @command{mysqld} to open. This is the real value allowed by the system and might be different from the value you gave @command{mysqld} as a startup option. The value is 0 on systems where MySQL can't change the number of open files. This variable was added in MySQL 3.23.20.
optimizer_prune_level
Controls the heuristics applied during query optimization to prune less-promising partial plans from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows retrieved by intermediate plans. This variable was added in MySQL 5.0.1.
optimizer_search_depth
The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value. If set to the maximum number of tables used in a query plus 2, the optimizer switches to the original algorithm used before MySQL 5.0.1 that performs an exhaustive search. This variable was added in MySQL 5.0.1.
pid_file
The pathname of the process ID (PID) file. This variable can be set with the --pid-file option. This variable was added in MySQL 3.23.23.
port
The port on which the server listens for TCP/IP connections. This variable can be set with the --port option.
preload_buffer_size
The size of the buffer that is allocated when preloading indexes. This variable was added in MySQL 4.1.1.
protocol_version
The version of the client/server protocol used by the MySQL server. This variable was added in MySQL 3.23.18.
query_alloc_block_size
The allocation size of memory blocks that are allocated for objects created during query parsing and execution. If you have problems with memory fragmentation, it might help to increase this a bit. This variable was added in MySQL 4.0.16.
query_cache_limit
Don't cache results that are bigger than this. The default value is 1MB. This variable was added in MySQL 4.0.1.
query_cache_min_res_unit
The minimum size for blocks allocated by the query cache. The default value is 4KB. Tuning information for this variable is given in section 5.11.3 Query Cache Configuration. This variable is present from MySQL 4.1.
query_cache_size
The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. Note that this amount of memory will be allocated even if query_cache_type is set to 0. This variable was added in MySQL 4.0.1.
query_cache_type
Set query cache type. Setting the GLOBAL value sets the type for all clients that connect thereafter. Individual clients can set the SESSION value to affect their own use of the query cache.
Option Description
0 or OFF Don't cache or retrieve results. Note that this will not deallocate the query cache buffer. To do that, you should set query_cache_size to 0.
1 or ON Cache all query results except for those that begin with SELECT SQL_NO_CACHE.
2 or DEMAND Cache results only for queries that begin with SELECT SQL_CACHE.
This variable was added in MySQL 4.0.3.
query_cache_wlock_invalidate
Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing queries for the table if the query results are present in the query cache. Setting this variable to 1 causes acquisition of a WRITE lock for a table to invalidate any queries in the query cache that refer to the table. This forces other clients that attempt to access the table to wait while the lock is in effect. This variable was added in MySQL 4.0.19.
query_prealloc_size
The size of the persistent buffer used for query parsing and execution. This buffer is not freed between queries. If you are running complex queries, a larger query_prealloc_size value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations. This variable was added in MySQL 4.0.16.
range_alloc_block_size
The size of blocks that are allocated when doing range optimization. This variable was added in MySQL 4.0.16.
read_buffer_size
Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you might want to increase this value. This variable was added in MySQL 4.0.3. Previously, it was named record_buffer.
read_only
When the variable is set to ON for a replication slave server, it causes the slave to allow no updates except from slave threads or from users with the SUPER privilege. This can be useful to ensure that a slave server accepts no updates from clients. This variable was added in MySQL 4.0.14.
relay_log_purge
Disables or enables automatic purging of relay logs as soon as they are not needed any more. The default value is 1 (enabled). This variable was added in MySQL 4.1.1.
read_rnd_buffer_size
When reading rows in sorted order after a sort, the rows are read through this buffer to avoid disk seeks. Setting the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries. This variable was added in MySQL 4.0.3. Previously, it was named record_rnd_buffer.
safe_show_database
Don't show databases for which the user has no database or table privileges. This can improve security if you're concerned about people being able to see what databases other users have. See also skip_show_database. This variable was removed in MySQL 4.0.5. Instead, use the SHOW DATABASES privilege to control access by MySQL accounts to database names.
secure_auth
If the MySQL server has been started with the --secure-auth option, it blocks connections from all accounts that have passwords stored in the old (pre-4.1) format. In that case, the value of this variable is ON, otherwise it is OFF. You should enable this option if you want to prevent all usage of passwords in old format (and hence insecure communication over the network). This variable was added in MySQL 4.1.1. Server startup will fail with an error if this option is enabled and the privilege tables are in pre-4.1 format. When used as a client-side option, the client refuses to connect to a server if the server requires a password in old format for the client account.
server_id
The value of the --server-id option. It is used for master and slave replication servers. This variable was added in MySQL 3.23.26.
shared_memory
Whether or not the server allows shared-memory connections. Currently, only Windows servers support this. This variable was added in MySQL 4.1.1.
shared_memory_base_name
Whether or not the server allows shared-memory connections. Currently, only Windows servers support this. This variable was added in MySQL 4.1.0.
skip_external_locking
This is OFF if @command{mysqld} uses external locking. This variable was added in MySQL 4.0.3. Previously, it was named skip_locking.
skip_networking
This is ON if the server allows only local (non-TCP/IP) connections. On Unix, local connections use a Unix socket file. On Windows, local connections use a named pipe. On NetWare, only TCP/IP connections are supported, so do not set this variable to ON. This variable was added in MySQL 3.22.23.
skip_show_database
This prevents people from using the SHOW DATABASES statement if they don't have the SHOW DATABASES privilege. This can improve security if you're concerned about people being able to see what databases other users have. See also safe_show_database. This variable was added in MySQL 3.23.4. As of MySQL 4.0.2, its effect also depends on the SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database.
slave_compressed_protocol
Whether to use compression of the slave/master protocol if both the slave and the master support it. This variable was added in MySQL 4.0.3.
slave_net_timeout
The number of seconds to wait for more data from a master/slave connection before aborting the read. This variable was added in MySQL 3.23.40.
slow_launch_time
If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable. This variable was added in MySQL 3.23.15.
socket
On Unix, this is the Unix socket file used for local client connections. On Windows, this is the name of the named pipe used for local client connections.
sort_buffer_size
Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster ORDER BY or GROUP BY operations. See section A.4.4 Where MySQL Stores Temporary Files.
sql_mode
The current server SQL mode. This variable was added in MySQL 3.23.41. It can be set dynamically as of MySQL 4.1.1. See section 5.2.2 The Server SQL Mode.
sql_slave_skip_counter
The number of events from the master that a slave server should skip. It was added in MySQL 3.23.33.
storage_engine
This variable is a synonym for table_type. It was added in MySQL 4.1.2.
sync_binlog
If positive, the MySQL server will synchronize its binary log to disk (fdatasync()) after every sync_binlog'th write to this binary log. Note that there is one write to the binary log per statement if in autocommit mode, and otherwise one write per transaction. The default value is 0 which does no sync'ing to disk. A value of 1 is the safest choice, because in case of crash you will lose at most one statement/transaction from the binary log; but it is also the slowest choice (unless the disk has a battery-backed cache, which makes sync'ing very fast). This variable was added in MySQL 4.1.3.
sync_frm
This was added as a command-line option in MySQL 4.0.18, and is also a settable global variable since MySQL 4.1.3. If set to 1, when a non-temporary table is created it will synchronize its `.frm' file to disk (fdatasync()); this is slower but safer in case of crash. Default is 1.
system_time_zone
The server system time zone. When the server starts, it attempts to determine the time zone of the host machine. For example, if the TZ environment variable is set, its value is used to set system_time_zone. This variable was added in MySQL 4.1.3.
table_cache
The number of open tables for all threads. Increasing this value increases the number of file descriptors that @command{mysqld} requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See section 5.2.4 Server Status Variables. If the value of Opened_tables is large and you don't do FLUSH TABLES a lot (which just forces all tables to be closed and reopened), then you should increase the value of the table_cache variable. For more information about the table cache, see section 7.4.8 How MySQL Opens and Closes Tables.
table_type
The default table type (storage engine). To set the table type at server startup, use the --default-table-type option. This variable was added in MySQL 3.23.0. See section 5.2.1 @command{mysqld} Command-Line Options.
thread_cache_size
How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there aren't already thread_cache_size threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. (Normally this doesn't give a notable performance improvement if you have a good thread implementation.) By examining the difference between the Connections and Threads_created status variables (see section 5.2.4 Server Status Variables for details) you can see how efficient the thread cache is. This variable was added in MySQL 3.23.16.
thread_concurrency
On Solaris, @command{mysqld} calls thr_setconcurrency() with this value. This function allows applications to give the threads system a hint about the desired number of threads that should be run at the same time. This variable was added in MySQL 3.23.7.
thread_stack
The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. The default is large enough for normal operation. See section 7.1.4 The MySQL Benchmark Suite.
time_zone
The current time zone. The initial value of this is 'SYSTEM' (use the value of system_time_zone), but can be specified explicitly at server startup time with the --default-time-zone option. This variable was added in MySQL 4.1.3.
timezone
The time zone for the server. This is set from the TZ environment variable when @command{mysqld} is started. The time zone also can be set by giving a --timezone argument to @command{mysqld_safe}. This variable was added in MySQL 3.23.15. As of MySQL 4.1.3, it is obsolete and has been replaced by the system_time_zone variable. See section A.4.6 Time Zone Problems.
tmp_table_size
If an in-memory temporary table exceeds this size, MySQL automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size if you do many advanced GROUP BY queries and you have lots of memory.
tmpdir
The directory used for temporary files and temporary tables. Starting from MySQL 4.1, this variable can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (`:') on Unix and semicolon characters (`;') on Windows, NetWare, and OS/2. This feature can be used to spread the load between several physical disks. If the MySQL server is acting as a replication slave, you should not set tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication will fail. This variable was added in MySQL 3.22.4.
transaction_alloc_block_size
The allocation size of memory blocks that are allocated for storing queries that are part of a transaction to be stored in the binary log when doing a commit. This variable was added in MySQL 4.0.16.
transaction_prealloc_size
The size of the persistent buffer for transaction_alloc_blocks that is not freed between queries. By making this big enough to fit all queries in a common transaction, you can avoid a lot of malloc() calls. This variable was added in MySQL 4.0.16.
tx_isolation
The default transaction isolation level. This variable was added in MySQL 4.0.3.
updatable_views_with_limit
This variable controls whether updates can be made using a view that does not contain a primary key in the underlying table, if the update contains a LIMIT clause. (Such updates often are generated by GUI tools.) An update is an UPDATE or DELETE statement. Primary key here means a PRIMARY KEY, or a UNIQUE index in which no column can contain NULL. The variable can have two values:
  • 1 or YES: Issue a warning only (not an error message). This is the default value.
  • 0 or NO: Prohibit the update.
This variable was added in MySQL 5.0.2.
version
The version number for the server.
version_bdb
The BDB storage engine version. This variable was added in MySQL 3.23.31 with the name bdb_version and renamed to version_bdb in MySQL 4.1.1.
version_comment
The @command{configure} script has a --with-comment option that allows a comment to be specified when building MySQL. This variable contains the value of that comment. This variable was added in MySQL 4.0.17.
version_compile_machine
The type of machine MySQL was built on. This variable was added in MySQL 4.1.1.
version_compile_os
The type of operating system MySQL was built on. This variable was added in MySQL 4.0.19.
wait_timeout
The number of seconds the server waits for activity on a non-interactive connection before closing it. On thread startup, the session wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.

5.2.3.1 Dynamic System Variables

Beginning with MySQL 4.0.3, many server system variables are dynamic and can be set at runtime using SET GLOBAL or SET SESSION. You can also select their values using SELECT. See section 10.4 System Variables.

The following table shows the full list of all dynamic system variables. The last column indicates for each variable whether GLOBAL or SESSION (or both) apply.

Variable Name Value Type Type
autocommit boolean SESSION
big_tables boolean SESSION
binlog_cache_size numeric GLOBAL
bulk_insert_buffer_size numeric GLOBAL | SESSION
character_set_client string GLOBAL | SESSION
character_set_connection string GLOBAL | SESSION
character_set_results string GLOBAL | SESSION
character_set_server string GLOBAL | SESSION
collation_connection string GLOBAL | SESSION
collation_server string GLOBAL | SESSION
concurrent_insert boolean GLOBAL
connect_timeout numeric GLOBAL
convert_character_set string GLOBAL | SESSION
default_week_format numeric GLOBAL | SESSION
delay_key_write OFF | ON | ALL GLOBAL
delayed_insert_limit numeric GLOBAL
delayed_insert_timeout numeric GLOBAL
delayed_queue_size numeric GLOBAL
error_count numeric SESSION
expire_logs_days numeric GLOBAL
flush boolean GLOBAL
flush_time numeric GLOBAL
foreign_key_checks boolean SESSION
ft_boolean_syntax numeric GLOBAL
group_concat_max_len numeric GLOBAL | SESSION
identity numeric SESSION
insert_id boolean SESSION
interactive_timeout numeric GLOBAL | SESSION
join_buffer_size numeric GLOBAL | SESSION
key_buffer_size numeric GLOBAL
last_insert_id numeric SESSION
local_infile boolean GLOBAL
log_warnings numeric GLOBAL
long_query_time numeric GLOBAL | SESSION
low_priority_updates boolean GLOBAL | SESSION
max_allowed_packet numeric GLOBAL | SESSION
max_binlog_cache_size numeric GLOBAL
max_binlog_size numeric GLOBAL
max_connect_errors numeric GLOBAL
max_connections numeric GLOBAL
max_delayed_threads numeric GLOBAL
max_error_count numeric GLOBAL | SESSION
max_heap_table_size numeric GLOBAL | SESSION
max_insert_delayed_threads numeric GLOBAL
max_join_size numeric GLOBAL | SESSION
max_relay_log_size numeric GLOBAL
max_seeks_for_key numeric GLOBAL | SESSION
max_sort_length numeric GLOBAL | SESSION
max_tmp_tables numeric GLOBAL | SESSION
max_user_connections numeric GLOBAL
max_write_lock_count numeric GLOBAL
myisam_data_pointer_size numeric GLOBAL
myisam_max_extra_sort_file_size numeric GLOBAL | SESSION
myisam_max_sort_file_size numeric GLOBAL | SESSION
myisam_repair_threads numeric GLOBAL | SESSION
myisam_sort_buffer_size numeric GLOBAL | SESSION
net_buffer_length numeric GLOBAL | SESSION
net_read_timeout numeric GLOBAL | SESSION
net_retry_count numeric GLOBAL | SESSION
net_write_timeout numeric GLOBAL | SESSION
old_passwords numeric GLOBAL | SESSION
optimizer_prune_level numeric GLOBAL | SESSION
optimizer_search_depth numeric GLOBAL | SESSION
preload_buffer_size numeric GLOBAL | SESSION
query_alloc_block_size numeric GLOBAL | SESSION
query_cache_limit numeric GLOBAL
query_cache_size numeric GLOBAL
query_cache_type enumeration GLOBAL | SESSION
query_cache_wlock_invalidate boolean GLOBAL | SESSION
query_prealloc_size numeric GLOBAL | SESSION
range_alloc_block_size numeric GLOBAL | SESSION
read_buffer_size numeric GLOBAL | SESSION
read_only numeric GLOBAL
read_rnd_buffer_size numeric GLOBAL | SESSION
rpl_recovery_rank numeric GLOBAL
safe_show_database boolean GLOBAL
secure_auth boolean GLOBAL
server_id numeric GLOBAL
slave_compressed_protocol boolean GLOBAL
slave_net_timeout numeric GLOBAL
slow_launch_time numeric GLOBAL
sort_buffer_size numeric GLOBAL | SESSION
sql_auto_is_null boolean SESSION
sql_big_selects boolean SESSION
sql_big_tables boolean SESSION
sql_buffer_result boolean SESSION
sql_log_bin boolean SESSION
sql_log_off boolean SESSION
sql_log_update boolean SESSION
sql_low_priority_updates boolean GLOBAL | SESSION
sql_max_join_size numeric GLOBAL | SESSION
sql_mode enumeration GLOBAL | SESSION
sql_quote_show_create boolean SESSION
sql_safe_updates boolean SESSION
sql_select_limit numeric SESSION
sql_slave_skip_counter numeric GLOBAL
updatable_views_with_limit enumeration GLOBAL | SESSION
sql_warnings boolean SESSION
sync_binlog numeric GLOBAL
sync_frm boolean GLOBAL
storage_engine enumeration GLOBAL | SESSION
table_cache numeric GLOBAL
table_type enumeration GLOBAL | SESSION
thread_cache_size numeric GLOBAL
time_zone string GLOBAL | SESSION
timestamp boolean SESSION
tmp_table_size enumeration GLOBAL | SESSION
transaction_alloc_block_size numeric GLOBAL | SESSION
transaction_prealloc_size numeric GLOBAL | SESSION
tx_isolation enumeration GLOBAL | SESSION
unique_checks boolean SESSION
wait_timeout numeric GLOBAL | SESSION
warning_count numeric SESSION

Variables that are marked as ``string'' take a string value. Variables that are marked as ``numeric'' take a numeric value. Variables that are marked as ``boolean'' can be set to 0, 1, ON or OFF. Variables that are marked as ``enumeration'' normally should be set to one of the available values for the variable, but can also be set to the number that corresponds to the desired enumeration value. For enumeration-valued system variables, the first enumeration value corresponds to 0. This differs from ENUM columns, for which the first enumeration value corresponds to 1.

5.2.4 Server Status Variables

The server maintains many status variables that provide information about its operations. You can view these variables and their values by using the SHOW STATUS statement: 

mysql> SHOW STATUS;
+--------------------------+------------+
| Variable_name            | Value      |
+--------------------------+------------+
| Aborted_clients          | 0          |
| Aborted_connects         | 0          |
| Bytes_received           | 155372598  |
| Bytes_sent               | 1176560426 |
| Connections              | 30023      |
| Created_tmp_disk_tables  | 0          |
| Created_tmp_files        | 60         |
| Created_tmp_tables       | 8340       |
| Delayed_errors           | 0          |
| Delayed_insert_threads   | 0          |
| Delayed_writes           | 0          |
| Flush_commands           | 1          |
| Handler_delete           | 462604     |
| Handler_read_first       | 105881     |
| Handler_read_key         | 27820558   |
| Handler_read_next        | 390681754  |
| Handler_read_prev        | 6022500    |
| Handler_read_rnd         | 30546748   |
| Handler_read_rnd_next    | 246216530  |
| Handler_update           | 16945404   |
| Handler_write            | 60356676   |
| Key_blocks_used          | 14955      |
| Key_read_requests        | 96854827   |
| Key_reads                | 162040     |
| Key_write_requests       | 7589728    |
| Key_writes               | 3813196    |
| Max_used_connections     | 0          |
| Not_flushed_delayed_rows | 0          |
| Not_flushed_key_blocks   | 0          |
| Open_files               | 2          |
| Open_streams             | 0          |
| Open_tables              | 1          |
| Opened_tables            | 44600      |
| Qcache_free_blocks       | 36         |
| Qcache_free_memory       | 138488     |
| Qcache_hits              | 79570      |
| Qcache_inserts           | 27087      |
| Qcache_lowmem_prunes     | 3114       |
| Qcache_not_cached        | 22989      |
| Qcache_queries_in_cache  | 415        |
| Qcache_total_blocks      | 912        |
| Questions                | 2026873    |
| Select_full_join         | 0          |
| Select_full_range_join   | 0          |
| Select_range             | 99646      |
| Select_range_check       | 0          |
| Select_scan              | 30802      |
| Slave_open_temp_tables   | 0          |
| Slave_running            | OFF        |
| Slow_launch_threads      | 0          |
| Slow_queries             | 0          |
| Sort_merge_passes        | 30         |
| Sort_range               | 500        |
| Sort_rows                | 30296250   |
| Sort_scan                | 4650       |
| Table_locks_immediate    | 1920382    |
| Table_locks_waited       | 0          |
| Threads_cached           | 0          |
| Threads_connected        | 1          |
| Threads_created          | 30022      |
| Threads_running          | 1          |
| Uptime                   | 80380      |
+--------------------------+------------+

Many status variables are reset to 0 by the FLUSH STATUS statement.

The status variables have the following meanings. The Com_xxx statement counter variables were added beginning with MySQL 3.23.47. The Qcache_xxx query cache variables were added beginning with MySQL 4.0.1. Otherwise, variables with no version indicated have been present since at least MySQL 3.22.

Aborted_clients
The number of connections that were aborted because the client died without closing the connection properly. See section A.2.10 Communication Errors and Aborted Connections.
Aborted_connects
The number of tries to connect to the MySQL server that failed. See section A.2.10 Communication Errors and Aborted Connections.
Binlog_cache_disk_use
The number of transactions that used the temporary binary log cache but that exceeded the value of binlog_cache_size and used a temporary file to store statements from the transaction. This variable was added in MySQL 4.1.2.
Binlog_cache_use
The number of transactions that used the temporary binary log cache. This variable was added in MySQL 4.1.2.
Bytes_received
The number of bytes received from all clients. This variable was added in MySQL 3.23.7.
Bytes_sent
The number of bytes sent to all clients. This variable was added in MySQL 3.23.7.
Com_xxx
The number of times each xxx statement has been executed. There is one status variable for each type of statement. For example, Com_delete and Com_insert count DELETE and INSERT statements.
Connections
The number of connection attempts (successful or not) to the MySQL server.
Created_tmp_disk_tables
The number of temporary tables on disk created automatically by the server while executing statements. This variable was added in MySQL 3.23.24.
Created_tmp_files
How many temporary files @command{mysqld} has created. This variable was added in MySQL 3.23.28.
Created_tmp_tables
The number of in-memory temporary tables created automatically by the server while executing statements. If Created_tmp_disk_tables is big, you may want to increase the tmp_table_size value to cause temporary tables to be memory-based instead of disk-based.
Delayed_errors
The number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).
Delayed_insert_threads
The number of INSERT DELAYED handler threads in use.
Delayed_writes
The number of INSERT DELAYED rows written.
Flush_commands
The number of executed FLUSH statements.
Handler_commit
The number of internal COMMIT statements. This variable was added in MySQL 4.0.2.
Handler_discover
The MySQL server can ask the NDB Cluster storage engine if it knows about a table with a given name. This is called discovery. Handler_discover indicates the number of time tables have been discovered. This variable was added in MySQL 4.1.2.
Handler_delete
The number of times a row was deleted from a table.
Handler_read_first
The number of times the first entry was read from an index. If this is high, it suggests that the server is doing a lot of full index scans; for example, SELECT col1 FROM foo, assuming that col1 is indexed.
Handler_read_key
The number of requests to read a row based on a key. If this is high, it is a good indication that your queries and tables are properly indexed.
Handler_read_next
The number of requests to read the next row in key order. This will be incremented if you are querying an index column with a range constraint or if you are doing an index scan.
Handler_read_prev
The number of requests to read the previous row in key order. This read method is mainly used to optimize ORDER BY ... DESC. This variable was added in MySQL 3.23.6.
Handler_read_rnd
The number of requests to read a row based on a fixed position. This will be high if you are doing a lot of queries that require sorting of the result. You probably have a lot of queries that require MySQL to scan whole tables or you have joins that don't use keys properly.
Handler_read_rnd_next
The number of requests to read the next row in the data file. This will be high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have.
Handler_rollback
The number of internal ROLLBACK statements. This variable was added in MySQL 4.0.2.
Handler_update
The number of requests to update a row in a table.
Handler_write
The number of requests to insert a row in a table.
Key_blocks_not_flushed
The number of key blocks in the key cache that have changed but haven't yet been flushed to disk. This variable was added in MySQL 4.1.1. It used to be known as Not_flushed_key_blocks.
Key_blocks_unused
This variable was added in MySQL 4.1.2. section 5.2.3 Server System Variables.
Key_blocks_used
The number of used blocks in the key cache. You can use this value to determine how much of the key cache is in use; see the discussion of key_buffer_size in section 5.2.3 Server System Variables.
Key_read_requests
The number of requests to read a key block from the cache.
Key_reads
The number of physical reads of a key block from disk. If Key_reads is big, then your key_buffer_size value is probably too small. The cache miss rate can be calculated as Key_reads/Key_read_requests.
Key_write_requests
The number of requests to write a key block to the cache.
Key_writes
The number of physical writes of a key block to disk.
Last_query_cost
The total cost of the last compiled query as computed by the query optimizer. Useful for comparing the cost of different query plans for the same query. The default value of -1 means that no query has been compiled yet. This variable was added in MySQL 5.0.1.
Max_used_connections
The maximum number of connections that have been in use simultaneously since the server started.
Not_flushed_delayed_rows
The number of rows waiting to be written in INSERT DELAY queues.
Not_flushed_key_blocks
The old name for Key_blocks_not_flushed before MySQL 4.1.1.
Open_files
The number of files that are open.
Open_streams
The number of streams that are open (used mainly for logging).
Open_tables
The number of tables that are open.
Opened_tables
The number of tables that have been opened. If Opened_tables is big, your table_cache value is probably too small.
Qcache_free_blocks
The number of free memory blocks in query cache.
Qcache_free_memory
The amount of free memory for query cache.
Qcache_hits
The number of cache hits.
Qcache_inserts
The number of queries added to the cache.
Qcache_lowmem_prunes
The number of queries that were deleted from the cache because of low memory.
Qcache_not_cached
The number of non-cached queries (not cachable, or due to query_cache_type).
Qcache_queries_in_cache
The number of queries registered in the cache.
Qcache_total_blocks
The total number of blocks in the query cache.
Questions
The number of queries that have been sent to the server.
Rpl_status
The status of failsafe replication (not yet implemented).
Select_full_join
The number of joins that do not use indexes. If this value is not 0, you should carefully check the indexes of your tables. This variable was added in MySQL 3.23.25.
Select_full_range_join
The number of joins that used a range search on a reference table. This variable was added in MySQL 3.23.25.
Select_range
The number of joins that used ranges on the first table. (It's normally not critical even if this is big.) This variable was added in MySQL 3.23.25.
Select_range_check
The number of joins without keys that check for key usage after each row. (If this is not 0, you should carefully check the indexes of your tables.) This variable was added in MySQL 3.23.25.
Select_scan
The number of joins that did a full scan of the first table. This variable was added in MySQL 3.23.25.
Slave_open_temp_tables
The number of temporary tables currently open by the slave SQL thread. This variable was added in MySQL 3.23.29.
Slave_running
This is ON if this server is a slave that is connected to a master. This variable was added in MySQL 3.23.16.
Slow_launch_threads
The number of threads that have taken more than slow_launch_time seconds to create. This variable was added in MySQL 3.23.15.
Slow_queries
The number of queries that have taken more than long_query_time seconds. See section 5.9.5 The Slow Query Log.
Sort_merge_passes
The number of merge passes the sort algorithm has had to do. If this value is large, you should consider increasing the value of the sort_buffer_size system variable. This variable was added in MySQL 3.23.28.
Sort_range
The number of sorts that were done with ranges. This variable was added in MySQL 3.23.25.
Sort_rows
The number of sorted rows. This variable was added in MySQL 3.23.25.
Sort_scan
The number of sorts that were done by scanning the table. This variable was added in MySQL 3.23.25.
Ssl_xxx
Variables used for SSL connections. These variables were added in MySQL 4.0.0.
Table_locks_immediate
The number of times that a table lock was acquired immediately. This variable was added as of MySQL 3.23.33.
Table_locks_waited
The number of times that a table lock could not be acquired immediately and a wait was needed. If this is high, and you have performance problems, you should first optimize your queries, and then either split your table or tables or use replication. This variable was added as of MySQL 3.23.33.
Threads_cached
The number of threads in the thread cache. This variable was added in MySQL 3.23.17.
Threads_connected
The number of currently open connections.
Threads_created
The number of threads created to handle connections. If Threads_created is big, you may want to increase the thread_cache_size value. The cache hit rate can be calculated as Threads_created/Connections. This variable was added in MySQL 3.23.31.
Threads_running
The number of threads that are not sleeping.
Uptime
The number of seconds the server has been up.

5.3 The MySQL Server Shutdown Process

The server shutdown process can be summarized like this:

  1. The shutdown process is initiated
  2. The server creates a shutdown thread if necessary
  3. The server stops accepting new connections
  4. The server terminates current activity
  5. Storage engines are shut down or closed
  6. The server exits

A more detailed description of the process follows:

  1. The shutdown process is initiated Server shutdown can be initiated several ways. For example, a user with the SHUTDOWN privilege can execute a @command{mysqladmin shutdown} command. @command{mysqladmin} can be used on any platform supported by MySQL. Other operating sytem-specific shutdown initiation methods are possible as well: The server shuts down on Unix when it receives a SIGTERM signal. A server running as a service on Windows shuts down when the services manager tells it to.
  2. The server creates a shutdown thread if necessary Depending on how shutdown was initiated, the server might create a thread to handle the shutdown process. If shutdown was requested by a client, a shutdown thread is created. If shutdown is the result of receiving a SIGTERM signal, the signal thread might handle shutdown itself, or it might create a separate thread to do so. If the server tries to create a shutdown thread and cannot (for example, if memory is exhausted), it issues a diagnostic message that will appear in the error log: 
    Error: Can't create thread to kill server
  3. The server stops accepting new connections To prevent new activity from being initiated during shutdown, the server stops accepting new client connections. It does this by closing the network connections to which it normally listens for connections: the TCP/IP port, the Unix socket file, the Windows named pipe.
  4. The server terminates current activity For each thread that is associated with a client connection, the connection to the client is broken and the thread is marked as killed. Threads die when they notice that they are so marked. Threads for idle connections die quickly. Threads that currently are processing queries check their state periodically and take longer to die. For additional information about thread termination, see section 14.5.4.3 KILL Syntax, in particular for the instructions about killed REPAIR TABLE or OPTIMIZE TABLE operations on MyISAM tables. For threads that have an open transaction, the tranaction is rolled back. Note that if a thread is updating a non-transactional table, an operation such as a multiple-row UPDATE or INSERT may leave the table partially updated, because the operation can terminate before completion. If the server is a master replication server, threads associated with currently connected slaves are treated like other client threads. That is, each one is marked as killed and exits when it next checks its state. If the server is a slave replication server, the I/O and SQL threads, if active, are stopped before client threads are marked as killed. The SQL thread is allowed to finish its current statement (to avoid causing replication problems) then stops. If the SQL thread was in the middle of a transaction at this point, the transaction is rolled back.
  5. Storage engines are shut down or closed At this stage, the table cache is flushed and all open tables are closed. Each storage engine performs any actions necessary for tables that it manages. For example, MyISAM flushes any pending index writes for a table. InnoDB flushes its buffer pool to disk, writes the current LSN to the tablespace, and terminates its own internal threads.
  6. The server exits

5.4 General Security Issues

This section describes some general security issues to be aware of and what you can do to make your MySQL installation more secure against attack or misuse. For information specifically about the access control system that MySQL uses for setting up user accounts and checking database access, see section 5.5 The MySQL Access Privilege System.

5.4.1 General Security Guidelines

Anyone using MySQL on a computer connected to the Internet should read this section to avoid the most common security mistakes.

In discussing security, we emphasize the necessity of fully protecting the entire server host (not just the MySQL server) against all types of applicable attacks: eavesdropping, altering, playback, and denial of service. We do not cover all aspects of availability and fault tolerance here.

MySQL uses security based on Access Control Lists (ACLs) for all connections, queries, and other operations that users can attempt to perform. There is also some support for SSL-encrypted connections between MySQL clients and servers. Many of the concepts discussed here are not specific to MySQL at all; the same general ideas apply to almost all applications.

When running MySQL, follow these guidelines whenever possible:

5.4.2 Making MySQL Secure Against Attackers

When you connect to a MySQL server, you should use a password. The password is not transmitted in clear text over the connection. Password handling during the client connection sequence was upgraded in MySQL 4.1.1 to be very secure. If you are using an older version of MySQL, or are still using pre-4.1.1-style passwords, the encryption algorithm is less strong and with some effort a clever attacker who can sniff the traffic between the client and the server can crack the password. (See section 5.5.9 Password Hashing in MySQL 4.1 for a discussion of the different password handling methods.) If the connection between the client and the server goes through an untrusted network, you should use an SSH tunnel to encrypt the communication.

All other information is transferred as text that can be read by anyone who is able to watch the connection. If you are concerned about this, you can use the compressed protocol (in MySQL 3.22 and above) to make traffic much more difficult to decipher. To make the connection even more secure, you should use SSH to get an encrypted TCP/IP connection between a MySQL server and a MySQL client. You can find an Open Source SSH client at http://www.openssh.org/, and a commercial SSH client at http://www.ssh.com/.

If you are using MySQL 4.0 or newer, you can also use internal OpenSSL support. See section 5.6.7 Using Secure Connections.

To make a MySQL system secure, you should strongly consider the following suggestions:

5.4.3 Startup Options for @command{mysqld} Concerning Security

The following @command{mysqld} options affect security:

--local-infile[={0|1}]
If you start the server with --local-infile=0, clients cannot use LOCAL in LOAD DATA statements. See section 5.4.4 Security Issues with LOAD DATA LOCAL.
--safe-show-database
With this option, the SHOW DATABASES statement displays the names of only those databases for which the user has some kind of privilege. As of MySQL 4.0.2, this option is deprecated and doesn't do anything (it is enabled by default), because there is now a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis. See section 14.5.1.2 GRANT and REVOKE Syntax.
--safe-user-create
If this is enabled, a user cannot create new users with the GRANT statement unless the user has the INSERT privilege for the mysql.user table. If you want a user to have the ability to create new users with those privileges that the user has right to grant, you should grant the user the following privilege: 
mysql> GRANT INSERT(user) ON mysql.user TO 'user_name'@'host_name';
This will ensure that the user can't change any privilege columns directly, but has to use the GRANT statement to give privileges to other users.
--secure-auth
Disallow authentication for accounts that have old (pre-4.1) passwords. This option is available as of MySQL 4.1.1.
--skip-grant-tables
This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing a @command{mysqladmin flush-privileges} or @command{mysqladmin reload} command, or by issuing a FLUSH PRIVILEGES statement.)
--skip-name-resolve
Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost.
--skip-networking
Don't allow TCP/IP connections over the network. All connections to @command{mysqld} must be made via Unix socket files. This option is unsuitable when using a MySQL version prior to 3.23.27 with the MIT-pthreads package, because Unix socket files were not supported by MIT-pthreads at that time.
--skip-show-database
With this option, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. Without this option, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database.

5.4.4 Security Issues with LOAD DATA LOCAL

The LOAD DATA statement can load a file that is located on the server host, or it can load a file that is located on the client host when the LOCAL keyword is specified.

There are two potential security issues with supporting the LOCAL version of LOAD DATA statements:

To deal with these problems, we changed how LOAD DATA LOCAL is handled as of MySQL 3.23.49 and MySQL 4.0.2 (4.0.13 on Windows):

5.5 The MySQL Access Privilege System

MySQL has an advanced but non-standard security/privilege system. This section describes how it works.

5.5.1 What the Privilege System Does

The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as SELECT, INSERT, UPDATE, and DELETE.

Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.

5.5.2 How the Privilege System Works

The MySQL privilege system ensures that all users may perform only the operations allowed to them. As a user, when you connect to a MySQL server, your identity is determined by the host from which you connect and the username you specify. The system grants privileges according to your identity and what you want to do.

MySQL considers both your hostname and username in identifying you because there is little reason to assume that a given username belongs to the same person everywhere on the Internet. For example, the user joe who connects from office.com need not be the same person as the user joe who connects from elsewhere.com. MySQL handles this by allowing you to distinguish users on different hosts that happen to have the same name: You can grant joe one set of privileges for connections from office.com, and a different set of privileges for connections from elsewhere.com.

MySQL access control involves two stages:

If your privileges are changed (either by yourself or someone else) while you are connected, those changes will not necessarily take effect immediately for the next statement you issue. See section 5.5.7 When Privilege Changes Take Effect for details.

The server stores privilege information in the grant tables of the mysql database (that is, in the database named mysql). The MySQL server reads the contents of these tables into memory when it starts and re-reads them under the circumstances indicated in section 5.5.7 When Privilege Changes Take Effect. Access-control decisions are based on the in-memory copies of the grant tables.

Normally, you manipulate the contents of the grant tables indirectly by using the GRANT and REVOKE statements to set up accounts and control the privileges available to each one. See section 14.5.1.2 GRANT and REVOKE Syntax. The discussion here describes the underlying structure of the grant tables and how the server uses their contents when interacting with clients.

The server uses the user, db, and host tables in the mysql database at both stages of access control. The columns in these grant tables are shown here:

Table Name user db host
Scope columns Host Host Host
User Db Db
Password User
Privilege columns Select_priv Select_priv Select_priv
Insert_priv Insert_priv Insert_priv
Update_priv Update_priv Update_priv
Delete_priv Delete_priv Delete_priv
Index_priv Index_priv Index_priv
Alter_priv Alter_priv Alter_priv
Create_priv Create_priv Create_priv
Drop_priv Drop_priv Drop_priv
Grant_priv Grant_priv Grant_priv
Create_view_priv Create_view_priv Create_view_priv
Show_view_priv Show_view_priv Show_view_priv
References_priv References_priv References_priv
Reload_priv
Shutdown_priv
Process_priv
File_priv
Show_db_priv
Super_priv
Create_tmp_table_priv Create_tmp_table_priv Create_tmp_table_priv
Lock_tables_priv Lock_tables_priv Lock_tables_priv
Execute_priv
Repl_slave_priv
Repl_client_priv
ssl_type
ssl_cipher
x509_issuer
x509_subject
max_questions
max_updates
max_connections

The ssl_type, ssl_cipher, x509_issuer, and x509_subject columns were added in MySQL 4.0.0.

The Create_tmp_table_priv, Execute_priv, Lock_tables_priv, Repl_client_priv, Repl_slave_priv, Show_db_priv, Super_priv, max_questions, max_updates, and max_connections columns were added in MySQL 4.0.2.

The Create_view_priv and Show_view_priv columns were added in MySQL 5.0.1.

During the second stage of access control (request verification), the server may, if the request involves tables, additionally consult the tables_priv and columns_priv tables that provide finer control at the table and column levels. The columns in these tables are shown here:

Table Name tables_priv columns_priv
Scope columns Host Host
Db Db
User User
Table_name Table_name
Column_name
Privilege columns Table_priv Column_priv
Column_priv
Other columns Timestamp Timestamp
Grantor

The Timestamp and Grantor columns currently are unused and are discussed no further here.

Each grant table contains scope columns and privilege columns:

Scope columns contain strings. They are declared as shown here; the default value for each is the empty string:

Column Name Type
Host CHAR(60)
User CHAR(16)
Password CHAR(16)
Db CHAR(64)
Table_name CHAR(60)
Column_name CHAR(60)

Before MySQL 3.23, the Db column is CHAR(32) in some tables and CHAR(60) in others.

For access-checking purposes, comparisons of Host values are case-insensitive. User, Password, Db, and Table_name values are case sensitive. Column_name values are case insensitive in MySQL 3.22.12 or later.

In the user, db, and host tables, each privilege is listed in a separate column that is declared as ENUM('N','Y') DEFAULT 'N'. In other words, each privilege can disabled or enabled, with the default being disabled.

In the tables_priv and columns_priv tables, the privilege columns are declared as SET columns. Values in these columns can contain any combination of the privileges controlled by the table:

Table Name Column Name Possible Set Elements
tables_priv Table_priv 'Select', 'Insert', 'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'
tables_priv Column_priv 'Select', 'Insert', 'Update', 'References'
columns_priv Column_priv 'Select', 'Insert', 'Update', 'References'

Briefly, the server uses the grant tables as follows:

Administrative privileges (such as RELOAD or SHUTDOWN) are specified only in the user table. This is because administrative operations are operations on the server itself and are not database-specific, so there is no reason to list these privileges in the other grant tables. In fact, to determine whether you can perform an administrative operation, the server need consult only the user table.

The FILE privilege also is specified only in the user table. It is not an administrative privilege as such, but your ability to read or write files on the server host is independent of the database you are accessing.

The @command{mysqld} server reads the contents of the grant tables into memory when it starts. You can tell it to re-read the tables by issuing a FLUSH PRIVILEGES statement or executing a @command{mysqladmin flush-privileges} or @command{mysqladmin reload} command. Changes to the grant tables take effect as indicated in section 5.5.7 When Privilege Changes Take Effect.

When you modify the contents of the grant tables, it is a good idea to make sure that your changes set up privileges the way you want. One way to check the privileges for a given account is to use the SHOW GRANTS statement. For example, to determine the privileges that are granted to an account with Host and User values of pc84.example.com and bob, issue this statement: 

mysql> SHOW GRANTS FOR 'bob'@'pc84.example.com';

A useful diagnostic tool is the @command{mysqlaccess} script, which Yves Carlier has provided for the MySQL distribution. Invoke @command{mysqlaccess} with the --help option to find out how it works. Note that @command{mysqlaccess} checks access using only the user, db, and host tables. It does not check table or column privileges specified in the tables_priv or columns_priv tables.

For additional help in diagnosing privilege-related problems, see section 5.5.8 Causes of Access denied Errors. For general advice on security issues, see section 5.4 General Security Issues.

5.5.3 Privileges Provided by MySQL

Information about account privileges is stored in the user, db, host, tables_priv, and columns_priv tables in the mysql database. The MySQL server reads the contents of these tables into memory when it starts and re-reads them under the circumstances indicated in section 5.5.7 When Privilege Changes Take Effect. Access-control decisions are based on the in-memory copies of the grant tables.

The names used in this manual to refer to the privileges provided by MySQL are shown in the following table, along with the table column name associated with each privilege in the grant tables and the context in which the privilege applies. Further information about the meaning of each privilege may be found at section 14.5.1.2 GRANT and REVOKE Syntax.

Privilege Column Context
ALTER Alter_priv tables
DELETE Delete_priv tables
INDEX Index_priv tables
INSERT Insert_priv tables
SELECT Select_priv tables
UPDATE Update_priv tables
CREATE Create_priv databases, tables, or indexes
DROP Drop_priv databases or tables
GRANT Grant_priv databases or tables
REFERENCES References_priv databases or tables
CREATE TEMPORARY TABLES Create_tmp_table_priv server administration
EXECUTE Execute_priv server administration
FILE File_priv file access on server host
LOCK TABLES Lock_tables_priv server administration
PROCESS Process_priv server administration
RELOAD Reload_priv server administration
REPLICATION CLIENT Repl_client_priv server administration
REPLICATION SLAVE Repl_slave_priv server administration
SHOW DATABASES Show_db_priv server administration
SHUTDOWN Shutdown_priv server administration
SUPER Super_priv server administration

The CREATE TEMPORARY TABLES, EXECUTE, LOCK TABLES, REPLICATION CLIENT, REPLICATION SLAVE, SHOW DATABASES, and SUPER privileges were added in MySQL 4.0.2.

The EXECUTE and REFERENCES privileges currently are unused.

The SELECT, INSERT, UPDATE, and DELETE privileges allow you to perform operations on rows in existing tables in a database.

SELECT statements require the SELECT privilege only if they actually retrieve rows from a table. Some SELECT statements do not access tables and can be executed without permission for any database. For example, you can use the @command{mysql} client as a simple calculator to evaluate expressions that make no reference to tables: 

mysql> SELECT 1+1;
mysql> SELECT PI()*2;

The CREATE and DROP privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables. If you grant the DROP privilege for the mysql database to a user, that user can drop the database in which the MySQL access privileges are stored!

The INDEX privilege allows you to create or drop (remove) indexes. INDEX applies to existing tables. If you have the CREATE privilege for a table, you can include index definitions in the CREATE TABLE statement.

The ALTER privilege allows you to use ALTER TABLE to change the structure of or rename tables.

The GRANT privilege allows you to give to other users those privileges that you yourself possess.

The FILE privilege gives you permission to read and write files on the server host using the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements. A user who has the FILE privilege can read any file on the server host that is either world-readable or readable by the MySQL server. (This implies the user can read any file in any database directory, because the server can access any of those files.) The FILE privilege also allows the user to create new files in any directory where the MySQL server has write access. Existing files cannot be overwritten.

The remaining privileges are used for administrative operations. Many of them can be performed by using the @command{mysqladmin} program or by issuing SQL statements. The following table shows which @command{mysqladmin} commands each administrative privilege allows you to execute:

Privilege Commands Permitted to Privilege Holders
RELOAD flush-hosts, flush-logs, flush-privileges, flush-status, flush-tables, flush-threads, refresh, reload
SHUTDOWN shutdown
PROCESS processlist
SUPER kill

The reload command tells the server to re-read the grant tables into memory. flush-privileges is a synonym for reload. The refresh command closes and reopens the log files and flushes all tables. The other flush-xxx commands perform functions similar to refresh, but are more specific and may be preferable in some instances. For example, if you want to flush just the log files, flush-logs is a better choice than refresh.

The shutdown command shuts down the server. This command can be issued only from @command{mysqladmin}. There is no corresponding SQL statement.

The processlist command displays information about the threads executing within the server (that is, about the statements being executed by clients associated with other accounts). The kill command terminates server threads. You can always display or kill your own threads, but you need the PROCESS privilege to display threads initiated by other users and the SUPER privilege to kill them. See section 14.5.4.3 KILL Syntax. Prior to MySQL 4.0.2 when SUPER was introduced, the PROCESS privilege controls the ability to both see and terminate threads for other clients.

The CREATE TEMPORARY TABLES privilege allows the use of the keyword TEMPORARY in CREATE TABLE statements.

The LOCK TABLES privilege allows the use of explicit LOCK TABLES statements to lock tables for which you have the SELECT privilege. This includes the use of write locks, which prevents anyone else from reading the locked table.

The REPLICATION CLIENT privilege allows the use of SHOW MASTER STATUS and SHOW SLAVE STATUS.

The REPLICATION SLAVE privilege should be granted to accounts that are used by slave servers when they connect to the current server as their master. Without this privilege, the slave cannot request updates that have been made to databases on the master server.

The SHOW DATABASES privilege allows the account to see database names by issuing the SHOW DATABASE statement. Accounts that do not have this privilege see only databases for which they have some privileges, and cannot use the statement at all if the server was started with the --skip-show-database option.

It is a good idea in general to grant privileges to only those accounts that need them, but you should exercise particular caution in granting administrative privileges:

There are some things that you cannot do with the MySQL privilege system:

5.5.4 Connecting to the MySQL Server

MySQL client programs generally expect you to specify connection parameters when you want to access a MySQL server:

For example, the @command{mysql} client can be started as follows from a command-line prompt (indicated here by shell>): 

shell> mysql -h host_name -u user_name -pyour_pass

Alternate forms of the -h, -u, and -p options are --host=host_name, --user=user_name, and --password=your_pass. Note that there is no space between -p or --password= and the password following it.

If you use a -p or --password option but do not specify the password value, the client program will prompt you to enter the password. The password is not displayed as you enter it. This is more secure than giving the password on the command line. Any user on your system may be able to see a password specified on the command line by executing a command such as @command{ps auxww}. See section 5.6.6 Keeping Your Password Secure.

MySQL client programs use default values for any connection parameter option that you do not specify:

Thus, for a Unix user with a login name of joe, all of the following commands are equivalent: 

shell> mysql -h localhost -u joe
shell> mysql -h localhost
shell> mysql -u joe
shell> mysql

Other MySQL clients behave similarly.

You can specify different default values to be used when you make a connection so that you need not enter them on the command line each time you invoke a client program. This can be done in a couple of ways:

5.5.5 Access Control, Stage 1: Connection Verification

When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, then enters Stage 2 and waits for requests.

Your identity is based on two pieces of information:

Identity checking is performed using the three user table scope columns (Host, User, and Password). The server accepts the connection only if the Host and User columns in some user table record match the client hostname and username, and the client supplies the password specified in that record.

Host values in the user table may be specified as follows:

Because you can use IP wildcard values in the Host column (for example, '144.155.166.%' to match every host on a subnet), someone could try to exploit this capability by naming a host 144.155.166.somewhere.com. To foil such attempts, MySQL disallows matching on hostnames that start with digits and a dot. Thus, if you have a host named something like 1.2.foo.com, its name will never match the Host column of the grant tables. An IP wildcard value can match only IP numbers, not hostnames.

In the User column, wildcard characters are not allowed, but you can specify a blank value, which matches any name. If the user table entry that matches an incoming connection has a blank username, the user is considered to be an anonymous user with no name, not a user with the name that the client actually specified. This means that a blank username is used for all further access checking for the duration of the connection (that is, during Stage 2).

The Password column can be blank. This is not a wildcard and does not mean that any password matches. It means that the user must connect without specifying a password.

Non-blank Password values in the user table represent encrypted passwords. MySQL does not store passwords in plaintext form for anyone to see. Rather, the password supplied by a user who is attempting to connect is encrypted (using the PASSWORD() function). The encrypted password then is used during the connection process when checking whether the password is correct. (This is done without the encrypted password ever traveling over the connection.) From MySQL's point of view, the encrypted password is the REAL password, so you should not give anyone access to it! In particular, don't give non-administrative users read access to the tables in the mysql database!

From version 4.1 on, MySQL employs a stronger authentication method that has better password protection during the connection process than in earlier versions. It is secure even if TCP/IP packets are sniffed or the mysql database is captured. Password encryption is discussed further in section 5.5.9 Password Hashing in MySQL 4.1.

The following examples show how various combinations of Host and User values in the user table apply to incoming connections:

Host Value User Value Connections Matched by Entry
'thomas.loc.gov' 'fred' fred, connecting from thomas.loc.gov
'thomas.loc.gov' '' Any user, connecting from thomas.loc.gov
'%' 'fred' fred, connecting from any host
'%' '' Any user, connecting from any host
'%.loc.gov' 'fred' fred, connecting from any host in the loc.gov domain
'x.y.%' 'fred' fred, connecting from x.y.net, x.y.com, x.y.edu, and so on. (this is probably not useful)
'144.155.166.177' 'fred' fred, connecting from the host with IP address 144.155.166.177
'144.155.166.%' 'fred' fred, connecting from any host in the 144.155.166 class C subnet
'144.155.166.0/255.255.255.0' 'fred' Same as previous example

It is possible for the client hostname and username of an incoming connection to match more than one entry in the user table. The preceding set of examples demonstrates this: Several of the entries shown match a connection from thomas.loc.gov by fred.

When multiple matches are possible, the server must determine which of them to use. It resolves this issue as follows:

To see how this works, suppose that the user table looks like this: 

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| %         | root     | ...
| %         | jeffrey  | ...
| localhost | root     | ...
| localhost |          | ...
+-----------+----------+-

When the server reads in the table, it orders the entries with the most-specific Host values first. Literal hostnames and IP numbers are the most specific. The pattern '%' means ``any host'' and is least specific. Entries with the same Host value are ordered with the most-specific User values first (a blank User value means ``any user'' and is least specific). For the user table just shown, the result after sorting looks like this: 

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| localhost | root     | ...
| localhost |          | ...
| %         | jeffrey  | ...
| %         | root     | ...
+-----------+----------+-

When a client attempts to connect, the server looks through the sorted entries and uses the first match found. For a connection from localhost by jeffrey, two of the entries in the table match: the one with Host and User values of 'localhost' and '', and the one with values of '%' and 'jeffrey'. The 'localhost' entry appears first in sorted order, so that is the one the server uses.

Here is another example. Suppose that the user table looks like this: 

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| %              | jeffrey  | ...
| thomas.loc.gov |          | ...
+----------------+----------+-

The sorted table looks like this: 

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| thomas.loc.gov |          | ...
| %              | jeffrey  | ...
+----------------+----------+-

A connection by jeffrey from thomas.loc.gov is matched by the first entry, whereas a connection by jeffrey from whitehouse.gov is matched by the second.

It is a common misconception to think that, for a given username, all entries that explicitly name that user will be used first when the server attempts to find a match for the connection. This is simply not true. The previous example illustrates this, where a connection from thomas.loc.gov by jeffrey is first matched not by the entry containing 'jeffrey' as the User column value, but by the entry with no username! As a result, jeffrey will be authenticated as an anonymous user, even though he specified a username when connecting.

If you are able to connect to the server, but your privileges are not what you expect, you probably are being authenticated as some other account. To find out what account the server used to authenticate you, use the CURRENT_USER() function. It returns a value in user_name@host_name format that indicates the User and Host values from the matching user table record. Suppose that jeffrey connects and issues the following query: 

mysql> SELECT CURRENT_USER();
+----------------+
| CURRENT_USER() |
+----------------+
| @localhost     |
+----------------+

The result shown here indicates that the matching user table entry had a blank User column value. In other words, the server is treating jeffrey as an anonymous user.

The CURRENT_USER() function is available as of MySQL 4.0.6. See section 13.8.3 Information Functions. Another thing you can do to diagnose authentication problems is to print out the user table and sort it by hand to see where the first match is being made.

5.5.6 Access Control, Stage 2: Request Verification

Once you establish a connection, the server enters Stage 2 of access control. For each request that comes in on the connection, the server determines what operation you want to perform, then checks whether you have sufficient privileges to do so. This is where the privilege columns in the grant tables come into play. These privileges can come from any of the user, db, host, tables_priv, or columns_priv tables. (You may find it helpful to refer to section 5.5.2 How the Privilege System Works, which lists the columns present in each of the grant tables.)

The user table grants privileges that are assigned to you on a global basis and that apply no matter what the current database is. For example, if the user table grants you the DELETE privilege, you can delete rows from any table in any database on the server host! In other words, user table privileges are superuser privileges. It is wise to grant privileges in the user table only to superusers such as database administrators. For other users, you should leave the privileges in the user table set to 'N' and grant privileges at more specific levels only. You can grant privileges for particular databases, tables, or columns.

The db and host tables grant database-specific privileges. Values in the scope columns of these tables can take the following forms:

The server reads in and sorts the db and host tables at the same time that it reads the user table. The server sorts the db table based on the Host, Db, and User scope columns, and sorts the host table based on the Host and Db scope columns. As with the user table, sorting puts the most-specific values first and least-specific values last, and when the server looks for matching entries, it uses the first match that it finds.

The tables_priv and columns_priv tables grant table-specific and column-specific privileges. Values in the scope columns of these tables can take the following form:

The server sorts the tables_priv and columns_priv tables based on the Host, Db, and User columns. This is similar to db table sorting, but simpler because only the Host column can contain wildcards.

The request verification process is described here. (If you are familiar with the access-checking source code, you will notice that the description here differs slightly from the algorithm used in the code. The description is equivalent to what the code actually does; it differs only to make the explanation simpler.)

For requests that require administrative privileges such as SHUTDOWN or RELOAD, the server checks only the user table entry because that is the only table that specifies administrative privileges. Access is granted if the entry allows the requested operation and denied otherwise. For example, if you want to execute @command{mysqladmin shutdown} but your user table entry doesn't grant the SHUTDOWN privilege to you, the server denies access without even checking the db or host tables. (They contain no Shutdown_priv column, so there is no need to do so.)

For database-related requests (INSERT, UPDATE, and so on), the server first checks the user's global (superuser) privileges by looking in the user table entry. If the entry allows the requested operation, access is granted. If the global privileges in the user table are insufficient, the server determines the user's database-specific privileges by checking the db and host tables:

  1. The server looks in the db table for a match on the Host, Db, and User columns. The Host and User columns are matched to the connecting user's hostname and MySQL username. The Db column is matched to the database that the user wants to access. If there is no entry for the Host and User, access is denied.
  2. If there is a matching db table entry and its Host column is not blank, that entry defines the user's database-specific privileges.
  3. If the matching db table entry's Host column is blank, it signifies that the host table enumerates which hosts should be allowed access to the database. In this case, a further lookup is done in the host table to find a match on the Host and Db columns. If no host table entry matches, access is denied. If there is a match, the user's database-specific privileges are computed as the intersection (not the union!) of the privileges in the db and host table entries; that is, the privileges that are 'Y' in both entries. (This way you can grant general privileges in the db table entry and then selectively restrict them on a host-by-host basis using the host table entries.)

After determining the database-specific privileges granted by the db and host table entries, the server adds them to the global privileges granted by the user table. If the result allows the requested operation, access is granted. Otherwise, the server successively checks the user's table and column privileges in the tables_priv and columns_priv tables, adds those to the user's privileges, and allows or denies access based on the result.

Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarized like this: 

global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges

It may not be apparent why, if the global user entry privileges are initially found to be insufficient for the requested operation, the server adds those privileges to the database, table, and column privileges later. The reason is that a request might require more than one type of privilege. For example, if you execute an INSERT INTO ... SELECT statement, you need both the INSERT and the SELECT privileges. Your privileges might be such that the user table entry grants one privilege and the db table entry grants the other. In this case, you have the necessary privileges to perform the request, but the server cannot tell that from either table by itself; the privileges granted by the entries in both tables must be combined.

The host table is not affected by the GRANT or REVOKE statements, so it is unused in most MySQL installations. If you modify it directly, you can use it for some specialized purposes, such as to to maintain a list of secure servers. For example, at TcX, the host table contains a list of all machines on the local network. These are granted all privileges.

You can also use the host table to indicate hosts that are not secure. Suppose that you have a machine public.your.domain that is located in a public area that you do not consider secure. You can allow access to all hosts on your network except that machine by using host table entries like this: 

+--------------------+----+-
| Host               | Db | ...
+--------------------+----+-
| public.your.domain | %  | ... (all privileges set to 'N')
| %.your.domain      | %  | ... (all privileges set to 'Y')
+--------------------+----+-

Naturally, you should always test your entries in the grant tables (for example, by using SHOW GRANTS or @command{mysqlaccess}) to make sure that your access privileges are actually set up the way you think they are.

5.5.7 When Privilege Changes Take Effect

When @command{mysqld} starts, all grant table contents are read into memory and become effective for access control at that point.

When the server reloads the grant tables, privileges for existing client connections are affected as follows:

If you modify the grant tables using GRANT, REVOKE, or SET PASSWORD, the server notices these changes and reloads the grant tables into memory again immediately.

If you modify the grant tables directly using statements such as INSERT, UPDATE, or DELETE, your changes have no effect on privilege checking until you either restart the server or tell it to reload the tables. To reload the grant tables manually, issue a FLUSH PRIVILEGES statement or execute a @command{mysqladmin flush-privileges} or @command{mysqladmin reload} command.

If you change the grant tables directly but forget to reload them, your changes will have no effect until you restart the server. This may leave you wondering why your changes don't seem to make any difference!

5.5.8 Causes of Access denied Errors

If you encounter problems when you try to connect to the MySQL server, the following items describe some courses of action you can take to correct the problem.

5.5.9 Password Hashing in MySQL 4.1

MySQL user accounts are listed in the user table of the mysql database. Each MySQL account is assigned a password, although what is stored in the Password column of the user table is not the plaintext version of the password, but a hash value computed from it. Password hash values are computed by the PASSWORD() function.

MySQL uses passwords in two phases of client/server communication:

In other words, the server uses hash values during authentication when a client first attempts to connect. The server generates hash values if a connected client invokes the PASSWORD() function or uses a GRANT or SET PASSWORD statement to set or change a password.

The password hashing mechanism was updated in MySQL 4.1 to provide better security and to reduce the risk of passwords being intercepted. However, this new mechanism is understood only by the 4.1 server and 4.1 clients, which can result in some compatibility problems. A 4.1 client can connect to a pre-4.1 server, because the client understands both the old and new password hashing mechanisms. However, a pre-4.1 client that attempts to connect to a 4.1 server may run into difficulties. For example, a 4.0 @command{mysql} client that attempts to connect to a 4.1 server may fail with the following error message: 

shell> mysql -h localhost -u root
Client does not support authentication protocol requested
by server; consider upgrading MySQL client

The following discussion describes the differences between the old and new password mechanisms, and what you should do if you upgrade your server to 4.1 but need to maintain backward compatibility with pre-4.1 clients. Additional information can be found in section A.2.3 Client does not support authentication protocol.

Note: This discussion contrasts 4.1 behavior with pre-4.1 behavior, but the 4.1 behavior described here actually begins with 4.1.1. MySQL 4.1.0 is an ``odd'' release because it has a slightly different mechanism than that implemented in 4.1.1 and up. Differences between 4.1.0 and more recent versions are described further in section 5.5.9.2 Password Hashing in MySQL 4.1.0.

Prior to MySQL 4.1, password hashes computed by the PASSWORD() function are 16 bytes long. Such hashes look like this: 

mysql> SELECT PASSWORD('mypass');
+--------------------+
| PASSWORD('mypass') |
+--------------------+
| 6f8c114b58f2ce9e   |
+--------------------+

The Password column of the user table (in which these hashes are stored) also is 16 bytes long before MySQL 4.1.

As of MySQL 4.1, the PASSWORD() function has been modified to produce a longer 41-byte hash value: 

mysql> SELECT PASSWORD('mypass');
+-----------------------------------------------+
| PASSWORD('mypass')                            |
+-----------------------------------------------+
| *43c8aa34cdc98eddd3de1fe9a9c2c2a9f92bb2098d75 |
+-----------------------------------------------+

Accordingly, the Password column in the user table also must be 41 bytes long to store these values:

A widened Password column can store password hashes in both the old and new formats. The format of any given password hash value can be determined two ways:

The longer password hash format has better cryptographic properties, and client authentication based on long hashes is more secure than that based on the older short hashes.

The differences between short and long password hashes are relevant both for how the server uses passwords during authentication and for how it generates password hashes for connected clients that perform password-changing operations.

The way in which the server uses password hashes during authentication is affected by the width of the Password column:

For short-hash accounts, the authentication process is actually a bit more secure for 4.1 clients than for older clients. In terms of security, the gradient from least to most secure is:

The way in which the server generates password hashes for connected clients is affected by the width of the Password column and by the --old-passwords option. A 4.1 server generates long hashes only if certain conditions are met: The Password column must be wide enough to hold long values and the --old-passwords option must not be given. These conditions apply as follows:

The purpose of the --old-passwords option is to allow you to maintain backward compatibility with pre-4.1 clients under circumstances where the server would otherwise generate long password hashes. The option doesn't affect authentication (4.1 clients can still use accounts that have long password hashes), but it does prevent creation of a long password hash in the user table as the result of a password-changing operation. Were that to occur, the account no longer could be used by pre-4.1 clients. Without the --old-passwords option, the following undesirable scenario is possible:

This scenario illustrates that, if you must support older pre-4.1 clients, it is dangerous to run a 4.1 server without using the --old-passwords option. By running the server with --old-passwords, password-changing operations will not generate long password hashes and thus do not cause accounts to become inaccessible to older clients. (Those clients cannot inadvertently lock themselves out by changing their password and ending up with a long password hash.)

The downside of the --old-passwords option is that any passwords you create or change will use short hashes, even for 4.1 clients. Thus, you lose the additional security provided by long password hashes. If you want to create an account that has a long hash (for example, for use by 4.1 clients), you must do so while running the server without --old-passwords.

The following scenarios are possible for running a 4.1 server:

Scenario 1: Short Password column in user table:

Scenario 2: Long Password column; server not started with --old-passwords option:

As indicated earlier, a danger in this scenario is that it is possible for accounts that have a short password hash to become inaccessible to pre-4.1 clients. A change to such an account's password made via GRANT, PASSWORD(), or SET PASSWORD results in the account being given a long password hash. From that point on, no pre-4.1 client can authenticate to that account until the client upgrades to 4.1.

To deal with this problem, you can change a password in a special way. For example, normally you use SET PASSWORD as follows to change an account password: 

mysql> SET PASSWORD FOR 'some_user'@'some_host' = PASSWORD('mypass');

To change the password but create a short hash, use the OLD_PASSWORD() function instead: 

mysql> SET PASSWORD FOR 'some_user'@'some_host' = OLD_PASSWORD('mypass');

OLD_PASSWORD() is useful for situations in which you explicitly want to generate a short hash.

Scenario 3: Long Password column; server started with --old-passwords option:

In this scenario, you cannot create accounts that have long password hashes, because the --old-passwords option prevents generation of long hashes. Also, if you create an account with a long hash before using the --old-passwords option, changing the account's password while --old-passwords is in effect results in the account being given a short password, causing it to lose the security benefits of a longer hash.

The disadvantages for these scenarios may be summarized as follows:

In scenario 1, you cannot take advantage of longer hashes that provide more secure authentication.

In scenario 2, accounts with short hashes become inaccessible to pre-4.1 clients if you change their passwords without explicitly using OLD_PASSWORD().

In scenario 3, --old-passwords prevents accounts with short hashes from becoming inaccessible, but password-changing operations cause accounts with long hashes to revert to short hashes, and you cannot change them back to long hashes while --old-passwords is in effect.

5.5.9.1 Implications of Password Hashing Changes for Application Programs

An upgrade to MySQL 4.1 can cause a compatibility issue for applications that use PASSWORD() to generate passwords for their own purposes. Applications really should not do this, because PASSWORD() should be used only to manage passwords for MySQL accounts. But some applications use PASSWORD() for their own purposes anyway.

If you upgrade to 4.1 and run the server under conditions where it generates long password hashes, an application that uses PASSWORD() for its own passwords will break. The recommended course of action is to modify the application to use another function, such as SHA1() or MD5(), to produce hashed values. If that is not possible, you can use the OLD_PASSWORD() function, which is provided to generate short hashes in the old format. But note that OLD_PASSWORD() may one day no longer be supported.

If the server is running under circumstances where it generates short hashes, OLD_PASSWORD() is available but is equivalent to PASSWORD().

5.5.9.2 Password Hashing in MySQL 4.1.0

Password hashing in MySQL 4.1.0 differs from hashing in 4.1.1 and up. The 4.1.0 differences are:

These differences make authentication in 4.1.0 incompatible with that of releases that follow it. If you have upgraded to MySQL 4.1.0, it is recommended that you upgrade to a newer version as soon as possible. After you do, reassign any long passwords in the user table so that they are compatible with the 41-byte format.

5.6 MySQL User Account Management

This section describes how to set up accounts for clients of your MySQL server. It discusses the following topics:

5.6.1 MySQL Usernames and Passwords

A MySQL account is defined in terms of a username and the client host or hosts from which the user can connect to the server. The account also has a password. There are several distinctions between the way usernames and passwords are used by MySQL and the way they are used by your operating system:

When you install MySQL, the grant tables are populated with an initial set of accounts. These accounts have names and access privileges that are described in section 2.4.3 Securing the Initial MySQL Accounts, which also discusses how to assign passwords to them. Thereafter, you normally set up, modify, and remove MySQL accounts using the GRANT and REVOKE statements. See section 14.5.1.2 GRANT and REVOKE Syntax.

When you connect to a MySQL server with a command-line client, you should specify the username and password for the account that you want to use: 

shell> mysql --user=monty --password=guess db_name

If you prefer short options, the command looks like this: 

shell> mysql -u monty -pguess db_name

There must be no space between the -p option and the following password value. See section 5.5.4 Connecting to the MySQL Server.

The preceding commands include the password value on the command line, which can be a security risk. See section 5.6.6 Keeping Your Password Secure. To avoid this, specify the --password or -p option without any following password value: 

shell> mysql --user=monty --password db_name
shell> mysql -u monty -p db_name

Then the client program will print a prompt and wait for you to enter the password. (In these examples, db_name is not interpreted as a password, because it is separated from the preceding password option by a space.)

On some systems, the library call that MySQL uses to prompt for a password automatically limits the password to eight characters. That is a problem with the system library, not with MySQL. Internally, MySQL doesn't have any limit for the length of the password. To work around the problem, change your MySQL password to a value that is eight or fewer characters long, or put your password in an option file.

5.6.2 Adding New User Accounts to MySQL

You can create MySQL accounts in two ways:

The preferred method is to use GRANT statements, because they are more concise and less error-prone. GRANT is available as of MySQL 3.22.11; its syntax is described in section 14.5.1.2 GRANT and REVOKE Syntax.

Another option for creating accounts is to use one of several available third-party programs that offer capabilities for MySQL account administration. phpMyAdmin is one such program.

The following examples show how to use the @command{mysql} client program to set up new users. These examples assume that privileges are set up according to the defaults described in section 2.4.3 Securing the Initial MySQL Accounts. This means that to make changes, you must connect to the MySQL server as the MySQL root user, and the root account must have the INSERT privilege for the mysql database and the RELOAD administrative privilege.

First, use the @command{mysql} program to connect to the server as the MySQL root user: 

shell> mysql --user=root mysql

If you have assigned a password to the root account, you'll also need to supply a --password or -p option for this @command{mysql} command and also for those later in this section.

After connecting to the server as root, you can add new accounts. The following statements use GRANT to set up four new accounts: 

mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'localhost'
    ->     IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'%'
    ->     IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT RELOAD,PROCESS ON *.* TO 'admin'@'localhost';
mysql> GRANT USAGE ON *.* TO 'dummy'@'localhost';

The accounts created by these GRANT statements have the following properties:

As an alternative to GRANT, you can create the same accounts directly by issuing INSERT statements and then telling the server to reload the grant tables: 

shell> mysql --user=root mysql
mysql> INSERT INTO user
    ->     VALUES('localhost','monty',PASSWORD('some_pass'),
    ->     'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user
    ->     VALUES('%','monty',PASSWORD('some_pass'),
    ->     'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user SET Host='localhost',User='admin',
    ->     Reload_priv='Y', Process_priv='Y';
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('localhost','dummy','');
mysql> FLUSH PRIVILEGES;

The reason for using FLUSH PRIVILEGES when you create accounts with INSERT is to tell the server to re-read the grant tables. Otherwise, the changes will go unnoticed until you restart the server. With GRANT, FLUSH PRIVILEGES is unnecessary.

The reason for using the PASSWORD() function with INSERT is to encrypt the password. The GRANT statement encrypts the password for you, so PASSWORD() is unnecessary.

The 'Y' values enable privileges for the accounts. Depending on your MySQL version, you may have to use a different number of 'Y' values in the first two INSERT statements. (Versions prior to 3.22.11 have fewer privilege columns, and versions from 4.0.2 on have more.) For the admin account, the more readable extended INSERT syntax using SET that is available starting with MySQL 3.22.11 is used.

In the INSERT statement for the dummy account, only the Host, User, and Password columns in the user table record are assigned values. None of the privilege columns are set explicitly, so MySQL assigns them all the default value of 'N'. This is equivalent to what GRANT USAGE does.

Note that to set up a superuser account, it is necessary only to create a user table entry with the privilege columns set to 'Y'. user table privileges are global, so no entries in any of the other grant tables are needed.

The next examples create three accounts and give them access to specific databases. Each of them has a username of custom and password of obscure.

To create the accounts with GRANT, use the following statements: 

shell> mysql --user=root mysql
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
    ->     ON bankaccount.*
    ->     TO 'custom'@'localhost'
    ->     IDENTIFIED BY 'obscure';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
    ->     ON expenses.*
    ->     TO 'custom'@'whitehouse.gov'
    ->     IDENTIFIED BY 'obscure';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
    ->     ON customer.*
    ->     TO 'custom'@'server.domain'
    ->     IDENTIFIED BY 'obscure';

The three accounts can be used as follows:

To set up the custom accounts without GRANT, use INSERT statements as follows to modify the grant tables directly: 

shell> mysql --user=root mysql
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('localhost','custom',PASSWORD('obscure'));
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('whitehouse.gov','custom',PASSWORD('obscure'));
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('server.domain','custom',PASSWORD('obscure'));
mysql> INSERT INTO db
    ->     (Host,Db,User,Select_priv,Insert_priv,
    ->     Update_priv,Delete_priv,Create_priv,Drop_priv)
    ->     VALUES('localhost','bankaccount','custom',
    ->     'Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
    ->     (Host,Db,User,Select_priv,Insert_priv,
    ->     Update_priv,Delete_priv,Create_priv,Drop_priv)
    ->     VALUES('whitehouse.gov','expenses','custom',
    ->     'Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
    ->     (Host,Db,User,Select_priv,Insert_priv,
    ->     Update_priv,Delete_priv,Create_priv,Drop_priv)
    ->     VALUES('server.domain','customer','custom',
    ->     'Y','Y','Y','Y','Y','Y');
mysql> FLUSH PRIVILEGES;

The first three INSERT statements add user table entries that allow the user custom to connect from the various hosts with the given password, but grant no global privileges (all privileges are set to the default value of 'N'). The next three INSERT statements add db table entries that grant privileges to custom for the bankaccount, expenses, and customer databases, but only when accessed from the proper hosts. As usual when you modify the grant tables directly, you tell the server to reload them with FLUSH PRIVILEGES so that the privilege changes take effect.

If you want to give a specific user access from all machines in a given domain (for example, mydomain.com), you can issue a GRANT statement that uses the `%' wildcard character in the host part of the account name: 

mysql> GRANT ...
    ->     ON *.*
    ->     TO 'myname'@'%.mydomain.com'
    ->     IDENTIFIED BY 'mypass';

To do the same thing by modifying the grant tables directly, do this: 

mysql> INSERT INTO user (Host,User,Password,...)
    ->     VALUES('%.mydomain.com','myname',PASSWORD('mypass'),...);
mysql> FLUSH PRIVILEGES;

5.6.3 Removing User Accounts from MySQL

To remove an account, use the DROP USER statement, which was added in MySQL 4.1.1. For older versions of MySQL, use DELETE instead. The account removal procedure is described in section 14.5.1.1 DROP USER Syntax.

5.6.4 Limiting Account Resources

Before MySQL 4.0.2, the only available method for limiting use of MySQL server resources is to set the max_user_connections system variable to a non-zero value. But that method is strictly global. It does not allow for management of individual accounts. Also, it limits only the number of simultaneous connections made using a single account, not what a client can do once connected. Both types of control are interest to many MySQL administrators, particularly those for Internet Service Providers.

Starting from MySQL 4.0.2, you can limit the following server resources for individual accounts:

Any statement that a client can issue counts against the query limit. Only statements that modify databases or tables count against the update limit.

An account in this context is a single record in the user table. Each account is uniquely identified by its User and Host column values.

As a prerequisite for using this feature, the user table in the mysql database must contain the resource-related columns. Resource limits are stored in the max_questions, max_updates, and max_connections columns. If your user table doesn't have these columns, it must be upgraded; see section 2.5.8 Upgrading the Grant Tables.

To set resource limits with a GRANT statement, use a WITH clause that names each resource to be limited and a per-hour count indicating the limit value. For example, to create a new account that can access the customer database, but only in a limited fashion, issue this statement: 

mysql> GRANT ALL ON customer.* TO 'francis'@'localhost'
    ->     IDENTIFIED BY 'frank'
    ->     WITH MAX_QUERIES_PER_HOUR 20
    ->          MAX_UPDATES_PER_HOUR 10
    ->          MAX_CONNECTIONS_PER_HOUR 5;

The limit types need not all be named in the WITH clause, but those named can be present in any order. The value for each limit should be an integer representing a count per hour. If the GRANT statement has no WITH clause, the limits are each set to the default value of zero (that is, no limit).

To set or change limits for an existing account, use a GRANT USAGE statement at the global level (ON *.*). The following statement changes the query limit for francis to 100: 

mysql> GRANT USAGE ON *.* TO 'francis'@'localhost'
    ->     WITH MAX_QUERIES_PER_HOUR 100;

This statement leaves the account's existing privileges unchanged and modifies only the limit values specified.

To remove an existing limit, set its value to zero. For example, to remove the limit on how many times per hour francis can connect, use this statement: 

mysql> GRANT USAGE ON *.* TO 'francis'@'localhost'
    ->     WITH MAX_CONNECTIONS_PER_HOUR 0;

Resource-use counting takes place when any account has a non-zero limit placed on its use of any of the resources.

As the server runs, it counts the number of times each account uses resources. If an account reaches its limit on number of connections within the last hour, further connections for the account are rejected until that hour is up. Similarly, if the account reaches its limit on the number of queries or updates, further queries or updates are rejected until the hour is up. In all such cases, an appropriate error message is issued.

Resource counting is done per account, not per client. For example, if your account has a query limit of 50, you cannot increase your limit to 100 by making two simultaneous client connections to the server. Queries issued on both connections are counted together.

The current resource-use counts can be reset globally for all accounts, or individually for a given count:

5.6.5 Assigning Account Passwords

Passwords may be assigned from the command line by using the @command{mysqladmin} command: 

shell> mysqladmin -u user_name -h host_name password "newpwd"

The account for which this command resets the password is the one with a user table record that matches user_name in the User column and the client host from which you connect in the Host column.

Another way to assign a password to an account is to issue a SET PASSWORD statement: 

mysql> SET PASSWORD FOR 'jeffrey'@'%' = PASSWORD('biscuit');

Only users such as root with update access to the mysql database can change the password for other users. If you are not connected as an anonymous user, you can change your own password by omitting the FOR clause: 

mysql> SET PASSWORD = PASSWORD('biscuit');

You can also use a GRANT USAGE statement at the global level (ON *.*) to assign a password to an account without affecting the account's current privileges: 

mysql> GRANT USAGE ON *.* TO 'jeffrey'@'%' IDENTIFIED BY 'biscuit';

Although it is generally preferable to assign passwords using one of the preceding methods, you can also do so by modifying the user table directly:

When you assign an account a password using SET PASSWORD, INSERT, or UPDATE, you must use the PASSWORD() function to encrypt it. (The only exception is that you need not use PASSWORD() if the password is empty.) PASSWORD() is necessary because the user table stores passwords in encrypted form, not as plaintext. If you forget that fact, you are likely to set passwords like this: 

shell> mysql -u root mysql
mysql> INSERT INTO user (Host,User,Password)
    -> VALUES('%','jeffrey','biscuit');
mysql> FLUSH PRIVILEGES;

The result is that the literal value 'biscuit' is stored as the password in the user table, not the encrypted value. When jeffrey attempts to connect to the server using this password, the value is encrypted and compared to the value stored in the user table. However, the stored value is the literal string 'biscuit', so the comparison fails and the server rejects the connection: 

shell> mysql -u jeffrey -pbiscuit test
Access denied

If you set passwords using the GRANT ... IDENTIFIED BY statement or the @command{mysqladmin password} command, they both take care of encrypting the password for you. The PASSWORD() function is unnecessary.

Note: PASSWORD() encryption is different from Unix password encryption. See section 5.6.1 MySQL Usernames and Passwords.

5.6.6 Keeping Your Password Secure

On an administrative level, you should never grant access to the mysql.user table to any non-administrative accounts. Passwords in the user table are stored in encrypted form, but in versions of MySQL earlier than 4.1, knowing the encrypted password for an account makes it possible to connect to the server using that account.

When you run a client program to connect to the MySQL server, it is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed here, along with an assessment of the risks of each method:

All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly protected option file.

5.6.7 Using Secure Connections

Beginning with version 4.0.0, MySQL has support for secure (encrypted) connections between MySQL clients and the server using the Secure Sockets Layer (SSL) protocol. This section discusses how to use SSL connections. It also describes a way to set up SSH on Windows.

The standard configuration of MySQL is intended to be as fast as possible, so encrypted connections are not used by default. Doing so would make the client/server protocol much slower. Encrypting data is a CPU-intensive operation that requires the computer to do additional work and can delay other MySQL tasks. For applications that require the security provided by encrypted connections, the extra computation is warranted.

MySQL allows encryption to be enabled on a per-connection basis. You can choose a normal unencrypted connection or a secure encrypted SSL connection according the requirements of individual applications.

5.6.7.1 Basic SSL Concepts

To understand how MySQL uses SSL, it's necessary to explain some basic SSL and X509 concepts. People who are already familiar with them can skip this part.

By default, MySQL uses unencrypted connections between the client and the server. This means that someone with access to the network could watch all your traffic and look at the data being sent or received. They could even change the data while it is in transit between client and server. To improve security a little, you can compress client/server traffic by using the --compress option when invoking client programs. However, this will not foil a determined attacker.

When you need to move information over a network in a secure fashion, an unencrypted connection is unacceptable. Encryption is the way to make any kind of data unreadable. In fact, today's practice requires many additional security elements from encryption algorithms. They should resist many kind of known attacks such as changing the order of encrypted messages or replaying data twice.

SSL is a protocol that uses different encryption algorithms to ensure that data received over a public network can be trusted. It has mechanisms to detect any data change, loss, or replay. SSL also incorporates algorithms that provide identity verification using the X509 standard.

X509 makes it possible to identify someone on the Internet. It is most commonly used in e-commerce applications. In basic terms, there should be some company called a ``Certificate Authority'' (or CA) that assigns electronic certificates to anyone who needs them. Certificates rely on asymmetric encryption algorithms that have two encryption keys (a public key and a secret key). A certificate owner can show the certificate to another party as proof of identity. A certificate consists of its owner's public key. Any data encrypted with this public key can be decrypted only using the corresponding secret key, which is held by the owner of the certificate.

If you need more information about SSL, X509, or encryption, use your favorite Internet search engine to search for keywords in which you are interested.

5.6.7.2 Requirements

To use SSL connections between the MySQL server and client programs, your system must be able to support OpenSSL and your version of MySQL must be 4.0.0 or newer.

To get secure connections to work with MySQL, you must do the following:

  1. Install the OpenSSL library. We have tested MySQL with OpenSSL 0.9.6. If you need OpenSSL, visit http://www.openssl.org.
  2. When you configure MySQL, run the @command{configure} script with the --with-vio and --with-openssl options.
  3. Make sure that you have upgraded your grant tables to include the SSL-related columns in the mysql.user table. This is necessary if your grant tables date from a version prior to MySQL 4.0.0. The upgrade procedure is described in section 2.5.8 Upgrading the Grant Tables.
  4. To check whether a running @command{mysqld} server supports OpenSSL, examine the value of the have_openssl system variable: 
    mysql> SHOW VARIABLES LIKE 'have_openssl';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| have_openssl  | YES   |
+---------------+-------+
    If the value is YES, the server supports OpenSSL connections.

5.6.7.3 Setting Up SSL Certificates for MySQL

Here is an example for setting up SSL certificates for MySQL: 

DIR=`pwd`/openssl
PRIV=$DIR/private

mkdir $DIR $PRIV $DIR/newcerts
cp /usr/share/ssl/openssl.cnf $DIR
replace ./demoCA $DIR -- $DIR/openssl.cnf

# Create necessary files: $database, $serial and $new_certs_dir 
# directory (optional)

touch $DIR/index.txt
echo "01" > $DIR/serial

#
# Generation of Certificate Authority(CA)
#

openssl req -new -x509 -keyout $PRIV/cakey.pem -out $DIR/cacert.pem \
    -config $DIR/openssl.cnf

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# ................++++++
# .........++++++
# writing new private key to '/home/monty/openssl/private/cakey.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL admin
# Email Address []:

#
# Create server request and key
#
openssl req -new -keyout $DIR/server-key.pem -out \
    $DIR/server-req.pem -days 3600 -config $DIR/openssl.cnf

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# ..++++++
# ..........++++++
# writing new private key to '/home/monty/openssl/server-key.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL server
# Email Address []:
# 
# Please enter the following 'extra' attributes
# to be sent with your certificate request
# A challenge password []:
# An optional company name []:

#
# Remove the passphrase from the key (optional)
#

openssl rsa -in $DIR/server-key.pem -out $DIR/server-key.pem

#
# Sign server cert
#
openssl ca  -policy policy_anything -out $DIR/server-cert.pem \
    -config $DIR/openssl.cnf -infiles $DIR/server-req.pem

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Enter PEM pass phrase:
# Check that the request matches the signature
# Signature ok
# The Subjects Distinguished Name is as follows
# countryName           :PRINTABLE:'FI'
# organizationName      :PRINTABLE:'MySQL AB'
# commonName            :PRINTABLE:'MySQL admin'
# Certificate is to be certified until Sep 13 14:22:46 2003 GMT
# (365 days)
# Sign the certificate? [y/n]:y
# 
# 
# 1 out of 1 certificate requests certified, commit? [y/n]y
# Write out database with 1 new entries
# Data Base Updated

#
# Create client request and key
#
openssl req -new -keyout $DIR/client-key.pem -out \
    $DIR/client-req.pem -days 3600 -config $DIR/openssl.cnf

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# .....................................++++++
# .............................................++++++
# writing new private key to '/home/monty/openssl/client-key.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL user
# Email Address []:
# 
# Please enter the following 'extra' attributes
# to be sent with your certificate request
# A challenge password []:
# An optional company name []:

#
# Remove a passphrase from the key (optional)
#
openssl rsa -in $DIR/client-key.pem -out $DIR/client-key.pem

#
# Sign client cert
#

openssl ca  -policy policy_anything -out $DIR/client-cert.pem \
    -config $DIR/openssl.cnf -infiles $DIR/client-req.pem

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Enter PEM pass phrase:
# Check that the request matches the signature
# Signature ok
# The Subjects Distinguished Name is as follows
# countryName           :PRINTABLE:'FI'
# organizationName      :PRINTABLE:'MySQL AB'
# commonName            :PRINTABLE:'MySQL user'
# Certificate is to be certified until Sep 13 16:45:17 2003 GMT
# (365 days)
# Sign the certificate? [y/n]:y
# 
# 
# 1 out of 1 certificate requests certified, commit? [y/n]y
# Write out database with 1 new entries
# Data Base Updated

#
# Create a my.cnf file that you can use to test the certificates
#

cnf=""
cnf="$cnf [client]"
cnf="$cnf ssl-ca=$DIR/cacert.pem"
cnf="$cnf ssl-cert=$DIR/client-cert.pem"
cnf="$cnf ssl-key=$DIR/client-key.pem"
cnf="$cnf [mysqld]"
cnf="$cnf ssl-ca=$DIR/cacert.pem"
cnf="$cnf ssl-cert=$DIR/server-cert.pem"
cnf="$cnf ssl-key=$DIR/server-key.pem"
echo $cnf | replace " " '
' > $DIR/my.cnf

To test SSL connections, start the server as follows, where $DIR is the pathname to the directory where the sample `my.cnf' option file is located: 

shell> mysqld --defaults-file=$DIR/my.cnf &

Then invoke a client program using the same option file: 

shell> mysql --defaults-file=$DIR/my.cnf

If you have a MySQL source distribution, you can also test your setup by modifying the preceding `my.cnf' file to refer to the demonstration certificate and key files in the `SSL' directory of the distribution.

5.6.7.4 SSL GRANT Options

MySQL can check X509 certificate attributes in addition to the usual authentication that is based on the username and password. To specify SSL-related options for a MySQL account, use the REQUIRE clause of the GRANT statement. See section 14.5.1.2 GRANT and REVOKE Syntax.

There are different possibilities for limiting connection types for an account:

The SUBJECT, ISSUER, and CIPHER options can be combined in the REQUIRE clause like this: 

mysql> GRANT ALL PRIVILEGES ON test.* TO 'root'@'localhost'
    -> IDENTIFIED BY 'goodsecret'
    -> REQUIRE SUBJECT '/C=EE/ST=Some-State/L=Tallinn/
       O=MySQL demo client certificate/
       CN=Tonu Samuel/[email protected]'
    -> AND ISSUER '/C=FI/ST=Some-State/L=Helsinki/
       O=MySQL Finland AB/CN=Tonu Samuel/[email protected]'
    -> AND CIPHER 'EDH-RSA-DES-CBC3-SHA';

Note that the SUBJECT and ISSUER values each should be entered as a single string.

Starting from MySQL 4.0.4, the AND keyword is optional between REQUIRE options.

The order of the options does not matter, but no option can be specified twice.

5.6.7.5 SSL Command-Line Options

The following list describes options that are used for specifying the use of SSL, certificate files, and key files. These options are available beginning with MySQL 4.0. They may be given on the command line or in an option file.

--ssl
For the server, this option specifies that the server allows SSL connections. For a client program, it allows the client to connect to the server using SSL. This option is not sufficient in itself to cause an SSL connection to be used. You must also specify the --ssl-ca, --ssl-cert, and --ssl-key options. This option is more often used in its opposite form to indicate that SSL should not be used. To do this, specify the option as --skip-ssl or --ssl=0. Note that use of --ssl doesn't require an SSL connection. For example, if the server or client is compiled without SSL support, a normal unencrypted connection will be used. The secure way to ensure that an SSL connection will be used is to create an account on the server that includes a REQUIRE SSL clause in the GRANT statement. Then use this account to connect to the server, with both a server and client that have SSL support enabled.
--ssl-ca=file_name
The path to a file with a list of trusted SSL CAs.
--ssl-capath=directory_name
The path to a directory that contains trusted SSL CA certificates in pem format.
--ssl-cert=file_name
The name of the SSL certificate file to use for establishing a secure connection.
--ssl-cipher=cipher_list
A list of allowable ciphers to use for SSL encryption. cipher_list has the same format as the openssl ciphers command. Example: --ssl-cipher=ALL:-AES:-EXP
--ssl-key=file_name
The name of the SSL key file to use for establishing a secure connection.

5.6.7.6 Connecting to MySQL Remotely from Windows with SSH

Here is a note about how to connect to get a secure connection to remote MySQL server with SSH (by David Carlson [email protected]):

  1. Install an SSH client on your Windows machine. As a user, the best non-free one I've found is from SecureCRT from http://www.vandyke.com/. Another option is f-secure from http://www.f-secure.com/. You can also find some free ones on Google at http://directory.google.com/Top/Computers/Security/Products_and_Tools/Cryptography/SSH/Clients/Windows/.
  2. Start your Windows SSH client. Set Host_Name = yourmysqlserver_URL_or_IP. Set userid=your_userid to log in to your server. This userid value may not be the same as the username of your MySQL account.
  3. Set up port forwarding. Either do a remote forward (Set local_port: 3306, remote_host: yourmysqlservername_or_ip, remote_port: 3306 ) or a local forward (Set port: 3306, host: localhost, remote port: 3306).
  4. Save everything, otherwise you'll have to redo it the next time.
  5. Log in to your server with the SSH session you just created.
  6. On your Windows machine, start some ODBC application (such as Access).
  7. Create a new file in Windows and link to MySQL using the ODBC driver the same way you normally do, except type in localhost for the MySQL host server, not yourmysqlservername.

You should now have an ODBC connection to MySQL, encrypted using SSH.

5.7 Disaster Prevention and Recovery

This section discusses how to make database backups and how to perform table maintenance. The syntax of the SQL statements described here is given in section 14.5 Database Administration Statements. Much of the information here pertains primarily to MyISAM tables. InnoDB backup procedures are given in section 16.9 Backing Up and Recovering an InnoDB Database.

5.7.1 Database Backups

Because MySQL tables are stored as files, it is easy to do a backup. To get a consistent backup, do a LOCK TABLES on the relevant tables, followed by FLUSH TABLES for the tables. See section 14.4.5 LOCK TABLES and UNLOCK TABLES Syntax and section 14.5.4.2 FLUSH Syntax. You need only a read lock; this allows other clients to continue to query the tables while you are making a copy of the files in the database directory. The FLUSH TABLES statement is needed to ensure that the all active index pages are written to disk before you start the backup.

If you want to make an SQL-level backup of a table, you can use SELECT INTO ... OUTFILE or BACKUP TABLE. For SELECT INTO ... OUTFILE, the output file cannot already exist. For BACKUP TABLE, the same is true as of MySQL 3.23.56 and 4.0.12, because this would be a security risk. See section 14.1.7 SELECT Syntax and section 14.5.2.2 BACKUP TABLE Syntax.

Another way to back up a database is to use the @command{mysqldump} program or the @command{mysqlhotcopy script}. See @ref{mysqldump, , @command{mysqldump}} and @ref{mysqlhotcopy, , @command{mysqlhotcopy}}.

  1. Do a full backup of your database: 
    shell> mysqldump --tab=/path/to/some/dir --opt db_name
    Or: 
    shell> mysqlhotcopy db_name /path/to/some/dir
    You can also simply copy all table files (`*.frm', `*.MYD', and `*.MYI' files) as long as the server isn't updating anything. The @command{mysqlhotcopy} script uses this method. (But note that these methods will not work if your database contains InnoDB tables. InnoDB does not store table contents in database directories, and @command{mysqlhotcopy} works only for MyISAM and ISAM tables.)
  2. Stop @command{mysqld} if it's running, then start it with the --log-bin[=file_name] option. See section 5.9.4 The Binary Log. The binary log files provide you with the information you need to replicate changes to the database that are made subsequent to the point at which you executed @command{mysqldump}.

If your MySQL server is a slave replication server, then regardless of the backup method you choose, you should also back up the `master.info' and `relay-log.info' files when you back up your slave's data. These files are always needed to resume replication after you restore the slave's data. If your slave is subject to replicating LOAD DATA INFILE commands, you should also back up any `SQL_LOAD-*' files that may exist in the directory specified by the --slave-load-tmpdir option. (This location defaults to the value of the tmpdir variable if not specified.) The slave needs these files to resume replication of any interrupted LOAD DATA INFILE operations.

If you have to restore MyISAM tables, try to recover them using REPAIR TABLE or @command{myisamchk -r} first. That should work in 99.9% of all cases. If @command{myisamchk} fails, try the following procedure. Note that it will work only if you have enabled binary logging by starting MySQL with the --log-bin option; see section 5.9.4 The Binary Log.

  1. Restore the original @command{mysqldump} backup, or binary backup.
  2. Execute the following command to re-run the updates in the binary logs: 
    shell> mysqlbinlog hostname-bin.[0-9]* | mysql
    In your case, you may want to re-run only certain binary logs, from certain positions (usually you want to re-run all binary logs from the date of the restored backup, excepting possibly some incorrect queries). See @ref{mysqlbinlog, , @command{mysqlbinlog}} for more information on the @command{mysqlbinlog} utility and how to use it. If you are using the update logs instead, you can process their contents like this: 
    shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql
    ls is used to sort the update log filenames into the right order.

You can also do selective backups of individual files:

If you have performance problems with your server while making backups, one strategy that can help is to set up replication and perform backups on the slave rather than on the master. See section 6.1 Introduction to Replication.

If you are using a Veritas filesystem, you can make a backup like this:

  1. From a client program, execute FLUSH TABLES WITH READ LOCK.
  2. From another shell, execute mount vxfs snapshot.
  3. From the first client, execute UNLOCK TABLES.
  4. Copy files from the snapshot.
  5. Unmount the snapshot.

5.7.2 Table Maintenance and Crash Recovery

The following text discusses how to use @command{myisamchk} to check or repair MyISAM tables (tables with `.MYI' and `.MYD' files). The same concepts apply to using @command{isamchk} to check or repair ISAM tables (tables with `.ISM' and `.ISD' files). See section 15 MySQL Storage Engines and Table Types.

You can use the @command{myisamchk} utility to get information about your database tables or to check, repair, or optimize them. The following sections describe how to invoke @command{myisamchk} (including a description of its options), how to set up a table maintenance schedule, and how to use @command{myisamchk} to perform its various functions.

Even though table repair with @command{myisamchk} is quite secure, it's always a good idea to make a backup before doing a repair (or any maintenance operation that could make a lot of changes to a table)

@command{myisamchk} operations that affect indexes can cause FULLTEXT indexes to be rebuilt with full-text parameters that are incompatible with the values used by the MySQL server. To avoid this, read the instructions in @ref{myisamchk general options, , @command{myisamchk} general options}.

In many cases, you may find it simpler to do MyISAM table maintenance using the SQL statements that perform operations that @command{myisamchk} can do:

These statements were introduced in different versions, but all are available from MySQL 3.23.14 on. See section 14.5.2.1 ANALYZE TABLE Syntax, section 14.5.2.3 CHECK TABLE Syntax, section 14.5.2.5 OPTIMIZE TABLE Syntax, and section 14.5.2.6 REPAIR TABLE Syntax. The statements can be used directly, or by means of the @command{mysqlcheck} client program, which provides a command-line interface to them.

One advantage of these statements over @command{myisamchk} is that the server does all the work. With @command{myisamchk}, you must make sure that the server does not use the tables at the same time. Otherwise, there can be unwanted interaction betweeen @command{myisamchk} and the server.

5.7.2.1 @command{myisamchk} Invocation Syntax

Invoke @command{myisamchk} like this: 

shell> myisamchk [options] tbl_name

The options specify what you want @command{myisamchk} to do. They are described in the following sections. You can also get a list of options by invoking @command{myisamchk --help}.

With no options, @command{myisamchk} simply checks your table as the default operation. To get more information or to tell @command{myisamchk} to take corrective action, specify options as described in the following discussion.

tbl_name is the database table you want to check or repair. If you run @command{myisamchk} somewhere other than in the database directory, you must specify the path to the database directory, because @command{myisamchk} has no idea where the database is located. In fact, @command{myisamchk} doesn't actually care whether the files you are working on are located in a database directory. You can copy the files that correspond to a database table into some other location and perform recovery operations on them there.

You can name several tables on the @command{myisamchk} command line if you wish. You can also specify a table by naming its index file (the file with the `.MYI' suffix). This allows you to specify all tables in a directory by using the pattern `*.MYI'. For example, if you are in a database directory, you can check all the MyISAM tables in that directory like this: 

shell> myisamchk *.MYI

If you are not in the database directory, you can check all the tables there by specifying the path to the directory: 

shell> myisamchk /path/to/database_dir/*.MYI

You can even check all tables in all databases by specifying a wildcard with the path to the MySQL data directory: 

shell> myisamchk /path/to/datadir/*/*.MYI

The recommended way to quickly check all MyISAM and ISAM tables is: 

shell> myisamchk --silent --fast /path/to/datadir/*/*.MYI
shell> isamchk --silent /path/to/datadir/*/*.ISM

If you want to check all MyISAM and ISAM tables and repair any that are corrupted, you can use the following commands: 

shell> myisamchk --silent --force --fast --update-state \
          -O key_buffer=64M -O sort_buffer=64M \
          -O read_buffer=1M -O write_buffer=1M \
          /path/to/datadir/*/*.MYI
shell> isamchk --silent --force -O key_buffer=64M \
          -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M \
          /path/to/datadir/*/*.ISM

These commands assume that you have more than 64MB free. For more information about memory allocation with @command{myisamchk}, see section 5.7.2.6 @command{myisamchk} Memory Usage.

You must ensure that no other program is using the tables while you are running @command{myisamchk}. Otherwise, when you run @command{myisamchk}, it may display the following error message: 

warning: clients are using or haven't closed the table properly

This means that you are trying to check a table that has been updated by another program (such as the @command{mysqld} server) that hasn't yet closed the file or that has died without closing the file properly.

If @command{mysqld} is running, you must force it to flush any table modifications that are still buffered in memory by using FLUSH TABLES. You should then ensure that no one is using the tables while you are running @command{myisamchk}. The easiest way to avoid this problem is to use CHECK TABLE instead of @command{myisamchk} to check tables.

5.7.2.2 General Options for @command{myisamchk}

The options described in this section can be used for any type of table maintenance operation performed by @command{myisamchk}. The sections following this one describe options that pertain only to specific operations, such as table checking or repairing.

--help, -?
Display a help message and exit.
--debug=debug_options, -# debug_options
Write a debugging log. The debug_options string often is 'd:t:o,file_name'.
--silent, -s
Silent mode. Write output only when errors occur. You can use -s twice (-ss) to make @command{myisamchk} very silent.
--verbose, -v
Verbose mode. Print more information. This can be used with -d and -e. Use -v multiple times (-vv, -vvv) for even more output.
--version, -V
Display version information and exit.
--wait, -w
Instead of terminating with an error if the table is locked, wait until the table is unlocked before continuing. Note that if you are running @command{mysqld} with the --skip-external-locking option, the table can be locked only by another @command{myisamchk} command.

You can also set the following variables by using --var_name=value options:

Variable Default Value
decode_bits 9
ft_max_word_len version-dependent
ft_min_word_len 4
ft_stopword_file built-in list
key_buffer_size 523264
myisam_block_size 1024
read_buffer_size 262136
sort_buffer_size 2097144
sort_key_blocks 16
write_buffer_size 262136

It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. However, this syntax is deprecated as of MySQL 4.0.

The possible @command{myisamchk} variables and their default values can be examined with @command{myisamchk --help}:

sort_buffer_size is used when the keys are repaired by sorting keys, which is the normal case when you use --recover.

key_buffer_size is used when you are checking the table with --extend-check or when the keys are repaired by inserting keys row by row into the table (like when doing normal inserts). Repairing through the key buffer is used in the following cases:

Repairing through the key buffer takes much less disk space than using sorting, but is also much slower.

If you want a faster repair, set the key_buffer_size and sort_buffer_size variables to about 25% of your available memory. You can set both variables to large values, because only one of them is used at a time.

myisam_block_size is the size used for index blocks. It is available as of MySQL 4.0.0.

The ft_min_word_len and ft_max_word_len variables are available as of MySQL 4.0.0. ft_stopword_file is available as of MySQL 4.0.19.

ft_min_word_len and ft_max_word_len indicate the minimum and maximum word length for FULLTEXT indexes. ft_stopword_file names the stopword file. These need to be set under the following circumstances.

If you use @command{myisamchk} to perform an operation that modifies table indexes (such as repair or analyze), the FULLTEXT indexes are rebuilt using the default full-text parameter values for minimum and maximum word length and the stopword file unless you specify otherwise. This can result in queries failing.

The problem occurs because these parameters are known only by the server. They are not stored in MyISAM index files. To avoid the problem if you have modified the minimum or maximum word length or the stopword file in the server, specify the same ft_min_word_len, ft_max_word_len, and ft_stopword_file values to @command{myisamchk} that you use for @command{mysqld}. For example, if you have set the minimum word length to 3, you can repair a table with @command{myisamchk} like this: 

shell> myisamchk --recover --ft_min_word_len=3 tbl_name.MYI

To ensure that @command{myisamchk} and the server use the same values for full-text parameters, you can place each one in both the [mysqld] and [myisamchk] sections of an option file: 

[mysqld]
ft_min_word_len=3

[myisamchk]
ft_min_word_len=3

An alternative to using @command{myisamchk} is to use the REPAIR TABLE, ANALYZE TABLE, OPTIMIZE TABLE, or ALTER TABLE. These statements are performed by the server, which knows the proper full-text parameter values to use.

5.7.2.3 Check Options for @command{myisamchk}

@command{myisamchk} supports the following options for table checking operations:

--check, -c
Check the table for errors. This is the default operation if you specify no option that selects an operation type explicitly.
--check-only-changed, -C
Check only tables that have changed since the last check.
--extend-check, -e
Check the table very thoroughly. This is quite slow if the table has many indexes. This option should only be used in extreme cases. Normally, @command{myisamchk} or @command{myisamchk --medium-check} should be able to determine whether there are any errors in the table. If you are using --extend-check and have plenty of memory, setting the key_buffer_size variable to a large value will help the repair operation run faster.
--fast, -F
Check only tables that haven't been closed properly.
--force, -f
Do a repair operation automatically if @command{myisamchk} finds any errors in the table. The repair type is the same as that specified with the --repair or -r option.
--information, -i
Print informational statistics about the table that is checked.
--medium-check, -m
Do a check that is faster than an --extend-check operation. This finds only 99.99% of all errors, which should be good enough in most cases.
--read-only, -T
Don't mark the table as checked. This is useful if you use @command{myisamchk} to check a table that is in use by some other application that doesn't use locking, such as @command{mysqld} when run with the --skip-external-locking option.
--update-state, -U
Store information in the `.MYI' file to indicate when the table was checked and whether the table crashed. This should be used to get full benefit of the --check-only-changed option, but you shouldn't use this option if the @command{mysqld} server is using the table and you are running it with the --skip-external-locking option.

5.7.2.4 Repair Options for @command{myisamchk}

@command{myisamchk} supports the following options for table repair operations:

--backup, -B
Make a backup of the `.MYD' file as `file_name-time.BAK'
--character-sets-dir=path
The directory where character sets are installed. See section 5.8.1 The Character Set Used for Data and Sorting.
--correct-checksum
Correct the checksum information for the table.
--data-file-length=#, -D #
Maximum length of the data file (when re-creating data file when it's ``full'').
--extend-check, -e
Do a repair that tries to to recover every possible row from the data file. Normally this will also find a lot of garbage rows. Don't use this option unless you are totally desperate.
--force, -f
Overwrite old temporary files (files with names like `tbl_name.TMD') instead of aborting.
--keys-used=#, -k #
For @command{myisamchk}, the option value indicates which indexes to update. Each binary bit of the option value corresponds to a table index, where the first index is bit 0. For @command{isamchk}, the option value indicates that only the first # of the table indexes should be updated. In either case, an option value of 0 disables updates to all indexes, which can be used to get faster inserts. Deactivated indexes can be reactivated by using @command{myisamchk -r} or (@command{isamchk -r}).
--no-symlinks, -l
Do not follow symbolic links. Normally @command{myisamchk} repairs the table that a symlink points to. This option doesn't exist as of MySQL 4.0, because versions from 4.0 on will not remove symlinks during repair operations.
--parallel-recover, -p
Uses the same technique as -r and -n, but creates all the keys in parallel, using different threads. This option was added in MySQL 4.0.2. This is alpha code. Use at your own risk!
--quick, -q
Achieve a faster repair by not modifying the data file. You can specify this option twice to force @command{myisamchk} to modify the original data file in case of duplicate keys.
--recover, -r
Do a repair that can fix almost any problem except unique keys that aren't unique (which is an extremely unlikely error with ISAM/MyISAM tables). If you want to recover a table, this is the option to try first. You should try -o only if @command{myisamchk} reports that the table can't be recovered by -r. (In the unlikely case that -r fails, the data file is still intact.) If you have lots of memory, you should increase the value of sort_buffer_size.
--safe-recover, -o
Do a repair using an old recovery method that reads through all rows in order and updates all index trees based on the rows found. This is an order of magnitude slower than -r, but can handle a couple of very unlikely cases that -r cannot. This recovery method also uses much less disk space than -r. Normally, you should repair first with -r, and then with -o only if -r fails. If you have lots of memory, you should increase the value of key_buffer_size.
--set-character-set=name
Change the character set used by the table indexes.
--sort-recover, -n
Force @command{myisamchk} to use sorting to resolve the keys even if the temporary files should be very big.
--tmpdir=path, -t path
Path of the directory to be used for storing temporary files. If this is not set, @command{myisamchk} uses the value of the TMPDIR environment variable. Starting from MySQL 4.1, tmpdir can be set to a list of directory paths that will be used successively in round-robin fashion for creating temporary files. The separator character between directory names should be colon (`:') on Unix and semicolon (`;') on Windows, NetWare, and OS/2.
--unpack, -u
Unpack a table that was packed with @command{myisampack}.

5.7.2.5 Other Options for @command{myisamchk}

@command{myisamchk} supports the following options for actions other than table checks and repairs:

--analyze, -a
Analyze the distribution of keys. This improves join performance by enabling the join optimizer to better choose the order in which to join the tables and which keys it should use. To obtain information about the distribution, use a @command{myisamchk --description --verbose tbl_name} command or the SHOW KEYS FROM tbl_name statement.
--description, -d
Print some descriptive information about the table.
--set-auto-increment[=value], -A[value]
Force AUTO_INCREMENT numbering for new records to start at the given value (or higher, if there are already records with AUTO_INCREMENT values this large). If value is not specified, AUTO_INCREMENT number for new records begins with the largest value currently in the table, plus one.
--sort-index, -S
Sort the index tree blocks in high-low order. This optimizes seeks and makes table scanning by key faster.
--sort-records=#, -R #
Sort records according to a particular index. This makes your data much more localized and may speed up range-based SELECT and ORDER BY operations that use this index. (The first time you use this option to sort a table, it may be very slow.) To determine a table's index numbers, use SHOW KEYS, which displays a table's indexes in the same order that @command{myisamchk} sees them. Indexes are numbered beginning with 1.

5.7.2.6 @command{myisamchk} Memory Usage

Memory allocation is important when you run @command{myisamchk}. @command{myisamchk} uses no more memory than you specify with the -O options. If you are going to use @command{myisamchk} on very large tables, you should first decide how much memory you want it to use. The default is to use only about 3MB to perform repairs. By using larger values, you can get @command{myisamchk} to operate faster. For example, if you have more than 32MB RAM, you could use options such as these (in addition to any other options you might specify): 

shell> myisamchk -O sort=16M -O key=16M -O read=1M -O write=1M ...

Using -O sort=16M should probably be enough for most cases.

Be aware that @command{myisamchk} uses temporary files in TMPDIR. If TMPDIR points to a memory filesystem, you may easily get out of memory errors. If this happens, set TMPDIR to point at some directory located on a filesystem with more space and run @command{myisamchk} again.

When repairing, @command{myisamchk} will also need a lot of disk space:

If you have a problem with disk space during repair, you can try to use --safe-recover instead of --recover.

5.7.2.7 Using @command{myisamchk} for Crash Recovery

If you run @command{mysqld} with --skip-external-locking (which is the default on some systems, such as Linux), you can't reliably use @command{myisamchk} to check a table when @command{mysqld} is using the same table. If you can be sure that no one is accessing the tables through @command{mysqld} while you run @command{myisamchk}, you only have to do @command{mysqladmin flush-tables} before you start checking the tables. If you can't guarantee this, then you must stop @command{mysqld} while you check the tables. If you run @command{myisamchk} while @command{mysqld} is updating the tables, you may get a warning that a table is corrupt even when it isn't.

If you are not using --skip-external-locking, you can use @command{myisamchk} to check tables at any time. While you do this, all clients that try to update the table will wait until @command{myisamchk} is ready before continuing.

If you use @command{myisamchk} to repair or optimize tables, you must always ensure that the @command{mysqld} server is not using the table (this also applies if you are using --skip-external-locking). If you don't take down @command{mysqld}, you should at least do a @command{mysqladmin flush-tables} before you run @command{myisamchk}. Your tables may become corrupted if the server and @command{myisamchk} access the tables simultaneously.

This section describes how to check for and deal with data corruption in MySQL databases. If your tables get corrupted frequently you should try to find the reason why. See section A.4.2 What to Do If MySQL Keeps Crashing.

The MyISAM table section contains reason for why a table could be corrupted. See section 15.1.4 MyISAM Table Problems.

When performing crash recovery, it is important to understand that each MyISAM table tbl_name in a database corresponds to three files in the database directory:

File Purpose
`tbl_name.frm' Definition (format) file
`tbl_name.MYD' Data file
`tbl_name.MYI' Index file

Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.

@command{myisamchk} works by creating a copy of the `.MYD' data file row by row. It ends the repair stage by removing the old `.MYD' file and renaming the new file to the original file name. If you use --quick, @command{myisamchk} does not create a temporary `.MYD' file, but instead assumes that the `.MYD' file is correct and only generates a new index file without touching the `.MYD' file. This is safe, because @command{myisamchk} automatically detects whether the `.MYD' file is corrupt and aborts the repair if it is. You can also specify the --quick option twice to @command{myisamchk}. In this case, @command{myisamchk} does not abort on some errors (such as duplicate-key errors) but instead tries to resolve them by modifying the `.MYD' file. Normally the use of two --quick options is useful only if you have too little free disk space to perform a normal repair. In this case, you should at least make a backup before running @command{myisamchk}.

5.7.2.8 How to Check MyISAM Tables for Errors

To check a MyISAM table, use the following commands:

myisamchk tbl_name
This finds 99.99% of all errors. What it can't find is corruption that involves only the data file (which is very unusual). If you want to check a table, you should normally run @command{myisamchk} without options or with either the -s or --silent option.
myisamchk -m tbl_name
This finds 99.999% of all errors. It first checks all index entries for errors and then reads through all rows. It calculates a checksum for all keys in the rows and verifies that the checksum matches the checksum for the keys in the index tree.
myisamchk -e tbl_name
This does a complete and thorough check of all data (-e means ``extended check''). It does a check-read of every key for each row to verify that they indeed point to the correct row. This may take a long time for a large table that has many indexes. Normally, @command{myisamchk} stops after the first error it finds. If you want to obtain more information, you can add the --verbose (-v) option. This causes @command{myisamchk} to keep going, up through a maximum of 20 errors.
myisamchk -e -i tbl_name
Like the previous command, but the -i option tells @command{myisamchk} to print some informational statistics, too.

In most cases, a simple @command{myisamchk} with no arguments other than the table name is sufficient to check a table.

5.7.2.9 How to Repair Tables

The discussion in this section describes how to use @command{myisamchk} on MyISAM tables (extensions `.MYI' and `.MYD'). If you are using ISAM tables (extensions `.ISM' and `.ISD'), you should use @command{isamchk} instead; the concepts are similar.

If you are using MySQL 3.23.16 and above, you can (and should) use the CHECK TABLE and REPAIR TABLE statements to check and repair MyISAM tables. See section 14.5.2.3 CHECK TABLE Syntax and section 14.5.2.6 REPAIR TABLE Syntax.

The symptoms of a corrupted table include queries that abort unexpectedly and observable errors such as these:

To get more information about the error you can run @command{perror} ###, where ### is the error number. The following example shows how to use @command{perror} to find the meanings for the most common error numbers that indicate a problem with a table: 

shell> perror 126 127 132 134 135 136 141 144 145
126 = Index file is crashed / Wrong file format
127 = Record-file is crashed
132 = Old database file
134 = Record was already deleted (or record file crashed)
135 = No more room in record file
136 = No more room in index file
141 = Duplicate unique key or constraint on write or update
144 = Table is crashed and last repair failed
145 = Table was marked as crashed and should be repaired

Note that error 135 (no more room in record file) and error 136 (no more room in index file) are not errors that can be fixed by a simple repair. In this case, you have to use ALTER TABLE to increase the MAX_ROWS and AVG_ROW_LENGTH table option values: 

ALTER TABLE tbl_name MAX_ROWS=xxx AVG_ROW_LENGTH=yyy;

If you don't know the current table option values, use SHOW CREATE TABLE tbl_name.

For the other errors, you must repair your tables. @command{myisamchk} can usually detect and fix most problems that occur.

The repair process involves up to four stages, described here. Before you begin, you should change location to the database directory and check the permissions of the table files. On Unix, make sure that they are readable by the user that @command{mysqld} runs as (and to you, because you need to access the files you are checking). If it turns out you need to modify files, they must also be writable by you.

The options that you can use for table maintenance with @command{myisamchk} and @command{isamchk} are described in several of the earlier subsections of section 5.7.2 Table Maintenance and Crash Recovery.

The following section is for the cases where the above command fails or if you want to use the extended features that @command{myisamchk} and @command{isamchk} provide.

If you are going to repair a table from the command line, you must first stop the @command{mysqld} server. Note that when you do @command{mysqladmin shutdown} on a remote server, the @command{mysqld} server will still be alive for a while after @command{mysqladmin} returns, until all queries are stopped and all keys have been flushed to disk.

Stage 1: Checking your tables

Run @command{myisamchk *.MYI} or @command{myisamchk -e *.MYI} if you have more time. Use the -s (silent) option to suppress unnecessary information.

If the @command{mysqld} server is down, you should use the --update-state option to tell @command{myisamchk} to mark the table as 'checked'.

You have to repair only those tables for which @command{myisamchk} announces an error. For such tables, proceed to Stage 2.

If you get weird errors when checking (such as out of memory errors), or if @command{myisamchk} crashes, go to Stage 3.

Stage 2: Easy safe repair

Note: If you want a repair operation to go much faster, you should set the values of the sort_buffer_size and key_buffer_size variables each to about 25% of your available memory when running @command{myisamchk} or @command{isamchk}.

First, try @command{myisamchk -r -q tbl_name} (-r -q means ``quick recovery mode''). This will attempt to repair the index file without touching the data file. If the data file contains everything that it should and the delete links point at the correct locations within the data file, this should work, and the table is fixed. Start repairing the next table. Otherwise, use the following procedure:

  1. Make a backup of the data file before continuing.
  2. Use @command{myisamchk -r tbl_name} (-r means ``recovery mode''). This will remove incorrect records and deleted records from the data file and reconstruct the index file.
  3. If the preceding step fails, use @command{myisamchk --safe-recover tbl_name}. Safe recovery mode uses an old recovery method that handles a few cases that regular recovery mode doesn't (but is slower).

If you get weird errors when repairing (such as out of memory errors), or if @command{myisamchk} crashes, go to Stage 3.

Stage 3: Difficult repair

You should reach this stage only if the first 16KB block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it's necessary to create a new index file. Do so as follows:

  1. Move the data file to some safe place.
  2. Use the table description file to create new (empty) data and index files: 
    shell> mysql db_name
mysql> SET AUTOCOMMIT=1;
mysql> TRUNCATE TABLE tbl_name;
mysql> quit
    If your version of MySQL doesn't have TRUNCATE TABLE, use DELETE FROM tbl_name instead.
  3. Copy the old data file back onto the newly created data file. (Don't just move the old file back onto the new file; you want to retain a copy in case something goes wrong.)

Go back to Stage 2. @command{myisamchk -r -q} should work now. (This shouldn't be an endless loop.)

As of MySQL 4.0.2, you can also use REPAIR TABLE tbl_name USE_FRM, which performs the whole procedure automatically.

Stage 4: Very difficult repair

You should reach this stage only if the `.frm' description file has also crashed. That should never happen, because the description file isn't changed after the table is created:

  1. Restore the description file from a backup and go back to Stage 3. You can also restore the index file and go back to Stage 2. In the latter case, you should start with @command{myisamchk -r}.
  2. If you don't have a backup but know exactly how the table was created, create a copy of the table in another database. Remove the new data file, then move the `.frm' description and `.MYI' index files from the other database to your crashed database. This gives you new description and index files, but leaves the `.MYD' data file alone. Go back to Stage 2 and attempt to reconstruct the index file.

5.7.2.10 Table Optimization

To coalesce fragmented records and eliminate wasted space resulting from deleting or updating records, run @command{myisamchk} in recovery mode: 

shell> myisamchk -r tbl_name

You can optimize a table in the same way by using the SQL OPTIMIZE TABLE statement. OPTIMIZE TABLE does a repair of the table and a key analysis, and also sorts the index tree to give faster key lookups. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use OPTIMIZE TABLE. See section 14.5.2.5 OPTIMIZE TABLE Syntax.

@command{myisamchk} also has a number of other options you can use to improve the performance of a table:

For a full description of the options, see section 5.7.2.1 @command{myisamchk} Invocation Syntax.

5.7.3 Setting Up a Table Maintenance Schedule

It is a good idea to perform table checks on a regular basis rather than waiting for problems to occur. One way to check and repair MyISAM tables is with the CHECK TABLE and REPAIR TABLE statements. These are available starting with MySQL 3.23.16. See section 14.5.2.3 CHECK TABLE Syntax and section 14.5.2.6 REPAIR TABLE Syntax.

Another way to check tables is to use @command{myisamchk}. For maintenance purposes, you can use @command{myisamchk -s}. The -s option (short for --silent) causes @command{myisamchk} to run in silent mode, printing messages only when errors occur.

It's also a good idea to check tables when the server starts. For example, whenever the machine has done a restart in the middle of an update, you usually need to check all the tables that could have been affected. (These are ``expected crashed tables.'') To check MyISAM tables automatically, start the server with the --myisam-recover option, available as of MySQL 3.23.25. If your server is too old to support this option, you could add a test to @command{mysqld_safe} that runs @command{myisamchk} to check all tables that have been modified during the last 24 hours if there is an old `.pid' (process ID) file left after a restart. (The `.pid' file is created by @command{mysqld} when it starts and removed when it terminates normally. The presence of a `.pid' file at system startup time indicates that @command{mysqld} terminated abnormally.)

An even better test would be to check any table whose last-modified time is more recent than that of the `.pid' file.

You should also check your tables regularly during normal system operation. At MySQL AB, we run a @command{cron} job to check all our important tables once a week, using a line like this in a `crontab' file: 

35 0 * * 0 /path/to/myisamchk --fast --silent /path/to/datadir/*/*.MYI

This prints out information about crashed tables so that we can examine and repair them when needed.

Because we haven't had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough for us.

We recommend that to start with, you execute @command{myisamchk -s} each night on all tables that have been updated during the last 24 hours, until you come to trust MySQL as much as we do.

Normally, MySQL tables need little maintenance. If you are changing MyISAM tables with dynamic-sized rows (tables with VARCHAR, BLOB, or TEXT columns) or have tables with many deleted rows you may want to defragment/reclaim space from the tables from time to time (once a month?).

You can do this by using OPTIMIZE TABLE on the tables in question. Or, if you can stop the @command{mysqld} server for a while, change location into the data directory and use this command while the server is stopped: 

shell> myisamchk -r -s --sort-index -O sort_buffer_size=16M */*.MYI

For ISAM tables, the command is similar: 

shell> isamchk -r -s --sort-index -O sort_buffer_size=16M */*.ISM

5.7.4 Getting Information About a Table

To obtain a description of a table or statistics about it, use the commands shown here. We explain some of the information in more detail later:

Sample output for some of these commands follows. They are based on a table with these data and index file sizes: 

-rw-rw-r--   1 monty    tcx     317235748 Jan 12 17:30 company.MYD
-rw-rw-r--   1 davida   tcx      96482304 Jan 12 18:35 company.MYM

Example of @command{myisamchk -d} output: 

MyISAM file:     company.MYI
Record format:   Fixed length
Data records:    1403698  Deleted blocks:         0
Recordlength:    226

table description:
Key Start Len Index   Type
1   2     8   unique  double
2   15    10  multip. text packed stripped
3   219   8   multip. double
4   63    10  multip. text packed stripped
5   167   2   multip. unsigned short
6   177   4   multip. unsigned long
7   155   4   multip. text
8   138   4   multip. unsigned long
9   177   4   multip. unsigned long
    193   1           text

Example of @command{myisamchk -d -v} output: 

MyISAM file:         company
Record format:       Fixed length
File-version:        1
Creation time:       1999-10-30 12:12:51
Recover time:        1999-10-31 19:13:01
Status:              checked
Data records:            1403698  Deleted blocks:              0
Datafile parts:          1403698  Deleted data:                0
Datafile pointer (bytes):      3  Keyfile pointer (bytes):     3
Max datafile length:  3791650815  Max keyfile length: 4294967294
Recordlength:                226

table description:
Key Start Len Index   Type                  Rec/key     Root Blocksize
1   2     8   unique  double                      1 15845376      1024
2   15    10  multip. text packed stripped        2 25062400      1024
3   219   8   multip. double                     73 40907776      1024
4   63    10  multip. text packed stripped        5 48097280      1024
5   167   2   multip. unsigned short           4840 55200768      1024
6   177   4   multip. unsigned long            1346 65145856      1024
7   155   4   multip. text                     4995 75090944      1024
8   138   4   multip. unsigned long              87 85036032      1024
9   177   4   multip. unsigned long             178 96481280      1024
    193   1           text

Example of @command{myisamchk -eis} output: 

Checking MyISAM file: company
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:  98%  Packed:   17%

Records:          1403698    M.recordlength:     226
Packed:             0%
Recordspace used:     100%   Empty space:          0%
Blocks/Record:   1.00
Record blocks:    1403698    Delete blocks:        0
Recorddata:     317235748    Deleted data:         0
Lost space:             0    Linkdata:             0

User time 1626.51, System time 232.36
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 627, Swaps 0
Blocks in 0 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 639, Involuntary context switches 28966

Example of @command{myisamchk -eiv} output: 

Checking MyISAM file: company
Data records: 1403698   Deleted blocks:       0
- check file-size
- check delete-chain
block_size 1024:
index  1:
index  2:
index  3:
index  4:
index  5:
index  6:
index  7:
index  8:
index  9:
No recordlinks
- check index reference
- check data record references index: 1
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 2
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
- check data record references index: 3
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
- check data record references index: 5
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 6
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 7
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 8
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 9
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:   9%  Packed:   17%

- check records and index references
[LOTS OF ROW NUMBERS DELETED]

Records:         1403698   M.recordlength:   226   Packed:           0%
Recordspace used:    100%  Empty space:        0%  Blocks/Record: 1.00
Record blocks:   1403698   Delete blocks:      0
Recorddata:    317235748   Deleted data:       0
Lost space:            0   Linkdata:           0

User time 1639.63, System time 251.61
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0
Blocks in 4 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 10604, Involuntary context switches 122798

Explanations for the types of information @command{myisamchk} produces are given here. ``Keyfile'' refers to the index file. ``Record'' and ``row'' are synonymous.

If a table has been compressed with @command{myisampack}, @command{myisamchk -d} prints additional information about each table column. See @ref{myisampack, , @command{myisampack}}, for an example of this information and a description of what it means.

5.8 MySQL Localization and International Usage

This section describes how to configure the server to use different character sets. It also discusses how to set the server's time zone and enable per-connection time zone support.

5.8.1 The Character Set Used for Data and Sorting

By default, MySQL uses the ISO-8859-1 (Latin1) character set with sorting according to Swedish/Finnish rules. These defaults are suitable for the United States and most of western Europe.

All MySQL binary distributions are compiled with --with-extra-charsets=complex. This adds code to all standard programs that enables them to handle latin1 and all multi-byte character sets within the binary. Other character sets will be loaded from a character-set definition file when needed.

The character set determines what characters are allowed in names. It also determines how strings are sorted by the ORDER BY and GROUP BY clauses of the SELECT statement.

You can change the character set with the --default-character-set option when you start the server. The character sets available depend on the --with-charset=charset and --with-extra-charsets= list-of-charsets | complex | all | none options to @command{configure}, and the character set configuration files listed in `SHAREDIR/charsets/Index'. @xref{configure options, , @command{configure} options}.

As of MySQL 4.1.1, you can also change the character set collation with the --default-collation option when you start the server. The collation must be a legal collation for the default character set. (Use the SHOW COLLATION statement to determine which collations are available for each character set.) @xref{configure options, , @command{configure} options}.

If you change the character set when running MySQL, that may also change the sort order. Consequently, you must run @command{myisamchk -r -q --set-character-set=charset} on all tables, or your indexes may not be ordered correctly.

When a client connects to a MySQL server, the server indicates to the client what the server's default character set is. The client will switch to use this character set for this connection.

You should use mysql_real_escape_string() when escaping strings for an SQL query. mysql_real_escape_string() is identical to the old mysql_escape_string() function, except that it takes the MYSQL connection handle as the first parameter so that the appropriate character set can be taken into account when escaping characters.

If the client is compiled with different paths than where the server is installed and the user who configured MySQL didn't include all character sets in the MySQL binary, you must tell the client where it can find the additional character sets it will need if the server runs with a different character set than the client.

You can do this by specifying a --character-sets-dir option to indicate the path to the directory in which the dynamic MySQL character sets are stored. For example, you can put the following in an option file: 

[client]
character-sets-dir=/usr/local/mysql/share/mysql/charsets

You can force the client to use specific character set as follows: 

[client]
default-character-set=charset

This is normally unnecessary, however.

5.8.1.1 Using the German Character Set

In MySQL 4.0, to get German sorting order, you should start @command{mysqld} with a --default-character-set=latin1_de option. This affects server behavior in several ways:

In MySQL 4.1 and up, character set and collation are specified separately. You should select the latin1 character set and either the latin1_german1_ci or latin1_german2_ci collation. For example, to start the server with the latin1_german1_ci collation, use the --character-set-server=latin1 and --collation-server=latin1_german1_ci options.

For information on the differences between these two collations, see section 11.11.2 West European Character Sets.

5.8.2 Setting the Error Message Language

By default, @command{mysqld} produces error messages in English, but they can also be displayed in any of these other languages: Czech, Danish, Dutch, Estonian, French, German, Greek, Hungarian, Italian, Japanese, Korean, Norwegian, Norwegian-ny, Polish, Portuguese, Romanian, Russian, Slovak, Spanish, or Swedish.

To start @command{mysqld} with a particular language for error messages, use the --language or -L option. The option value can be a language name or the full path to the error message file. For example: 

shell> mysqld --language=swedish

Or: 

shell> mysqld --language=/usr/local/share/swedish

The language name should be specified in lowercase.

The language files are located (by default) in the `share/LANGUAGE' directory under the MySQL base directory.

To change the error message file, you should edit the `errmsg.txt' file, and then execute the following command to generate the `errmsg.sys' file: 

shell> comp_err errmsg.txt errmsg.sys

If you upgrade to a newer version of MySQL, remember to repeat your changes with the new `errmsg.txt' file.

5.8.3 Adding a New Character Set

This section discusses the procedure for adding add another character set to MySQL. You must have a MySQL source distribution to use these instructions.

To choose the proper procedure, decide whether the character set is simple or complex:

For example, latin1 and danish are simple character sets, whereas big5 and czech are complex character sets.

In the following procedures, the name of your character set is represented by MYSET.

For a simple character set, do the following:

  1. Add MYSET to the end of the `sql/share/charsets/Index' file. Assign a unique number to it.
  2. Create the file `sql/share/charsets/MYSET.conf'. (You can use a copy of `sql/share/charsets/latin1.conf' as the basis for this file.) The syntax for the file is very simple:
    • Comments start with a `#' character and proceed to the end of the line.
    • Words are separated by arbitrary amounts of whitespace.
    • When defining the character set, every word must be a number in hexadecimal format.
    • The ctype array takes up the first 257 words. The to_lower[], to_upper[] and sort_order[] arrays take up 256 words each after that.
    See section 5.8.4 The Character Definition Arrays.
  3. Add the character set name to the CHARSETS_AVAILABLE and COMPILED_CHARSETS lists in configure.in.
  4. Reconfigure, recompile, and test.

For a complex character set, do the following:

  1. Create the file `strings/ctype-MYSET.c' in the MySQL source distribution.
  2. Add MYSET to the end of the `sql/share/charsets/Index' file. Assign a unique number to it.
  3. Look at one of the existing `ctype-*.c' files (such as `strings/ctype-big5.c') to see what needs to be defined. Note that the arrays in your file must have names like ctype_MYSET, to_lower_MYSET, and so on. These correspond to the arrays for a simple character set. See section 5.8.4 The Character Definition Arrays.
  4. Near the top of the file, place a special comment like this: 
    /*
 * This comment is parsed by configure to create ctype.c,
 * so don't change it unless you know what you are doing.
 *
 * .configure. number_MYSET=MYNUMBER
 * .configure. strxfrm_multiply_MYSET=N
 * .configure. mbmaxlen_MYSET=N
 */
    The @command{configure} program uses this comment to include the character set into the MySQL library automatically. The strxfrm_multiply and mbmaxlen lines are explained in the following sections. You need include them only if you need the string collating functions or the multi-byte character set functions, respectively.
  5. You should then create some of the following functions:
    • my_strncoll_MYSET()
    • my_strcoll_MYSET()
    • my_strxfrm_MYSET()
    • my_like_range_MYSET()
    See section 5.8.5 String Collating Support.
  6. Add the character set name to the CHARSETS_AVAILABLE and COMPILED_CHARSETS lists in configure.in.
  7. Reconfigure, recompile, and test.

The `sql/share/charsets/README' file includes additional instructions.

If you want to have the character set included in the MySQL distribution, mail a patch to the MySQL internals mailing list. See section 1.7.1.1 The MySQL Mailing Lists.

5.8.4 The Character Definition Arrays

to_lower[] and to_upper[] are simple arrays that hold the lowercase and uppercase characters corresponding to each member of the character set. For example: 

to_lower['A'] should contain 'a'
to_upper['a'] should contain 'A'

sort_order[] is a map indicating how characters should be ordered for comparison and sorting purposes. Quite often (but not for all character sets) this is the same as to_upper[], which means that sorting will be case-insensitive. MySQL will sort characters based on the values of sort_order[] elements. For more complicated sorting rules, see the discussion of string collating in section 5.8.5 String Collating Support.

ctype[] is an array of bit values, with one element for one character. (Note that to_lower[], to_upper[], and sort_order[] are indexed by character value, but ctype[] is indexed by character value + 1. This is an old legacy convention to be able to handle EOF.)

You can find the following bitmask definitions in `m_ctype.h': 

#define _U      01      /* Uppercase */
#define _L      02      /* Lowercase */
#define _N      04      /* Numeral (digit) */
#define _S      010     /* Spacing character */
#define _P      020     /* Punctuation */
#define _C      040     /* Control character */
#define _B      0100    /* Blank */
#define _X      0200    /* heXadecimal digit */

The ctype[] entry for each character should be the union of the applicable bitmask values that describe the character. For example, 'A' is an uppercase character (_U) as well as a hexadecimal digit (_X), so ctype['A'+1] should contain the value: 

_U + _X = 01 + 0200 = 0201

5.8.5 String Collating Support

If the sorting rules for your language are too complex to be handled with the simple sort_order[] table, you need to use the string collating functions.

Right now the best documentation for this is the character sets that are already implemented. Look at the big5, czech, gbk, sjis, and tis160 character sets for examples.

You must specify the strxfrm_multiply_MYSET=N value in the special comment at the top of the file. N should be set to the maximum ratio the strings may grow during my_strxfrm_MYSET (it must be a positive integer).

5.8.6 Multi-Byte Character Support

If you want to add support for a new character set that includes multi-byte characters, you need to use the multi-byte character functions.

Right now the best documentation on this consists of the character sets that are already implemented. Look at the euc_kr, gb2312, gbk, sjis, and ujis character sets for examples. These are implemented in the `ctype-charset.c' files in the `strings' directory.

You must specify the mbmaxlen_MYSET=N value in the special comment at the top of the source file. N should be set to the size in bytes of the largest character in the set.

5.8.7 Problems With Character Sets

If you try to use a character set that is not compiled into your binary, you might run into the following problems:

For MyISAM tables, you can check the character set name and number for a table with @command{myisamchk -dvv tbl_name}.

5.8.8 MySQL Server Time Zone Support

Before MySQL 4.1.3, you can set the time zone for the server with the --timezone=timezone_name option to @command{mysqld_safe}. You can also set it by setting the TZ environment variable before you start @command{mysqld}.

The allowable values for --timezone or TZ are system-dependent. Consult your operating system documentation to see what values are acceptable.

Beginning with MySQL 4.1.3, the server maintains several time zone settings:

The current values of the global and per-connection time zones can be retrieved like this: 

mysql> SELECT @@global.time_zone, @@session.time_zone;

timezone values can be given as strings indicating an offset from UTC, such as '+10:00' or '-6:00'. If the time zone-related tables in the mysql database have been created and populated, you can also used named time zones, such as 'Europe/Helsinki', 'US/Eastern', or 'MET'. The value 'SYSTEM' indicates that the time zone should be the same as the system time zone. Time zone names are not case sensitive.

The MySQL installation procedure creates the time zone tables in the mysql database, but does not load them. You must do so manually. (If you are upgrading to MySQL 4.1.3 or later from an earlier version, you should create the tables by upgrading your mysql database. Use the instructions in section 2.5.8 Upgrading the Grant Tables.)

Note: Currently, time time zone tables can be populated only on Unix. This problem will be addressed soon for Windows.

The @command{mysql_tzinfo_to_sql} program is used to load the time zone tables. You will need to know the directory under which your system's time zone files are stored. A likely location is `/usr/share/zoneinfo'. Pass the directory name on the command line to @command{mysql_tzinfo_to_sql}, and send the output into the @command{mysql} program. For example: 

shell> mysql_tzinfo_to_sql /usr/share/zoneinfo | mysql -u root mysql

@command{mysql_tzinfo_to_sql} reads your system's time zone files and generates SQL statements from them. @command{mysql} processes those statements to load the time zone tables.

@command{mysql_tzinfo_to_sql} also can be used to load a single time zone file, and to generate leap second information.

To load a single time zone file tz_file that corresponds to a time zone name tz_name, invoke @command{mysql_tzinfo_to_sql} like this: 

shell> mysql_tzinfo_to_sql tz_file tz_name | mysql -u root mysql

If your time zone needs to account for leap seconds, initialize the leap second information like this, where tz_file is the name of your time zone file: 

shell> mysql_tzinfo_to_sql --leap tz_file | mysql -u root mysql

5.9 The MySQL Log Files

MySQL has several different log files that can help you find out what's going on inside @command{mysqld}:

Log File Types of Information Logged to File
The error log Logs problems encountered starting, running, or stopping @command{mysqld}.
The isam log Logs all changes to the ISAM tables. Used only for debugging the isam code.
The query log Logs established client connections and executed statements.
The update log Logs statements that change data. This log is deprecated.
The binary log Logs all statements that change data. Also used for replication.
The slow log Logs all queries that took more than long_query_time seconds to execute or didn't use indexes.

By default, all logs are created in the @command{mysqld} data directory. You can force @command{mysqld} to close and reopen the log files (or in some cases switch to a new log) by flushing the logs. Log flushing occurs when you issue a FLUSH LOGS statement or execute @command{mysqladmin flush-logs} or @command{mysqladmin refresh}. See section 14.5.4.2 FLUSH Syntax.

If you are using MySQL replication capabilities, slave replication servers maintain additional log files called relay logs. These are discussed in section 6 Replication in MySQL.

5.9.1 The Error Log

The error log file contains information indicating when @command{mysqld} was started and stopped and also any critical errors that occur while the server is running.

If @command{mysqld} dies unexpectedly and @command{mysqld_safe} needs to restart it, @command{mysqld_safe} will write a @code{restarted mysqld} message to the error log. If @command{mysqld} notices a table that needs to be automatically checked or repaired, it writes a message to the error log.

On some operating systems, the error log will contain a stack trace if @command{mysqld} dies. The trace can be used to determine where @command{mysqld} died. See section D.1.4 Using a Stack Trace.

Beginning with MySQL 4.0.10, you can specify where @command{mysqld} stores the error log file with the --log-error[=file_name] option. If no file_name value is given, @command{mysqld} uses the name `host_name.err' and writes the file in the data directory. (Prior to MySQL 4.0.10, the Windows error log name is `mysql.err'.) If you execute FLUSH LOGS, the error log is renamed with a suffix of -old and @command{mysqld} creates a new empty log file.

In older MySQL versions on Unix, error log handling was done by @command{mysqld_safe} which redirected the error file to host_name.err. You could change this filename by specifying a --err-log=file_name option to @command{mysqld_safe}.

If you don't specify --log-error, or (on Windows) if you use the --console option, errors are written to stderr, the standard error output. Usually this is your terminal.

On Windows, error output is always written to the .err file if --console is not given.

5.9.2 The General Query Log

If you want to know what happens within @command{mysqld}, you should start it with the --log[=file_name] or -l [file_name] option. If no file_name value is given, the default name is `host_name.log' This will log all connections and statements to the log file. This log can be very useful when you suspect an error in a client and want to know exactly what the client sent to @command{mysqld}.

Older versions of the @command{mysql.server} script (from MySQL 3.23.4 to 3.23.8) pass a --log option to @command{safe_mysqld} to enable the general query log. If you need better performance when you start using MySQL in a production environment, you can remove the --log option from @command{mysql.server} or change it to --log-bin. See section 5.9.4 The Binary Log.

@command{mysqld} writes statements to the query log in the order that it receives them. This may be different from the order in which they are executed. This is in contrast to the update log and the binary log, which are written after the query is executed, but before any locks are released.

Server restarts and log flushing do not cause a new general query log file to be generated (although flushing closes and reopens it). On Unix, you can rename the file and create a new one by using the following commands: 

shell> mv hostname.log hostname-old.log
shell> mysqladmin flush-logs
shell> cp hostname-old.log to-backup-directory
shell> rm hostname-old.log

On Windows, you cannot rename the log file while the server has it open. You must stop the server and rename the log. Then restart the server to create a new log.

5.9.3 The Update Log

Note: The update log has been deprecated and replaced by the binary log. See section 5.9.4 The Binary Log. The binary log can do anything the old update log could do, and more. The update log is unavailable as of MySQL 5.0.0.

When started with the --log-update[=file_name] option, @command{mysqld} writes a log file containing all SQL statements that update data. If no file_name value is given, the default name is name of the host machine. If a filename is given, but it doesn't contain a leading path, the file is written in the data directory. If `file_name' doesn't have an extension, @command{mysqld} creates log files with names of the form file_name.###, where ### is a number that is incremented each time you start the server or flush the logs.

Note: For this naming scheme to work, you must not create your own files with the same names as those that might be used for the log file sequence.

Update logging is smart because it logs only statements that really update data. So, an UPDATE or a DELETE with a WHERE that finds no rows is not written to the log. It even skips UPDATE statements that set a column to the value it already has.

The update logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that statements are logged in execution order.

If you want to update a database from update log files, you could do the following (assuming that your update logs have names of the form `file_name.###'): 

shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql

ls is used to sort the update log filenames into the right order.

This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.

5.9.4 The Binary Log

The binary log has replaced the old update log, which is unavailable starting from MySQL 5.0. The binary log contains all information that is available in the update log in a more efficient format and in a manner that is transactionally safe.

The binary log contains all statements which updated data or (starting from MySQL 4.1.3) could potentially have updated it (for example, a DELETE which matched no rows).

The binary log also contains information about how long each statement took that updated the database. It doesn't contain statements that don't modify any data. If you want to log all statements (for example, to identify a problem query) you should use the general query log. See section 5.9.2 The General Query Log.

The primary purpose of the binary log is to be able to update the database during a restore operation as fully as possible, because the binary log will contain all updates done after a backup was made.

The binary log is also used on master replication servers as a record of the statements to be sent to slave servers. See section 6 Replication in MySQL.

Running the server with the binary log enabled makes performance about 1% slower. However, the benefits of the binary log for restore operations and in allowing you to set up replication generally outweigh this minor performance decrement.

When started with the --log-bin[=file_name] option, @command{mysqld} writes a log file containing all SQL commands that update data. If no file_name value is given, the default name is the name of the host machine followed by -bin. If file name is given, but it doesn't contain a path, the file is written in the data directory.

If you supply an extension in the log name (for example, --log-bin=file_name.extension), the extension is silently removed and ignored.

@command{mysqld} appends a numeric extension to the binary log name. The number is incremented each time you start the server or flush the logs. A new binary log also is created automatically when the current log's size reaches max_binlog_size. A binary log may become larger than max_binlog_size if you are using large transactions: A transaction is written to the binary log in one piece, never split between binary logs.

To be able to know which different binary log files have been used, @command{mysqld} also creates a binary log index file that contains the name of all used binary log files. By default this has the same name as the binary log file, with the extension '.index'. You can change the name of the binary log index file with the --log-bin-index[=file_name] option. You should not manually edit this file while @command{mysqld} is running; doing so would confuse @command{mysqld}.

You can delete all binary log files with the RESET MASTER statement, or only some of them with PURGE MASTER LOGS. See section 14.5.4.5 RESET Syntax and section 14.6.1 SQL Statements for Controlling Master Servers.

You can use the following options to @command{mysqld} to affect what is logged to the binary log. See also the discussion that follows this option list.

--binlog-do-db=db_name
Tells the master that it should log updates to the binary log if the current database (that is, the one selected by USE) is db_name. All other databases that are not explicitly mentioned are ignored. If you use this, you should ensure that you only do updates in the current database. An example of what does not work as you might expect: If the server is started with binlog-do-db=sales, and you do USE prices; UPDATE sales.january SET amount=amount+1000;, this statement will not be written into the binary log.
--binlog-ignore-db=db_name
Tells the master that updates where the current database (that is, the one selected by USE) is db_name should not be stored in the binary log. If you use this, you should ensure that you only do updates in the current database. An example of what does not work as you might expect: If the server is started with binlog-ignore-db=sales, and you do USE prices; UPDATE sales.january SET amount=amount+1000;, this statement will be written into the binary log.

To log or ignore multiple databases, specify the appropriate option multiple times, once for each database.

The rules for logging or ignoring updates to the binary log are evaluated in the following order:

  1. Are there binlog-do-db or binlog-ignore-db rules?
    • No: Write the statement to the binary log and exit.
    • Yes: Go to the next step.
  2. There are some rules (binlog-do-db or binlog-ignore-db or both). Is there a current database (has any database been selected by USE?)?
    • No: Do not write the statement, and exit.
    • Yes: Go to the next step.
  3. There is a current database. Are there some binlog-do-db rules?
    • Yes: Does the current database match any of the binlog-do-db rules?
      • Yes: Write the statement and exit.
      • No: Do not write the statement, and exit.
    • No: Go to the next step.
  4. There are some binlog-ignore-db rules. Does the current database match any of the binlog-ignore-db rules?
    • Yes: Do not write the statement, and exit.
    • No: Write the query and exit.

For example, a slave running with only binlog-do-db=sales will not write to the binary log any statement whose current database is different from sales (in other words, binlog-do-db can sometimes mean ``ignore other databases'').

If you are using replication, you should not delete old binary log files until you are sure that no slave still needs to use them. One way to do this is to do @command{mysqladmin flush-logs} once a day and then remove any logs that are more than three days old. You can remove them manually, or preferably using PURGE MASTER LOGS (see section 14.6.1 SQL Statements for Controlling Master Servers), which will also safely update the binary log index file for you (and which can take a date argument since MySQL 4.1)

A client with the SUPER privilege can disable binary logging of its own statements by using a SET SQL_LOG_BIN=0 statement. See section 14.5.3.1 SET Syntax.

You can examine the binary log file with the @command{mysqlbinlog} utility. This can be useful when you want to reprocess statements in the log. For example, you can update a MySQL server from the binary log as follows: 

shell> mysqlbinlog log-file | mysql -h server_name

See @ref{mysqlbinlog, , @command{mysqlbinlog}} for more information on the @command{mysqlbinlog} utility and how to use it.

If you are using transactions, you must use the MySQL binary log for backups instead of the old update log.

The binary logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order.

Updates to non-transactional tables are stored in the binary log immediately after execution. For transactional tables such as BDB or InnoDB tables, all updates (UPDATE, DELETE, or INSERT) that change tables are cached until a COMMIT statement is received by the server. At that point, @command{mysqld} writes the whole transaction to the binary log before the COMMIT is executed. When the thread that handles the transaction starts, it allocates a buffer of binlog_cache_size to buffer queries. If a statement is bigger than this, the thread opens a temporary file to store the transaction. The temporary file is deleted when the thread ends.

The max_binlog_cache_size (default 4GB) can be used to restrict the total size used to cache a multiple-statement transaction. If a transaction is larger than this, it will fail and roll back.

If you are using the update log or binary log, concurrent inserts will be converted to normal inserts when using CREATE ... SELECT or INSERT ... SELECT. This is to ensure that you can re-create an exact copy of your tables by applying the log on a backup.

The binary log format is different in versions 3.23, 4.0, and 5.0.0. Those format changes were required to implement enhancements to replication. MySQL 4.1 has the same binary log format as 4.0. See section 6.5 Replication Compatibility Between MySQL Versions.

By default, the binary log is not synchronized to disk at each write. So if the operating system or machine (not only the MySQL server) crashes there is a chance that the last statements of the binary log are lost. To prevent this, you can make the binary log be synchronized to disk after every Nth binary log write, with the sync_binlog global variable (1 being the safest value, but also the slowest). See section 5.2.3 Server System Variables. Even with this set to 1, there is still the chance of an inconsistency between the tables content and the binary log content in case of crash. For example, if using InnoDB tables, and the MySQL server processes a COMMIT statement, it writes the whole transaction to the binary log and then commits this transaction into InnoDB. If it crashes between those two operations, at restart the transaction will be rolled back by InnoDB but still exist in the binary log. This problem can be solved with the --innodb-safe-binlog option (available starting from MySQL 4.1.3), which adds consistency between the content of InnoDB tables and the binary log. For this option to really bring safety to you, the MySQL server should also be configured to synchronize to disk, at every transaction, the binary log (sync_binlog=1) and (which is true by default) the InnoDB logs. The effect of this option is that at restart after a crash, after doing a rollback of transactions, the MySQL server will cut rolled back InnoDB transactions from the binary log. This ensures that the binary log reflects the exact data of InnoDB tables, and so, that the slave remains in sync with the master (not receiving a statement which has been rolled back). Note that --innodb-safe-binlog can be used even if the MySQL server updates other storage engines than InnoDB. Only statements/transactions affecting InnoDB tables are subject to being removed from the binary log at InnoDB's crash recovery. If at crash recovery the MySQL server discovers that the binary log is shorter than it should have been (that is, it lacks at least one successfully committed InnoDB transaction), which should not happen if sync_binlog=1 and the disk/filesystem do an actual sync when they are requested to (some don't), it will print an error message ("The binary log <name> is shorter than its expected size"). In this case, this binary log is not correct, replication should be restarted from a fresh master's data snapshot.

The binary log format has some limitations which apply when the client sessions change their character set and collation variables. See section 6.7 Replication Features and Known Problems.

5.9.5 The Slow Query Log

When started with the --log-slow-queries[=file_name] option, @command{mysqld} writes a log file containing all SQL statements that took more than long_query_time seconds to execute. The time to acquire the initial table locks are not counted as execution time.

If no file_name value is given, the default is the name of the host machine with a suffix of -slow.log. If a filename is given, but doesn't contain a path, the file is written in the data directory.

A statement is logged to the slow query log after it has been executed and after all locks have been released. Log order may be different from execution order.

The slow query log can be used to find queries that take a long time to execute and are therefore candidates for optimization. However, examining a long slow query log can become a difficult task. To make this easier, you can pipe the slow query log through the @command{mysqldumpslow} command to get a summary of the queries that appear in the log.

If you also use the --log-long-format when logging slow queries, then queries that are not using indexes are logged as well. See section 5.2.1 @command{mysqld} Command-Line Options.

5.9.6 Log File Maintenance

The MySQL Server can create a number of different log files that make it easy to see what is going on. See section 5.9 The MySQL Log Files. However, you must clean up these files regularly to ensure that the logs don't take up too much disk space.

When using MySQL with logging enabled, you will want to back up and remove old log files from time to time and tell MySQL to start logging to new files. See section 5.7.1 Database Backups.

On a Linux (Red Hat) installation, you can use the mysql-log-rotate script for this. If you installed MySQL from an RPM distribution, the script should have been installed automatically. You should be careful with this script if you are using the binary log for replication! (You should not remove binary logs until you are certain that their contents have been processed by all slaves.)

On other systems, you must install a short script yourself that you start from @command{cron} to handle log files.

You can force MySQL to start using new log files by using @command{mysqladmin flush-logs} or by using the SQL statement FLUSH LOGS. If you are using MySQL 3.21, you must use @command{mysqladmin refresh}.

A log flushing operation does the following:

If you are using only an update log, you only have to rename the log file and then flush the logs before making a backup. For example, you can do something like this: 

shell> cd mysql-data-directory
shell> mv mysql.log mysql.old
shell> mysqladmin flush-logs

Then make a backup and remove `mysql.old'.

5.10 Running Multiple MySQL Servers on the Same Machine

In some cases, you might want to run multiple @command{mysqld} servers on the same machine. You might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you may want to give different users access to different @command{mysqld} servers that they manage themselves. (For example, you might be an Internet Service Provider that wants to provide independent MySQL installations for different customers.)

To run multiple servers on a single machine, each server must have unique values for several operating parameters. These can be set on the command line or in option files. See section 4.3 Specifying Program Options.

At least the following options must be different for each server:

--port=port_num
--port controls the port number for TCP/IP connections.
--socket=path
--socket controls the Unix socket file path on Unix and the name of the named pipe on Windows. On Windows, it's necessary to specify distinct pipe names only for those servers that support named pipe connections.
--shared-memory-base-name=name
This option currently is used only on Windows. It designates the shared memory name used by a Windows server to allow clients to connect via shared memory. This option is new in MySQL 4.1.
--pid-file=path
This option is used only on Unix. It indicates the name of the file in which the server writes its process ID.

If you use the following log file options, they must be different for each server:

Log file options are described in section 5.9.6 Log File Maintenance.

If you want more performance, you can also specify the following options differently for each server, to spread the load between several physical disks:

Having different temporary directories is also recommended, to make it easier to determine which MySQL server created any given temporary file.

Generally, each server should also use a different data directory, which is specified using the --datadir=path option.

Warning: Normally you should never have two servers that update data in the same databases! This may lead to unpleasant surprises if your operating system doesn't support fault-free system locking! If (despite this warning) you run multiple servers using the same data directory and they have logging enabled, you must use the appropriate options to specify log filenames that are unique to each server. Otherwise, the servers will try to log to the same files. Please note that this kind of setup will only work with ISAM, MyISAM and MERGE tables, and not with any of the other storage engines.

The warning against sharing a data directory among servers also applies in an NFS environment. Allowing multiple MySQL servers to access a common data directory over NFS is a bad idea!

Make it easy for yourself: Forget about sharing a data directory among servers over NFS. A better solution is to have one computer that contains several CPUs and use an operating system that handles threads efficiently.

If you have multiple MySQL installations in different locations, normally you can specify the base installation directory for each server with the --basedir=path option to cause each server to use a different data directory, log files, and PID file. (The defaults for all these values are determined relative to the base directory). In that case, the only other options you need to specify are the --socket and --port options. For example, suppose that you install different versions of MySQL using `tar' file binary distributions. These will install in different locations, so you can start the server for each installation using the command @command{bin/mysqld_safe} under its corresponding base directory. @command{mysqld_safe} will determine the proper --basedir option to pass to @command{mysqld}, and you need specify only the --socket and --port options to @command{mysqld_safe}. (For versions of MySQL older than 4.0, use @command{safe_mysqld} rather than @command{mysqld_safe}.)

As discussed in the following sections, it is possible to start additional servers by setting environment variables or by specifying appropriate command-line options. However, if you need to run multiple servers on a more permanent basis, it will be more convenient to use option files to specify for each server those option values that must be unique to it.

5.10.1 Running Multiple Servers on Windows

You can run multiple servers on Windows by starting them manually from the command line, each with appropriate operating parameters. On Windows NT-based systems, you also have the option of installing several servers as Windows services and running them that way. General instructions for running MySQL servers from the command line or as services are given in section 2.2.1 Installing MySQL on Windows. This section describes how to make sure that you start each server with different values for those startup options that must be unique per server, such as the data directory. These options are described in section 5.10 Running Multiple MySQL Servers on the Same Machine.

5.10.1.1 Starting Multiple Windows Servers at the Command Line

To start multiple servers manually from the command line, you can specify the appropriate options on the command line or in an option file. It's more convenient to place the options in an option file, but it's necessary to make sure that each server gets its own set of options. To do this, create an option file for each server and tell the server the filename with a --defaults-file option when you run it.

Suppose that you want to run @command{mysqld} on port 3307 with a data directory of `C:\mydata1', and @command{mysqld-max} on port 3308 with a data directory of `C:\mydata2'. (To do this, make sure that before you start the servers, each data directory exists and has its own copy of the mysql database that contains the grant tables.)

Then create two option files. For example, create one file named `C:\my-opts1.cnf' that looks like this: 

[mysqld]
datadir = C:/mydata1
port = 3307

Create a second file named `C:\my-opts2.cnf' that looks like this: 

[mysqld]
datadir = C:/mydata2
port = 3308

Then start each server with its own option file: 

C:\> C:\mysql\bin\mysqld --defaults-file=C:\my-opts1.cnf
C:\> C:\mysql\bin\mysqld-max --defaults-file=C:\my-opts2.cnf

On NT, each server will start in the foreground (no new prompt appears until the server exits later); you'll need to issue those two commands in separate console windows.

To shut down the servers, you must connect to the appropriate port number: 

C:\> C:\mysql\bin\mysqladmin --port=3307 shutdown
C:\> C:\mysql\bin\mysqladmin --port=3308 shutdown

Servers configured as just described will allow clients to connect over TCP/IP. If your version of Windows supports named pipes and you also want to allow named pipe connections, use the @command{mysqld-nt} or @command{mysqld-max-nt} servers and specify options that enable the named pipe and specify its name. Each server that supports named pipe connections must use a unique pipe name. For example, the `C:\my-opts1.cnf' file might be written like this: 

[mysqld]
datadir = C:/mydata1
port = 3307
enable-named-pipe
socket = mypipe1

Then start the server this way: 

C:\> C:\mysql\bin\mysqld-nt --defaults-file=C:\my-opts1.cnf

Modify `C:\my-opts2.cnf' similarly for use by the second server.

5.10.1.2 Starting Multiple Windows Servers as Services

On NT-based systems, a MySQL server can be run as a Windows service. The procedures for installing, controlling, and removing a single MySQL service are described in section 2.2.1.7 Starting MySQL as a Windows Service.

As of MySQL 4.0.2, you can install multiple servers as services. In this case, you must make sure that each server uses a different service name in addition to all the other parameters that must be unique per server.

For the following instructions, assume that you want to run the @command{mysqld-nt} server from two different versions of MySQL that are installed at `C:\mysql-4.0.8' and `C:\mysql-4.0.17', respectively. (This might be the case if you're running 4.0.8 as your production server, but want to test 4.0.17 before upgrading to it.)

The following principles apply when installing a MySQL service with the --install or --install-manual option:

Note: Before MySQL 4.0.17, only a server installed using the default service name (MySQL) or one installed explicitly with a service name of @command{mysqld} will read the [mysqld] group in the standard option files. As of 4.0.17, all servers read the [mysqld] group if they read the standard option files, even if they are installed using another service name. This allows you to use the [mysqld] group for options that should be used by all MySQL services, and an option group named after each service for use by the server installed with that service name.

Based on the preceding information, you have several ways to set up multiple services. The following instructions describe some examples. Before trying any of them, be sure that you shut down and remove any existing MySQL services first.

To remove multiple services, use @command{mysqld --remove} for each one, specifying a service name following the --remove option. If the service name is the default (MySQL), you can omit it.

5.10.2 Running Multiple Servers on Unix

The easiest way is to run multiple servers on Unix is to compile them with different TCP/IP ports and Unix socket files so that each one is listening on different network interfaces. Also, by compiling in different base directories for each installation, that automatically results in different compiled-in data directory, log file, and PID file locations for each of your servers.

Assume that an existing server is configured for the default TCP/IP port number (3306) and Unix socket file (`/tmp/mysql.sock'). To configure a new server to have different operating parameters, use a @command{configure} command something like this: 

shell> ./configure --with-tcp-port=port_number \
             --with-unix-socket-path=file_name \
             --prefix=/usr/local/mysql-4.0.17

Here, port_number and file_name must be different from the default TCP/IP port number and Unix socket file pathname, and the --prefix value should specify an installation directory different than the one under which the existing MySQL installation is located.

If you have a MySQL server listening on a given port number, you can use the following command to find out what operating parameters it is using for several important configurable variables, including the base directory and Unix socket filename: 

shell> mysqladmin --host=host_name --port=port_number variables

With the information displayed by that command, you can tell what option values not to use when configuring an additional server.

Note that if you specify localhost as a hostname, @command{mysqladmin} will default to using a Unix socket file connection rather than TCP/IP. In MySQL 4.1, you can explicitly specify the connection protocol to use by using the --protocol={TCP | SOCKET | PIPE | MEMORY} option.

You don't have to compile a new MySQL server just to start with a different Unix socket file and TCP/IP port number. It is also possible to specify those values at runtime. One way to do so is by using command-line options: 

shell> mysqld_safe --socket=file_name --port=port_number

To start a second server, provide different --socket and --port option values, and pass a a --datadir=path option to @command{mysqld_safe} so that the server uses a different data directory.

Another way to achieve a similar effect is to use environment variables to set the Unix socket filename and TCP/IP port number: 

shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
shell> MYSQL_TCP_PORT=3307
shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
shell> mysql_install_db --user=mysql
shell> mysqld_safe --datadir=/path/to/datadir &

This is a quick way of starting a second server to use for testing. The nice thing about this method is that the environment variable settings will apply to any client programs that you invoke from the same shell. Thus, connections for those clients automatically will be directed to the second server!

section E Environment Variables includes a list of other environment variables you can use to affect @command{mysqld}.

For automatic server execution, your startup script that is executed at boot time should execute the following command once for each server with an appropriate option file path for each command: 

mysqld_safe --defaults-file=path

Each option file should contain option values specific to a given server.

On Unix, the @command{mysqld_multi} script is another way to start multiple servers. @xref{mysqld_multi, , @command{mysqld_multi}}.

5.10.3 Using Client Programs in a Multiple-Server Environment

When you want to connect with a client program to a MySQL server that is listening to different network interfaces than those compiled into your client, you can use one of the following methods:

5.11 The MySQL Query Cache

From version 4.0.1 on, MySQL Server features a query cache. When in use, the query cache stores the text of a SELECT query together with the corresponding result that was sent to the client. If the identical query is received later, the server retrieves the results from the query cache rather than parsing and executing the query again.

The query cache is extremely useful in an environment where (some) tables don't change very often and you have a lot of identical queries. This is a typical situation for many Web servers that generate a lot of dynamic pages based on database content.

Note: The query cache does not return stale data. When tables are modified, any relevant entries in the query cache are flushed.

Note: The query cache does not work in an environment where you have many @command{mysqld} servers updating the same MyISAM tables.

Some performance data for the query cache follow. These results were generated by running the MySQL benchmark suite on a Linux Alpha 2 x 500MHz system with 2GB RAM and a 64MB query cache.

To disable the query cache at server startup, set the query_cache_size system variable to 0. By disabling the query cache code, there is no noticeable overhead. Query cache capabilities can be excluded from the server entirely by using the --without-query-cache option to @command{configure} when compiling MySQL.

5.11.1 How the Query Cache Operates

This section describes how the query cache works when it is operational. section 5.11.3 Query Cache Configuration describes how to control whether or not it is operational.

Queries are compared before parsing, so the following two queries are regarded as different by the query cache: 

SELECT * FROM tbl_name
Select * from tbl_name

Queries must be exactly the same (byte for byte) to be seen as identical. In addition, query strings that are identical may be treated as different for other reasons. Queries that use different databases, different protocol versions, or different default character sets are considered different queries and are cached separately.

If a query result is returned from query cache, the server increments the Qcache_hits status variable, not Com_select. See section 5.11.4 Query Cache Status and Maintenance.

If a table changes, then all cached queries that use the table become invalid and are removed from the cache. This includes queries that use MERGE tables that map to the changed table. A table can be changed by many types of statements, such as INSERT, UPDATE, DELETE, TRUNCATE, ALTER TABLE, DROP TABLE, or DROP DATABASE.

Transactional InnoDB tables that have been changed are invalidated when a COMMIT is performed.

In MySQL 4.0, the query cache is disabled within transactions (it does not return results). Beginning with MySQL 4.1.1, the query cache also works within transactions when using InnoDB tables (it uses the table version number to detect whether or not its contents are still current).

Before MySQL 5.0, a query that begins with a leading comment might be cached, but could not be fetched from the cache. This problem is fixed in MySQL 5.0.

The query cache works for SELECT SQL_CALC_FOUND_ROWS ... and SELECT FOUND_ROWS() type queries. FOUND_ROWS() returns the correct value even if the preceding query was fetched from the cache because the number of found rows is also stored in the cache.

A query cannot be cached if it contains any of the following functions:

BENCHMARK() CONNECTION_ID() CURDATE()
CURRENT_DATE() CURRENT_TIME() CURRENT_TIMESTAMP()
CURTIME() DATABASE() ENCRYPT() with one parameter
FOUND_ROWS() GET_LOCK() LAST_INSERT_ID()
LOAD_FILE() MASTER_POS_WAIT() NOW()
RAND() RELEASE_LOCK() SYSDATE()
UNIX_TIMESTAMP() with no parameters USER()

A query also will not be cached under these conditions:

5.11.2 Query Cache SELECT Options

There are two query cache-related options that may be specified in a SELECT statement:

SQL_CACHE
The query result is cached if the value of the query_cache_type system variable is ON or DEMAND.
SQL_NO_CACHE
The query result is not cached.

Examples: 

SELECT SQL_CACHE id, name FROM customer;
SELECT SQL_NO_CACHE id, name FROM customer;

5.11.3 Query Cache Configuration

The have_query_cache server system variable indicates whether the query cache is available: 

mysql> SHOW VARIABLES LIKE 'have_query_cache';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| have_query_cache | YES   |
+------------------+-------+

Several other system variables control query cache operation. These can be set in an option file or on the command line when starting @command{mysqld}. The query cache-related system variables all have names that begin with query_cache_. They are described briefly in section 5.2.3 Server System Variables, with additional configuration information given here.

To set the size of the query cache, set the query_cache_size system variable. Setting it to 0 disables the query cache. The default cache size is 0; that is, the query cache is disabled.

If the query cache is enabled, the query_cache_type variable influences how it works. This variable can be set to the following values:

Setting the GLOBAL value of query_cache_type determines query cache behavior for all clients that connect after the change is made. Individual clients can control cache behavior for their own connection by setting the SESSION value of query_cache_type. For example, a client can disable use of the query cache for its own queries like this: 

mysql> SET SESSION query_cache_type = OFF;

To control the maximum size of individual query results that can be cached, set the query_cache_limit variable. The default value is 1MB.

The result of a query (the data sent to the client) is stored in the query cache during result retrieval. Therefore the data usually is not handled in one big chunk. The query cache allocates blocks for storing this data on demand, so when one block is filled, a new block is allocated. Because memory allocation operation is costly (timewise), the query cache allocates blocks with a minimum size given by the query_cache_min_res_unit system variable. When a query is executed, the last result block is trimmed to the actual data size so that unused memory is freed. Depending on the types of queries your server executes, you might find it helpful to tune the value of query_cache_min_res_unit:

query_cache_min_res_unit is present from MySQL 4.1.

5.11.4 Query Cache Status and Maintenance

You can check whether the query cache is present in your MySQL server using the following statement: 

mysql> SHOW VARIABLES LIKE 'have_query_cache';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| have_query_cache | YES   |
+------------------+-------+

You can defragment the query cache to better utilize its memory with the FLUSH QUERY CACHE statement. The statement does not remove any queries from the cache.

The RESET QUERY CACHE statement removes all query results from the query cache. The FLUSH TABLES statement also does this.

To monitor query cache performance, use SHOW STATUS to view the cache status variables: 

mysql> SHOW STATUS LIKE 'Qcache%';
+-------------------------+--------+
| Variable_name           | Value  |
+-------------------------+--------+
| Qcache_free_blocks      | 36     |
| Qcache_free_memory      | 138488 |
| Qcache_hits             | 79570  |
| Qcache_inserts          | 27087  |
| Qcache_lowmem_prunes    | 3114   |
| Qcache_not_cached       | 22989  |
| Qcache_queries_in_cache | 415    |
| Qcache_total_blocks     | 912    |
+-------------------------+--------+

Descriptions of each of these variables are given in section 5.2.4 Server Status Variables. Some uses for them are described here.

The total number of SELECT queries is equal to: 

  Com_select
+ Qcache_hits
+ queries with errors found by parser

The Com_select value is equal to: 

  Qcache_inserts
+ Qcache_not_cached
+ queries with errors found during columns/rights check

The query cache uses variable-length blocks, so Qcache_total_blocks and Qcache_free_blocks may indicate query cache memory fragmentation. After FLUSH QUERY CACHE, only a single free block remains.

Every cached query requires a minimum of two blocks (one for the query text and one or more for the query results). Also, every table that is used by a query requires one block. However, if two or more queries use the same table, only one block needs to be allocated.

The information provided by the Qcache_lowmem_prunes status variable can help you tune the query cache size. It counts the number of queries that have been removed from the cache to free up memory for caching new queries. The query cache uses a least recently used (LRU) strategy to decide which queries to remove from the cache. Tuning information is given in section 5.11.3 Query Cache Configuration.

6 Replication in MySQL

Replication capabilities allowing the databases on one MySQL server to be duplicated on another were introduced in MySQL 3.23.15. This chapter describes the various replication features provided by MySQL. It introduces replication concepts, shows how to set up replication servers, and serves as a reference to the available replication options. It also provides a list of frequently asked questions (with answers), and troubleshooting advice for solving problems.

For a description of the syntax of replication-related SQL statements, see section 14.6 Replication Statements.

We suggest that you visit our Web site at http://www.mysql.com often and read updates to this chapter. Replication is constantly being improved, and we update the manual frequently with the most current information.

6.1 Introduction to Replication

MySQL 3.23.15 and up features support for one-way replication. One server acts as the master, while one or more other servers act as slaves. The master server writes updates to its binary log files, and maintains an index of the files to keep track of log rotation. These logs serve as a record of updates to be sent to slave servers. When a slave server connects to the master server, it informs the master of its last position within the logs since the last successfully propagated update. The slave catches up any updates that have occurred since then, and then blocks and waits for the master to notify it of new updates.

A slave server can also serve as a master if you want to set up chained replication servers.

Note that when you are using replication, all updates to the tables that are replicated should be performed on the master server. Otherwise, you must always be careful to avoid conflicts between updates that users make to tables on the master and updates that they make to tables on the slave.

One-way replication has benefits for robustness, speed, and system administration:

6.2 Replication Implementation Overview

MySQL replication is based on the master server keeping track of all changes to your databases (updates, deletes, and so on) in the binary logs. Therefore, to use replication, you must enable binary logging on the master server. See section 5.9.4 The Binary Log.

Each slave server receives from the master the saved updates that the master has recorded in its binary log, so that the slave can execute the same updates on its copy of the data.

It is very important to realize that the binary log is simply a record starting from the fixed point in time at which you enable binary logging. Any slaves that you set up will need copies of the databases on your master as they existed at the moment you enabled binary logging on the master. If you start your slaves with databases that are not the same as what was on the master when the binary log was started, your slaves may fail.

One way to copy the master's data to the slave is to use the LOAD DATA FROM MASTER statement. Be aware that LOAD DATA FROM MASTER is available only as of MySQL 4.0.0 and currently works only if all the tables on the master are MyISAM type. Also, this statement acquires a global read lock, so no updates on the master are possible while the tables are being transferred to the slave. When we implement lock-free hot table backup (in MySQL 5.0), this global read lock will no longer be necessary.

Due to these limitations, we recommend that at this point you use LOAD DATA FROM MASTER only if the dataset on the master is relatively small, or if a prolonged read lock on the master is acceptable. While the actual speed of LOAD DATA FROM MASTER may vary from system to system, a good rule of thumb for how long it will take is 1 second per 1MB of data. That is only a rough estimate, but you should get close to it if both master and slave are equivalent to 700MHz Pentium performance and are connected through a 100MBit/s network.

After the slave has been set up with a copy of the master's data, it will simply connect to the master and wait for updates to process. If the master goes away or the slave loses connectivity with your master, it will keep trying to connect periodically until it is able to reconnect and resume listening for updates. The retry interval is controlled by the --master-connect-retry option. The default is 60 seconds.

Each slave keeps track of where it left off. The master server has no knowledge of how many slaves there are or which ones are up to date at any given time.

6.3 Replication Implementation Details

MySQL replication capabilities are implemented using three threads (one on the master server and two on the slave). When START SLAVE is issued, the slave creates an I/O thread. The I/O thread connects to the master and asks it to send the statements recorded in its binary logs. The master creates a thread to send the binary log contents to the slave. This thread can be identified as the Binlog Dump thread in the output of SHOW PROCESSLIST on the master. The slave I/O thread reads what the master Binlog Dump thread sends and simply copies it to some local files in the slave's data directory called relay logs. The third thread is the SQL thread, which the slave creates to read the relay logs and execute the updates they contain.

In the preceding description, there are three threads per slave. For a master that has multiple slaves, it creates one thread for each currently connected slave, and each slave has its own I/O and SQL threads.

For versions of MySQL before 4.0.2, replication involves only two threads (one on the master and one on the slave). The slave I/O and SQL threads are combined as a single thread, and no relay log files are used.

The advantage of using two slave threads is that statement reading and execution are separated into two independent tasks. The task of reading statements is not slowed down if statement execution is slow. For example, if the slave server has not been running for a while, its I/O thread can quickly fetch all the binary log contents from the master when the slave starts, even if the SQL thread lags far behind and may take hours to catch up. If the slave stops before the SQL thread has executed all the fetched statements, the I/O thread has at least fetched everything so that a safe copy of the statements is locally stored in the slave's relay logs for execution when next the slave starts. This allows the binary logs to be purged on the master, because it no longer need wait for the slave to fetch their contents.

The SHOW PROCESSLIST statement provides information that tells you what is happening on the master and on the slave regarding replication.

The following example illustrates how the three threads show up in SHOW PROCESSLIST. The output format is that used by SHOW PROCESSLIST as of MySQL version 4.0.15, when the content of the State column was changed to be more meaningful compared to earlier versions.

On the master server, the output from SHOW PROCESSLIST looks like this: 

mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
     Id: 2
   User: root
   Host: localhost:32931
     db: NULL
Command: Binlog Dump
   Time: 94
  State: Has sent all binlog to slave; waiting for binlog to
         be updated
   Info: NULL

Here, thread 2 is a replication thread for a connected slave. The information indicates that all outstanding updates have been sent to the slave and that the master is waiting for more updates to occur.

On the slave server, the output from SHOW PROCESSLIST looks like this: 

mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
     Id: 10
   User: system user
   Host:
     db: NULL
Command: Connect
   Time: 11
  State: Waiting for master to send event
   Info: NULL
*************************** 2. row ***************************
     Id: 11
   User: system user
   Host:
     db: NULL
Command: Connect
   Time: 11
  State: Has read all relay log; waiting for the slave I/O
         thread to update it
   Info: NULL

This information indicates that thread 10 is the I/O thread that is communicating with the master server, and thread 11 is the SQL thread that is processing the updates stored in the relay logs. Currently, both threads are idle, waiting for further updates.

Note that the value in the Time column can tell how late the slave is compared to the master. See section 6.9 Replication FAQ.

6.3.1 Replication Master Thread States

The following list shows the most common states you will see in the State column for the master's Binlog Dump thread. If you don't see any Binlog Dump threads on a master server, replication is not running. That is, no slaves currently are connected.

Sending binlog event to slave
Binary logs consist of events, where an event is usually an update statement plus some other information. The thread has read an event from the binary log and is sending it to the slave.
Finished reading one binlog; switching to next binlog
The thread has finished reading a binary log file and is opening the next one to send to the slave.
Has sent all binlog to slave; waiting for binlog to be updated
The thread has read all outstanding updates from the binary logs and sent them to the slave. It is idle, waiting for new events to appear in the binary log resulting from new update statements being executed on the master.
Waiting to finalize termination
A very brief state that occurs as the thread is stopping.

6.3.2 Replication Slave I/O Thread States

The following list shows the most common states you will see in the State column for a slave server I/O thread. Beginning with MySQL 4.1.1, this state also appears in the Slave_IO_State column displayed by the SHOW SLAVE STATUS statement. This means that you can get a good view of what is happening by using only SHOW SLAVE STATUS.

Connecting to master
The thread is attempting to connect to the master.
Checking master version
A very brief state that occurs just after the connection to the master is established.
Registering slave on master
A very brief state that occurs just after the connection to the master is established.
Requesting binlog dump
A very brief state that occurs just after the connection to the master is established. The thread sends to the master a request for the contents of its binary logs, starting from the requested binary log filename and position.
Waiting to reconnect after a failed binlog dump request
If the binary log dump request failed (due to disconnection), the thread goes into this state while it sleeps, then tries to reconnect periodically. The interval between retries can be specified using the --master-connect-retry option.
Reconnecting after a failed binlog dump request
The thread is trying to reconnect to the master.
Waiting for master to send event
The thread has connected to the master and is waiting for binary log events to arrive. This can last for a long time if the master is idle. If the wait lasts for slave_read_timeout seconds, a timeout will occur. At that point, the thread will consider the connection to be broken and make an attempt to reconnect.
Queueing master event to the relay log
The thread has read an event and is copying it to the relay log so that the SQL thread can process it.
Waiting to reconnect after a failed master event read
An error occurred while reading (due to disconnection). The thread is sleeping for master-connect-retry seconds before attempting to reconnect.
Reconnecting after a failed master event read
The thread is trying to reconnect to the master. When connection is established again, the state will become Waiting for master to send event.
Waiting for the slave SQL thread to free enough relay log space
You are using a non-zero relay_log_space_limit value, and the relay logs have grown so much that their combined size exceeds this value. The I/O thread is waiting until the SQL thread frees enough space by processing relay log contents so that it can delete some relay log files.
Waiting for slave mutex on exit
A very brief state that occurs as the thread is stopping.

6.3.3 Replication Slave SQL Thread States

The following list shows the most common states you will see in the State column for a slave server SQL thread:

Reading event from the relay log
The thread has read an event from the relay log so that it can process it.
Has read all relay log; waiting for the slave I/O thread to update it
The thread has processed all events in the relay log files and is waiting for the I/O thread to write new events to the relay log.
Waiting for slave mutex on exit
A very brief state that occurs as the thread is stopping.

The State column for the I/O thread may also show the text of a statement. This indicates that the thread has read an event from the relay log, extracted the statement from it, and is executing it.

6.3.4 Replication Relay and Status Files

By default, relay logs are named using filenames of the form `host_name-relay-bin.nnn', where host_name is the name of the slave server host and nnn is a sequence number. Successive relay log files are created using successive sequence numbers, beginning with 000001 (001 in MySQL 4.0 or older). The slave keeps track of relay logs currently in use in an index file. The default relay log index filename is `host_name-relay-bin.index'. By default, these files are created in the slave's data directory. The default filenames may be overridden with the --relay-log and --relay-log-index server options. See section 6.8 Replication Startup Options.

Relay logs have the same format as binary logs, so you can use @command{mysqlbinlog} to read them. A relay log is automatically deleted by the SQL thread as soon as it has executed all its events and no longer needs it). There is no explicit mechanism for deleting relay logs, because the SQL thread takes care of doing so. However, from MySQL 4.0.14, FLUSH LOGS rotates relay logs, which will influence when the SQL thread deletes them.

A new relay log is created under the following conditions:

A slave replication server creates two additional small files in the data directory. These are status files and are named `master.info' and `relay-log.info' by default. They contain information like that shown in the output of the SHOW SLAVE STATUS statement (see section 14.6.2 SQL Statements for Controlling Slave Servers for a description of this statement). As disk files, they survive a slave server's shutdown. The next time the slave starts up, it reads these files to determine how far it has proceeded in reading binary logs from the master and in processing its own relay logs.

The `master.info' file is updated by the I/O thread. Before MySQL 4.1, the correspondence between the lines in the file and the columns displayed by SHOW SLAVE STATUS is as follows:

Line Description
1 Master_Log_File
2 Read_Master_Log_Pos
3 Master_Host
4 Master_User
5 Password (not shown by SHOW SLAVE STATUS)
6 Master_Port
7 Connect_Retry

As of MySQL 4.1, the file includes a line count and information about SSL options:

Line Description
1 Number of lines in the file
2 Master_Log_File
3 Read_Master_Log_Pos
4 Master_Host
5 Master_User
6 Password (not shown by SHOW SLAVE STATUS)
7 Master_Port
8 Connect_Retry
9 Master_SSL_Allowed
10 Master_SSL_CA_File
11 Master_SSL_CA_Path
12 Master_SSL_Cert
13 Master_SSL_Cipher
14 Master_SSL_Key

The `relay-log.info' file is updated by the SQL thread. The correspondence between the lines in the file and the columns displayed by SHOW SLAVE STATUS is as follows:

Line Description
1 Relay_Log_File
2 Relay_Log_Pos
3 Relay_Master_Log_File
4 Exec_Master_Log_Pos

When you back up your slave's data, you should back up these two small files as well, along with the relay log files. They are needed to resume replication after you restore the slave's data. If you lose the relay logs but still have the `relay-log.info' file, you can check it to determine how far the SQL thread has executed in the master binary logs. Then you can use CHANGE MASTER TO with the MASTER_LOG_FILE and MASTER_LOG_POS options to tell the slave to re-read the binary logs from that point. This requires that the binary logs still exist on the master server.

If your slave is subject to replicating LOAD DATA INFILE statements, you should also back up any `SQL_LOAD-*' files that exist in the directory that the slave uses for this purpose. The slave needs these files to resume replication of any interrupted LOAD DATA INFILE operations. The directory location is specified using the --slave-load-tmpdir option. Its default value, if not specified, is the value of the tmpdir variable.

6.4 How to Set Up Replication

Here is a quick description of how to set up complete replication of your current MySQL server. It assumes that you want to replicate all your databases and have not configured replication before. You will need to shut down your master server briefly to complete the steps outlined here.

The procedure is written in terms of setting up a single slave, but you can use it to set up multiple slaves.

While this method is the most straightforward way to set up a slave, it is not the only one. For example, if you already have a snapshot of the master's data, and the master already has its server ID set and binary logging enabled, you can set up a slave without shutting down the master or even blocking updates to it. For more details, please see section 6.9 Replication FAQ.

If you want to administer a MySQL replication setup, we suggest that you read this entire chapter through and try all statements mentioned in section 14.6.1 SQL Statements for Controlling Master Servers and section 14.6.2 SQL Statements for Controlling Slave Servers. You should also familiarize yourself with replication startup options described in section 6.8 Replication Startup Options.

Note that this procedure and some of the replication SQL statements in later sections refer to the SUPER privilege. Prior to MySQL 4.0.2, use the PROCESS privilege instead.

  1. Make sure that you have a recent version of MySQL installed on the master and slaves, and that these versions are compatible according to the table shown in section 6.5 Replication Compatibility Between MySQL Versions. Please do not report bugs until you have verified that the problem is present in the latest release.
  2. Set up an account on the master server that the slave server can use to connect. This account must be given the REPLICATION SLAVE privilege. If the account is used only for replication (which is recommended), you don't need to grant any additional privileges. Suppose that your domain is mydomain.com and you want to create an account with a username of repl such that slave servers can use the account to access the master server from any host in your domain using a password of slavepass. To create the account, this use GRANT statement: 
    mysql> GRANT REPLICATION SLAVE ON *.*
    -> TO 'repl'@'%.mydomain.com' IDENTIFIED BY 'slavepass';
    For MySQL versions older than 4.0.2, the REPLICATION SLAVE privilege does not exist. Grant the FILE privilege instead: 
    mysql> GRANT FILE ON *.*
    -> TO 'repl'@'%.mydomain.com' IDENTIFIED BY 'slavepass';
    If you plan to use the LOAD TABLE FROM MASTER or LOAD DATA FROM MASTER statements from the slave host, you will need to grant this account additional privileges:
    • Grant the account the SUPER and RELOAD global privileges.
    • Grant the SELECT privilege for all tables that you want to load. Any master tables from which the account cannot SELECT will be ignored by LOAD DATA FROM MASTER.
  3. If you are using only MyISAM tables, flush all the tables and block write statements by executing a FLUSH TABLES WITH READ LOCK statement. 
    mysql> FLUSH TABLES WITH READ LOCK;
    Leave the client running from which you issue the FLUSH TABLES statement so that the read lock remains in effect. (If you exit the client, the lock is released.) Then take a snapshot of the data on your master server. The easiest way to create a snapshot is to use an archiving program to make a binary backup of the databases in your master's data directory. For example, use @command{tar} on Unix, or @command{PowerArchiver}, @command{WinRAR}, @command{WinZip}, or any similar software on Windows. To use @command{tar} to create an archive that includes all databases, change location into the master server's data directory, then execute this command: 
    shell> tar -cvf /tmp/mysql-snapshot.tar .
    If you want the archive to include only a database called this_db, use this command instead: 
    shell> tar -cvf /tmp/mysql-snapshot.tar ./this_db
    Then copy the archive file to the `/tmp' directory on the slave server host. On that machine, change location into the slave's data directory, and unpack the archive file using this command: 
    shell> tar -xvf /tmp/mysql-snapshot.tar
    You may not want to replicate the mysql database if the slave server has a different set of user accounts from those that exist on the master. In this case, you should exclude it from the archive. You also need not include any log files in the archive, or the `master.info' or `relay-log.info' files. While the read lock placed by FLUSH TABLES WITH READ LOCK is in effect, read the value of the current binary log name and offset on the master: 
    mysql > SHOW MASTER STATUS;
+---------------+----------+--------------+------------------+
| File          | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+---------------+----------+--------------+------------------+
| mysql-bin.003 | 73       | test         | manual,mysql     |
+---------------+----------+--------------+------------------+
    The File column shows the name of the log, while Position shows the offset. In this example, the binary log value is mysql-bin.003 and the offset is 73. Record the values. You will need to use them later when you are setting up the slave. They represent the replication coordinates at which the slave should begin processing new updates from the master. After you have taken the snapshot and recorded the log name and offset, you can re-enable write activity on the master: 
    mysql> UNLOCK TABLES;
    If you are using InnoDB tables, ideally you should use the InnoDB Hot Backup tool. It takes a consistent snapshot without acquiring any locks on the master server, and records the log name and offset corresponding to the snapshot to be later used on the slave. InnoDB Hot Backup is a non-free (commercial) additional tool that is not included in the standard MySQL distribution. See the InnoDB Hot Backup home page at http://www.innodb.com/manual.php for detailed information and screenshots. Without the Hot Backup tool, the quickest way to take a binary snapshot of InnoDB tables is to shut down the master server and copy the InnoDB data files, log files, and table definition files (`.frm' files). To record the current log file name and offset, you should issue the following statements before you shut down the server: 
    mysql> FLUSH TABLES WITH READ LOCK;
mysql> SHOW MASTER STATUS;
    Then record the log name and the offset from the output of SHOW MASTER STATUS as was shown earlier. After recording the log name and the offset, shut down the server without unlocking the tables to make sure that the server goes down with the snapshot corresponding to the current log file and offset: 
    shell> mysqladmin -u root shutdown
    An alternative that works for both MyISAM and InnoDB tables is to take an SQL dump of the master instead of a binary copy as described in the preceding discussion. For this, you can use @command{mysqldump --master-data} on your master and later load the SQL dump file into your slave. However, this is slower than doing a binary copy. If the master has been previously running without --log-bin enabled, the log name and position values displayed by SHOW MASTER STATUS or @command{mysqldump} will be empty. In that case, the values that you will need to use later when specifying the slave's log file and position are the empty string ('') and 4.
  4. Make sure that the [mysqld] section of the `my.cnf' file on the master host includes a log-bin option. The section should also have a server-id=master_id option, where master_id must be a positive integer value from 1 to 2^32 - 1. For example: 
    [mysqld]
log-bin
server-id=1
    If those options are not present, add them and restart the server.
  5. Stop the server that is to be used as a slave server and add the following to its `my.cnf' file: 
    [mysqld]
server-id=slave_id
    The slave_id value, like the master_id value, must be a positive integer value from 1 to 2^32 - 1. In addition, it is very important that the ID of the slave be different from the ID of the master. For example: 
    [mysqld]
server-id=2
    If you are setting up multiple slaves, each one must have a unique server-id value that differs from that of the master and from each of the other slaves. Think of server-id values as something similar to IP addresses: These IDs uniquely identify each server instance in the community of replication partners. If you don't specify a server-id value, it will be set to 1 if you have not defined master-host, else it will be set to 2. Note that in the case of server-id omission, a master will refuse connections from all slaves, and a slave will refuse to connect to a master. Thus, omitting server-id is good only for backup with a binary log.
  6. If you made a binary backup of the master server's data, copy it to the slave server's data directory before starting the slave. Make sure that the privileges on the files and directories are correct. The user that the server MySQL runs as must able to read and write the files, just as on the master. If you made a backup using @command{mysqldump}, start the slave first (see next step).
  7. Start the slave server. If it has been replicating previously, start the slave server with the --skip-slave-start option so that it doesn't immediately try to connect to its master. You also may want to start the slave server with the --log-warnings option (enabled by default as of MySQL 4.0.19 and 4.1.2), to get more messages in the error log about problems (for example, network or connection problems). As of MySQL 4.0.21 and 4.1.3, aborted connections are not logged to the error log unless the value is greater than 1.
  8. If you made a backup of the master server's data using @command{mysqldump}, load the dump file into the slave server: 
    shell> mysql -u root -p < dump_file.sql
  9. Execute the following statement on the slave, replacing the option values with the actual values relevant to your system: 
    mysql> CHANGE MASTER TO
    ->     MASTER_HOST='master_host_name',
    ->     MASTER_USER='replication_user_name',
    ->     MASTER_PASSWORD='replication_password',
    ->     MASTER_LOG_FILE='recorded_log_file_name',
    ->     MASTER_LOG_POS=recorded_log_position;
    The following table shows the maximum length for the string options:
    MASTER_HOST 60
    MASTER_USER 16
    MASTER_PASSWORD 32
    MASTER_LOG_FILE 255
  10. Start the slave threads: 
    mysql> START SLAVE;

After you have performed this procedure, the slave should connect to the master and catch up on any updates that have occurred since the snapshot was taken.

If you have forgotten to set the server-id value for the master, slaves will not be able to connect to it.

If you have forgotten to set the server-id value for the slave, you will get the following error in its error log: 

Warning: You should set server-id to a non-0 value if master_host is set;
we force server id to 2, but this MySQL server will not act as a slave.

You will also find error messages in the slave's error log if it is not able to replicate for any other reason.

Once a slave is replicating, you will find in its data directory one file named `master.info' and another named `relay-log.info'. The slave uses these two files to keep track of how much of the master's binary log it has processed. Do not remove or edit these files, unless you really know what you are doing and understand the implications. Even in that case, it is preferred that you use the CHANGE MASTER TO statement.

Note: The content of `master.info' overrides some options specified on the command line or in `my.cnf'. See section 6.8 Replication Startup Options for more details.

Once you have a snapshot, you can use it to set up other slaves by following the slave portion of the procedure just described. You do not need to take another snapshot of the master; you can use the same one for each slave.

6.5 Replication Compatibility Between MySQL Versions

The original binary log format was developed in MySQL 3.23. It changed in MySQL 4.0, and again in MySQL 5.0. This has consequences when you upgrade servers in a replication setup, as described in section 6.6 Upgrading a Replication Setup.

As far as replication is concerned, any MySQL 4.1.x version and any 4.0.x version are identical, because they all use the same binary log format. Thus, any servers from these versions are compatible, and replication between them should work seamlessly. The exceptions to this compatibility is that versions from MySQL 4.0.0 to 4.0.2 were very early development versions that should not be used anymore. (These were the alpha versions in the 4.0 release series. Compatibility for them is still documented in the manual included with their distributions.)

The following table indicates master/slave replication compatibility between different versions of MySQL.

Master Master Master
3.23.33 and up 4.0.3 and up or any 4.1.x 5.0.0
Slave 3.23.33 and up yes no no
Slave 4.0.3 and up yes yes no
Slave 5.0.0 yes yes yes

As a general rule, we recommended using recent MySQL versions, because replication capabilities are continually being improved. We also recommend using the same version for both the master and the slave.

6.6 Upgrading a Replication Setup

When you upgrade servers that participate in a replication setup, the procedure for upgrading depends on the current server versions and the version to which you are upgrading.

6.6.1 Upgrading Replication to 4.0 or 4.1

This section applies to upgrading replication from MySQL 3.23 to 4.0 or 4.1. A 4.0 server should be 4.0.3 or newer, as mentioned in section 6.5 Replication Compatibility Between MySQL Versions.

When you upgrade a master from MySQL 3.23 to MySQL 4.0 or 4.1, you should first ensure that all the slaves of this master are already at 4.0 or 4.1. If that is not the case, you should first upgrade your slaves: Shut down each one, upgrade it, restart it, and restart replication.

The upgrade can safely be done using the following procedure, assuming that you have a 3.23 master to upgrade and the slaves are 4.0 or 4.1. Note that after the master has been upgraded, you should not restart replication using any old 3.23 binary logs, because this will unfortunately confuse the 4.0 or 4.1 slave.

  1. Block all updates on the master by issuing a FLUSH TABLES WITH READ LOCK statement.
  2. Wait until all the slaves have caught up with all changes from the master server. Use SHOW MASTER STATUS on the master to obtain its current binary log file and position. Then, for each slave, use those values with a SELECT MASTER_POS_WAIT() statement. The statement will block on the slave and return when the slave has caught up. Then run STOP SLAVE on the slave.
  3. Stop the master server and upgrade it to MySQL 4.0 or 4.1.
  4. Restart the master server and record the name of its newly created binary log. You can obtain the name of the file by issuing a SHOW MASTER STATUS statement on the master. Then issue these statements on each slave: 
    mysql> CHANGE MASTER TO MASTER_LOG_FILE='binary_log_name',
    ->     MASTER_LOG_POS=4;
mysql> START SLAVE;

6.6.2 Upgrading Replication to 5.0

This section applies to upgrading replication from MySQL 3.23, 4.0, or 4.1 to 5.0.0. A 4.0 server should be 4.0.3 or newer, as mentioned in section 6.5 Replication Compatibility Between MySQL Versions.

First, note that MySQL 5.0.0 is an alpha release. It is intended to work better than older versions (easier upgrade, replication of some important session variables such as sql_mode; see section C.1.3 Changes in release 5.0.0 (22 Dec 2003: Alpha)). However it has not yet been extensively tested. As with any alpha release, we recommend that you not use it in critical production environments yet.

When you upgrade a master from MySQL 3.23, 4.0, or 4.1 to 5.0.0, you should first ensure that all the slaves of this master are already 5.0.0. If that's not the case, you should first upgrade your slaves. To upgrade each slave, just shut it down, upgrade it to 5.0.0, restart it, and restart replication. The 5.0.0 slave will be able to read its old relay logs that were written before the upgrade and execute the statements they contain. Relay logs created by the slave after the upgrade will be in 5.0.0 format.

After the slaves have been upgraded, shut down your master, upgrade it to 5.0.0, and restart it. The 5.0.0 master will be able to read its old binary logs that were written before the upgrade and send them to the 5.0.0 slaves. The slaves will recognize the old format and handle it properly. Binary logs created by master after the upgrade will be in 5.0.0 format. These too will be recognized by the 5.0.0 slaves.

In other words, there are no measures to take when upgrading to 5.0.0, except that slaves must be 5.0.0 before you can upgrade the master to 5.0.0. Note that downgrading from 5.0.0 to older versions does not work so automatically: You must ensure that any 5.0.0 binary logs or relay logs have been fully processed, so that you can remove them before proceeding with the downgrade.

6.7 Replication Features and Known Problems

The following list explains what is supported and what is not. Additional InnoDB-specific information about replication is given in section 16.7.5 InnoDB and MySQL Replication.

The following table lists replication problems in MySQL 3.23 that are fixed in MySQL 4.0:

6.8 Replication Startup Options

On both the master and the slave, you must use the server-id option to establish a unique replication ID for each server. You should pick a unique positive integer in the range from 1 to 2^32 - 1 for each master and slave. Example: server-id=3

The options that you can use on the master server for controlling binary logging are described in section 5.9.4 The Binary Log.

The following table describes the options you can use on slave replication servers. You can specify them on the command line or in an option file.

Some slave server replication options are handled in a special way, in the sense that they are ignored if a `master.info' file exists when the slave starts and contains values for the options. The following options are handled this way:

As of MySQL 4.1.1, the following options also are handled specially:

The `master.info' file format in 4.1.1 changed to include values corresponding to the SSL options. In addition, the 4.1.1 file format includes as its first line the number of lines in the file. If you upgrade an older server to 4.1.1, the new server upgrades the `master.info' file to the new format automatically when it starts. However, if you downgrade a 4.1.1 or newer server to a version older than 4.1.1, you should manually remove the first line before starting the older server for the first time. Note that, in this case, the downgraded server no longer can use an SSL connection to communicate with the master.

If no `master.info' file exists when the slave server starts, it uses values for those options that are specified in option files or on the command line. This will occur when you start the server as a replication slave for the very first time, or when you have run RESET SLAVE and shut down and restarted the slave server.

If the `master.info' file exists when the slave server starts, the server ignores those options. Instead, it uses the values found in the `master.info' file.

If you restart the slave server with different values of the startup options that correspond to values in the `master.info' file, the different values have no effect, because the server continues to use the `master.info' file. To use different values, you must either restart after removing the `master.info' file or (preferably) use the CHANGE MASTER TO statement to reset the values while the slave is running.

Suppose that you specify this option in your `my.cnf' file: 

[mysqld]
master-host=some_host

The first time you start the server as a replication slave, it reads and uses that option from the `my.cnf' file. The server then records the value in the `master.info' file. The next time you start the server, it reads the master host value from the `master.info' file only and ignores the value in the option file. If you modify the `my.cnf' file to specify a different master host of some_other_host, the change still will have no effect. You should use CHANGE MASTER TO instead.

Because the server gives an existing `master.info' file precedence over the startup options just described, you might prefer not to use startup options for these values at all, and instead specify them by using the CHANGE MASTER TO statement. See section 14.6.2.1 CHANGE MASTER TO Syntax.

This example shows a more extensive use of startup options to configure a slave server: 

[mysqld]
server-id=2
master-host=db-master.mycompany.com
master-port=3306
master-user=pertinax
master-password=freitag
master-connect-retry=60
report-host=db-slave.mycompany.com

The following list describes startup options for controlling replication: Many of these options can be reset while the server is running by using the CHANGE MASTER TO statement. Others, such as the --replicate-* options, can be set only when the slave server starts. We plan to fix this.

--log-slave-updates
Normally, updates received from a master server by a slave are not logged to its binary log. This option tells the slave to log the updates performed by its SQL thread to the slave's own binary log. For this option to have any effect, the slave must also be started with the --log-bin option to enable binary logging. --log-slave-updates is used when you want to chain replication servers. For example, you might want a setup like this: 
A -> B -> C
That is, A serves as the master for the slave B, and B serves as the master for the slave C. For this to work, B must be both a master and a slave. You must start both A and B with --log-bin to enable binary logging, and B with the --log-slave-updates option.
--log-warnings
Makes the slave print more messages to the error log about what it is doing. For example, it will warn you that it succeeded in reconnecting after a network/connection failure, and warn you about how each slave thread started. This option is enabled by default as of MySQL 4.0.19 and 4.1.2; to disable it, use --skip-log-warnings. As of MySQL 4.0.21 and 4.1.3, aborted connections are not logged to the error log unless the value is greater than 1. This option is not limited to replication use only. It produces warnings across a spectrum of server activities.
--master-connect-retry=seconds
The number of seconds the slave thread sleeps before retrying to connect to the master in case the master goes down or the connection is lost. The value in the `master.info' file takes precedence if it can be read. If not set, the default is 60.
--master-host=host
The hostname or IP number of the master replication server. If this option is not given, the slave thread will not be started. The value in `master.info' takes precedence if it can be read.
--master-info-file=file_name
The name to use for the file in which the slave records information about the master. The default name is `mysql.info' in the data directory.
--master-password=password
The password of the account that the slave thread uses for authentication when connecting to the master. The value in the `master.info' file takes precedence if it can be read. If not set, an empty password is assumed.
--master-port=port_number
The TCP/IP port the master is listening on. The value in the `master.info' file takes precedence if it can be read. If not set, the compiled-in setting is assumed. If you have not tinkered with @command{configure} options, this should be 3306.
--master-ssl
--master-ssl-ca=file_name
--master-ssl-capath=directory_name
--master-ssl-cert=file_name
--master-ssl-cipher=cipher_list
--master-ssl-key=file_name
These options are used for setting up a secure replication connection to the master server using SSL. Their meanings are the same as the corresponding --ssl, --ssl-ca, --ssl-capath, --ssl-cert, --ssl-cipher, --ssl-key options described in section 5.6.7.5 SSL Command-Line Options. The values in the `master.info' file take precedence if they can be read. These options are operational as of MySQL 4.1.1.
--master-user=username
The username of the account that the slave thread uses for authentication when connecting to the master. The account must have the REPLICATION SLAVE privilege. (Prior to MySQL 4.0.2, it must have the FILE privilege instead.) The value in the `master.info' file takes precedence if it can be read. If the master user is not set, user test is assumed.
--max-relay-log-size=#
To rotate the relay log automatically. See section 5.2.3 Server System Variables. This option is available as of MySQL 4.0.14.
--read-only
This option causes the slave to allow no updates except from slave threads or from users with the SUPER privilege. This can be useful to ensure that a slave server accepts no updates from clients. This option is available as of MySQL 4.0.14.
--relay-log=file_name
The name for the relay log. The default name is host_name-relay-bin.nnn, where host_name is the name of the slave server host and nnn indicates that relay logs are created in numbered sequence. You can specify the option to create hostname-independent relay log names, or if your relay logs tend to be big (and you don't want to decrease max_relay_log_size) and you need to put them in some area different from the data directory, or if you want to increase speed by balancing load between disks.
--relay-log-index=file_name
The location and name that should be used for the relay log index file. The default name is host_name-relay-bin.index, where host_name is the name of the slave server.
--relay-log-info-file=file_name
The name to use for the file in which the slave records information about the relay logs. The default name is `relay-log.info' in the data directory.
--relay-log-purge={0|1}
Disables or enables automatic purging of relay logs as soon as they are not needed any more. The default value is 1 (enabled). This is a global variable that can be changed dynamically with SET GLOBAL relay_log_purge. This option is available as of MySQL 4.1.1.
--relay-log-space-limit=#
Places an upper limit on the total size of all relay logs on the slave (a value of 0 means ``unlimited''). This is useful for a slave server host that has limited disk space. When the limit is reached, the I/O thread stops reading binary log events from the master server until the SQL thread has caught up and deleted some now unused relay logs. Note that this limit is not absolute: There are cases where the SQL thread needs more events before it can delete relay logs. In that case, the I/O thread will exceed the limit until it becomes possible for the SQL thread to delete some relay logs. Not doing so would cause a deadlock (which is what happens before MySQL 4.0.13). You should not set --relay-log-space-limit to less than twice the value of --max-relay-log-size (or --max-binlog-size if --max-relay-log-size is 0). In that case, there is a chance that the I/O thread will wait for free space because --relay-log-space-limit is exceeded, but the SQL thread will have no relay log to purge and be unable to satisfy the I/O thread. This forces the I/O thread to temporarily ignore --relay-log-space-limit.
--replicate-do-db=db_name
Tells the slave to restrict replication to statements where the default database (that is, the one selected by USE) is db_name. To specify more than one database, use this option multiple times, once for each database. Note that this will not replicate cross-database statements such as UPDATE some_db.some_table SET foo='bar' while having selected a different database or no database. If you need cross-database updates to work, make sure that you have MySQL 3.23.28 or later, and use --replicate-wild-do-table=db_name.%. Please read the notes that follow this option list. An example of what does not work as you might expect: If the slave is started with --replicate-do-db=sales and you issue the following statements on the master, the UPDATE statement will not be replicated: 
USE prices;
UPDATE sales.january SET amount=amount+1000;
If you need cross-database updates to work, use --replicate-wild-do-table=db_name.% instead. The main reason for this ``just-check-the-default-database'' behavior is that it's difficult from the statement alone to know whether or not it should be replicated (for example, if you are using multiple-table DELETE or multiple-table UPDATE statements that go across multiple databases). It's also very fast to just check the default database.
--replicate-do-table=db_name.tbl_name
Tells the slave thread to restrict replication to the specified table. To specify more than one table, use this option multiple times, once for each table. This will work for cross-database updates, in contrast to --replicate-do-db. Please read the notes that follow this option list.
--replicate-ignore-db=db_name
Tells the slave to not replicate any statement where the default database (that is, the one selected by USE) is db_name. To specify more than one database to ignore, use this option multiple times, once for each database. You should not use this option if you are using cross-database updates and you don't want these updates to be replicated. Please read the notes that follow this option list. An example of what does not work as you might expect: If the slave is started with --replicate-ignore-db=sales and you issue the following statements on the master, the UPDATE statement will be replicated: 
USE prices;
UPDATE sales.january SET amount=amount+1000;
If you need cross-database updates to work, use --replicate-wild-ignore-table=db_name.% instead.
--replicate-ignore-table=db_name.tbl_name
Tells the slave thread to not replicate any statement that updates the specified table (even if any other tables might be updated by the same statement). To specify more than one table to ignore, use this option multiple times, once for each table. This will work for cross-database updates, in contrast to --replicate-ignore-db. Please read the notes that follow this option list.
--replicate-wild-do-table=db_name.tbl_name
Tells the slave thread to restrict replication to statements where any of the updated tables match the specified database and table name patterns. Patterns can contain the `%' and `_' wildcard characters, which have the same meaning as for the LIKE pattern-matching operator. To specify more than one table, use this option multiple times, once for each table. This will work for cross-database updates. Please read the notes that follow this option list. Example: --replicate-wild-do-table=foo%.bar% will replicate only updates that use a table where the database name starts with foo and the table name starts with bar. If the table name pattern is %, it matches any table name and the option also applies to database-level statements (CREATE DATABASE, DROP DATABASE, and ALTER DATABASE). For example, if you use --replicate-wild-do-table=foo%.%, database-level statements statements are replicated if the database name matches the pattern foo%. To include literal wildcard characters in the database or table name patterns, escape them with a backslash. For example, to replicate all tables of a database that is named my_own%db, but not replicate tables from the my1ownAABCdb database, you should escape the `_' and `%' characters like this: --replicate-wild-do-table=my\_own\%db. If you're using the option on the command line, you might need to double the backslashes or quote the option value, depending on your command interpreter. For example, with the @command{bash} shell, you would need to type --replicate-wild-do-table=my\\_own\\%db.
--replicate-wild-ignore-table=db_name.tbl_name
Tells the slave thread to not replicate a statement where any table matches the given wildcard pattern. To specify more than one table to ignore, use this option multiple times, once for each table. This will work for cross-database updates. Please read the notes that follow this option list. Example: --replicate-wild-ignore-table=foo%.bar% will not replicate updates that use a table where the database name starts with foo and the table name starts with bar. For information about how matching works, see the description of the --replicate-wild-ignore-table option. The rules for including literal wildcard characters in the option value are the same as for --replicate-wild-ignore-table as well.
--replicate-rewrite-db=from_name->to_name
Tells the slave to translate the default database (that is, the one selected by USE) to to_name if it was from_name on the master. Only statements involving tables are affected (not statements such as CREATE DATABASE, DROP DATABASE, and ALTER DATABASE), and only if from_name was the default database on the master. This will not work for cross-database updates. Note that the database name translation is done before --replicate-* rules are tested. If you use this option on the command line and the `>' character is special to your command interpreter, quote the option value. For example: 
shell> mysqld --replicate-rewrite-db="olddb->newdb"
--replicate-same-server-id
To be used on slave servers. Usually you can should the default setting of 0, to prevent infinite loops in circular replication. If set to 1, this slave will not skip events having its own server id; normally this is useful only in rare configurations. Cannot be set to 1 if --log-slave-updates is used. Be careful that starting from MySQL 4.1, by default the slave I/O thread does not even write binary log events to the relay log if they have the slave's server id (this optimization helps save disk usage compared to 4.0). So if you want to use --replicate-same-server-id in 4.1 versions, be sure to start the slave with this option before you make the slave read its own events which you want the slave SQL thread to execute.
--report-host=host
The hostname or IP number of the slave to be reported to the master during slave registration. This value will appear in the output of SHOW SLAVE HOSTS on the master server. Leave the value unset if you do not want the slave to register itself with the master. Note that it is not sufficient for the master to simply read the IP number of the slave from the TCP/IP socket after the slave connects. Due to NAT and other routing issues, that IP may not be valid for connecting to the slave from the master or other hosts. This option is available as of MySQL 4.0.0.
--report-port=port_number
The TCP/IP port for connecting to the slave, to be reported to the master during slave registration. Set it only if the slave is listening on a non-default port or if you have a special tunnel from the master or other clients to the slave. If you are not sure, leave this option unset. This option is available as of MySQL 4.0.0.
--skip-slave-start
Tells the slave server not to start the slave threads when the server starts. To start the threads later, use a START SLAVE statement.
--slave_compressed_protocol={0|1}
If this option is set to 1, use compression of the slave/master protocol if both the slave and the master support it.
--slave-load-tmpdir=file_name
The name of the directory where the slave creates temporary files. This option is by default equal to the value of the tmpdir system variable. When the slave SQL thread replicates a LOAD DATA INFILE statement, it extracts the to-be-loaded file from the relay log into temporary files, then loads these into the table. If the file loaded on the master was huge, the temporary files on the slave will be huge, too. Therefore, it might be advisable to use this option to tell the slave to put temporary files in a directory located in some filesystem that has a lot of available space. In that case, you may also use the --relay-log option to place the relay logs in that filesystem, because the relay logs will be huge as well. --slave-load-tmpdir should point to a disk-based filesystem, not a memory-based one: The slave needs the temporary files used to replicate LOAD DATA INFILE to survive a machine's restart. The directory also should not be one that is cleared by the operating system during the system startup process.
--slave-net-timeout=seconds
The number of seconds to wait for more data from the master before aborting the read, considering the connection broken, and trying to reconnect. The first retry occurs immediately after the timeout. The interval between retries is controlled by the --master-connect-retry option.
--slave-skip-errors= [err_code1,err_code2,... | all]
Normally, replication stops when an error occurs, which gives you the opportunity to resolve the inconsistency in the data manually. This option tells the slave SQL thread to continue replication when a statement returns any of the errors listed in the option value. Do not use this option unless you fully understand why you are getting the errors. If there are no bugs in your replication setup and client programs, and no bugs in MySQL itself, an error that stops replication should never occur. Indiscriminate use of this option will result in slaves becoming hopelessly out of sync with the master, and you will have no idea why. For error codes, you should use the numbers provided by the error message in your slave error log and in the output of SHOW SLAVE STATUS. The server error codes are listed in section 23 Error Handling in MySQL. You can (but should not) also use the very non-recommended value of all which will ignore all error messages and keep barging along regardless of what happens. Needless to say, if you use it, we make no promises regarding your data integrity. Please do not complain if your data on the slave is not anywhere close to what it is on the master in this case. You have been warned. Examples: 
--slave-skip-errors=1062,1053
--slave-skip-errors=all

The --replicate-* rules are evaluated as follows to determine whether a statement will be executed by the slave or ignored:

  1. Are there some --replicate-do-db or --replicate-ignore-db rules?
    • Yes: Test them as for --binlog-do-db and --binlog-ignore-db (see section 5.9.4 The Binary Log). What is the result of the test?
      • Ignore the statement: Ignore it and exit.
      • Execute the statement: Don't execute it immediately, defer the decision, go to the next step.
    • No: Go to the next step.
  2. Are there some --replicate-*-table rules?
    • No: Execute the query and exit.
    • Yes: Go to the next step. Only tables that are to be updated are compared to the rules (INSERT INTO sales SELECT * FROM prices: only sales will be compared to the rules). If several tables are to be updated (multiple-table statement), the first matching table (matching ``do'' or ``ignore'') wins. That is, the first table is compared to the rules. Then, if no decision could be mad, the second table is compared to the rules, and so forth.
  3. Are there some --replicate-do-table rules?
    • Yes: Does the table match any of them?
      • Yes: Execute the query and exit.
      • No: Go to the next step.
    • No: Go to the next step.
  4. Are there some --replicate-ignore-table rules?
    • Yes: Does the table match any of them?
      • Yes: Ignore the query and exit.
      • No: Go to the next step.
    • No: Go to the next step.
  5. Are there some --replicate-wild-do-table rules?
    • Yes: Does the table match any of them?
      • Yes: Execute the query and exit.
      • No: Go to the next step.
    • No: Go to the next step.
  6. Are there some --replicate-wild-ignore-table rules?
    • Yes: Does the table match any of them?
      • Yes: Ignore the query and exit.
      • No: Go to the next step.
    • No: Go to the next step.
  7. No --replicate-*-table rule was matched. Is there another table to test against these rules?
    • Yes: Loop.
    • No: We have tested all tables to be updated and could not match any rule. Are there --replicate-do-table or --replicate-wild-do-table rules?
      • Yes: Ignore the query and exit.
      • No: Execute the query and exit.

6.9 Replication FAQ

Q: How do I configure a slave if the master is already running and I do not want to stop it?

A: There are several options. If you have taken a backup of the master at some point and recorded the binary log name and offset (from the output of SHOW MASTER STATUS ) corresponding to the snapshot, use the following procedure:

  1. Make sure that the slave is assigned a unique server ID.
  2. Execute the following statement on the slave, filling in appropriate values for each option: 
    mysql> CHANGE MASTER TO
    ->     MASTER_HOST='master_host_name',
    ->     MASTER_USER='master_user_name',
    ->     MASTER_PASSWORD='master_pass',
    ->     MASTER_LOG_FILE='recorded_log_file_name',
    ->     MASTER_LOG_POS=recorded_log_position;
  3. Execute START SLAVE on the slave.

If you do not have a backup of the master server already, here is a quick procedure for creating one. All steps should be performed on the master host.

  1. Issue this statement: 
    mysql> FLUSH TABLES WITH READ LOCK;
  2. With the lock still in place, execute this command (or a variation of it): 
    shell> tar zcf /tmp/backup.tar.gz /var/lib/mysql
  3. Issue this statement and make sure to record the output, which you will need later: 
    mysql> SHOW MASTER STATUS;
  4. Release the lock: 
    mysql> UNLOCK TABLES;

An alternative is to make an SQL dump of the master instead of a binary copy as in the preceding procedure. To do this, you can use @command{mysqldump --master-data} on your master and later load the SQL dump into your slave. However, this is slower than making a binary copy.

No matter which of the two methods you use, afterward follow the instructions for the case when you have a snapshot and have recorded the log name and offset. You can use the same snapshot to set up several slaves. Once you have the snapshot of the master, you can wait to set up a slave as long as the binary logs of the master are left intact. The two practical limitations on the length of time you can wait are the amount of disk space available to retain binary logs on the master and the length of time it will take the slave to catch up.

You can also use LOAD DATA FROM MASTER. This is a convenient statement that transfers a snapshot to the slave and adjusts the log name and offset all at once. In the future, LOAD DATA FROM MASTER will be the recommended way to set up a slave. Be warned, however, that it works only for MyISAM tables and it may hold a read lock for a long time. It is not yet implemented as efficiently as we would like. If you have large tables, the preferred method at this time is still to make a binary snapshot on the master server after executing FLUSH TABLES WITH READ LOCK.

Q: Does the slave need to be connected to the master all the time?

A: No, it does not. The slave can go down or stay disconnected for hours or even days, then reconnect and catch up on the updates. For example, you can set up a master/slave relationship over a dial-up link where the link is up only sporadically and for short periods of time. The implication of this is that, at any given time, the slave is not guaranteed to be in sync with the master unless you take some special measures. In the future, we will have the option to block the master until at least one slave is in sync.

Q: How do I know how late a slave is compared to the master? In other words, how do I know the date of the last query replicated by the slave?

A: If the slave is 4.1.1 or newer, read the Seconds_Behind_Master column in SHOW SLAVE STATUS. For older versions, the following applies. This is possible only if SHOW PROCESSLIST on the slave shows that the SQL thread is running (or for MySQL 3.23, that the slave thread is running), and that the thread has executed at least one event from the master. See section 6.3 Replication Implementation Details.

When the slave SQL thread executes an event read from the master, it modifies its own time to the event timestamp (this is why TIMESTAMP is well replicated). In the Time column in the output of SHOW PROCESSLIST, the number of seconds displayed for the slave SQL thread is the number of seconds between the timestamp of the last replicated event and the real time of the slave machine. You can use this to determine the date of the last replicated event. Note that if your slave has been disconnected from the master for one hour, and then reconnects, you may immediately see Time values like 3600 for the slave SQL thread in SHOW PROCESSLIST. This would be because the slave is executing statements that are one hour old.

Q: How do I force the master to block updates until the slave catches up?

A: Use the following procedure:

  1. On the master, execute these statements: 
    mysql> FLUSH TABLES WITH READ LOCK;
mysql> SHOW MASTER STATUS;
    Record the log name and the offset from the output of the SHOW statement. These are the replication coordinates.
  2. On the slave, issue the following statement, where the arguments to the MASTER_POS_WAIT() function are the replication coordinate values obtained in the previous step: 
    mysql> SELECT MASTER_POS_WAIT('log_name', log_offset);
    The SELECT statement will block until the slave reaches the specified log file and offset. At that point, the slave will be in sync with the master and the statement will return.
  3. On the master, issue the following statement to allow the master to begin processing updates again: 
    mysql> UNLOCK TABLES;

Q: What issues should I be aware of when setting up two-way replication?

A: MySQL replication currently does not support any locking protocol between master and slave to guarantee the atomicity of a distributed (cross-server) update. In other words, it is possible for client A to make an update to co-master 1, and in the meantime, before it propagates to co-master 2, client B could make an update to co-master 2 that will make the update of client A work differently than it did on co-master 1. Thus, when the update of client A makes it to co-master 2, it will produce tables that are different than what you have on co-master 1, even after all the updates from co-master 2 have also propagated. This means that you should not co-chain two servers in a two-way replication relationship unless you are sure that your updates can safely happen in any order, or unless you take care of mis-ordered updates somehow in the client code.

You must also realize that two-way replication actually does not improve performance very much (if at all), as far as updates are concerned. Both servers need to do the same number of updates each, as you would have one server do. The only difference is that there will be a little less lock contention, because the updates originating on another server will be serialized in one slave thread. Even this benefit might be offset by network delays.

Q: How can I use replication to improve performance of my system?

A: You should set up one server as the master and direct all writes to it. Then configure as many slaves as you have the budget and rackspace for, and distribute the reads among the master and the slaves. You can also start the slaves with the --skip-innodb, --skip-bdb, --low-priority-updates, and --delay-key-write=ALL options to get speed improvements on the slave end. In this case, the slave will use non-transactional MyISAM tables instead of InnoDB and BDB tables to get more speed.

Q: What should I do to prepare client code in my own applications to use performance-enhancing replication?

A: If the part of your code that is responsible for database access has been properly abstracted/modularized, converting it to run with a replicated setup should be very smooth and easy. Just change the implementation of your database access to send all writes to the master, and to send reads to either the master or a slave. If your code does not have this level of abstraction, setting up a replicated system will give you the opportunity and motivation to it clean up. You should start by creating a wrapper library or module with the following functions:

safe_ in each function name means that the function will take care of handling all the error conditions. You can use different names for the functions. The important thing is to have a unified interface for connecting for reads, connecting for writes, doing a read, and doing a write.

You should then convert your client code to use the wrapper library. This may be a painful and scary process at first, but it will pay off in the long run. All applications that use the approach just described will be able to take advantage of a master/slave configuration, even one involving multiple slaves. The code will be a lot easier to maintain, and adding troubleshooting options will be trivial. You will just need to modify one or two functions; for example, to log how long each statement took, or which statement among your many thousands gave you an error.

If you have written a lot of code already, you may want to automate the conversion task by using the @command{replace} utility that comes with standard MySQL distributions, or just write your own conversion script. Ideally, your code already uses consistent programming style conventions. If not, then you are probably better off rewriting it anyway, or at least going through and manually regularizing it to use a consistent style.

Q: When and how much can MySQL replication improve the performance of my system?

A: MySQL replication is most beneficial for a system with frequent reads and infrequent writes. In theory, by using a single-master/multiple-slave setup, you can scale the system by adding more slaves until you either run out of network bandwidth, or your update load grows to the point that the master cannot handle it.

In order to determine how many slaves you can get before the added benefits begin to level out, and how much you can improve performance of your site, you need to know your query patterns, and to determine empirically by benchmarking the relationship between the throughput for reads (reads per second, or max_reads) and for writes (max_writes) on a typical master and a typical slave. The example here shows a rather simplified calculation of what you can get with replication for a hypothetical system.

Let's say that system load consists of 10% writes and 90% reads, and we have determined by benchmarking that max_reads is 1200 - 2 * max_writes. In other words, the system can do 1,200 reads per second with no writes, the average write is twice as slow as the average read, and the relationship is linear. Let us suppose that the master and each slave have the same capacity, and that we have one master and N slaves. Then we have for each server (master or slave):

reads = 1200 - 2 * writes

reads = 9 * writes / (N + 1) (reads are split, but writes go to all servers)

9 * writes / (N + 1) + 2 * writes = 1200

writes = 1200 / (2 + 9/(N+1))

The last equation indicates that the maximum number of writes for N slaves, given a maximum possible read rate of 1,200 per minute and a ratio of nine reads per write.

This analysis yields the following conclusions:

Note that these computations assume infinite network bandwidth and neglect several other factors that could turn out to be significant on your system. In many cases, you may not be able to perform a computation similar to the just shown that will accurately predict what will happen on your system if you add N replication slaves. However, answering the following questions should help you decide whether and how much replication will improve the performance of your system:

Q: How can I use replication to provide redundancy/high availability?

A: With the currently available features, you would have to set up a master and a slave (or several slaves), and write a script that will monitor the master to see whether it is up. Then instruct your applications and the slaves to change master in case of failure. Some suggestions:

We are currently working on integrating an automatic master election system into MySQL, but until it is ready, you will have to create your own monitoring tools.

6.10 Troubleshooting Replication

If you have followed the instructions, and your replication setup is not working, first check the following:

6.11 Reporting Replication Bugs

When you have determined that there is no user error involved, and replication still either does not work at all or is unstable, it is time to send us a bug report. We need to get as much information as possible from you to be able to track down the bug. Please do spend some time and effort preparing a good bug report.

If you have a repeatable test case that demonstrates the bug, please enter it into our bugs database at http://bugs.mysql.com/. If you have a phantom problem (one that you cannot duplicate ``at will''), use the following procedure:

  1. Verify that no user error is involved. For example, if you update the slave outside of the slave thread, the data will go out of sync, and you can have unique key violations on updates. In this case, the slave thread will stop and wait for you to clean up the tables manually to bring them in sync. This is not a replication problem. It is a problem of outside interference that causes replication to fail.
  2. Run the slave with the --log-slave-updates and --log-bin options. They will cause the slave to log the updates that it receives from the master into its own binary logs.
  3. Save all evidence before resetting the replication state. If we have no information or only sketchy information, it becomes difficult or impossible for us to track down the problem. The evidence you should collect is:
    • All binary logs from the master
    • All binary logs from the slave
    • The output of SHOW MASTER STATUS from the master at the time you have discovered the problem
    • The output of SHOW SLAVE STATUS from the master at the time you have discovered the problem
    • Error logs from the master and the slave
  4. Use @command{mysqlbinlog} to examine the binary logs. The following should be helpful to find the trouble query, for example: 
    shell> mysqlbinlog -j pos_from_slave_status \
           /path/to/log_from_slave_status | head

Once you have collected the evidence for the phantom problem, try hard to isolate it into a separate test case first. Then enter the problem into our bugs database at http://bugs.mysql.com/ with as much information as possible.

7 MySQL Optimization

Optimization is a complex task because ultimately it requires understanding of the entire system to be optimized. Although it may be possible to perform some local optimizations with little knowledge of your system or application, the more optimal you want your system to become, the more you will have to know about it.

This chapter tries to explain and give some examples of different ways to optimize MySQL. Remember, however, that there are always additional ways to make the system even faster, although they may require increasing effort to achieve.

7.1 Optimization Overview

The most important factor in making a system fast is its basic design. You also need to know what kinds of things your system will be doing, and what your bottlenecks are.

The most common system bottlenecks are:

7.1.1 MySQL Design Limitations and Tradeoffs

When using the MyISAM storage engine, MySQL uses extremely fast table locking that allows multiple readers or a single writer. The biggest problem with this storage engine occurs when you have a steady stream of mixed updates and slow selects on a single table. If this is a problem for certain tables, you can use another table type for them. See section 15 MySQL Storage Engines and Table Types.

MySQL can work with both transactional and non-transactional tables. To be able to work smoothly with non-transactional tables (which can't roll back if something goes wrong), MySQL has the following rules (when not running in strict mode or if you use the IGNORE specifier to INSERT or UPDATE).

If you are using non-transactional tables, you should not use MySQL to check column content. In general, the safest (and often fastest) way is to let the application ensure that it passes only legal values to the database.

For more information about this, see section 1.8.6 How MySQL Deals with Constraints and section 14.1.4 INSERT Syntax or section 5.2.2 The Server SQL Mode.

7.1.2 Designing Applications for Portability

Because all SQL servers implement different parts of standard SQL, it takes work to write portable SQL applications. It is very easy to achieve portability for very simple selects and inserts, but becomes more difficult the more capabilities you require. If you want an application that is fast with many database systems, it becomes even harder!

To make a complex application portable, you need to determine which SQL servers it must work with, then determine what features those servers support.

All database systems have some weak points. That is, they have different design compromises that lead to different behavior.

You can use the MySQL crash-me program to find functions, types, and limits that you can use with a selection of database servers. crash-me does not check for every possible feature, but it is still reasonably comprehensive, performing about 450 tests.

An example of the type of information crash-me can provide is that you shouldn't have column names longer than 18 characters if you want to be able to use Informix or DB2.

The crash-me program and the MySQL benchmarks are all very database independent. By taking a look at how they are written, you can get a feeling for what you have to do to make your own applications database independent. The programs can be found in the `sql-bench' directory of MySQL source distributions. They are written in Perl and use the DBI database interface. Use of DBI in itself solves part of the portability problem because it provides database-independent access methods.

For crash-me results, visit http://dev.mysql.com/tech-resources/crash-me.php. See http://dev.mysql.com/tech-resources/benchmarks/ for the results from the benchmarks.

If you strive for database independence, you need to get a good feeling for each SQL server's bottlenecks. For example, MySQL is very fast in retrieving and updating records for MyISAM tables, but will have a problem in mixing slow readers and writers on the same table. Oracle, on the other hand, has a big problem when you try to access rows that you have recently updated (until they are flushed to disk). Transactional databases in general are not very good at generating summary tables from log tables, because in this case row locking is almost useless.

To make your application really database independent, you need to define an easily extendable interface through which you manipulate your data. As C++ is available on most systems, it makes sense to use a C++ class-based interface to the databases.

If you use some feature that is specific to a given database system (such as the REPLACE statement, which is specific to MySQL), you should implement the same feature for other SQL servers by coding an alternative method. Although the alternative may be slower, it will allow the other servers to perform the same tasks.

With MySQL, you can use the /*! */ syntax to add MySQL-specific keywords to a query. The code inside /**/ will be treated as a comment (and ignored) by most other SQL servers.

If high performance is more important than exactness, as in some Web applications, it is possible to create an application layer that caches all results to give you even higher performance. By letting old results ``expire'' after a while, you can keep the cache reasonably fresh. This provides a method to handle high load spikes, in which case you can dynamically increase the cache and set the expiration timeout higher until things get back to normal.

In this case, the table creation information should contain information of the initial size of the cache and how often the table should normally be refreshed.

An alternative to implementing an application cache is to use the MySQL query cache. By enabling the query cache, the server handles the details of determining whether a query result can be reused. This simplifies your application. See section 5.11 The MySQL Query Cache.

7.1.3 What We Have Used MySQL For

This section describes an early application for MySQL.

During MySQL initial development, the features of MySQL were made to fit our largest customer, which handled data warehousing for a couple of the largest retailers in Sweden.

From all stores, we got weekly summaries of all bonus card transactions, and were expected to provide useful information for the store owners to help them find how their advertising campaigns were affecting their own customers.

The volume of data was quite huge (about seven million summary transactions per month), and we had data for 4-10 years that we needed to present to the users. We got weekly requests from our customers, who wanted to get ``instant'' access to new reports from this data.

We solved this problem by storing all information per month in compressed ``transaction'' tables. We had a set of simple macros that generated summary tables grouped by different criteria (product group, customer id, store, and so on) from the tables in which the transactions were stored. The reports were Web pages that were dynamically generated by a small Perl script. This script parsed a Web page, executed the SQL statements in it, and inserted the results. We would have used PHP or mod_perl instead, but they were not available at the time.

For graphical data, we wrote a simple tool in C that could process SQL query results and produce GIF images based on those results. This tool also was dynamically executed from the Perl script that parses the Web pages.

In most cases, a new report could be created simply by copying an existing script and modifying the SQL query in it. In some cases, we needed to add more columns to an existing summary table or generate a new one. This also was quite simple because we kept all transaction-storage tables on disk. (This amounted to about 50GB of transaction tables and 200GB of other customer data.)

We also let our customers access the summary tables directly with ODBC so that the advanced users could experiment with the data themselves.

This system worked well and we had no problems handling the data with quite modest Sun Ultra SPARCstation hardware (2x200MHz). Eventually the system was migrated to Linux.

7.1.4 The MySQL Benchmark Suite

This section should contain a technical description of the MySQL benchmark suite (and crash-me), but that description has not yet been written. Currently, you can get a good idea of the benchmarks by looking at the code and results in the `sql-bench' directory in any MySQL source distribution.

This benchmark suite is meant to tell any user what operations a given SQL implementation performs well or poorly.

Note that this benchmark is single-threaded, so it measures the minimum time for the operations performed. We plan to add multi-threaded tests to the benchmark suite in the future.

To use the benchmark suite, the following requirements must be satisfied:

After you obtain a MySQL source distribution, you will find the benchmark suite located in its `sql-bench' directory. To run the benchmark tests, build MySQL, then change location into the `sql-bench' directory and execute the run-all-tests script: 

shell> cd sql-bench
shell> perl run-all-tests --server=server_name

server_name is one of the supported servers. To get a list of all options and supported servers, invoke this command: 

shell> perl run-all-tests --help

The crash-me script also is located in the `sql-bench' directory. crash-me tries to determine what features a database supports and what its capabilities and limitations are by actually running queries. For example, it determines:

You can find the results from crash-me for many different database servers at http://dev.mysql.com/tech-resources/crash-me.php. For more information about benchmark results, visit http://dev.mysql.com/tech-resources/benchmarks/.

7.1.5 Using Your Own Benchmarks

You should definitely benchmark your application and database to find out where the bottlenecks are. By fixing a bottleneck (or by replacing it with a ``dummy module''), you can then easily identify the next bottleneck. Even if the overall performance for your application currently is acceptable, you should at least make a plan for each bottleneck, and decide how to solve it if someday you really need the extra performance.

For an example of portable benchmark programs, look at the MySQL benchmark suite. See section 7.1.4 The MySQL Benchmark Suite. You can take any program from this suite and modify it for your needs. By doing this, you can try different solutions to your problem and test which really is fastest for you.

Another free benchmark suite is the Open Source Database Benchmark, available at http://osdb.sourceforge.net/.

It is very common for a problem to occur only when the system is very heavily loaded. We have had many customers who contact us when they have a (tested) system in production and have encountered load problems. In most cases, performance problems turn out to be due to issues of basic database design (for example, table scans are not good at high load) or problems with the operating system or libraries. Most of the time, these problems would be a lot easier to fix if the systems were not already in production.

To avoid problems like this, you should put some effort into benchmarking your whole application under the worst possible load! You can use Super Smack for this. It is available at http://jeremy.zawodny.com/mysql/super-smack/. As the name suggests, it can bring a system to its knees if you ask it, so make sure to use it only on your development systems.

7.2 Optimizing SELECT Statements and Other Queries

First, one factor affects all statements: The more complex your permission setup is, the more overhead you will have.

Using simpler permissions when you issue GRANT statements enables MySQL to reduce permission-checking overhead when clients execute statements. For example, if you don't grant any table-level or column-level privileges, the server need not ever check the contents of the tables_priv and columns_priv tables. Similarly, if you place no resource limits on any accounts, the server does not have to perform resource counting. If you have a very high query volume, it may be worth the time to use a simplified grant structure to reduce permission-checking overhead.

If your problem is with some specific MySQL expression or function, you can use the BENCHMARK() function from the @command{mysql} client program to perform a timing test. Its syntax is BENCHMARK(loop_count,expression). For example: 

mysql> SELECT BENCHMARK(1000000,1+1);
+------------------------+
| BENCHMARK(1000000,1+1) |
+------------------------+
|                      0 |
+------------------------+
1 row in set (0.32 sec)

This result was obtained on a Pentium II 400MHz system. It shows that MySQL can execute 1,000,000 simple addition expressions in 0.32 seconds on that system.

All MySQL functions should be very optimized, but there may be some exceptions. BENCHMARK() is a great tool to find out if this is a problem with your query.

7.2.1 EXPLAIN Syntax (Get Information About a SELECT)

EXPLAIN tbl_name

Or: 

EXPLAIN SELECT select_options

The EXPLAIN statement can be used either as a synonym for DESCRIBE or as a way to obtain information about how MySQL will execute a SELECT statement:

This section provides information about the second use of EXPLAIN.

With the help of EXPLAIN, you can see when you must add indexes to tables to get a faster SELECT that uses indexes to find records.

If you have a problem with incorrect index usage, you should run ANALYZE TABLE to update table statistics such as cardinality of keys, which can affect the choices the optimizer makes. See section 14.5.2.1 ANALYZE TABLE Syntax.

You can also see whether the optimizer joins the tables in an optimal order. To force the optimizer to use a join order corresponding to the order in which the tables are named in the SELECT statement, begin the statement with SELECT STRAIGHT_JOIN rather than just SELECT.

EXPLAIN returns a row of information for each table used in the SELECT statement. The tables are listed in the output in the order that MySQL would read them while processing the query. MySQL resolves all joins using a single-sweep multi-join method. This means that MySQL reads a row from the first table, then finds a matching row in the second table, then in the third table, and so on. When all tables are processed, it outputs the selected columns and backtracks through the table list until a table is found for which there are more matching rows. The next row is read from this table and the process continues with the next table.

In MySQL version 4.1, the EXPLAIN output format was changed to work better with constructs such as UNION statements, subqueries, and derived tables. Most notable is the addition of two new columns: id and select_type. You will not see these columns when using servers older than MySQL 4.1.

Each output row from EXPLAIN provides information about one table, and each row consists of the following columns:

id
The SELECT identifier. This is the sequential number of the SELECT within the query.
select_type
The type of SELECT, which can be any of the following:
SIMPLE
Simple SELECT (not using UNION or subqueries)
PRIMARY
Outermost SELECT
UNION
Second or later SELECT statement in a UNION
DEPENDENT UNION
Second or later SELECT statement in a UNION, dependent on outer subquery
SUBQUERY
First SELECT in subquery
DEPENDENT SUBQUERY
First SELECT in subquery, dependent on outer subquery
DERIVED
Derived table SELECT (subquery in FROM clause)
table
The table to which the row of output refers.
type
The join type. The different join types are listed here, ordered from the best type to the worst:
system
The table has only one row (= system table). This is a special case of the const join type.
const
The table has at most one matching row, which will be read at the start of the query. Because there is only one row, values from the column in this row can be regarded as constants by the rest of the optimizer. const tables are very fast because they are read only once! const is used when you compare all parts of a PRIMARY KEY or UNIQUE index with constant values. In the following queries, tbl_name can be used as a const table: 
SELECT * FROM tbl_name WHERE primary_key=1;

SELECT * FROM tbl_name
WHERE primary_key_part1=1 AND primary_key_part2=2;
eq_ref
One row will be read from this table for each combination of rows from the previous tables. Other than the const types, this is the best possible join type. It is used when all parts of an index are used by the join and the index is a PRIMARY KEY or UNIQUE index. eq_ref can be used for indexed columns that are compared using the = operator. The comparison value can be a constant or an expression that uses columns from tables that are read before this table. In the following examples, MySQL can use an eq_ref join to process ref_table: 
SELECT * FROM ref_table,other_table
WHERE ref_table.key_column=other_table.column;

SELECT * FROM ref_table,other_table
WHERE ref_table.key_column_part1=other_table.column
AND ref_table.key_column_part2=1;
ref
All rows with matching index values will be read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key or if the key is not a PRIMARY KEY or UNIQUE index (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this is a good join type. ref can be used for indexed columns that are compared using the = operator. In the following examples, MySQL can use a ref join to process ref_table: 
SELECT * FROM ref_table WHERE key_column=expr;

SELECT * FROM ref_table,other_table
WHERE ref_table.key_column=other_table.column;

SELECT * FROM ref_table,other_table
WHERE ref_table.key_column_part1=other_table.column
AND ref_table.key_column_part2=1;
ref_or_null
This join type is like ref, but with the addition that MySQL will do an extra search for rows that contain NULL values. This join type optimization is new for MySQL 4.1.1 and is mostly used when resolving subqueries. In the following examples, MySQL can use a ref_or_null join to process ref_table: 
SELECT * FROM ref_table
WHERE key_column=expr OR key_column IS NULL;
See section 7.2.7 How MySQL Optimizes IS NULL.
index_merge
This join type indicates that the Index Merge optimization is used. In this case, the key column contains a list of indexes used, and key_len contains a list of the longest key parts for the indexes used. For more information, see section 7.2.6 Index Merge Optimization.
unique_subquery
This type replaces ref for some IN subqueries of the following form: 
value IN (SELECT primary_key FROM single_table WHERE some_expr)
unique_subquery is just an index lookup function that replaces the subquery completely for better efficiency.
index_subquery
This join type is similar to unique_subquery. It replaces IN subqueries, but it works for non-unique indexes in subqueries of the following form: 
value IN (SELECT key_column FROM single_table WHERE some_expr)
range
Only rows that are in a given range will be retrieved, using an index to select the rows. The key column indicates which index is used. The key_len contains the longest key part that was used. The ref column will be NULL for this type. range can be used for when a key column is compared to a constant using any of the =, <>, >, >=, <, <=, IS NULL, <=>, BETWEEN, or IN operators: 
SELECT * FROM tbl_name
WHERE key_column = 10;

SELECT * FROM tbl_name
WHERE key_column BETWEEN 10 and 20;

SELECT * FROM tbl_name
WHERE key_column IN (10,20,30);

SELECT * FROM tbl_name
WHERE key_part1= 10 AND key_part2 IN (10,20,30);
index
This join type is the same as ALL, except that only the index tree is scanned. This usually is faster than ALL, because the index file usually is smaller than the data file. MySQL can use this join type when the query uses only columns that are part of a single index.
ALL
A full table scan will be done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked const, and usually very bad in all other cases. Normally, you can avoid ALL by adding indexes that allow row retrieval from the table based on constant values or column values from earlier tables.
possible_keys
The possible_keys column indicates which indexes MySQL could use to find the rows in this table. Note that this column is totally independent of the order of the tables as displayed in the output from EXPLAIN. That means that some of the keys in possible_keys might not be usable in practice with the generated table order. If this column is NULL, there are no relevant indexes. In this case, you may be able to improve the performance of your query by examining the WHERE clause to see whether it refers to some column or columns that would be suitable for indexing. If so, create an appropriate index and check the query with EXPLAIN again. See section 14.2.2 ALTER TABLE Syntax. To see what indexes a table has, use SHOW INDEX FROM tbl_name.
key
The key column indicates the key (index) that MySQL actually decided to use. The key is NULL if no index was chosen. To force MySQL to use or ignore an index listed in the possible_keys column, use FORCE INDEX, USE INDEX, or IGNORE INDEX in your query. See section 14.1.7 SELECT Syntax. For MyISAM and BDB tables, running ANALYZE TABLE will help the optimizer choose better indexes. For MyISAM tables, @command{myisamchk --analyze} will do the same. See section 14.5.2.1 @code{ANALYZE TABLE} Syntax and section 5.7.2 Table Maintenance and Crash Recovery.
key_len
The key_len column indicates the length of the key that MySQL decided to use. The length is NULL if the key column says NULL. Note that the value of key_len allows you to determine how many parts of a multiple-part key MySQL will actually use.
ref
The ref column shows which columns or constants are used with the key to select rows from the table.
rows
The rows column indicates the number of rows MySQL believes it must examine to execute the query.
Extra
This column contains additional information about how MySQL will resolve the query. Here is an explanation of the different text strings that can appear in this column:
Distinct
MySQL will stop searching for more rows for the current row combination after it has found the first matching row.
Not exists
MySQL was able to do a LEFT JOIN optimization on the query and will not examine more rows in this table for the previous row combination after it finds one row that matches the LEFT JOIN criteria. Here is an example of the type of query that can be optimized this way: 
SELECT * FROM t1 LEFT JOIN t2 ON t1.id=t2.id
WHERE t2.id IS NULL;
Assume that t2.id is defined as NOT NULL. In this case, MySQL will scan t1 and look up the rows in t2 using the values of t1.id. If MySQL finds a matching row in t2, it knows that t2.id can never be NULL, and will not scan through the rest of the rows in t2 that have the same id value. In other words, for each row in t1, MySQL needs to do only a single lookup in t2, regardless of how many rows actually match in t2.
range checked for each record (index map: #)
MySQL found no good index to use, but found that some of indexes might be used once column values from preceding tables are known. For each row combination in the preceding tables, MySQL will check whether it is possible to use a range or index_merge access method to retrieve rows. The applicability criteria are as described in section 7.2.5 Range Optimization and section 7.2.6 Index Merge Optimization, with the exception that all column values for the preceding table are known and considered to be constants. This is not very fast, but is faster than performing a join with no index at all.
Using filesort
MySQL will need to do an extra pass to find out how to retrieve the rows in sorted order. The sort is done by going through all rows according to the join type and storing the sort key and pointer to the row for all rows that match the WHERE clause. The keys then are sorted and the rows are retrieved in sorted order. See section 7.2.10 How MySQL Optimizes ORDER BY.
Using index
The column information is retrieved from the table using only information in the index tree without having to do an additional seek to read the actual row. This strategy can be used when the query uses only columns that are part of a single index.
Using temporary
To resolve the query, MySQL will need to create a temporary table to hold the result. This typically happens if the query contains GROUP BY and ORDER BY clauses that list columns differently.
Using where
A WHERE clause will be used to restrict which rows to match against the next table or send to the client. Unless you specifically intend to fetch or examine all rows from the table, you may have something wrong in your query if the Extra value is not Using where and the table join type is ALL or index.
If you want to make your queries as fast as possible, you should look out for Extra values of Using filesort and Using temporary.
Using sort_union(...)
Using union(...)
Using intersect(...)
These indicate how index scans are merged for the index_merge join type. See section 7.2.6 Index Merge Optimization for more information.
Using index for group-by
Similar to the Using index way of accessing a table, Using index for group-by indicates that MySQL found an index that can be used to retrieve all columns of a GROUP BY or DISTINCT query without any extra disk access to the actual table. Additionally, the index will be used in the most efficient way so that for each group, only a few index entries will be read. For details, see section 7.2.11 How MySQL Optimizes GROUP BY.

You can get a good indication of how good a join is by taking the product of the values in the rows column of the EXPLAIN output. This should tell you roughly how many rows MySQL must examine to execute the query. If you restrict queries with the max_join_size system variable, this product also is used to determine which multiple-table SELECT statements to execute. See section 7.5.2 Tuning Server Parameters.

The following example shows how a multiple-table join can be optimized progressively based on the information provided by EXPLAIN.

Suppose that you have the SELECT statement shown here and you plan to examine it using EXPLAIN: 

EXPLAIN SELECT tt.TicketNumber, tt.TimeIn,
            tt.ProjectReference, tt.EstimatedShipDate,
            tt.ActualShipDate, tt.ClientID,
            tt.ServiceCodes, tt.RepetitiveID,
            tt.CurrentProcess, tt.CurrentDPPerson,
            tt.RecordVolume, tt.DPPrinted, et.COUNTRY,
            et_1.COUNTRY, do.CUSTNAME
        FROM tt, et, et AS et_1, do
        WHERE tt.SubmitTime IS NULL
            AND tt.ActualPC = et.EMPLOYID
            AND tt.AssignedPC = et_1.EMPLOYID
            AND tt.ClientID = do.CUSTNMBR;

For this example, make the following assumptions:

Initially, before any optimizations have been performed, the EXPLAIN statement produces the following information: 

table type possible_keys key  key_len ref  rows  Extra
et    ALL  PRIMARY       NULL NULL    NULL 74
do    ALL  PRIMARY       NULL NULL    NULL 2135
et_1  ALL  PRIMARY       NULL NULL    NULL 74
tt    ALL  AssignedPC,   NULL NULL    NULL 3872
           ClientID,
           ActualPC
      range checked for each record (key map: 35)

Because type is ALL for each table, this output indicates that MySQL is generating a Cartesian product of all the tables; that is, every combination of rows. This will take quite a long time, because the product of the number of rows in each table must be examined. For the case at hand, this product is 74 * 2135 * 74 * 3872 = 45,268,558,720 rows. If the tables were bigger, you can only imagine how long it would take.

One problem here is that MySQL can use indexes on columns more efficiently if they are declared the same. (For ISAM tables, indexes may not be used at all unless the columns are declared the same.) In this context, VARCHAR and CHAR are the same unless they are declared as different lengths. Because tt.ActualPC is declared as CHAR(10) and et.EMPLOYID is declared as CHAR(15), there is a length mismatch.

To fix this disparity between column lengths, use ALTER TABLE to lengthen ActualPC from 10 characters to 15 characters: 

mysql> ALTER TABLE tt MODIFY ActualPC VARCHAR(15);

Now tt.ActualPC and et.EMPLOYID are both VARCHAR(15). Executing the EXPLAIN statement again produces this result: 

table type   possible_keys key     key_len ref         rows    Extra
tt    ALL    AssignedPC,   NULL    NULL    NULL        3872    Using
             ClientID,                                         where
             ActualPC
do    ALL    PRIMARY       NULL    NULL    NULL        2135
      range checked for each record (key map: 1)
et_1  ALL    PRIMARY       NULL    NULL    NULL        74
      range checked for each record (key map: 1)
et    eq_ref PRIMARY       PRIMARY 15      tt.ActualPC 1

This is not perfect, but is much better: The product of the rows values is now less by a factor of 74. This version is executed in a couple of seconds.

A second alteration can be made to eliminate the column length mismatches for the tt.AssignedPC = et_1.EMPLOYID and tt.ClientID = do.CUSTNMBR comparisons: 

mysql> ALTER TABLE tt MODIFY AssignedPC VARCHAR(15),
    ->                MODIFY ClientID   VARCHAR(15);

Now EXPLAIN produces the output shown here: 

table type   possible_keys key      key_len ref           rows Extra
et    ALL    PRIMARY       NULL     NULL    NULL          74
tt    ref    AssignedPC,   ActualPC 15      et.EMPLOYID   52   Using
             ClientID,                                         where
             ActualPC
et_1  eq_ref PRIMARY       PRIMARY  15      tt.AssignedPC 1
do    eq_ref PRIMARY       PRIMARY  15      tt.ClientID   1

This is almost as good as it can get.

The remaining problem is that, by default, MySQL assumes that values in the tt.ActualPC column are evenly distributed, and that is not the case for the tt table. Fortunately, it is easy to tell MySQL to analyze the key distribution: 

mysql> ANALYZE TABLE tt;

Now the join is perfect, and EXPLAIN produces this result: 

table type   possible_keys key     key_len ref           rows Extra
tt    ALL    AssignedPC    NULL    NULL    NULL          3872 Using
             ClientID,                                        where
             ActualPC
et    eq_ref PRIMARY       PRIMARY 15      tt.ActualPC   1
et_1  eq_ref PRIMARY       PRIMARY 15      tt.AssignedPC 1
do    eq_ref PRIMARY       PRIMARY 15      tt.ClientID   1

Note that the rows column in the output from EXPLAIN is an educated guess from the MySQL join optimizer. You should check whether the numbers are even close to the truth. If not, you may get better performance by using STRAIGHT_JOIN in your SELECT statement and trying to list the tables in a different order in the FROM clause.

7.2.2 Estimating Query Performance

In most cases, you can estimate the performance by counting disk seeks. For small tables, you can usually find a row in one disk seek (because the index is probably cached). For bigger tables, you can estimate that, using B-tree indexes, you will need this many seeks to find a row: log(row_count) / log(index_block_length / 3 * 2 / (index_length + data_pointer_length)) + 1.

In MySQL, an index block is usually 1024 bytes and the data pointer is usually 4 bytes. For a 500,000-row table with an index length of 3 bytes (medium integer), the formula indicates log(500,000)/log(1024/3*2/(3+4)) + 1 = 4 seeks.

This index would require storage of about 500,000 * 7 * 3/2 = 5.2MB (assuming a typical index buffer fill ratio of 2/3), so you will probably have much of the index in memory and you will probably need only one or two calls to read data to find the row.

For writes, however, you will need four seek requests (as above) to find where to place the new index and normally two seeks to update the index and write the row.

Note that the preceding discussion doesn't mean that your application performance will slowly degenerate by log N! As long as everything is cached by the OS or SQL server, things will become only marginally slower as the table gets bigger. After the data gets too big to be cached, things will start to go much slower until your applications is only bound by disk-seeks (which increase by log N). To avoid this, increase the key cache size as the data grows. For MyISAM tables, the key cache size is controlled by the key_buffer_size system variable. See section 7.5.2 Tuning Server Parameters.

7.2.3 Speed of SELECT Queries

In general, when you want to make a slow SELECT ... WHERE query faster, the first thing to check is whether you can add an index. All references between different tables should usually be done with indexes. You can use the EXPLAIN statement to determine which indexes are used for a SELECT. See section 7.4.5 How MySQL Uses Indexes and section 7.2.1 EXPLAIN Syntax (Get Information About a SELECT).

Some general tips for speeding up queries on MyISAM tables:

7.2.4 How MySQL Optimizes WHERE Clauses

This section discusses optimizations that can be made for processing WHERE clauses. The examples use SELECT statements, but the same optimizations apply for WHERE clauses in DELETE and UPDATE statements.

Note that work on the MySQL optimizer is ongoing, so this section is incomplete. MySQL does many optimizations, not all of which are documented here.

Some of the optimizations performed by MySQL are listed here:

Some examples of queries that are very fast: 

SELECT COUNT(*) FROM tbl_name;

SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;

SELECT MAX(key_part2) FROM tbl_name
    WHERE key_part1=constant;

SELECT ... FROM tbl_name
    ORDER BY key_part1,key_part2,... LIMIT 10;

SELECT ... FROM tbl_name
    ORDER BY key_part1 DESC, key_part2 DESC, ... LIMIT 10;

The following queries are resolved using only the index tree, assuming that the indexed columns are numeric: 

SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;

SELECT COUNT(*) FROM tbl_name
    WHERE key_part1=val1 AND key_part2=val2;

SELECT key_part2 FROM tbl_name GROUP BY key_part1;

The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass: 

SELECT ... FROM tbl_name
    ORDER BY key_part1,key_part2,... ;

SELECT ... FROM tbl_name
    ORDER BY key_part1 DESC, key_part2 DESC, ... ;

7.2.5 Range Optimization

The range access method uses a single index to retrieve a subset of table records that are contained within one or several index value intervals. It can be used for a single-part or multiple-part index. A detailed description of how intervals are extracted from the WHERE clause is given in the following sections.

7.2.5.1 Range Access Method for Single-Part Indexes

For a single-part index, index value intervals can be conveniently represented by corresponding conditions in the WHERE clause, so we'll talk about ``range conditions'' instead of intervals.

The definition of a range condition for a single-part index is as follows:

``Constant value'' in the preceding descriptions means one of the following:

Here are some examples of queries with range conditions in the WHERE clause: 

SELECT * FROM t1 WHERE key_col > 1 AND key_col < 10;

SELECT * FROM t1 WHERE key_col = 1 OR key_col IN (15,18,20);

SELECT * FROM t1 WHERE key_col LIKE 'ab%' OR key_col BETWEEN 
'bar' AND 'foo';

Note that some non-constant values may be converted to constants during the constant propagation phase.

MySQL tries to extract range conditions from the WHERE clause for each of the possible indexes. During the extraction process, conditions that can't be used for constructing the range condition are dropped, conditions that produce overlapping ranges are combined, and conditions that produce empty ranges are removed.

For example, consider the following statement, where key1 is an indexed column and nonkey is not indexed: 

SELECT * FROM t1 WHERE
   (key1 < 'abc' AND (key1 LIKE 'abcde%' OR key1 LIKE '%b')) OR
   (key1 < 'bar' AND nonkey = 4) OR
   (key1 < 'uux' AND key1 > 'z');

The extraction process for key key1 is as follows:

  1. Start with original WHERE clause: 
    (key1 < 'abc' AND (key1 LIKE 'abcde%' OR key1 LIKE '%b')) OR
(key1 < 'bar' AND nonkey = 4) OR
(key1 < 'uux' AND key1 > 'z')
  2. Remove nonkey = 4 and key1 LIKE '%b' because they cannot be used for a range scan. The right way to remove them is to replace them with TRUE, so that we don't miss any matching records when doing the range scan. Having replaced them with TRUE, we get: 
    (key1 < 'abc' AND (key1 LIKE 'abcde%' OR TRUE)) OR
(key1 < 'bar' AND TRUE) OR
(key1 < 'uux' AND key1 > 'z')
  3. Collapse conditions that are always true or false:
    • (key1 LIKE 'abcde%' OR TRUE) is always true
    • (key1 < 'uux' AND key1 > 'z') is always false
    Replacing these conditions with constants, we get: 
    (key1 < 'abc' AND TRUE) OR (key1 < 'bar' AND TRUE) OR (FALSE)
    Removing unnecessary TRUE and FALSE constants, we obtain 
    (key1 < 'abc') OR (key1 < 'bar')
  4. Combining overlapping intervals into one yields the final condition to be used for the range scan: 
    (key1 < 'bar')

In general (and as demonstrated in the example), the condition used for a range scan is less restrictive than the WHERE clause. MySQL will perform an additional check to filter out rows that satisfy the range condition but not the full WHERE clause.

The range condition extraction algorithm can handle nested AND/OR constructs of arbitrary depth, and its output doesn't depend on the order in which conditions appear in WHERE clause.

7.2.5.2 Range Access Method for Multiple-Part Indexes

Range conditions on a multiple-part index are an extension of range conditions for a single-part index. A range condition on a multiple-part index restricts index records to lie within one or several key tuple intervals. Key tuple intervals are defined over a set of key tuples, using ordering from the index.

For example, consider a multiple-part index defined as key1(key_part1, key_part2, key_part3), and the following set of key tuples listed in key order: 

key_part1  key_part2  key_part3
  NULL       1          'abc'
  NULL       1          'xyz'
  NULL       2          'foo'
   1         1          'abc'
   1         1          'xyz'
   1         2          'abc'
   2         1          'aaa'

The condition key_part1 = 1 defines this interval: 

(1, -inf, -inf) <= (key_part1, key_part2, key_part3) < (1, +inf, +inf)

The interval covers the 4th, 5th, and 6th tuples in the preceding data set and can be used by the range access method.

By contrast, the condition key_part3 = 'abc' does not define a single interval and cannot be used by the range access method.

The following descriptions indicate how range conditions work for multiple-part indexes in greater detail.

section 7.2.5.1 Range Access Method for Single-Part Indexes describes how optimizations are performed to combine or eliminate intervals for range conditions on single-part index. Analogous steps are performed for range conditions on multiple-part keys.

7.2.6 Index Merge Optimization

The Index Merge (index_merge) method is used to retrieve rows with several ref, ref_or_null, or range scans and merge the results into one. This method is employed when the table condition is a disjunction of conditions for which ref, ref_or_null, or range could be used with different keys.

This ``join'' type optimization is new in MySQL 5.0.0, and represents a significant change in behavior with regard to indexes, because the old rule was that the server is only ever able to use at most one index for each referenced table.

In EXPLAIN output, this method appears as index_merge in the type column. In this case, the key column contains a list of indexes used, and key_len contains a list of the longest key parts for those indexes.

Examples: 

SELECT * FROM tbl_name WHERE key_part1 = 10 OR key_part2 = 20;

SELECT * FROM tbl_name
    WHERE (key_part1 = 10 OR key_part2 = 20) AND non_key_part=30;

SELECT * FROM t1, t2
    WHERE (t1.key1 IN (1,2) OR t1.key2 LIKE 'value%')
    AND t2.key1=t1.some_col;

SELECT * FROM t1, t2
    WHERE t1.key1=1
    AND (t2.key1=t1.some_col OR t2.key2=t1.some_col2);

The Index Merge method has several access algorithms (seen in the Extra field of EXPLAIN output):

The following sections describe these methods in greater detail.

Note: The Index Merge optimization algorithm has the following known deficiencies:

The choice between different possible variants of the index_merge access method and other access methods is based on cost estimates of various available options.

7.2.6.1 Index Merge Intersection Access Algorithm

This access algorithm can be employed when a WHERE clause was converted to several range conditions on different keys combined with AND, and each condition is one of the following:

Here are some examples: 

SELECT * FROM innodb_table WHERE primary_key < 10 AND key_col1=20;

SELECT * FROM tbl_name
WHERE (key1_part1=1 AND key1_part2=2) AND key2=2;

The Index Merge intersection algorithm performs simultaneous scans on all used indexes and produces the intersection of row sequences that it receives from the merged index scans.

If all columns used in the query are covered by the used indexes, full table records will not be retrieved (EXPLAIN output will contain Using index in Extra field in this case). Here is an example of such query: 

SELECT COUNT(*) FROM t1 WHERE key1=1 AND key2=1;

If the used indexes don't cover all columns used in the query, full records will be retrieved only when the range conditions for all used keys are satisfied.

If one of the merged conditions is a condition over a primary key of an InnoDB or BDB table, it is not used for record retrieval, but is used to filter out records retrieved using other conditions.

7.2.6.2 Index Merge Union Access Algorithm

The applicability criteria for this algorithm are similar to those of the Index Merge method intersection algorithm. The algorithm can be employed when the table WHERE clause was converted to several range conditions on different keys combined with OR, and each condition is one of the following:

Here are some examples: 

SELECT * FROM t1 WHERE key1=1 OR key2=2 OR key3=3;

SELECT * FROM innodb_table WHERE (key1=1 AND key2=2) OR
  (key3='foo' AND key4='bar') AND key5=5;

7.2.6.3 Index Merge Sort-Union Access Algorithm

This access algorithm is employed when the WHERE clause was converted to several range conditions combined by OR, but for which the Index Merge method union algorithm is not applicable.

Here are some examples: 

SELECT * FROM tbl_name WHERE key_col1 < 10 OR key_col2 < 20;

SELECT * FROM tbl_name
     WHERE (key_col1 > 10 OR key_col2 = 20) AND nonkey_col=30;

The difference between the sort-union algorithm and the union algorithm is that the sort-union algorithm must first fetch row IDs for all records and sort them before returning any records.

7.2.7 How MySQL Optimizes IS NULL

MySQL can do the same optimization on col_name IS NULL that it can do with col_name = constant_value. For example, MySQL can use indexes and ranges to search for NULL with IS NULL. 

SELECT * FROM tbl_name WHERE key_col IS NULL;

SELECT * FROM tbl_name WHERE key_col <=> NULL;

SELECT * FROM tbl_name
    WHERE key_col=const1 OR key_col=const2 OR key_col IS NULL;

If a WHERE clause includes a col_name IS NULL condition for a column that is declared as NOT NULL, that expression will be optimized away. This optimization does not occur in cases when the column might produce NULL anyway; for example, if it comes from a table on the right side of a LEFT JOIN.

MySQL 4.1.1 and up can additionally optimize the combination col_name = expr AND col_name IS NULL, a form that is common in resolved subqueries. EXPLAIN will show ref_or_null when this optimization is used.

This optimization can handle one IS NULL for any key part.

Some examples of queries that are optimized, assuming that there is an index on columns a and b of table t2: 

SELECT * FROM t1 WHERE t1.a=expr OR t1.a IS NULL;

SELECT * FROM t1, t2 WHERE t1.a=t2.a OR t2.a IS NULL;

SELECT * FROM t1, t2
    WHERE (t1.a=t2.a OR t2.a IS NULL) AND t2.b=t1.b;

SELECT * FROM t1, t2
    WHERE t1.a=t2.a AND (t2.b=t1.b OR t2.b IS NULL);

SELECT * FROM t1, t2
    WHERE (t1.a=t2.a AND t2.a IS NULL AND ...)
    OR (t1.a=t2.a AND t2.a IS NULL AND ...);

ref_or_null works by first doing a read on the reference key, and then a separate search for rows with a NULL key value.

Note that the optimization can handle only one IS NULL level. In the following query, MySQL will use key lookups only on the expression (t1.a=t2.a AND t2.a IS NULL) and not be able to use the key part on b: 

SELECT * FROM t1, t2
     WHERE (t1.a=t2.a AND t2.a IS NULL)
     OR (t1.b=t2.b AND t2.b IS NULL);

7.2.8 How MySQL Optimizes DISTINCT

DISTINCT combined with ORDER BY will need a temporary table in many cases.

Note that because DISTINCT may use GROUP BY, you should be aware of how MySQL works with columns in ORDER BY or HAVING clauses that are not part of the selected columns. See section 13.9.3 GROUP BY with Hidden Fields.

In most cases, a DISTINCT clause can be considered as a special case of GROUP BY. For example, the following two queries are equivalent: 

SELECT DISTINCT(c1, c2, c3) FROM t1 WHERE c1 > const;

SELECT c1, c2, c3 FROM t1 WHERE c1 > const GROUP BY c1, c2, c3;

Due to this equivalence, the optimizations applicable to GROUP BY queries can be also applied to queries with a DISTINCT clause. Thus, for more details on the optimization possibilities for DISTINCT queries, see section 7.2.11 How MySQL Optimizes GROUP BY.

When combining LIMIT row_count with DISTINCT, MySQL stops as soon as it finds row_count unique rows.

If you don't use columns from all tables named in a query, MySQL stops scanning the not-used tables as soon as it finds the first match. In the following case, assuming that t1 is used before t2 (which you can check with EXPLAIN), MySQL stops reading from t2 (for any particular row in t1) when the first row in t2 is found: 

SELECT DISTINCT t1.a FROM t1, t2 where t1.a=t2.a;

7.2.9 How MySQL Optimizes LEFT JOIN and RIGHT JOIN

A LEFT JOIN B join_condition is implemented in MySQL as follows:

RIGHT JOIN is implemented analogously to LEFT JOIN, with the roles of the tables reversed.

The join optimizer calculates the order in which tables should be joined. The table read order forced by LEFT JOIN and STRAIGHT_JOIN helps the join optimizer do its work much more quickly, because there are fewer table permutations to check. Note that this means that if you do a query of the following type, MySQL will do a full scan on b because the LEFT JOIN forces it to be read before d: 

SELECT *
    FROM a,b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d ON (d.key=a.key)
    WHERE b.key=d.key;

The fix in this case is to rewrite the query as follows: 

SELECT *
    FROM b,a LEFT JOIN c ON (c.key=a.key) LEFT JOIN d ON (d.key=a.key)
    WHERE b.key=d.key;

Starting from 4.0.14, MySQL does the following LEFT JOIN optimization: If the WHERE condition is always false for the generated NULL row, the LEFT JOIN is changed to a normal join.

For example, the WHERE clause would be false in the following query if t2.column1 would be NULL: 

SELECT * FROM t1 LEFT JOIN t2 ON (column1) WHERE t2.column2=5;

Therefore, it's safe to convert the query to a normal join: 

SELECT * FROM t1, t2 WHERE t2.column2=5 AND t1.column1=t2.column1;

This can be made faster because MySQL can now use table t2 before table t1 if this would result in a better query plan. To force a specific table order, use STRAIGHT_JOIN.

7.2.10 How MySQL Optimizes ORDER BY

In some cases, MySQL can use an index to satisfy an ORDER BY clause without doing any extra sorting.

The index can also be used even if the ORDER BY doesn't match the index exactly, as long as all the unused index parts and all the extra are ORDER BY columns are constants in the WHERE clause. The following queries will use the index to resolve the ORDER BY part: 

SELECT * FROM t1 ORDER BY key_part1,key_part2,... ;
SELECT * FROM t1 WHERE key_part1=constant ORDER BY key_part2;
SELECT * FROM t1 ORDER BY key_part1 DESC, key_part2 DESC;
SELECT * FROM t1
    WHERE key_part1=1 ORDER BY key_part1 DESC, key_part2 DESC;

In some cases, MySQL cannot use indexes to resolve the ORDER BY, although it still will use indexes to find the rows that match the WHERE clause. These cases include the following:

With EXPLAIN SELECT ... ORDER BY, you can check whether MySQL can use indexes to resolve the query. It cannot if you see Using filesort in the Extra column. See section 7.2.1 EXPLAIN Syntax (Get Information About a SELECT).

In those cases where MySQL must sort the result, it uses the following filesort algorithm before MySQL 4.1:

  1. Read all rows according to key or by table scanning. Rows that don't match the WHERE clause are skipped.
  2. For each row, store a pair of values in a buffer (the sort key and the row pointer). The size of the buffer is the value of the sort_buffer_size system variable.
  3. When the buffer gets full, run a qsort (quicksort) on it and store the result in a temporary file. Save a pointer to the sorted block. (If all pairs fit into the sort buffer, no temporary file is created.)
  4. Repeat the preceding steps until all rows have been read.
  5. Do a multi-merge of up to MERGEBUFF (7) regions to one block in another temporary file. Repeat until all blocks from the first file are in the second file.
  6. Repeat the following until there are fewer than MERGEBUFF2 (15) blocks left.
  7. On the last multi-merge, only the pointer to the row (the last part of the sort key) is written to a result file.
  8. Read the rows in sorted order by using the row pointers in the result file. To optimize this, we read in a big block of row pointers, sort them, and use them to read the rows in sorted order into a row buffer. The size of the buffer is the value of the read_rnd_buffer_size system variable. The code for this step is in the `sql/records.cc' source file.

One problem with this approach is that it reads rows twice: One time when evaluating the WHERE clause, and again after sorting the pair values. And even if the rows were accessed successively the first time (for example, if a table scan is done), the second time they are accessed randomly. (The sort keys are ordered, but the row positions are not.)

In MySQL 4.1 and up, a filesort optimization is used that records not only the sort key value and row position, but also the columns required for the query. This avoids reading the rows twice. The modified filesort algorithm works like this:

  1. Read the rows that match the WHERE clause, as before.
  2. For each row, record a tuple of values consisting of the sort key value and row position, and also the columns required for the query.
  3. Sort the tuples by sort key value
  4. Retrieve the rows in sorted order, but read the required columns directly from the sorted tuples rather than by accessing the table a second time.

Using the modified filesort algorithm, the tuples are longer than the pairs used in the original method, and fewer of them fit in the sort buffer (the size of which is given by sort_buffer_size). As a result, it is possible for the extra I/O to make the modified approach slower, not faster. To avoid a slowdown, the optimization is used only if the total size of the extra columns in the sort tuple does not exceed the value of the max_length_for_sort_data system variable. (A symptom of setting the value of this variable too high is that you will see high disk activity and low CPU activity.)

If you want to increase ORDER BY speed, first see whether you can get MySQL to use indexes rather than an extra sorting phase. If this is not possible, you can try the following strategies:

By default, MySQL sorts all GROUP BY col1, col2, ... queries as if you specified ORDER BY col1, col2, ... in the query as well. If you include an ORDER BY clause explicitly that contains the same column list, MySQL optimizes it away without any speed penalty, although the sorting still occurs. If a query includes GROUP BY but you want to avoid the overhead of sorting the result, you can suppress sorting by specifying ORDER BY NULL. For example: 

INSERT INTO foo
SELECT a, COUNT(*) FROM bar GROUP BY a ORDER BY NULL;

7.2.11 How MySQL Optimizes GROUP BY

The most general way to satisfy a GROUP BY clause is to scan the whole table and create a new temporary table where all rows from each group are consecutive, and then use this temporary table to discover groups and apply aggregate functions (if any). In some cases, MySQL is able to do much better than that and to avoid creation of temporary tables by using index access.

The most important preconditions for using indexes for GROUP BY are that all GROUP BY columns reference attributes from the same index, and the index stores its keys in order (for example, this is a B-Tree index, and not a HASH index). Whether usage of temporary tables can be replaced by index access also depends on which parts of an index are used in a query, the conditions specified for these parts, and the selected aggregate functions.

There are two ways to execute a GROUP BY query via index access, as detailed in the following sections. In the first method, the grouping operation is applied together with all range predicates (if any). The second method first performs a range scan, and then groups the resulting tuples.

7.2.11.1 Loose index scan

The most efficient way is when the index is used to directly retrieve the group fields. With this access method, MySQL uses the property of some index types (for example, B-Trees) that the keys are ordered. This property allows use of lookup groups in an index without having to consider all keys in the index that satisfy all WHERE conditions. Since this access method considers only a fraction of the keys in an index, it is called ``loose index scan.'' When there is no WHERE clause, a loose index scan will read as many keys as the number of groups, which may be a much smaller number than all keys. If the WHERE clause contains range predicates (described in section 7.2.1 EXPLAIN Syntax (Get Information About a SELECT), under the range join type), a loose index scan looks up the first key of each group that satisfies the range conditions, and again reads the least possible number of keys. This is possible under the following conditions:

The EXPLAIN output for such queries shows Using index for group-by in the Extra column.

The following queries provide several examples that fall into this category, assuming there is an index idx(c1, c2, c3) on table t1(c1,c2,c3,c4): 

SELECT c1, c2 FROM t1 GROUP BY c1, c2;
SELECT DISTINCT(c1, c2) FROM t1;
SELECT c1, MIN(c2) FROM t1 GROUP BY c1;
SELECT c1, c2 FROM t1 WHERE c1 < const GROUP BY c1, c2;
SELECT MAX(c3), MIN(c3), c1, c2 FROM t1 WHERE c2 > const GROUP BY c1, c2;
SELECT c2 FROM t1 WHERE c1 < const GROUP BY c1, c2;
SELECT c1, c2 FROM t1 WHERE c3 = const GROUP BY c1, c2;

The following queries cannot be executed with this quick select method, for the reasons given:

7.2.11.2 Tight index scan

A tight index scan may be either a full index scan or a range index scan, depending on the query conditions.

When the conditions for a loose index scan are not met, it is still possible to avoid creation of temporary tables for GROUP BY queries. If there are range conditions in the WHERE clause, this method will read only the keys that satisfy these conditions. Otherwise, it performs an index scan. Since this method reads all keys in each range defined by the WHERE clause, or scans the whole index if there are no range conditions, we term it a ``tight index scan.'' Notice that with a tight index scan, the grouping operation is performed after all keys that satisfy the range conditions have been found.

For this method to work, it is sufficient that for all columns in a query referring to key parts before or in between the GROUP BY key parts, there is a constant equality condition. The constants from the equality conditions fill in the ``gaps'' in the search keys so that it is possible to form complete prefixes of the index. Then these index prefixes can be used for index lookups. If we require sorting of the GROUP BY result, and it is possible to form search keys that are prefixes of the index, MySQL also will avoid sorting because searching with prefixes in an ordered index already retrieves all keys in order.

The following queries will not work with the first method above, but will still work with the second index access method (assuming we have the aforementioned index idx on table t1):

7.2.12 How MySQL Optimizes LIMIT

In some cases, MySQL will handle a query differently when you are using LIMIT row_count and not using HAVING:

7.2.13 How to Avoid Table Scans

The output from EXPLAIN will show ALL in the type column when MySQL uses a table scan to resolve a query. This usually happens under the following conditions:

For small tables, a table scan often is appropriate. For large tables, try the following techniques to avoid having the optimizer incorrectly choose a table scan:

7.2.14 Speed of INSERT Statements

The time to insert a record is determined by the following factors, where the numbers indicate approximate proportions:

This does not take into consideration the initial overhead to open tables, which is done once for each concurrently running query.

The size of the table slows down the insertion of indexes by log N, assuming B-tree indexes.

You can use the following methods to speed up inserts:

7.2.15 Speed of UPDATE Statements

Update statements are optimized as a SELECT query with the additional overhead of a write. The speed of the write depends on the amount of data being updated and the number of indexes that are updated. Indexes that are not changed will not be updated.

Also, another way to get fast updates is to delay updates and then do many updates in a row later. Doing many updates in a row is much quicker than doing one at a time if you lock the table.

Note that for a MyISAM table that uses dynamic record format, updating a record to a longer total length may split the record. If you do this often, it is very important to use OPTIMIZE TABLE occasionally. See section 14.5.2.5 OPTIMIZE TABLE Syntax.

7.2.16 Speed of DELETE Statements

The time to delete individual records is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the key cache. See section 7.5.2 Tuning Server Parameters.

If you want to delete all rows in the table, use TRUNCATE TABLE tbl_name rather than DELETE FROM tbl_name. See section 14.1.9 TRUNCATE Syntax.

7.2.17 Other Optimization Tips

This section lists a number of miscellaneous tips for improving query processing speed:

7.3 Locking Issues

7.3.1 Locking Methods

Currently, MySQL supports table-level locking for ISAM, MyISAM, and MEMORY (HEAP) tables, page-level locking for BDB tables, and row-level locking for InnoDB tables.

In many cases, you can make an educated guess about which locking type is best for an application, but generally it's very hard to say that a given lock type is better than another. Everything depends on the application and different parts of an application may require different lock types.

To decide whether you want to use a storage engine with row-level locking, you will want to look at what your application does and what mix of select and update statements it uses. For example, most Web applications do lots of selects, very few deletes, updates based mainly on key values, and inserts into some specific tables. The base MySQL MyISAM setup is very well tuned for this.

Table locking in MySQL is deadlock-free for storage engines that use table-level locking. Deadlock avoidance is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order.

The table-locking method MySQL uses for WRITE locks works as follows:

The table-locking method MySQL uses for READ locks works as follows:

When a lock is released, the lock is made available to the threads in the write lock queue, then to the threads in the read lock queue.

This means that if you have many updates for a table, SELECT statements will wait until there are no more updates.

Starting in MySQL 3.23.33, you can analyze the table lock contention on your system by checking the Table_locks_waited and Table_locks_immediate status variables: 

mysql> SHOW STATUS LIKE 'Table%';
+-----------------------+---------+
| Variable_name         | Value   |
+-----------------------+---------+
| Table_locks_immediate | 1151552 |
| Table_locks_waited    | 15324   |
+-----------------------+---------+

As of MySQL 3.23.7 (3.23.25 for Windows), you can freely mix concurrent INSERT and SELECT statements for a MyISAM table without locks if the INSERT statements are non-conflicting. That is, you can insert rows into a MyISAM table at the same time other clients are reading from it. No conflict occurs if the data file contains no free blocks in the middle, because in that case, records always are inserted at the end of the data file. (Holes can result from rows having been deleted from or updated in the middle of the table.) If there are holes, concurrent inserts are re-enabled automatically when all holes have been filled with new data.

If you want to do many INSERT and SELECT operations on a table when concurrent inserts are not possible, you can insert rows in a temporary table and update the real table with the records from the temporary table once in a while. This can be done with the following code: 

mysql> LOCK TABLES real_table WRITE, insert_table WRITE;
mysql> INSERT INTO real_table SELECT * FROM insert_table;
mysql> TRUNCATE TABLE insert_table;
mysql> UNLOCK TABLES;

InnoDB uses row locks and BDB uses page locks. For the InnoDB and BDB storage engines, deadlock is possible. This is because InnoDB automatically acquires row locks and BDB acquires page locks during the processing of SQL statements, not at the start of the transaction.

Advantages of row-level locking:

Disadvantages of row-level locking:

Table locks are superior to page-level or row-level locks in the following cases:

Options other than row-level or page-level locking:

Versioning (such as we use in MySQL for concurrent inserts) where you can have one writer at the same time as many readers. This means that the database/table supports different views for the data depending on when you started to access it. Other names for this are time travel, copy on write, or copy on demand.

Copy on demand is in many cases much better than page-level or row-level locking. However, the worst case does use much more memory than when using normal locks.

Instead of using row-level locks, you can use application-level locks, such as GET_LOCK() and RELEASE_LOCK() in MySQL. These are advisory locks, so they work only in well-behaved applications.

7.3.2 Table Locking Issues

To achieve a very high lock speed, MySQL uses table locking (instead of page, row, or column locking) for all storage engines except InnoDB and BDB.

For InnoDB and BDB tables, MySQL only uses table locking if you explicitly lock the table with LOCK TABLES. For these table types, we recommend you to not use LOCK TABLES at all, because InnoDB uses automatic row-level locking and BDB uses page-level locking to ensure transaction isolation.

For large tables, table locking is much better than row locking for most applications, but there are some pitfalls.

Table locking enables many threads to read from a table at the same time, but if a thread wants to write to a table, it must first get exclusive access. During the update, all other threads that want to access this particular table must wait until the update is done.

Table updates normally are considered to be more important than table retrievals, so they are given higher priority. This should ensure that updates to a table are not ``starved'' even if there is heavy SELECT activity for the table.

Table locking causes problems in cases such as when a thread is waiting because the disk is full and free space needs to become available before the thread can proceed. In this case, all threads that want to access the problem table will also be put in a waiting state until more disk space is made available.

Table locking is also disadvantageous under the following scenario:

The following list describes some ways to avoid or reduce contention caused by table locking:

Here are some tips about table locking in MySQL:

7.4 Optimizing Database Structure

7.4.1 Design Choices

MySQL keeps row data and index data in separate files. Many (almost all) other databases mix row and index data in the same file. We believe that the MySQL choice is better for a very wide range of modern systems.

Another way to store the row data is to keep the information for each column in a separate area (examples are SDBM and Focus). This will cause a performance hit for every query that accesses more than one column. Because this degenerates so quickly when more than one column is accessed, we believe that this model is not good for general-purpose databases.

The more common case is that the index and data are stored together (as in Oracle/Sybase, et al). In this case, you will find the row information at the leaf page of the index. The good thing with this layout is that it, in many cases, depending on how well the index is cached, saves a disk read. The bad things with this layout are:

7.4.2 Make Your Data as Small as Possible

One of the most basic optimizations is to design your tables to take as little space on the disk as possible. This can give huge improvements because disk reads are faster, and smaller tables normally require less main memory while their contents are being actively processed during query execution. Indexing also is a lesser resource burden if done on smaller columns.

MySQL supports a lot of different table types and row formats. For each table, you can decide which storage/index method to use. Choosing the right table format for your application may give you a big performance gain. See section 15 MySQL Storage Engines and Table Types.

You can get better performance on a table and minimize storage space using the techniques listed here:

7.4.3 Column Indexes

All MySQL column types can be indexed. Use of indexes on the relevant columns is the best way to improve the performance of SELECT operations.

The maximum number of indexes per table and the maximum index length is defined per storage engine. See section 15 MySQL Storage Engines and Table Types. All storage engines support at least 16 indexes per table and a total index length of at least 256 bytes. Most storage engines have higher limits.

With col_name(length) syntax in an index specification, you can create an index that uses only the first length characters of a CHAR or VARCHAR column. Indexing only a prefix of column values like this can make the index file much smaller. See section 7.4.3 Column Indexes.

The MyISAM and (as of MySQL 4.0.14) InnoDB storage engines also support indexing on BLOB and TEXT columns. When indexing a BLOB or TEXT column, you must specify a prefix length for the index. For example: 

CREATE TABLE test (blob_col BLOB, INDEX(blob_col(10)));

Prefixes can be up to 255 bytes long (or 1000 bytes for MyISAM and InnoDB tables as of MySQL 4.1.2). Note that prefix limits are measured in bytes, whereas the prefix length in CREATE TABLE statements is interpreted as number of characters. Take this into account when specifying a prefix length for a column that uses a multi-byte character set.

As of MySQL 3.23.23, you can also create FULLTEXT indexes. They are used for full-text searches. Only the MyISAM table type supports FULLTEXT indexes and only for CHAR, VARCHAR, and TEXT columns. Indexing always happens over the entire column and partial (prefix) indexing is not supported. See section 13.6 Full-Text Search Functions for details.

As of MySQL 4.1.0, you can create indexes on spatial column types. Currently, spatial types are supported only by the MyISAM storage engine. Spatial indexes use R-trees.

The MEMORY (HEAP) storage engine supports hash indexes. As of MySQL 4.1.0, the engine also supports B-tree indexes.

7.4.4 Multiple-Column Indexes

MySQL can create indexes on multiple columns. An index may consist of up to 15 columns. For certain column types, you can index a prefix of the column (see section 7.4.3 Column Indexes).

A multiple-column index can be considered a sorted array containing values that are created by concatenating the values of the indexed columns.

MySQL uses multiple-column indexes in such a way that queries are fast when you specify a known quantity for the first column of the index in a WHERE clause, even if you don't specify values for the other columns.

Suppose that a table has the following specification: 

CREATE TABLE test (
    id INT NOT NULL,
    last_name CHAR(30) NOT NULL,
    first_name CHAR(30) NOT NULL,
    PRIMARY KEY (id),
    INDEX name (last_name,first_name));

The name index is an index over last_name and first_name. The index can be used for queries that specify values in a known range for last_name, or for both last_name and first_name. Therefore, the name index will be used in the following queries: 

SELECT * FROM test WHERE last_name='Widenius';

SELECT * FROM test
    WHERE last_name='Widenius' AND first_name='Michael';

SELECT * FROM test
    WHERE last_name='Widenius'
    AND (first_name='Michael' OR first_name='Monty');

SELECT * FROM test
    WHERE last_name='Widenius'
    AND first_name >='M' AND first_name < 'N';

However, the name index will not be used in the following queries: 

SELECT * FROM test WHERE first_name='Michael';

SELECT * FROM test
    WHERE last_name='Widenius' OR first_name='Michael';

The manner in which MySQL uses indexes to improve query performance is discussed further in the next section.

7.4.5 How MySQL Uses Indexes

Indexes are used to find rows with specific column values fast. Without an index, MySQL has to start with the first record and then read through the whole table to find the relevant rows. The larger the table, the more this costs. If the table has an index for the columns in question, MySQL can quickly determine the position to seek to in the middle of the data file without having to look at all the data. If a table has 1,000 rows, this is at least 100 times faster than reading sequentially. Note that if you need to access almost all 1,000 rows, it is faster to read sequentially, because that minimizes disk seeks.

Most MySQL indexes (PRIMARY KEY, UNIQUE, INDEX, and FULLTEXT) are stored in B-trees. Exceptions are that indexes on spatial column types use R-trees, and MEMORY (HEAP) tables support hash indexes.

Strings are automatically prefix- and end-space compressed. See section 14.2.5 CREATE INDEX Syntax.

In general, indexes are used as described in the following discussion. Characteristics specific to hash indexes (as used in MEMORY tables) are described at the end of this section.

Suppose that you issue the following SELECT statement: 

mysql> SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;

If a multiple-column index exists on col1 and col2, the appropriate rows can be fetched directly. If separate single-column indexes exist on col1 and col2, the optimizer tries to find the most restrictive index by deciding which index will find fewer rows and using that index to fetch the rows.

If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to find rows. For example, if you have a three-column index on (col1, col2, col3), you have indexed search capabilities on (col1), (col1, col2), and (col1, col2, col3).

MySQL can't use a partial index if the columns don't form a leftmost prefix of the index. Suppose that you have the SELECT statements shown here: 

SELECT * FROM tbl_name WHERE col1=val1;
SELECT * FROM tbl_name WHERE col2=val2;
SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;

If an index exists on (col1, col2, col3), only the first of the preceding queries uses the index. The second and third queries do involve indexed columns, but (col2) and (col2, col3) are not leftmost prefixes of (col1, col2, col3).

An index is used for columns that you compare with the =, >, >=, <, <=, or BETWEEN operators.

MySQL also uses indexes for LIKE comparisons if the argument to LIKE is a constant string that doesn't start with a wildcard character. For example, the following SELECT statements use indexes: 

SELECT * FROM tbl_name WHERE key_col LIKE 'Patrick%';
SELECT * FROM tbl_name WHERE key_col LIKE 'Pat%_ck%';

In the first statement, only rows with 'Patrick' <= key_col < 'Patricl' are considered. In the second statement, only rows with 'Pat' <= key_col < 'Pau' are considered.

The following SELECT statements will not use indexes: 

SELECT * FROM tbl_name WHERE key_col LIKE '%Patrick%';
SELECT * FROM tbl_name WHERE key_col LIKE other_col;

In the first statement, the LIKE value begins with a wildcard character. In the second statement, the LIKE value is not a constant.

MySQL 4.0 and up performs an additional LIKE optimization. If you use ... LIKE '%string%' and string is longer than three characters, MySQL will use the Turbo Boyer-Moore algorithm to initialize the pattern for the string and then use this pattern to perform the search quicker.

Searching using col_name IS NULL will use indexes if col_name is indexed.

Any index that doesn't span all AND levels in the WHERE clause is not used to optimize the query. In other words, to be able to use an index, a prefix of the index must be used in every AND group.

The following WHERE clauses use indexes: 

... WHERE index_part1=1 AND index_part2=2 AND other_column=3
    /* index = 1 OR index = 2 */
... WHERE index=1 OR A=10 AND index=2
    /* optimized like "index_part1='hello'" */
... WHERE index_part1='hello' AND index_part3=5
    /* Can use index on index1 but not on index2 or index3 */
... WHERE index1=1 AND index2=2 OR index1=3 AND index3=3;

These WHERE clauses do not use indexes: 

    /* index_part1 is not used */
... WHERE index_part2=1 AND index_part3=2 
    /* Index is not used in both AND parts */
... WHERE index=1 OR A=10                 
    /* No index spans all rows  */
... WHERE index_part1=1 OR index_part2=10

Sometimes MySQL will not use an index, even if one is available. One way this occurs is when the optimizer estimates that using the index would require MySQL to access a large percentage of the rows in the table. (In this case, a table scan is probably much faster, because it will require many fewer seeks.) However, if such a query uses LIMIT to only retrieve part of the rows, MySQL will use an index anyway, because it can much more quickly find the few rows to return in the result.

Hash indexes have somewhat different characteristics than those just discussed:

7.4.6 The MyISAM Key Cache

To minimize disk I/O, the MyISAM storage engine employs a strategy that is used by many database management systems. It exploits a cache mechanism to keep the most frequently accessed table blocks in memory:

This section first describes the basic operation of the MyISAM key cache. Then it discusses changes made in MySQL 4.1 that improve key cache performance and that enable you to better control cache operation:

The key cache mechanism also is used for ISAM tables. However, the significance of this fact is on the wane. ISAM table use has been decreasing since MySQL 3.23 when MyISAM was introduced. MySQL 4.1 carries this trend further; the ISAM storage engine is disabled by default.

You can control the size of the key cache by means of the key_buffer_size system variable. If this variable is set equal to zero, no key cache is used. The key cache also is not used if the key_buffer_size value is too small to allocate the minimal number of block buffers (8).

When the key cache is not operational, index files are accessed using only the native filesystem buffering provided by the operating system. (In other words, table index blocks are accessed using the same strategy as that employed for table data blocks.)

An index block is a contiguous unit of access to the MyISAM index files. Usually the size of an index block is equal to the size of nodes of the index B-tree. (Indexes are represented on disk using a B-tree data structure. Nodes at the bottom of the tree are leaf nodes. Nodes above the leaf nodes are non-leaf nodes.)

All block buffers in a key cache structure are the same size. This size can be equal to, greater than, or less than the size of a table index block. Usually one these two values is a multiple of the other.

When data from any table index block must be accessed, the server first checks whether it is available in some block buffer of the key cache. If it is, the server accesses data in the key cache rather than on disk. That is, it reads from the cache or writes into it rather than reading from or writing to disk. Otherwise, the server chooses a cache block buffer containing a different table index block (or blocks) and replaces the data there by a copy of required table index block. As soon as the new index block is in the cache, the index data can be accessed.

If it happens that a block selected for replacement has been modified, the block is considered ``dirty.'' In this case, before being replaced, its contents are flushed to the table index from which it came.

Usually the server follows an LRU (Least Recently Used) strategy: When choosing a block for replacement, it selects the least recently used index block. To be able to make such a choice easy, the key cache module maintains a special queue (LRU chain) of all used blocks. When a block is accessed, it is placed at the end of the queue. When blocks need to be replaced, blocks at the beginning of the queue are the least recently used and become the first candidates for eviction.

7.4.6.1 Shared Key Cache Access

Prior to MySQL 4.1, access to the key cache is serialized: No two threads can access key cache buffers simultaneously. The server processes a request for an index block only after it has finished processing the previous request. As a result, a request for an index block not present in any key cache buffer blocks access by other threads while a buffer is being updated to contain the requested index block.

Starting from version 4.1.0, the server supports shared access to the key cache:

Shared access to the key cache allows the server to improve throughput significantly.

7.4.6.2 Multiple Key Caches

Shared access to the key cache improves performance but does not eliminate contention among threads entirely. They still compete for control structures that manage access to the key cache buffers. To reduce key cache access contention further, MySQL 4.1.1 offers the feature of multiple key caches. This allows you to assign different table indexes to different key caches.

When there can be multiple key caches, the server must know which cache to use when processing queries for a given MyISAM table. By default, all MyISAM table indexes are cached in the default key cache. To assign table indexes to a specific key cache, use the CACHE INDEX statement.

For example, the following statement assigns indexes from the tables t1, t2, and t3 to the key cache named hot_cache: 

mysql> CACHE INDEX t1, t2, t3 IN hot_cache;
+---------+--------------------+----------+----------+
| Table   | Op                 | Msg_type | Msg_text |
+---------+--------------------+----------+----------+
| test.t1 | assign_to_keycache | status   | OK       |
| test.t2 | assign_to_keycache | status   | OK       |
| test.t3 | assign_to_keycache | status   | OK       |
+---------+--------------------+----------+----------+

Note: If the server has been built with the ISAM storage engine enabled, ISAM tables use the key cache mechanism. However, ISAM indexes use only the default key cache and cannot be reassigned to a different cache.

The key cache referred to in a CACHE INDEX statement can be created by setting its size with a SET GLOBAL parameter setting statement or by using server startup options. For example: 

mysql> SET GLOBAL keycache1.key_buffer_size=128*1024;

To destroy a key cache, set its size to zero: 

mysql> SET GLOBAL keycache1.key_buffer_size=0;

Key cache variables are structured system variables that have a name and components. For keycache1.key_buffer_size, keycache1 is the cache variable name and key_buffer_size is the cache component. See section 10.4.1 Structured System Variables for a description of the syntax used for referring to structured key cache system variables.

By default, table indexes are assigned to the main (default) key cache created at the server startup. When a key cache is destroyed, all indexes assigned to it are reassigned to the default key cache.

For a busy server, we recommend a strategy that uses three key caches:

One reason the use of three key caches is beneficial is that access to one key cache structure does not block access to the others. Queries that access tables assigned to one cache do not compete with queries that access tables assigned to another cache. Performance gains occur for other reasons as well:

7.4.6.3 Midpoint Insertion Strategy

By default, the key cache management system of MySQL 4.1 uses the LRU strategy for choosing key cache blocks to be evicted, but it also supports a more sophisticated method called the "midpoint insertion strategy."

When using the midpoint insertion strategy, the LRU chain is divided into two parts: a hot sub-chain and a warm sub-chain. The division point between two parts is not fixed, but the key cache management system takes care that the warm part is not ``too short,'' always containing at least key_cache_division_limit percent of the key cache blocks. key_cache_division_limit is a component of structured key cache variables, so its value is a parameter that can be set per cache.

When an index block is read from a table into the key cache, it is placed at the end of the warm sub-chain. After a certain number of hits (accesses of the block), it is promoted to the hot sub-chain. At present, the number of hits required to promote a block (3) is the same for all index blocks. In the future, we will allow the hit count to depend on the B-tree level of the node corresponding to an index block: Fewer hits will be required for promotion of an index block if it contains a non-leaf node from the upper levels of the index B-tree than if it contains a leaf node.

A block promoted into the hot sub-chain is placed at the end of the chain. The block then circulates within this sub-chain. If the block stays at the beginning of the sub-chain for a long enough time, it is demoted to the warm chain. This time is determined by the value of the key_cache_age_threshold component of the key cache.

The threshold value prescribes that, for a key cache containing N blocks, the block at the beginning of the hot sub-chain not accessed within the last N*key_cache_age_threshold/100 hits is to be moved to the beginning of the warm sub-chain. It then becomes the first candidate for eviction, because blocks for replacement always are taken from the beginning of the warm sub-chain.

The midpoint insertion strategy allows you to keep more-valued blocks always in the cache. If you prefer to use the plain LRU strategy, leave the key_cache_division_limit value set to its default of 100.

The midpoint insertion strategy helps to improve performance when execution of a query that requires an index scan effectively pushes out of the cache all the index blocks corresponding to valuable high-level B-tree nodes. To avoid this, you must use a midpoint insertion strategy with the key_cache_division_limit set to much less than 100. Then valuable frequently hit nodes will be preserved in the hot sub-chain during an index scan operation as well.

7.4.6.4 Index Preloading

If there are enough blocks in a key cache to hold blocks of an entire index, or at least the blocks corresponding to its non-leaf nodes, then it makes sense to preload the key cache with index blocks before starting to use it. Preloading allows you to put the table index blocks into a key cache buffer in the most efficient way: by reading the index blocks from disk sequentially.

Without preloading, the blocks still will be placed into the key cache as needed by queries. Although the blocks will stay in the cache, because there are enough buffers for all of them, they will be fetched from disk in a random order, not sequentially.

To preload an index into a cache, use the LOAD INDEX INTO CACHE statement. For example, the following statement preloads nodes (index blocks) of indexes of the tables t1 and t2: 

mysql> LOAD INDEX INTO CACHE t1, t2 IGNORE LEAVES;
+---------+--------------+----------+----------+
| Table   | Op           | Msg_type | Msg_text |
+---------+--------------+----------+----------+
| test.t1 | preload_keys | status   | OK       |
| test.t2 | preload_keys | status   | OK       |
+---------+--------------+----------+----------+

The IGNORE LEAVES modifier causes only blocks for the non-leaf nodes of the index to be preloaded. Thus, the statement shown preloads all index blocks from t1, but only blocks for the non-leaf nodes from t2.

If an index has been assigned to a key cache using a CACHE INDEX statement, preloading places index blocks into that cache. Otherwise, the index is loaded into the default key cache.

7.4.6.5 Key Cache Block Size

MySQL 4.1 introduces a new key_cache_block_size variable on a per-key cache basis. This variable specifies the size of the block buffers for a key cache. It is intended to allow tuning of the performance of I/O operations for index files.

The best performance for I/O operations is achieved when the size of read buffers is equal to the size of the native operating system I/O buffers. But setting the size of key nodes equal to the size of the I/O buffer does not always ensure the best overall performance. When reading the big leaf nodes, the server pulls in a lot of unnecessary data, effectively preventing reading other leaf nodes.

Currently, you cannot control the size of the index blocks in a table. This size is set by the server when the `.MYI' index file is created, depending on the size of the keys in the indexes present in the table definition. In most cases, it is set equal to the I/O buffer size. In the future, this will be changed and then key_cache_block_size variable will be fully employed.

7.4.6.6 Restructuring a Key Cache

A key cache can be restructured at any time by updating its parameter values. For example: 

mysql> SET GLOBAL cold_cache.key_buffer_size=4*1024*1024;

If you assign to either the key_buffer_size or key_cache_block_size key cache component a value that differs from the component's current value, the server destroys the cache's old structure and creates a new one based on the new values. If the cache contains any dirty blocks, the server saves them to disk before destroying and re-creating the cache. Restructuring does not occur if you set other key cache parameters.

When restructuring a key cache, the server first flushes the contents of any dirty buffers to disk. After that, the cache contents become unavailable. However, restructuring does not block queries that need to use indexes assigned to the cache. Instead, the server directly accesses the table indexes using native filesystem caching. Filesystem caching is not as efficient as using a key cache, so although queries will execute, a slowdown can be anticipated. Once the cache has been restructured, it becomes available again for caching indexes assigned to it, and the use of filesystem caching for the indexes ceases.

7.4.7 How MySQL Counts Open Tables

When you execute a @command{mysqladmin status} command, you'll see something like this: 

Uptime: 426 Running threads: 1 Questions: 11082
Reloads: 1 Open tables: 12

The Open tables value of 12 can be somewhat puzzling if you have only six tables.

MySQL is multi-threaded, so there may be many clients issuing queries for a given table simultaneously. To minimize the problem with multiple client threads having different states on the same table, the table is opened independently by each concurrent thread. This takes some memory but normally increases performance. With MyISAM tables, one extra file descriptor is required for the data file for each client that has the table open. (By contrast, the index file descriptor is shared between all threads.) The ISAM storage engine shares this behavior.

You can read more about this topic in the next section. See section 7.4.8 How MySQL Opens and Closes Tables.

7.4.8 How MySQL Opens and Closes Tables

The table_cache, max_connections, and max_tmp_tables system variables affect the maximum number of files the server keeps open. If you increase one or more of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. Many operating systems allow you to increase the open-files limit, although the method varies widely from system to system. Consult your operating system documentation to determine whether it is possible to increase the limit and how to do so.

table_cache is related to max_connections. For example, for 200 concurrent running connections, you should have a table cache size of at least 200 * N, where N is the maximum number of tables in a join. You also need to reserve some extra file descriptors for temporary tables and files.

Make sure that your operating system can handle the number of open file descriptors implied by the table_cache setting. If table_cache is set too high, MySQL may run out of file descriptors and refuse connections, fail to perform queries, and be very unreliable. You also have to take into account that the MyISAM storage engine needs two file descriptors for each unique open table. You can increase the number of file descriptors available for MySQL with the --open-files-limit startup option to @command{mysqld_safe}. See section A.2.17 File Not Found.

The cache of open tables will be kept at a level of table_cache entries. The default value is 64; this can be changed with the --table_cache option to @command{mysqld}. Note that MySQL may temporarily open even more tables to be able to execute queries.

An unused table is closed and removed from the table cache under the following circumstances:

When the table cache fills up, the server uses the following procedure to locate a cache entry to use:

When the cache is in a temporarily extended state and a table goes from a used to unused state, the table is closed and released from the cache.

A table is opened for each concurrent access. This means the table needs to be opened twice if two threads access the same table or if a thread accesses the table twice in the same query (for example, by joining the table to itself). Each concurrent open requires an entry in the table cache. The first open of any table takes two file descriptors: one for the data file and one for the index file. Each additional use of the table takes only one file descriptor, for the data file. The index file descriptor is shared among all threads.

If you are opening a table with the HANDLER tbl_name OPEN statement, a dedicated table object is allocated for the thread. This table object is not shared by other threads and is not closed until the thread calls HANDLER tbl_name CLOSE or the thread terminates. When this happens, the table is put back in the table cache (if the cache isn't full). See section 14.1.3 HANDLER Syntax.

You can determine whether your table cache is too small by checking the @command{mysqld} status variable Opened_tables: 

mysql> SHOW STATUS LIKE 'Opened_tables';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Opened_tables | 2741  |
+---------------+-------+

If the value is quite big, even when you haven't issued a lot of FLUSH TABLES statements, you should increase your table cache size. See section 5.2.3 Server System Variables and section 5.2.4 Server Status Variables.

7.4.9 Drawbacks to Creating Many Tables in the Same Database

If you have many MyISAM or ISAM tables in a database directory, open, close, and create operations will be slow. If you execute SELECT statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger.

7.5 Optimizing the MySQL Server

7.5.1 System Factors and Startup Parameter Tuning

We start with system-level factors, because some of these decisions must be made very early to achieve large performance gains. In other cases, a quick look at this section may suffice. However, it is always nice to have a sense of how much can be gained by changing things at this level.

The default operating system to use is very important! To get the best use of multiple-CPU machines, you should use Solaris (because its threads implementation works really well) or Linux (because the 2.4 kernel has really good SMP support). Note that older Linux kernels have a 2GB filesize limit by default. If you have such a kernel and a desperate need for files larger than 2GB, you should get the Large File Support (LFS) patch for the ext2 filesystem. Other filesystems such as ReiserFS and XFS do not have this 2GB limitation.

Before using MySQL in production, we advise you to test it on your intended platform.

Other tips:

7.5.2 Tuning Server Parameters

You can determine the default buffer sizes used by the @command{mysqld} server with this command (prior to MySQL 4.1, omit --verbose): 

shell> mysqld --verbose --help

This command produces a list of all @command{mysqld} options and configurable system variables. The output includes the default variable values and looks something like this: 

back_log                 current value: 5
bdb_cache_size           current value: 1048540
binlog_cache_size        current value: 32768
connect_timeout          current value: 5
delayed_insert_limit     current value: 100
delayed_insert_timeout   current value: 300
delayed_queue_size       current value: 1000
flush_time               current value: 0
interactive_timeout      current value: 28800
join_buffer_size         current value: 131072
key_buffer_size          current value: 1048540
long_query_time          current value: 10
lower_case_table_names   current value: 0
max_allowed_packet       current value: 1048576
max_binlog_cache_size    current value: 4294967295
max_connect_errors       current value: 10
max_connections          current value: 100
max_delayed_threads      current value: 20
max_heap_table_size      current value: 16777216
max_join_size            current value: 4294967295
max_sort_length          current value: 1024
max_tmp_tables           current value: 32
max_write_lock_count     current value: 4294967295
myisam_sort_buffer_size  current value: 8388608
net_buffer_length        current value: 16384
net_read_timeout         current value: 30
net_retry_count          current value: 10
net_write_timeout        current value: 60
read_buffer_size         current value: 131072
read_rnd_buffer_size     current value: 262144
slow_launch_time         current value: 2
sort_buffer              current value: 2097116
table_cache              current value: 64
thread_concurrency       current value: 10
thread_stack             current value: 131072
tmp_table_size           current value: 1048576
wait_timeout             current value: 28800

If there is a @command{mysqld} server currently running, you can see what values it actually is using for the system variables by connecting to it and issuing this statement: 

mysql> SHOW VARIABLES;

You can also see some statistical and status indicators for a running server by issuing this statement: 

mysql> SHOW STATUS;

System variable and status information also can be obtained using @command{mysqladmin}: 

shell> mysqladmin variables
shell> mysqladmin extended-status

You can find a full description for all system and status variables in section 5.2.3 Server System Variables and section 5.2.4 Server Status Variables.

MySQL uses algorithms that are very scalable, so you can usually run with very little memory. However, normally you will get better performance by giving MySQL more memory.

When tuning a MySQL server, the two most important variables to configure are key_buffer_size and table_cache. You should first feel confident that you have these set appropriately before trying to change any other variables.

The following examples indicate some typical variable values for different runtime configurations. The examples use the @command{mysqld_safe} script and use --var_name=value syntax to set the variable var_name to the value value. This syntax is available as of MySQL 4.0. For older versions of MySQL, take the following differences into account:

If you have at least 256MB of memory and many tables and want maximum performance with a moderate number of clients, you should use something like this: 

shell> mysqld_safe --key_buffer_size=64M --table_cache=256 \
           --sort_buffer_size=4M --read_buffer_size=1M &

If you have only 128MB of memory and only a few tables, but you still do a lot of sorting, you can use something like this: 

shell> mysqld_safe --key_buffer_size=16M --sort_buffer_size=1M

If there are very many simultaneous connections, swapping problems may occur unless @command{mysqld} has been configured to use very little memory for each connection. @command{mysqld} performs better if you have enough memory for all connections.

With little memory and lots of connections, use something like this: 

shell> mysqld_safe --key_buffer_size=512K --sort_buffer_size=100K \
           --read_buffer_size=100K &

Or even this: 

shell> mysqld_safe --key_buffer_size=512K --sort_buffer_size=16K \
           --table_cache=32 --read_buffer_size=8K \
           --net_buffer_length=1K &

If you are doing GROUP BY or ORDER BY operations on tables that are much larger than your available memory, you should increase the value of read_rnd_buffer_size to speed up the reading of rows after sorting operations.

When you have installed MySQL, the `support-files' directory will contain some different `my.cnf' sample files: `my-huge.cnf', `my-large.cnf', `my-medium.cnf', and `my-small.cnf'. You can use these as a basis for optimizing your system.

Note that if you specify an option on the command line for @command{mysqld} or @command{mysqld_safe}, it remains in effect only for that invocation of the server. To use the option every time the server runs, put it in an option file.

To see the effects of a parameter change, do something like this (prior to MySQL 4.1, omit --verbose): 

shell> mysqld --key_buffer_size=32M --verbose --help

The variable values are listed near the end of the output. Make sure that the --verbose and --help options are last. Otherwise, the effect of any options listed after them on the command line will not be reflected in the output.

For information on tuning the InnoDB storage engine, see section 16.12 InnoDB Performance Tuning Tips.

7.5.3 Controlling Query Optimizer Performance

The task of the query optimizer is to find an optimal plan for executing an SQL query. Because the difference in performance between ``good'' and ``bad'' plans can be orders of magnitude (that is, seconds versus hours or even days), most query optimizers, including that of MySQL, perform more or less exhaustive search for an optimal plan among all possible query evaluation plans. For join queries, the number of possible plans investigated by the MySQL optimizer grows exponentially with the number of tables referenced in a query. For small numbers of tables (typically less than 7-10) this is not a problem. However, when bigger queries are submitted, the time spent in query optimization may easily become the major bottleneck in the server performance.

MySQL 5.0.1 introduces a new more flexible method for query optimization that allows the user to control how exhaustive the optimizer is in its search for an optimal query evaluation plan. The general idea is that the fewer plans that are investigated by the optimizer, the less time it will spend in compiling a query. On the other hand, because the optimizer will skip some plans, it may miss finding an optimal plan.

The behavior of the optimizer with respect to the number of plans it evaluates can be controlled via two system variables:

7.5.4 How Compiling and Linking Affects the Speed of MySQL

Most of the following tests were performed on Linux with the MySQL benchmarks, but they should give some indication for other operating systems and workloads.

You get the fastest executables when you link with -static.

On Linux, you will get the fastest code when compiling with pgcc and -O3. You need about 200MB memory to compile `sql_yacc.cc' with these options, because @command{gcc/pgcc} needs a lot of memory to make all functions inline. You should also set CXX=gcc when configuring MySQL to avoid inclusion of the libstdc++ library, which is not needed. Note that with some versions of pgcc, the resulting code will run only on true Pentium processors, even if you use the compiler option indicating that you want the resulting code to work on all x586-type processors (such as AMD).

By just using a better compiler and better compiler options, you can get a 10-30% speed increase in your application. This is particularly important if you compile the MySQL server yourself.

We have tested both the Cygnus CodeFusion and Fujitsu compilers, but when we tested them, neither was sufficiently bug-free to allow MySQL to be compiled with optimizations enabled.

The standard MySQL binary distributions are compiled with support for all character sets. When you compile MySQL yourself, you should include support only for the character sets that you are going to use. This is controlled by the --with-charset option to @command{configure}.

Here is a list of some measurements that we have made:

Binary MySQL distributions for Linux that are provided by MySQL AB used to be compiled with pgcc. We had to go back to regular @command{gcc} due to a bug in pgcc that would generate code that does not run on AMD. We will continue using @command{gcc} until that bug is resolved. In the meantime, if you have a non-AMD machine, you can get a faster binary by compiling with pgcc. The standard MySQL Linux binary is linked statically to make it faster and more portable.

7.5.5 How MySQL Uses Memory

The following list indicates some of the ways that the @command{mysqld} server uses memory. Where applicable, the name of the system variable relevant to the memory use is given:

@command{ps} and other system status programs may report that @command{mysqld} uses a lot of memory. This may be caused by thread stacks on different memory addresses. For example, the Solaris version of @command{ps} counts the unused memory between stacks as used memory. You can verify this by checking available swap with swap -s. We have tested @command{mysqld} with several memory-leakage detectors (both commercial and open source), so there should be no memory leaks.

7.5.6 How MySQL Uses DNS

When a new client connects to @command{mysqld}, @command{mysqld} spawns a new thread to handle the request. This thread first checks whether the hostname is in the hostname cache. If not, the thread attempts to resolve the hostname:

You can disable DNS hostname lookups by starting @command{mysqld} with the --skip-name-resolve option. However, in this case, you can use only IP numbers in the MySQL grant tables.

If you have a very slow DNS and many hosts, you can get more performance by either disabling DNS lookups with --skip-name-resolve or by increasing the HOST_CACHE_SIZE define (default value: 128) and recompiling @command{mysqld}.

You can disable the hostname cache by starting the server with the --skip-host-cache option. To clear the hostname cache, issue a FLUSH HOSTS statement or execute the @command{mysqladmin flush-hosts} command.

If you want to disallow TCP/IP connections entirely, start @command{mysqld} with the --skip-networking option.

7.6 Disk Issues

7.6.1 Using Symbolic Links

You can move tables and databases from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space or increase the speed of your system by spreading your tables to different disk.

The recommended way to do this is to just symlink databases to a different disk. Symlink tables only as a last resort.

7.6.1.1 Using Symbolic Links for Databases on Unix

On Unix, the way to symlink a database is to first create a directory on some disk where you have free space and then create a symlink to it from the MySQL data directory. 

shell> mkdir /dr1/databases/test
shell> ln -s /dr1/databases/test /path/to/datadir

MySQL doesn't support linking one directory to multiple databases. Replacing a database directory with a symbolic link will work fine as long as you don't make a symbolic link between databases. Suppose that you have a database db1 under the MySQL data directory, and then make a symlink db2 that points to db1: 

shell> cd /path/to/datadir
shell> ln -s db1 db2

Now, for any table tbl_a in db1, there also appears to be a table tbl_a in db2. If one client updates db1.tbl_a and another client updates db2.tbl_a, there will be problems.

If you really need to do this, you can change one of the source files. The file to modify depends on your version of MySQL. For MySQL 4.0 and up, look for the following statement in the `mysys/my_symlink.c' file: 

if (!(MyFlags & MY_RESOLVE_LINK) || 
    (!lstat(filename,&stat_buff) && S_ISLNK(stat_buff.st_mode)))

Before MySQL 4.0, look for this statement in the `mysys/mf_format.c' file: 

if (flag & 32 || (!lstat(to,&stat_buff) && S_ISLNK(stat_buff.st_mode)))

Change the statement to this: 

if (1)

On Windows, you can use internal symbolic links to directories by compiling MySQL with -DUSE_SYMDIR. This allows you to put different databases on different disks. See section 7.6.1.3 Using Symbolic Links for Databases on Windows.

7.6.1.2 Using Symbolic Links for Tables on Unix

Before MySQL 4.0, you should not symlink tables unless you are very careful with them. The problem is that if you run ALTER TABLE, REPAIR TABLE, or OPTIMIZE TABLE on a symlinked table, the symlinks will be removed and replaced by the original files. This happens because these statements work by creating a temporary file in the database directory and replacing the original file with the temporary file when the statement operation is complete.

You should not symlink tables on systems that don't have a fully working realpath() call. (At least Linux and Solaris support realpath()). You can check whether your system supports symbolic links by issuing a SHOW VARIABLES LIKE 'have_symlink' statement.

In MySQL 4.0, symlinks are fully supported only for MyISAM tables. For other table types, you will probably get strange problems if you try to use symbolic links on files in the operating system with any of the preceding statements.

The handling of symbolic links for MyISAM tables in MySQL 4.0 works the following way:

SHOW CREATE TABLE doesn't report if a table has symbolic links prior to MySQL 4.0.15. This is also true for @command{mysqldump}, which uses SHOW CREATE TABLE to generate CREATE TABLE statements.

Table symlink operations that are not yet supported:

7.6.1.3 Using Symbolic Links for Databases on Windows

Beginning with MySQL 3.23.16, the @command{mysqld-max} and mysql-max-nt servers for Windows are compiled with the -DUSE_SYMDIR option. This allows you to put a database directory on a different disk by setting up a symbolic link to it. This is similar to the way that symbolic links work on Unix, although the procedure for setting up the link is different.

As of MySQL 4.0, symbolic links are enabled by default. If you don't need them, you can disable them with the skip-symbolic-links option: 

[mysqld]
skip-symbolic-links

Before MySQL 4.0, symbolic links are disabled by default. To enable them, you should put the following entry in your `my.cnf' or `my.ini' file: 

[mysqld]
symbolic-links

On Windows, you make a symbolic link to a MySQL database by creating a file in the data directory that contains the path to the destination directory. The file should be named `db_name.sym', where db_name is the database name.

Suppose that the MySQL data directory is `C:\mysql\data' and you want to have database foo located at `D:\data\foo'. Set up a symlink like this:

  1. Make sure that the `D:\data\foo' directory exists by creating it if necessary. If you already have a database directory named `foo' in the data directory, you should move it to `D:\data'. Otherwise, the symbolic link will be ineffective. To avoid problems, the server should not be running when you move the database directory.
  2. Create a file `C:\mysql\data\foo.sym' that contains the pathname D:\data\foo\.

After that, all tables created in the database foo will be created in `D:\data\foo'. Note that the symbolic link will not be used if a directory with the database name exists in the MySQL data directory.

8 MySQL Client and Utility Programs

There are many different MySQL client programs that connect to the server to access databases or perform administrative tasks. Other utilities are available as well. These do not communicate with the server but perform MySQL-related operations.

This chapter provides a brief overview of these programs and then a more detailed description of each one. The descriptions indicate how to invoke the programs and the options they understand. See section 4 Using MySQL Programs for general information on invoking programs and specifying program options.

8.1 Overview of the Client-Side Scripts and Utilities

The following list briefly describes the MySQL client programs and utilities:

myisampack
A utility that compresses MyISAM tables to produce smaller read-only tables. @xref{myisampack, , @command{myisampack}}.
mysql
The command-line tool for interactively entering SQL statements or executing them from a file in batch mode. @xref{mysql, , @command{mysql}}.
mysqlaccess
A script that checks the access privileges for a host, user, and database combination.
mysqladmin
A client that performs administrative operations, such as creating or dropping databases, reloading the grant tables, flushing tables to disk, and reopening log files. @command{mysqladmin} can also be used to retrieve version, process, and status information from the server. @xref{mysqladmin, , @command{mysqladmin}}.
mysqlbinlog
A utility for reading statements from a binary log. The log of executed statements contained in the binary log files can be used to help recover from a crash. @xref{mysqlbinlog, , @command{mysqlbinlog}}.
mysqlcc
A client that provides a graphical interface for interacting with the server. @xref{mysqlcc, , @command{mysqlcc}}.
mysqlcheck
A table-maintenance client that checks, repairs, analyzes, and optimizes tables. @xref{Using mysqlcheck, , Using @command{mysqlcheck}}.
mysqldump
A client that dumps a MySQL database into a file as SQL statements or as tab-separated text files. Enhanced freeware originally by Igor Romanenko. @xref{mysqldump, , @command{mysqldump}}.
mysqlhotcopy
A utility that quickly makes backups of MyISAM or ISAM tables while the server is running. @xref{mysqlhotcopy, , @command{mysqlhotcopy}}.
mysqlimport
A client that imports text files into their respective tables using LOAD DATA INFILE. @xref{mysqlimport, , @command{mysqlimport}}.
mysqlshow
A client that displays information about databases, tables, columns, and indexes. @xref{mysqlshow, , @command{mysqlshow}}.
perror
A utility that displays the meaning of system or MySQL error codes. @xref{perror, , @command{perror}}.
replace
A utility program that changes strings in place in files or on the standard input. @xref{replace utility, , @command{replace} utility}.

Each MySQL program takes many different options. However, every MySQL program provides a --help option that you can use to get a full description of the program's different options. For example, try @command{mysql --help}.

MySQL clients that communicate with the server using the mysqlclient library use the following environment variables:

MYSQL_UNIX_PORT @tab The default Unix socket file; used for connections to localhost
MYSQL_TCP_PORT @tab The default port number; used for TCP/IP connections
MYSQL_PWD @tab The default password
MYSQL_DEBUG @tab Debug trace options when debugging
TMPDIR @tab The directory where temporary tables and files are created

Use of MYSQL_PWD is insecure. See section 5.6.6 Keeping Your Password Secure.

You can override the default option values or values specified in environment variables for all standard programs by specifying options in an option file or on the command line. section 4.3 Specifying Program Options.

8.2 @command{myisampack}, the MySQL Compressed Read-only Table Generator

The @command{myisampack} utility compresses MyISAM tables. @command{myisampack} works by compressing each column in the table separately. Usually, @command{myisampack} packs the data file 40%-70%.

When the table is used later, the information needed to decompress columns is read into memory. This results in much better performance when accessing individual records, because you only have to uncompress exactly one record, not a much larger disk block as when using Stacker on MS-DOS.

MySQL uses mmap() when possible to perform memory mapping on compressed tables. If mmap() doesn't work, MySQL falls back to normal read/write file operations.

A similar utility, @command{pack_isam}, compresses ISAM tables. Because ISAM tables are deprecated, this section discusses only @command{myisampack}, but the general procedures for using @command{myisampack} are also true for @command{pack_isam} unless otherwise specified.

Please note the following:

Invoke @command{myisampack} like this: 

shell> myisampack [options] filename ...

Each filename should be the name of an index (`.MYI') file. If you are not in the database directory, you should specify the pathname to the file. It is permissible to omit the `.MYI' extension.

@command{myisampack} supports the following options:

--help, -?
Display a help message and exit.
--backup, -b
Make a backup of the table data file using the name `tbl_name.OLD'.
--debug[=debug_options], -# [debug_options]
Write a debugging log. The debug_options string often is 'd:t:o,file_name'.
--force, -f
Produce a packed table even if it becomes larger than the original or if the temporary file from an earlier invocation of @command{myisampack} exists. (@command{myisampack} creates a temporary file named `tbl_name.TMD' while it compresses the table. If you kill @command{myisampack}, the `.TMD' file might not be deleted.) Normally, @command{myisampack} exits with an error if it finds that `tbl_name.TMD' exists. With --force, @command{myisampack} packs the table anyway.
--join=big_tbl_name, -j big_tbl_name
Join all tables named on the command line into a single table big_tbl_name. All tables that are to be combined must have identical structure (same column names and types, same indexes, and so forth).
--packlength=#, -p #
Specify the record length storage size, in bytes. The value should be 1, 2, or 3. @command{myisampack} stores all rows with length pointers of 1, 2, or 3 bytes. In most normal cases, @command{myisampack} can determine the right length value before it begins packing the file, but it may notice during the packing process that it could have used a shorter length. In this case, @command{myisampack} will print a note that the next time you pack the same file, you could use a shorter record length.
--silent, -s
Silent mode. Write output only when errors occur.
--test, -t
Don't actually pack the table, just test packing it.
--tmp_dir=path, -T path
Use the named directory as the location in which to write the temporary table.
--verbose, -v
Verbose mode. Write information about the progress of the packing operation and its result.
--version, -V
Display version information and exit.
--wait, -w
Wait and retry if the table is in use. If the @command{mysqld} server was invoked with the --skip-external-locking option, it is not a good idea to invoke @command{myisampack} if the table might be updated by the server during the packing process.

The following sequence of commands illustrates a typical table compression session: 

shell> ls -l station.*
-rw-rw-r--   1 monty    my         994128 Apr 17 19:00 station.MYD
-rw-rw-r--   1 monty    my          53248 Apr 17 19:00 station.MYI
-rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

shell> myisamchk -dvv station

MyISAM file:     station
Isam-version:  2
Creation time: 1996-03-13 10:08:58
Recover time:  1997-02-02  3:06:43
Data records:              1192  Deleted blocks:              0
Datafile parts:            1192  Deleted data:                0
Datafile pointer (bytes):     2  Keyfile pointer (bytes):     2
Max datafile length:   54657023  Max keyfile length:   33554431
Recordlength:               834
Record format: Fixed length

table description:
Key Start Len Index   Type                 Root  Blocksize    Rec/key
1   2     4   unique  unsigned long        1024       1024          1
2   32    30  multip. text                10240       1024          1

Field Start Length Type
1     1     1
2     2     4
3     6     4
4     10    1
5     11    20
6     31    1
7     32    30
8     62    35
9     97    35
10    132   35
11    167   4
12    171   16
13    187   35
14    222   4
15    226   16
16    242   20
17    262   20
18    282   20
19    302   30
20    332   4
21    336   4
22    340   1
23    341   8
24    349   8
25    357   8
26    365   2
27    367   2
28    369   4
29    373   4
30    377   1
31    378   2
32    380   8
33    388   4
34    392   4
35    396   4
36    400   4
37    404   1
38    405   4
39    409   4
40    413   4
41    417   4
42    421   4
43    425   4
44    429   20
45    449   30
46    479   1
47    480   1
48    481   79
49    560   79
50    639   79
51    718   79
52    797   8
53    805   1
54    806   1
55    807   20
56    827   4
57    831   4

shell> myisampack station.MYI
Compressing station.MYI: (1192 records)
- Calculating statistics

normal:     20  empty-space:   16  empty-zero:     12  empty-fill:  11
pre-space:   0  end-space:     12  table-lookups:   5  zero:         7
Original trees:  57  After join: 17
- Compressing file
87.14%
Remember to run myisamchk -rq on compressed tables

shell> ls -l station.*
-rw-rw-r--   1 monty    my         127874 Apr 17 19:00 station.MYD
-rw-rw-r--   1 monty    my          55296 Apr 17 19:04 station.MYI
-rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

shell> myisamchk -dvv station

MyISAM file:     station
Isam-version:  2
Creation time: 1996-03-13 10:08:58
Recover time:  1997-04-17 19:04:26
Data records:               1192  Deleted blocks:              0
Datafile parts:             1192  Deleted data:                0
Datafile pointer (bytes):      3  Keyfile pointer (bytes):     1
Max datafile length:    16777215  Max keyfile length:     131071
Recordlength:                834
Record format: Compressed

table description:
Key Start Len Index   Type                 Root  Blocksize    Rec/key
1   2     4   unique  unsigned long       10240       1024          1
2   32    30  multip. text                54272       1024          1

Field Start Length Type                         Huff tree  Bits
1     1     1      constant                             1     0
2     2     4      zerofill(1)                          2     9
3     6     4      no zeros, zerofill(1)                2     9
4     10    1                                           3     9
5     11    20     table-lookup                         4     0
6     31    1                                           3     9
7     32    30     no endspace, not_always              5     9
8     62    35     no endspace, not_always, no empty    6     9
9     97    35     no empty                             7     9
10    132   35     no endspace, not_always, no empty    6     9
11    167   4      zerofill(1)                          2     9
12    171   16     no endspace, not_always, no empty    5     9
13    187   35     no endspace, not_always, no empty    6     9
14    222   4      zerofill(1)                          2     9
15    226   16     no endspace, not_always, no empty    5     9
16    242   20     no endspace, not_always              8     9
17    262   20     no endspace, no empty                8     9
18    282   20     no endspace, no empty                5     9
19    302   30     no endspace, no empty                6     9
20    332   4      always zero                          2     9
21    336   4      always zero                          2     9
22    340   1                                           3     9
23    341   8      table-lookup                         9     0
24    349   8      table-lookup                        10     0
25    357   8      always zero                          2     9
26    365   2                                           2     9
27    367   2      no zeros, zerofill(1)                2     9
28    369   4      no zeros, zerofill(1)                2     9
29    373   4      table-lookup                        11     0
30    377   1                                           3     9
31    378   2      no zeros, zerofill(1)                2     9
32    380   8      no zeros                             2     9
33    388   4      always zero                          2     9
34    392   4      table-lookup                        12     0
35    396   4      no zeros, zerofill(1)               13     9
36    400   4      no zeros, zerofill(1)                2     9
37    404   1                                           2     9
38    405   4      no zeros                             2     9
39    409   4      always zero                          2     9
40    413   4      no zeros                             2     9
41    417   4      always zero                          2     9
42    421   4      no zeros                             2     9
43    425   4      always zero                          2     9
44    429   20     no empty                             3     9
45    449   30     no empty                             3     9
46    479   1                                          14     4
47    480   1                                          14     4
48    481   79     no endspace, no empty               15     9
49    560   79     no empty                             2     9
50    639   79     no empty                             2     9
51    718   79     no endspace                         16     9
52    797   8      no empty                             2     9
53    805   1                                          17     1
54    806   1                                           3     9
55    807   20     no empty                             3     9
56    827   4      no zeros, zerofill(2)                2     9
57    831   4      no zeros, zerofill(1)                2     9

@command{myisampack} displays the following kinds of information:

normal
The number of columns for which no extra packing is used.
empty-space
The number of columns containing values that are only spaces; these will occupy one bit.
empty-zero
The number of columns containing values that are only binary zeros; these will occupy one bit.
empty-fill
The number of integer columns that don't occupy the full byte range of their type; these are changed to a smaller type. For example, a BIGINT column (eight bytes) can be stored as a TINYINT column (one byte) if all its values are in the range from -128 to 127.
pre-space
The number of decimal columns that are stored with leading spaces. In this case, each value will contain a count for the number of leading spaces.
end-space
The number of columns that have a lot of trailing spaces. In this case, each value will contain a count for the number of trailing spaces.
table-lookup
The column had only a small number of different values, which were converted to an ENUM before Huffman compression.
zero
The number of columns for which all values are zero.
Original trees
The initial number of Huffman trees.
After join
The number of distinct Huffman trees left after joining trees to save some header space.

After a table has been compressed, @command{myisamchk -dvv} prints additional information about each column:

Type
The column type. The value may contain any of the following descriptors:
constant
All rows have the same value.
no endspace
Don't store endspace.
no endspace, not_always
Don't store endspace and don't do endspace compression for all values.
no endspace, no empty
Don't store endspace. Don't store empty values.
table-lookup
The column was converted to an ENUM.
zerofill(n)
The most significant n bytes in the value are always 0 and are not stored.
no zeros
Don't store zeros.
always zero
Zero values are stored using one bit.
Huff tree
The number of the Huffman tree associated with the column.
Bits
The number of bits used in the Huffman tree.

After you run @command{myisampack}, you must run @command{myisamchk} to re-create any indexes. At this time, you can also sort the index blocks and create statistics needed for the MySQL optimizer to work more efficiently: 

shell> myisamchk -rq --sort-index --analyze tbl_name.MYI

A similar procedure applies for ISAM tables. After using @command{pack_isam}, use @command{isamchk} to re-create the indexes: 

shell> isamchk -rq --sort-index --analyze tbl_name.ISM

After you have installed the packed table into the MySQL database directory, you should execute @command{mysqladmin flush-tables} to force @command{mysqld} to start using the new table.

To unpack a packed table, use the --unpack option to @command{myisamchk} or @command{isamchk}.

8.3 @command{mysql}, the Command-Line Tool

@command{mysql} is a simple SQL shell (with GNU readline capabilities). It supports interactive and non-interactive use. When used interactively, query results are presented in an ASCII-table format. When used non-interactively (for example, as a filter), the result is presented in tab-separated format. The output format can be changed using command-line options.

If you have problems due to insufficient memory for large result sets, use the --quick option. This forces @command{mysql} to retrieve results from the server a row at a time rather than retrieving the entire result set and buffering it in memory before displaying it. This is done by using mysql_use_result() rather than mysql_store_result() to retrieve the result set.

Using @command{mysql} is very easy. Invoke it from the prompt of your command interpreter as follows: 

shell> mysql db_name

Or: 

shell> mysql --user=user_name --password=your_password db_name

Then type an SQL statement, end it with `;', \g, or \G and press Enter.

You can run a script simply like this: 

shell> mysql db_name < script.sql > output.tab

@command{mysql} supports the following options:

--help, -?
Display a help message and exit.
--batch, -B
Print results using tab as the column separator, with each row on a new line. With this option, @command{mysql} doesn't use the history file.
--character-sets-dir=path
The directory where character sets are installed. See section 5.8.1 The Character Set Used for Data and Sorting.
--compress, -C
Compress all information sent between the client and the server if both support compression.
--database=db_name, -D db_name
The database to use. This is useful mainly in an option file.
--debug[=debug_options], -# [debug_options]
Write a debugging log. The debug_options string often is 'd:t:o,file_name'. The default is 'd:t:o,/tmp/mysql.trace'.
--debug-info, -T
Print some debugging information when the program exits.
--default-character-set=charset
Use charset as the default character set. See section 5.8.1 The Character Set Used for Data and Sorting.
--execute=statement, -e statement
Execute the statement and quit. The default output format is like that produced with --batch.
--force, -f
Continue even if an SQL error occurs.
--host=host_name, -h host_name
Connect to the MySQL server on the given host.
--html, -H
Produce HTML output.
--ignore-space, -i
Ignore spaces after function names. The effect of this is described in the discussion for IGNORE_SPACE in section 5.2.2 The Server SQL Mode.
--local-infile[={0|1}]
Enable or disable LOCAL capability for LOAD DATA INFILE. With no value, the option enables LOCAL. It may be given as --local-infile=0 or --local-infile=1 to explicitly disable or enable LOCAL. Enabling LOCAL has no effect if the server does not also support it.
--named-commands, -G
Named commands are enabled. Long format commands are allowed as well as shortened \* commands. For example, quit and \q both are recognized.
--no-auto-rehash, -A
No automatic rehashing. This option causes @command{mysql} to start faster, but you must issue the rehash command if you want to use table and column name completion.
--no-beep, -b
Do not beep when errors occur.
--no-named-commands, -g
Named commands are disabled. Use the \* form only, or use named commands only at the beginning of a line ending with a semicolon (`;'). As of MySQL 3.23.22, @command{mysql} starts with this option enabled by default! However, even with this option, long-format commands still work from the first line.
--no-pager
Do not use a pager for displaying query output. Output paging is discussed further in @ref{mysql Commands, , @command{mysql} Commands}.
--no-tee
Do not copy output to a file. Tee files are discussed further in @ref{mysql Commands, , @command{mysql} Commands}.
--one-database, -O
Ignore statements except those for the default database named on the command line. This is useful for skipping updates to other databases in the binary log.
--pager[=command]
Use the given command for paging query output. If the command is omitted, the default pager is the value of your PAGER environment variable. Valid pagers are @command{less}, @command{more}, @command{cat [> filename]}, and so forth. This option works only on Unix. It does not work in batch mode. Output paging is discussed further in @ref{mysql Commands, , @command{mysql} Commands}.
--password[=password], -p[password]
The password to use when connecting to the server. If you use the short option form (-p), you cannot have a space between the option and the password. If no password is given on the command line, you will be prompted for one.
--port=port_num, -P port_num
The TCP/IP port number to use for the connection.
--prompt=format_str
Set the prompt to the specified format. The default is mysql>. The special sequences that the prompt can contain are described in @ref{mysql Commands, , @command{mysql} Commands}.
--protocol={TCP | SOCKET | PIPE | MEMORY}
The connection protocol to use. New in MySQL 4.1.
--quick, -q
Don't cache each query result, print each row as it is received. This may slow down the server if the output is suspended. With this option, @command{mysql} doesn't use the history file.
--raw, -r
Write column values without escape conversion. Often used with the --batch option.
--reconnect
If the connection to the server is lost, automatically try to reconnect. A single reconnect attempt is made each time the connection is lost. To suppress reconnection behavior, use --skip-reconnect. New in MySQL 4.1.0.
--safe-updates, --i-am-a-dummy, -U
Allow only UPDATE and DELETE statements that specify rows to affect using key values. If you have this option in an option file, you can override it by using --safe-updates on the command line. See @ref{mysql Tips, , @command{mysql} Tips} for more information about this option.
--sigint-ignore
Ignore SIGINT signals (typically the result of typing Control-C). This option was added in MySQL 4.1.6.
--silent, -s
Silent mode. Produce less output. This option can be given multiple times to produce less and less output.
--skip-column-names, -N
Don't write column names in results.
--skip-line-numbers, -L
Don't write line numbers for errors. Useful when you want to compare result files that include error messages.
--socket=path, -S path
The socket file to use for the connection.
--table, -t
Display output in table format. This is the default for interactive use, but can be used to produce table output in batch mode.
--tee=file_name
Append a copy of output to the given file. This option does not work in batch mode. Tee files are discussed further in @ref{mysql Commands, , @command{mysql} Commands}.
--unbuffered, -n
Flush the buffer after each query.
--user=user_name, -u user_name
The MySQL username to use when connecting to the server.
--verbose, -v
Verbose mode. Produce more output. This option can be given multiple times to produce more and more output. (For example, -v -v -v produces the table output format even in batch mode.)
--version, -V
Display version information and exit.
--vertical, -E
Print the rows of query output vertically. Without this option, you can specify vertical output for individual statements by terminating them with \G.
--wait, -w
If the connection cannot be established, wait and retry instead of aborting.
--xml, -X
Produce XML output.

You can also set the following variables by using --var_name=value options:

connect_timeout
The number of seconds before connection timeout. (Default value is 0.)
max_allowed_packet
The maximum packet length to send to or receive from the server. (Default value is 16MB.)
max_join_size
The automatic limit for rows in a join when using --safe-updates. (Default value is 1,000,000.)
net_buffer_length
The buffer size for TCP/IP and socket communication. (Default value is 16KB.)
select_limit
The automatic limit for SELECT statements when using --safe-updates. (Default value is 1,000.)

It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. However, this syntax is deprecated as of MySQL 4.0.

On Unix, the @command{mysql} client writes a record of executed statements to a history file. By default, the history file is named `.mysql_history' and is created in your home directory. To specify a different file, set the value of the MYSQL_HISTFILE environment variable.

If you do not want to maintain a history file, first remove `.mysql_history' if it exists, and then use either of the following techniques:

8.3.1 @command{mysql} Commands

@command{mysql} sends SQL statements that you issue to the server to be executed. There is also a set of commands that @command{mysql} itself interprets. For a list of these commands, type help or \h at the mysql> prompt: 

mysql> help

MySQL commands:
?         (\h)    Synonym for `help'.
clear     (\c)    Clear command.
connect   (\r)    Reconnect to the server.
                  Optional arguments are db and host.
delimiter (\d)    Set query delimiter.
edit      (\e)    Edit command with $EDITOR.
ego       (\G)    Send command to mysql server,
                  display result vertically.
exit      (\q)    Exit mysql. Same as quit.
go        (\g)    Send command to mysql server.
help      (\h)    Display this help.
nopager   (\n)    Disable pager, print to stdout.
notee     (\t)    Don't write into outfile.
pager     (\P)    Set PAGER [to_pager].
                  Print the query results via PAGER.
print     (\p)    Print current command.
prompt    (\R)    Change your mysql prompt.
quit      (\q)    Quit mysql.
rehash    (\#)    Rebuild completion hash.
source    (\.)    Execute an SQL script file.
                  Takes a file name as an argument.
status    (\s)    Get status information from the server.
system    (\!)    Execute a system shell command.
tee       (\T)    Set outfile [to_outfile].
                  Append everything into given outfile.
use       (\u)    Use another database.
                  Takes database name as argument.

Each command has both a long and short form. The long form is not case sensitive; the short form is. The long form can be followed by an optional semicolon terminator, but the short form should not.

The edit, nopager, pager, and system commands work only in Unix.

The status command provides some information about the connection and the server you are using. If you are running in --safe-updates mode, status also prints the values for the @command{mysql} variables that affect your queries.

To log queries and their output, use the tee command. All the data displayed on the screen will be appended into a given file. This can be very useful for debugging purposes also. You can enable this feature on the command line with the --tee option, or interactively with the tee command. The tee file can be disabled interactively with the notee command. Executing tee again re-enables logging. Without a parameter, the previous file will be used. Note that tee flushes query results to the file after each statement, just before @command{mysql} prints its next prompt.

Browsing or searching query results in interactive mode by using Unix programs such as @command{less}, @command{more}, or any other similar program is now possible with the --pager option. If you specify no value for the option, @command{mysql} checks the value of the PAGER environment variable and sets the pager to that. Output paging can be enabled interactively with the pager command and disabled with nopager. The command takes an optional argument; if given, the paging program is set to that. With no argument, the pager is set to the pager that was set on the command line, or stdout if no pager was specified.

Output paging works only in Unix because it uses the popen() function, which doesn't exist on Windows. For Windows, the tee option can be used instead to save query output, although this is not as convenient as pager for browsing output in some situations.

A few tips about the pager command:

You can also combine the tee and pager functions. Have a @command{tee} file enabled and pager set to @command{less}, and you will be able to browse the results using the @command{less} program and still have everything appended into a file the same time. The difference between the Unix @command{tee} used with the pager command and the @command{mysql} built-in tee command is that the built-in tee works even if you don't have the Unix @command{tee} available. The built-in tee also logs everything that is printed on the screen, whereas the Unix @command{tee} used with pager doesn't log quite that much. Additionally, tee file logging can be turned on and off interactively from within @command{mysql}. This is useful when you want to log some queries to a file, but not others.

From MySQL 4.0.2 on, the default mysql> prompt can be reconfigured. The string for defining the prompt can contain the following special sequences:

Option Description
\v The server version
\d The current database
\h The server host
\p The current TCP/IP host
\u Your username
\U Your full user_name@host_name account name
\\ A literal `\' backslash character
\n A newline character
\t A tab character
\ A space (a space follows the backslash)
\_ A space
\R The current time, in 24-hour military time (0-23)
\r The current time, standard 12-hour time (1-12)
\m Minutes of the current time
\y The current year, two digits
\Y The current year, four digits
\D The full current date
\s Seconds of the current time
\w The current day of the week in three-letter format (Mon, Tue, ...)
\P am/pm
\o The current month in numeric format
\O The current month in three-letter format (Jan, Feb, ...)
\c A counter that increments for each statement you issue
\S Semicolon
\' Single quote
\" Double quote

`\' followed by any other letter just becomes that letter.

If you specify the prompt command with no argument, @command{mysql} resets the prompt to the default of mysql>.

You can set the prompt in several ways:

8.3.2 Executing SQL Statements from a Text File

The @command{mysql} client typically is used interactively, like this: 

shell> mysql db_name

However, it's also possible to put your SQL statements in a file and then tell @command{mysql} to read its input from that file. To do so, create a text file `text_file' that contains the statements you wish to execute. Then invoke @command{mysql} as shown here: 

shell> mysql db_name < text_file

You can also start your text file with a USE db_name statement. In this case, it is unnecessary to specify the database name on the command line: 

shell> mysql < text_file

If you are already running @command{mysql}, you can execute an SQL script file using the source or \. command: 

mysql> source filename
mysql> \. filename

Sometimes you may want your script to display progress information to the user; for this you can insert some lines like 

SELECT '<info_to_display>' AS ' ';

which will output <info_to_display>.

For more information about batch mode, see section 3.5 Using @command{mysql} in Batch Mode.

8.3.3 @command{mysql} Tips

This section describes some techniques that can help you use @command{mysql} more effectively.

8.3.3.1 Displaying Query Results Vertically

Some query results are much more readable when displayed vertically, instead of in the usual horizontal table format. Queries can be displayed vertically by terminating the query with \G instead of a semicolon. For example, longer text values that include newlines often are much easier to read with vertical output: 

mysql> SELECT * FROM mails WHERE LENGTH(txt) < 300 LIMIT 300,1\G
*************************** 1. row ***************************
  msg_nro: 3068
     date: 2000-03-01 23:29:50
time_zone: +0200
mail_from: Monty
    reply: [email protected]
  mail_to: "Thimble Smith" <[email protected]>
      sbj: UTF-8
      txt: >>>>> "Thimble" == Thimble Smith writes:

Thimble> Hi.  I think this is a good idea.  Is anyone familiar
Thimble> with UTF-8 or Unicode? Otherwise, I'll put this on my
Thimble> TODO list and see what happens.

Yes, please do that.

Regards,
Monty
     file: inbox-jani-1
     hash: 190402944
1 row in set (0.09 sec)

8.3.3.2 Using the --safe-updates Option

For beginners, a useful startup option is --safe-updates (or --i-am-a-dummy, which has the same effect). This option was introduced in MySQL 3.23.11. It is helpful for cases when you might have issued a DELETE FROM tbl_name statement but forgotten the WHERE clause. Normally, such a statement will delete all rows from the table. With --safe-updates, you can delete rows only by specifying the key values that identify them. This helps prevent accidents.

When you use the --safe-updates option, @command{mysql} issues the following statement when it connects to the MySQL server: 

SET SQL_SAFE_UPDATES=1,SQL_SELECT_LIMIT=1000, SQL_MAX_JOIN_SIZE=1000000;

See section 14.5.3.1 SET Syntax.

The SET statement has the following effects:

To specify limits other than 1,000 and 1,000,000, you can override the defaults by using --select_limit and --max_join_size options: 

shell> mysql --safe-updates --select_limit=500 --max_join_size=10000

8.3.3.3 Disabling @command{mysql} Auto-Reconnect

If the @command{mysql} client loses its connection to the server while sending a query, it will immediately and automatically try to reconnect once to the server and send the query again. However, even if @command{mysql} succeeds in reconnecting, your first connection has ended and all your previous session objects and settings are lost: temporary tables, the autocommit mode, and user and session variables. This behavior may be dangerous for you, as in the following example where the server was shut down and restarted without you knowing it: 

mysql> SET @a=1;
Query OK, 0 rows affected (0.05 sec)

mysql> INSERT INTO t VALUES(@a);
ERROR 2006: MySQL server has gone away
No connection. Trying to reconnect...
Connection id:    1
Current database: test

Query OK, 1 row affected (1.30 sec)

mysql> SELECT * FROM t;
+------+
| a    |
+------+
| NULL |
+------+
1 row in set (0.05 sec)

The @a user variable has been lost with the connection, and after the reconnection it is undefined. If it is important to have @command{mysql} terminate with an error if the connection has been lost, you can start the @command{mysql} client with the --skip-reconnect option.

8.4 @command{mysqladmin}, Administering a MySQL Server

@command{mysqladmin} is a client for performing administrative operations. You can use it to check the server's configuration and current status, create and drop databases, and more.

Invoke @command{mysqladmin} like this: 

shell> mysqladmin [options] command [command-option] command ...

@command{mysqladmin} supports the following commands:

create db_name
Create a new database named db_name.
drop db_name
Delete the database named db_name and all its tables.
extended-status
Display the server status variables and their values.
flush-hosts
Flush all information in the host cache.
flush-logs
Flush all logs.
flush-privileges
Reload the grant tables (same as reload).
flush-status
Clear status variables.
flush-tables
Flush all tables.
flush-threads
Flush the thread cache. (Added in MySQL 3.23.16.)
kill id,id,...
Kill server threads.
password new-password
Set a new password. This changes the password to new-password for the account that you use with @command{mysqladmin} for connecting to the server.
ping
Check whether the server is alive. The return status from @command{mysqladmin} is 0 if the server is running, 1 if it is not. Beginning with MySQL 4.0.22, the status is 0 even in case of an error such as Access denied, because that means the server is running but disallowed the connection, which is different from the server not running.
processlist
Show a list of active server threads. This is like the output of the SHOW PROCESSLIST statement. If the --verbose option is given, the output is like that of SHOW FULL PROCESSLIST.
reload
Reload the grant tables.
refresh
Flush all tables and close and open log files.
shutdown
Stop the server.
start-slave
Start replication on a slave server. (Added in MySQL 3.23.16.)
status
Display a short server status message.
stop-slave
Stop replication on a slave server. (Added in MySQL 3.23.16.)
variables
Display the server system variables and their values.
version
Display version information from the server.

All commands can be shortened to any unique prefix. For example: 

shell> mysqladmin proc stat
+----+-------+-----------+----+-------------+------+-------+------+
| Id | User  | Host      | db | Command     | Time | State | Info |
+----+-------+-----------+----+-------------+------+-------+------+
| 6  | monty | localhost |    | Processlist | 0    |       |      |
+----+-------+-----------+----+-------------+------+-------+------+
Uptime: 10077  Threads: 1  Questions: 9  Slow queries: 0
Opens: 6 Flush tables: 1  Open tables: 2
Memory in use: 1092K  Max memory used: 1116K

The @command{mysqladmin status} command result displays the following values:

Uptime
The number of seconds the MySQL server has been running.
Threads
The number of active threads (clients).
Questions
The number of questions (queries) from clients since the server was started.
Slow queries
The number of queries that have taken more than long_query_time seconds. See section 5.9.5 The Slow Query Log.
Opens
The number of tables the server has opened.
Flush tables
The number of flush ..., refresh, and reload commands the server has executed.
Open tables
The number of tables that currently are open.
Memory in use
The amount of memory allocated directly by @command{mysqld} code. This value is displayed only when MySQL has been compiled with --with-debug=full.
Maximum memory used
The maximum amount of memory allocated directly by @command{mysqld} code. This value is displayed only when MySQL has been compiled with --with-debug=full.

If you execute @command{mysqladmin shutdown} when connecting to a local server using a Unix socket file, @command{mysqladmin} waits until the server's process ID file has been removed, to ensure that the server has stopped properly.

@command{mysqladmin} supports the following options:

--help, -?
Display a help message and exit.
--character-sets-dir=path
The directory where character sets are installed. See section 5.8.1 The Character Set Used for Data and Sorting.
--compress, -C
Compress all information sent between the client and the server if both support compression.
--count=#, -c #
The number of iterations to make. This works only with --sleep (-i).
--debug[=debug_options], -# [debug_options]
Write a debugging log. The debug_options string often is 'd:t:o,file_name'. The default is 'd:t:o,/tmp/mysqladmin.trace'.
--force, -f
Don't ask for confirmation for the drop database command. With multiple commands, continue even if an error occurs.
--host=host_name, -h host_name
Connect to the MySQL server on the given host.
--password[=password], -p[password]
The password to use when connecting to the server. If you use the short option form (-p), you cannot have a space between the option and the password. If no password is given on the command line, you will be prompted for one.
--port=port_num, -P port_num
The TCP/IP port number to use for the connection.
--protocol={TCP | SOCKET | PIPE | MEMORY}
The connection protocol to use. New in MySQL 4.1.
--relative, -r
Show the difference between the current and previous values when used with -i. Currently, this option works only with the extended-status command.
--silent, -s
Exit silently if a connection to the server cannot be established.
--sleep=delay, -i delay
Execute commands again and again, sleeping for delay seconds in between.
--socket=path, -S path
The socket file to use for the connection.
--user=user_name, -u user_name
The MySQL username to use when connecting to the server.
--verbose, -v
Verbose mode. Print out more information on what the program does.
--version, -V
Display version information and exit.
--vertical, -E
Print output vertically. This is similar to --relative, but prints output vertically.
--wait[=#], -w[#]
If the connection cannot be established, wait and retry instead of aborting. If an option value is given, it indicates the number of times to retry. The default is one time.

You can also set the following variables by using --var_name=value options:

connect_timeout
The number of seconds before connection timeout. (Default value is 0.)
shutdown_timeout
The number of seconds to wait for shutdown. (Default value is 0.)

It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. However, this syntax is deprecated as of MySQL 4.0.

8.5 The @command{mysqlbinlog} Binary Log Utility

The binary log files that the server generates are written in binary format. To examine these files in text format, use the @command{mysqlbinlog} utility. It is available as of MySQL 3.23.14.

Invoke @command{mysqlbinlog} like this: 

shell> mysqlbinlog [options] log-file ...

For example, to display the contents of the binary log `binlog.000003', use this command: 

shell> mysqlbinlog binlog.0000003

The output includes all statements contained in `binlog.000003', together with other information such as the time each statement took, the thread ID of the client that issued it, the timestamp when it was issued, and so forth.

Normally, you use @command{mysqlbinlog} to read binary log files directly and apply them to the local MySQL server. It is also possible to read binary logs from a remote server by using the --read-from-remote-server option.

When you read remote binary logs, the connection parameter options can be given to indicate how to connect to the server, but they are ignored unless you also specify the --read-from-remote-server option. These options are --host, --password, --port, --protocol, --socket, and --user.

You can also use @command{mysqlbinlog} to read relay log files written by a slave server in a replication setup. Relay logs have the same format as binary log files.

The binary log is discussed further in section 5.9.4 The Binary Log.

@command{mysqlbinlog} supports the following options:

--help, -?
Display a help message and exit.
--database=db_name, -d db_name
List entries for just this database (local log only).
--force-read, -f
With this option, if @command{mysqlbinlog} reads a binary log event that it does not recognize, it prints a warning, ignores the event, and continues. Without this option, @command{mysqlbinlog} stops if it reads such an event.
--host=host_name, -h host_name
Get the binary log from the MySQL server on the given host.
--local-load=path, -l path
Prepare local temporary files for LOAD DATA INFILE in the specified directory.
--offset=N, -o N
Skip the first N entries.
--password[=password], -p[password]
The password to use when connecting to the server. If you use the short option form (-p), you cannot have a space between the option and the password. If no password is given on the command line, you will be prompted for one.
--port=port_num, -P port_num
The TCP/IP port number to use for connecting to a remote server.
--position=N, -j N
Deprecated, use --start-position instead (starting from MySQL 4.1.4).
--protocol={TCP | SOCKET | PIPE | MEMORY}
The connection protocol to use. New in MySQL 4.1.
--read-from-remote-server, -R
Read the binary log from a MySQL server. Any connection parameter options are ignored unless this option is given as well. These options are --host, --password, --port, --protocol, --socket, and --user.
--result-file=name, -r name
Direct output to the given file.
--short-form, -s
Display only the statements contained in the log, without any extra information.
--socket=path, -S path
The socket file to use for the connection.
--start-datetime=datetime
Start reading the binary log at the first event having a datetime equal or posterior to the datetime argument. Available as of MySQL 4.1.4.
--stop-datetime=datetime
Stop reading the binary log at the first event having a datetime equal or posterior to the datetime argument. Available as of MySQL 4.1.4. Useful for point-in-time recovery.
--start-position=N
Start reading the binary log at the first event having a position equal to the N argument. Available as of MySQL 4.1.4 (previously named --position).
--stop-position=N
Stop reading the binary log at the first event having a position equal or greater than the N argument. Available as of MySQL 4.1.4.
--to-last-log, -t
Do not stop at the end of the requested binary log of the MySQL server, but rather continue printing until the end of the last binary log. If you send the output to the same MySQL server, this may lead to an endless loop. This option requires --read-from-remote-server. Available as of MySQL 4.1.2.
--user=user_name, -u user_name
The MySQL username to use when connecting to a remote server.
--version, -V
Display version information and exit.

You can also set the following variable by using --var_name=value options:

open_files_limit
Specify the number of open file descriptors to reserve.

You can pipe the output of @command{mysqlbinlog} into a @command{mysql} client to execute the statements contained in the binary log. This is used to recover from a crash when you have an old backup (see section 5.7.1 Database Backups): 

shell> mysqlbinlog hostname-bin.000001 | mysql

Or: 

shell> mysqlbinlog hostname-bin.[0-9]* | mysql

You can also redirect the output of @command{mysqlbinlog} to a text file instead, if you need to modify the statement log first (for example, to remove statements that you don't want to execute for some reason). After editing the file, execute the statements that it contains by using it as input to the @command{mysql} program.

@command{mysqlbinlog} has the --position option, which prints only those statements with an offset in the binary log greater than or equal to a given position (the given position must match the start of one event). It also has options to stop or start when it sees an event of a given date and time. This enables you to perform point-in-time recovery using the --stop-datetime option (to be able to say, for example, "roll forward my databases to how they were today at 10:30 AM").

If you have more than one binary log to execute on the MySQL server, the safe method is to process them all using a single connection to the server. Here is an example that demonstrates what may be unsafe: 

shell> mysqlbinlog hostname-bin.000001 | mysql # DANGER!!
shell> mysqlbinlog hostname-bin.000002 | mysql # DANGER!!

Processing binary logs this way using different connections to the server will cause problems if the first log file contains a CREATE TEMPORARY TABLE statement and the second log contains a statement that uses the temporary table. When the first @command{mysql} process terminates, the server will drop the temporary table. When the second @command{mysql} process attempts to use the table, the server will report ``unknown table.''

To avoid problems like this, use a single connection to execute the contents of all binary logs that you want to process. Here is one way to do that: 

shell> mysqlbinlog hostname-bin.000001 hostname-bin.000002 | mysql

Another approach is to do this: 

shell> mysqlbinlog hostname-bin.000001 >  /tmp/statements.sql
shell> mysqlbinlog hostname-bin.000002 >> /tmp/statements.sql
shell> mysql -e "source /tmp/statements.sql"

In MySQL 3.23, the binary log did not contain the data to load for LOAD DATA INFILE statements. To execute such a statement from a binary log file, the original data file was needed. Starting from MySQL 4.0.14, the binary log does contain the data, so @command{mysqlbinlog} can produce output that reproduces the LOAD DATA INFILE operation without the original data file. @command{mysqlbinlog} copies the data to a temporary file and writes a LOAD DATA LOCAL INFILE statement that refers to the file. The default location of the directory where these files are written is system-specific. To specify a directory explicitly, use the --local-load option.

Because @command{mysqlbinlog} converts LOAD DATA INFILE statements to LOAD DATA LOCAL INFILE statements (that is, it adds LOCAL), both the client and the server that you use to process the statements must be configured to allow LOCAL capability. See section 5.4.4 Security Issues with LOAD DATA LOCAL.

Warning: The temporary files created for LOAD DATA LOCAL statements are not automatically deleted because they are needed until you actually execute those statements. You should delete the temporary files yourself after you no longer need the statement log. The files can be found in the temporary file directory and have names like `original_file_name-#-#'.

In the future, we will fix this problem by allowing @command{mysqlbinlog} to connect directly to a @command{mysqld} server. Then it will be possible to safely remove the log files automatically as soon as the LOAD DATA INFILE statements have been executed.

Before MySQL 4.1, @command{mysqlbinlog} could not prepare output suitable for @command{mysql} if the binary log contained intertwined statements originating from different clients that used temporary tables of the same name. This is fixed in MySQL 4.1.

8.6 @command{mysqlcc}, the MySQL Control Center

@command{mysqlcc}, the MySQL Control Center, is a platform-independent client that provides a graphical user interface (GUI) to the MySQL database server. It supports interactive use, including syntax highlighting and tab completion. It provides database and table management, and allows server administration.

@command{mysqlcc} is not included with MySQL distributions, but can be downloaded separately at http://dev.mysql.com/downloads/. Currently, @command{mysqlcc} runs on Windows and Linux platforms.

Invoke @command{mysqlcc} by double-clicking its icon in a graphical environment. From the command line, invoke it like this: 

shell> mysqlcc [options]

@command{mysqlcc} supports the following options:

--help, -?
Display a help message and exit.
--blocking_queries, -b
Use blocking queries.
--compress, -C
Compress all information sent between the client and the server if both support compression.
--connection_name=name, -c name
This option is a synonym for --server.
--database=db_name, -d db_name
The database to use. This is useful mainly in an option file.
--history_size=#, -H #
The history size for the query window.
--host=host_name, -h host_name
Connect to the MySQL server on the given host.
--local-infile[={0|1}]
Enable or disable LOCAL capability for LOAD DATA INFILE. With no value, the option enables LOCAL. It may be given as --local-infile=0 or --local-infile=1 to explicitly disable or enable LOCAL. Enabling LOCAL has no effect if the server does not also support it.
--password[=password], -p[password]
The password to use when connecting to the server. If you use the short option form (-p), you cannot have a space between the option and the password. If no password is given on the command line, you will be prompted for one.
--plugins_path=name, -g name
The path to the directory where MySQL Control Center plugins are located.
--port=port_num, -P port_num
The TCP/IP port number to use for the connection.
--query, -q
Open a query window on startup.
--register, -r
Open the Register Server dialog on startup.
--server=name, -s name
The MySQL Control Center connection name.
--socket=path, -S path
The socket file to use for the connection.
--syntax, -y
Enable syntax highlighting and completion.
--syntax_file=name, -Y name
The syntax file for completion.
--translations_path=name, -T name
The path to the directory where MySQL Control Center translations are located.
--user=user_name, -u user_name
The MySQL username to use when connecting to the server.
--version, -V
Display version information and exit.

You can also set the following variables by using --var_name=value options:

connect_timeout
The number of seconds before connection timeout. (Default value is 0.)
max_allowed_packet
The maximum packet length to send to or receive from the server. (Default value is 16MB.)
max_join_size
The automatic limit for rows in a join. (Default value is 1,000,000.)
net_buffer_length
The buffer size for TCP/IP and socket communication. (Default value is 16KB.)
select_limit
The automatic limit for SELECT statements. (Default value is 1,000.)

It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. However, this syntax is deprecated as of MySQL 4.0.

8.7 The @command{mysqlcheck} Table Maintenance and Repair Program

The @command{mysqlcheck} client checks and repairs MyISAM tables. It can also optimize and analyze tables. @command{mysqlcheck} is available as of MySQL 3.23.38.

@command{mysqlcheck} is similar in function to @command{myisamchk}, but works differently. The main operational difference is that @command{mysqlcheck} must be used when the @command{mysqld} server is running, whereas @command{myisamchk} should be used when it is not. The benefit of using @command{mysqlcheck} is that you do not have to stop the server to check or repair your tables.

@command{mysqlcheck} uses the SQL statements CHECK TABLE, @code{REPAIR TABLE}, ANALYZE TABLE, and OPTIMIZE TABLE in a convenient way for the user. It determines which statements to use for the operation you want to perform, then sends the statements to the server to be executed.

There are three general ways to invoke @command{mysqlcheck}: 

shell> mysqlcheck [options] db_name [tables]
shell> mysqlcheck [options] --databases DB1 [DB2 DB3...]
shell> mysqlcheck [options] --all-databases

If you don't name any tables or use the --databases or --all-databases option, entire databases will be checked.

@command{mysqlcheck} has a special feature compared to the other clients. The default behavior of checking tables (--check) can be changed by renaming the binary. If you want to have a tool that repairs tables by default, you should just make a copy of @command{mysqlcheck} named @command{mysqlrepair}, or make a symbolic link to @command{mysqlcheck} named @command{mysqlrepair}. If you invoke mysqlrepair, it will repair tables by command.

The following names can be used to change @command{mysqlcheck} default behavior:

@command{mysqlrepair} The default option will be --repair
@command{mysqlanalyze} The default option will be --analyze
@command{mysqloptimize} The default option will be --optimize

@command{mysqlcheck} supports the following options:

--help, -?
Display a help message and exit.
--all-databases, -A
Check all tables in all databases. This is the same as using the --databases option and naming all the databases on the command line.
--all-in-1, -1
Instead of issuing a statement for each table, execute a single statement for each database that names all the tables from that database to be processed.
--analyze, -a
Analyze the tables.
--auto-repair
If a checked table is corrupted, automatically fix it. Any necessary repairs are done after all tables have been checked.
--character-sets-dir=path
The directory where character sets are installed. See section 5.8.1 The Character Set Used for Data and Sorting.
--check, -c
Check the tables for errors.
--check-only-changed, -C
Check only tables that have changed since the last check or that haven't been closed properly.
--compress
Compress all information sent between the client and the server if both support compression.
--databases, -B
Process all tables in the named databases. With this option, all name arguments are regarded as database names, not as table names.
--debug[=debug_options], -# [debug_options]
Write a debugging log. The debug_options string often is 'd:t:o,file_name'.
--default-character-set=charset
Use charset as the default character set. See section 5.8.1 The Character Set Used for Data and Sorting.
--extended, -e
If you are using this option to check tables, it ensures that they are 100% consistent but will take a long time. If you are using this option to repair tables, it runs an extended repair that may not only take a long time to execute, but may produce a lot of garbage rows also!
--fast, -F
Check only tables that haven't been closed properly.
--force, -f
Continue even if an SQL error occurs.
--host=host_name, -h host_name
Connect to the MySQL server on the given host.
--medium-check, -m
Do a check that is faster than an --extended operation. This finds only 99.99% of all errors, which should be good enough in most cases.
--optimize, -o
Optimize the tables.
--password[=password], -p[password]
The password to use when connecting to the server. If you use the short option form (-p), you cannot have a space between the option and the password. If no password is given on the command line, you will be prompted for one.
--port=port_num, -P port_num
The TCP/IP port number to use for the connection.
--protocol={TCP | SOCKET | PIPE | MEMORY}
The connection protocol to use. New in MySQL 4.1.
--quick, -q
If you are using this option to check tables, it prevents the check from scanning the rows to check for incorrect links. This is the fastest check method. If you are using this option to repair tables, it tries to repair only the index tree. This is the fastest repair method.
--repair, -r
Do a repair that can fix almost anything except unique keys that aren't unique.
--silent, -s
Silent mode. Print only error messages.
--socket=path, -S path
The socket file to use for the connection.
--tables
Overrides the --databases or -B option. All arguments following the option are regarded as table names.
--user=user_name, -u user_name
The MySQL username to use when connecting to the server.
--verbose, -v
Verbose mode. Print information about the various stages of program operation.
--version, -V
Display version information and exit.

8.8 The @command{mysqldump} Database Backup Program

The @command{mysqldump} client can be used to dump a database or a collection of databases for backup or for transferring the data to another SQL server (not necessarily a MySQL server). The dump will contain SQL statements to create the table and/or populate the table.

If you are doing a backup on the server, you should consider using the @command{mysqlhotcopy} instead. @xref{mysqlhotcopy, , @command{mysqlhotcopy}}.

There are three general ways to invoke @command{mysqldump}: 

shell> mysqldump [options] db_name [tables]
shell> mysqldump [options] --databases DB1 [DB2 DB3...]
shell> mysqldump [options] --all-databases

If you don't name any tables or use the --databases or --all-databases option, entire databases will be dumped.

To get a list of the options your version of @command{mysqldump} supports, execute @command{mysqldump --help}.

If you run @command{mysqldump} without the --quick or --opt option, @command{mysqldump} will load the whole result set into memory before dumping the result. This will probably be a problem if you are dumping a big database. As of MySQL 4.1, --opt is on by default, but can be disabled with --skip-opt.

If you are using a recent copy of the @command{mysqldump} program and you are going to generate a dump that will be reloaded into a very old MySQL server, you should not use the --opt or -e options.

Out-of-range numeric values such as -inf and inf, as well as NaN (not-a-number) values are dumped by @command{mysqldump} as NULL. You can see this using the following sample table: 

mysql> CREATE TABLE t (f DOUBLE);
mysql> INSERT INTO t VALUES(1e+111111111111111111111);
mysql> INSERT INTO t VALUES(-1e111111111111111111111);
mysql> SELECT f FROM t;
+------+
| f    |
+------+
|  inf |
| -inf |
+------+

For this table, @command{mysqldump} produces the following data output: 

--
-- Dumping data for table `t`
--

INSERT INTO t VALUES (NULL);
INSERT INTO t VALUES (NULL);

The significance of this behavior is that if you dump and restore the table, the new table has contents that differ from the original contents. Note that since MySQL 4.1.2 you cannot insert inf in the table, so this @command{mysqldump} behavior is only relevant when you deal with old servers.

@command{mysqldump} supports the following options:

--help, -?
Display a help message and exit.
--add-drop-table
Add a DROP TABLE statement before each CREATE TABLE statement.
--add-locks
Surround each table dump with LOCK TABLES and UNLOCK TABLES statements. This results in faster inserts when the dump file is reloaded. See section 7.2.14 Speed of INSERT Statements.
--all-databases, -A
Dump all tables in all databases. This is the same as using the --databases option and naming all the databases on the command line.
--allow-keywords
Allow creation of column names that are keywords. This works by prefixing each column name with the table name.
--comments[={0|1}]
If set to 0, suppresses additional information in the dump file such as program version, server version, and host. --skip-comments has the same effect as --comments=0. The default value is 1 to not suppress the extra information. New in MySQL 4.0.17.
--compatible=name
Produce output that is compatible with other database systems or with older MySQL servers. The value of name can be ansi, mysql323, mysql40, postgresql, oracle, mssql, db2, maxdb, no_key_options, no_table_options, or no_field_options. To use several values, separate them by commas. These values have the same meaning as the corresponding options for setting the server SQL mode. See section 5.2.2 The Server SQL Mode. This option requires a server version of 4.1.0 or higher. With older servers, it does nothing.
--complete-insert, -c
Use complete INSERT statements that include column names.
--compress, -C
Compress all information sent between the client and the server if both support compression.
--create-options
Include all MySQL-specific table options in the CREATE TABLE statements. Before MySQL 4.1.2, use --all instead.
--databases, -B
To dump several databases. Note the difference in usage. In this case, no tables are given. All name arguments on the command line are regarded as database names. A USE db_name statement is included in the output before each new database.
--debug[=debug_options], -# [debug_options]
Write a debugging log. The debug_options string often is 'd:t:o,file_name'.
--default-character-set=charset
Use charset as the default character set. See section 5.8.1 The Character Set Used for Data and Sorting. If not specified, @command{mysqldump} from MySQL 4.1.2 or later uses utf8; earlier versions use latin1.
--delayed
Insert rows using INSERT DELAYED statements.
--delete-master-logs
On a master replication server, delete the binary logs after performing the dump operation. This option automatically enables --first-slave. It was added in MySQL 3.23.57 (for MySQL 3.23) and MySQL 4.0.13 (for MySQL 4.0).
--disable-keys, -K
For each table, surround the INSERT statements with /*!40000 ALTER TABLE tbl_name DISABLE KEYS */; and /*!40000 ALTER TABLE tbl_name ENABLE KEYS */; statements. This makes loading the dump file into a MySQL 4.0 server faster because the indexes are created after all rows are inserted. This option is effective only for MyISAM tables.
--extended-insert, -e
Use multiple-row INSERT syntax that include several VALUES lists. This results in a smaller dump file and speeds up inserts when the file is reloaded.
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--lines-terminated-by=...
These options are used with the -T option and have the same meaning as the corresponding clauses for LOAD DATA INFILE. See section 14.1.5 LOAD DATA INFILE Syntax.
--first-slave, -x
Locks all tables across all databases.
--flush-logs, -F
Flush the MySQL server log files before starting the dump. Note that if you use this option in combination with the --all-databases (or -A) option, the logs are flushed for each database dumped.
--force, -f
Continue even if an SQL error occurs during a table dump.
--host=host_name, -h host_name
Dump data from the MySQL server on the given host. The default host is localhost.
--lock-tables, -l
Lock all tables before starting the dump. The tables are locked with READ LOCAL to allow concurrent inserts in the case of MyISAM tables. Please note that when dumping multiple databases, --lock-tables locks tables for each database separately. So, using this option will not guarantee that the tables in the dump file will be logically consistent between databases. Tables in different databases may be dumped in completely different states.
--master-data
This option is like --first-slave, but also produces CHANGE MASTER TO statements that will make your slave server start from the correct position in the master's binary logs if you use this SQL dump of the master to set up the slave.
--no-create-db, -n
This option suppresses the CREATE DATABASE /*!32312 IF NOT EXISTS*/ db_name statements that are otherwise included in the output if the --databases or --all-databases option is given.
--no-create-info, -t
Don't write CREATE TABLE statements that re-create each dumped table.
--no-data, -d
Don't write any row information for the table. This is very useful if you just want to get a dump of the structure for a table.
--opt
This option is shorthand; it is the same as specifying --add-drop-table --add-locks --create-options --disable-keys --extended-insert --lock-tables --quick --set-charset. It should give you a fast dump operation and produce a dump file that can be reloaded into a MySQL server quickly. As of MySQL 4.1, --opt is on by default, but can be disabled with --skip-opt. To disable only certain of the options enabled by --opt, use their --skip forms; for example, --skip-add-drop-table or --skip-quick.
--password[=password], -p[password]
The password to use when connecting to the server. If you use the short option form (-p), you cannot have a space between the option and the password. If no password is given on the command line, you will be prompted for one.
--port=port_num, -P port_num
The TCP/IP port number to use for the connection.
--protocol={TCP | SOCKET | PIPE | MEMORY}
The connection protocol to use. New in MySQL 4.1.
--quick, -q
This option is useful for dumping large tables. It forces @command{mysqldump} to retrieve rows for a table from the server a row at a time rather than retrieving the entire row set and buffering it in memory before writing it out.
--quote-names, -Q
Quote database, table, and column names within ``' characters. If the server SQL mode includes the ANSI_QUOTES option, names are quoted within `"' characters. As of MySQL 4.1.1, --quote-names is on by default, but can be disabled with --skip-quote-names.
--result-file=file, -r file
Direct output to a given file. This option should be used on Windows, because it prevents newline `\n' characters from being converted to `\r\n' carriage return/newline sequences.
--set-charset
Add SET NAMES default_character_set to the output. This option is enabled by default. To suppress the SET NAMES statement, use --skip-set-charset. This option was added in MySQL 4.1.2.
--single-transaction
This option issues a BEGIN SQL statement before dumping data from the server. It is mostly useful with InnoDB tables and READ COMMITTED transaction isolation level, because in this mode it will dump the consistent state of the database at the time then BEGIN was issued without blocking any applications. When using this option, you should keep in mind that only transactional tables will be dumped in a consistent state. For example, any MyISAM or HEAP tables dumped while using this option may still change state. The --single-transaction option was added in MySQL 4.0.2. This option is mutually exclusive with the --lock-tables option, because LOCK TABLES causes any pending transactions to be committed implicitly.
--socket=path, -S path
The socket file to use when connecting to localhost (which is the default host).
--skip-comments
See the description for the --comments option.
--tab=path, -T path
Produces tab-separated data files. For each dumped table, @command{mysqldump} creates a `tbl_name.sql' file that contains the CREATE TABLE statement that creates the table, and a `tbl_name.txt' file that contains its data. The option value is the directory in which to write the files. By default, the `.txt' data files are formatted using tab characters between column values and a newline at the end of each line. The format can be specified explicitly using the --fields-xxx and --lines--xxx options. Note: This option should be used only when @command{mysqldump} is run on the same machine as the @command{mysqld} server. You must use a MySQL account that has the FILE privilege, and the server must have permission to write files in the directory you specify.
--tables
Overrides the --databases or -B option. All arguments following the option are regarded as table names.
--user=user_name, -u user_name
The MySQL username to use when connecting to the server.
--verbose, -v
Verbose mode. Print out more information on what the program does.
--version, -V
Display version information and exit.
--where='where-condition', -w 'where-condition'
Dump only records selected by the given WHERE condition. Note that quotes around the condition are mandatory if it contains spaces or characters that are special to your command interpreter. Examples: 
"--where=user='jimf'"
"-wuserid>1"
"-wuserid<1"
--xml, -X
Write dump output as well-formed XML.

You can also set the following variables by using --var_name=value options:

max_allowed_packet
The maximum size of the buffer for client/server communication. The value of the variable can be up to 16MB before MySQL 4.0, and up to 1GB from MySQL 4.0 on. When creating multiple-row-insert statements (as with option --extended-insert or --opt), @command{mysqldump} will create rows up to max_allowed_packet length. If you increase this variable, you should also ensure that the max_allowed_packet variable in the MySQL server is at least this large.
net_buffer_length
The initial size of the buffer for client/server communication.

It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. However, this syntax is deprecated as of MySQL 4.0.

The most common use of @command{mysqldump} is probably for making a backup of entire databases. 

shell> mysqldump --opt db_name > backup-file.sql

You can read the dump file back into the server with: 

shell> mysql db_name < backup-file.sql

Or: 

shell> mysql -e "source /path-to-backup/backup-file.sql" db_name

@command{mysqldump} is also very useful for populating databases by copying data from one MySQL server to another: 

shell> mysqldump --opt db_name | mysql --host=remote_host -C db_name

It is possible to dump several databases with one command: 

shell> mysqldump --databases db_name1 [db_name2 ...] > my_databases.sql

If you want to dump all databases, use the --all-databases option: 

shell> mysqldump --all-databases > all_databases.sql

For more information on making backups, see section 5.7.1 Database Backups.

8.9 The @command{mysqlhotcopy} Database Backup Program

@command{mysqlhotcopy} is a Perl script that was originally written and contributed by Tim Bunce. It uses LOCK TABLES, FLUSH TABLES, and cp or scp to quickly make a backup of a database. It's the fastest way to make a backup of the database or single tables, but it can be run only on the same machine where the database directories are located. @command{mysqlhotcopy} works only for backing up MyISAM and ISAM tables. It runs on Unix, and as of MySQL 4.0.18 also on NetWare. 

shell> mysqlhotcopy db_name [/path/to/new_directory]

shell> mysqlhotcopy db_name_1 ... db_name_n /path/to/new_directory

shell> mysqlhotcopy db_name./regex/

@command{mysqlhotcopy} supports the following options:

--help, -?
Display a help message and exit.
--allowold
Don't abort if target already exists (rename it by adding an _old suffix).
--checkpoint=db_name.tbl_name
Insert checkpoint entries into the specified db_name.tbl_name.
--debug
Enable debug output.
--dryrun, -n
Report actions without doing them.
--flushlog
Flush logs after all tables are locked.
--keepold
Don't delete previous (now renamed) target when done.
--method=#
Method for copy (cp or scp).
--noindices
Don't include full index files in the backup. This makes the backup smaller and faster. The indexes can be reconstructed later with @command{myisamchk -rq} for MyISAM tables or @command{isamchk -rq} for ISAM tables.
--password=password, -ppassword
The password to use when connecting to the server. Note that the password value is not optional for this option, unlike for other MySQL programs.
--port=port_num, -P port_num
The TCP/IP port number to use when connecting to the local server.
--quiet, -q
Be silent except for errors.
--regexp=expr
Copy all databases with names matching the given regular expression.
--socket=path, -S path
The Unix socket file to use for the connection.
--suffix=str
The suffix for names of copied databases.
--tmpdir=path
The temporary directory (instead of `/tmp').
--user=user_name, -u user_name
The MySQL username to use when connecting to the server.

@command{mysqlhotcopy} reads the [client] and [mysqlhotcopy] option groups from option files.

To execute @command{mysqlhotcopy}, you must have access to the files for the tables that you are backing up, the SELECT privilege for those tables, and the RELOAD privilege (to be able to execute FLUSH TABLES).

Use perldoc for additional @command{mysqlhotcopy} documentation: 

shell> perldoc mysqlhotcopy

8.10 The @command{mysqlimport} Data Import Program

The @command{mysqlimport} client provides a command-line interface to the LOAD DATA INFILE SQL statement. Most options to @command{mysqlimport} correspond directly to clauses of LOAD DATA INFILE. See section 14.1.5 LOAD DATA INFILE Syntax.

Invoke @command{mysqlimport} like this: 

shell> mysqlimport [options] db_name textfile1 [textfile2 ...]

For each text file named on the command line, @command{mysqlimport} strips any extension from the filename and uses the result to determine the name of the table into which to import the file's contents. For example, files named `patient.txt', `patient.text', and `patient' all would be imported into a table named patient.

@command{mysqlimport} supports the following options:

--help, -?
Display a help message and exit.
--columns=column_list, -c column_list
This option takes a comma-separated list of column names as its value. The order of the column names indicates how to match up data file columns with table columns.
--compress, -C
Compress all information sent between the client and the server if both support compression.
--debug[=debug_options], -# [debug_options]
Write a debugging log. The debug_options string often is 'd:t:o,file_name'.
--delete, -D
Empty the table before importing the text file.
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--lines-terminated-by=...
These options have the same meaning as the corresponding clauses for LOAD DATA INFILE. See section 14.1.5 LOAD DATA INFILE Syntax.
--force, -f
Ignore errors. For example, if a table for a text file doesn't exist, continue processing any remaining files. Without --force, @command{mysqlimport} exits if a table doesn't exist.
--host=host_name, -h host_name
Import data to the MySQL server on the given host. The default host is localhost.
--ignore, -i
See the description for the --replace option.
--ignore-lines=n
Ignore the first n lines of the data file.
--local, -L
Read input files locally from the client host.
--lock-tables, -l
Lock all tables for writing before processing any text files. This ensures that all tables are synchronized on the server.
--password[=password], -p[password]
The password to use when connecting to the server. If you use the short option form (-p), you cannot have a space between the option and the password. If no password is given on the command line, you will be prompted for one.
--port=port_num, -P port_num
The TCP/IP port number to use for the connection.
--protocol={TCP | SOCKET | PIPE | MEMORY}
The connection protocol to use. New in MySQL 4.1.
--replace, -r
The --replace and --ignore options control handling of input records that duplicate existing records on unique key values. If you specify --replace, new rows replace existing rows that have the same unique key value. If you specify --ignore, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.
--silent, -s
Silent mode. Produce output only when errors occur.
--socket=path, -S path
The socket file to use when connecting to localhost (which is the default host).
--user=user_name, -u user_name
The MySQL username to use when connecting to the server.
--verbose, -v
Verbose mode. Print out more information what the program does.
--version, -V
Display version information and exit.

Here is a sample session that demonstrates use of @command{mysqlimport}: 

shell> mysql -e 'CREATE TABLE imptest(id INT, n VARCHAR(30))' test
shell> ed
a
100     Max Sydow
101     Count Dracula
.
w imptest.txt
32
q
shell> od -c imptest.txt
0000000   1   0   0  \t   M   a   x       S   y   d   o   w  \n   1   0
0000020   1  \t   C   o   u   n   t       D   r   a   c   u   l   a  \n
0000040
shell> mysqlimport --local test imptest.txt
test.imptest: Records: 2  Deleted: 0  Skipped: 0  Warnings: 0
shell> mysql -e 'SELECT * FROM imptest' test
+------+---------------+
| id   | n             |
+------+---------------+
|  100 | Max Sydow     |
|  101 | Count Dracula |
+------+---------------+

8.11 @command{mysqlshow}, Showing Databases, Tables, and Columns

The @command{mysqlshow} client can be used to quickly look at which databases exist, their tables, and a table's columns or indexes.

@command{mysqlshow} provides a command-line interface to several SQL SHOW statements. The same information can be obtained by using those statements directly. For example, you can issue them from the @command{mysql} client program. See section 14.5.3 SET and SHOW Syntax.

Invoke @command{mysqlshow} like this: 

shell> mysqlshow [options] [db_name [tbl_name [col_name]]]

Note that in newer MySQL versions, you see only those database, tables, or columns for which you have some privileges.

If the last argument contains shell or SQL wildcard characters (`*', `?', `%', or `_'), only those names that are matched by the wildcard are shown. If a database name contains any underscores, those should be escaped with a backslash (some Unix shells will require two) in order to get a list of the proper tables or columns. `*' and `?' characters are converted into SQL `%' and `_' wildcard characters. This might cause some confusion when you try to display the columns for a table with a `_' in the name, because in this case @command{mysqlshow} shows you only the table names that match the pattern. This is easily fixed by adding an extra `%' last on the command line as a separate argument.

@command{mysqlshow} supports the following options:

--help, -?
Display a help message and exit.
--character-sets-dir=path
The directory where character sets are installed. See section 5.8.1 The Character Set Used for Data and Sorting.
--compress, -C
Compress all information sent between the client and the server if both support compression.
--debug[=debug_options], -# [debug_options]
Write a debugging log. The debug_options string often is 'd:t:o,file_name'.
--default-character-set=charset
Use charset as the default character set. See section 5.8.1 The Character Set Used for Data and Sorting.
--host=host_name, -h host_name
Connect to the MySQL server on the given host.
--keys, -k
Show table indexes.
--password[=password], -p[password]
The password to use when connecting to the server. If you use the short option form (-p), you cannot have a space between the option and the password. If no password is given on the command line, you will be prompted for one.
--port=port_num, -P port_num
The TCP/IP port number to use for the connection.
--protocol={TCP | SOCKET | PIPE | MEMORY}
The connection protocol to use. New in MySQL 4.1.
--socket=path, -S path
The socket file to use when connecting to localhost (which is the default host).
--status, -i
Display extra information about each table.
--user=user_name, -u user_name
The MySQL username to use when connecting to the server.
--verbose, -v
Verbose mode. Print out more information what the program does. This option can be used multiple times to increase the amount of information.
--version, -V
Display version information and exit.

8.12 @command{perror}, Explaining Error Codes

For most system errors, MySQL displays, in addition to an internal text message, the system error code in one of the following styles: 

message ... (errno: #)
message ... (Errcode: #)

You can find out what the error code means by either examining the documentation for your system or by using the @command{perror} utility.

@command{perror} prints a description for a system error code or for a storage engine (table handler) error code.

Invoke @command{perror} like this: 

shell> perror [options] errorcode ...

Example: 

shell> perror 13 64
Error code  13:  Permission denied
Error code  64:  Machine is not on the network

Note that the meaning of system error messages may be dependent on your operating system. A given error code may mean different things on different operating systems.

8.13 The @command{replace} String-Replacement Utility

The @command{replace} utility program changes strings in place in files or on the standard input. It uses a finite state machine to match longer strings first. It can be used to swap strings. For example, the following command swaps a and b in the given files, `file1' and `file2': 

shell> replace a b b a -- file1 file2 ...

Use the -- option to indicate where the string-replacement list ends and the filenames begin.

Any file named on the command line is modified in place, so you may want to make a copy of the original before converting it.

If no files are named on the command line, @command{replace} reads the standard input and writes to the standard output. In this case, no -- option is needed.

The @command{replace} program is used by @command{msql2mysql}. @xref{msql2mysql, , @command{msql2mysql}}.

@command{replace} supports the following options:

-?, -I
Display a help message and exit.
-# debug_options
Write a debugging log. The debug_options string often is 'd:t:o,file_name'.
-s
Silent mode. Print out less information what the program does.
-v
Verbose mode. Print out more information what the program does.
-V
Display version information and exit.

9 MySQL Language Reference

MySQL has a very complete, but intuitive and easy to learn SQL interface. The next several chapters of the manual comprise a language reference. They describe the various commands, types, and functions you will need to know in order to use MySQL efficiently and effectively. These chapters also serve as a reference to all functionality included in MySQL. The material they contain is grouped by topic:

In order to use this material most effectively, you may find it useful to refer to the various indexes.

10 Language Structure

This chapter discusses the rules for writing the following elements of SQL statements when using MySQL:

10.1 Literal Values

This section describes how to write literal values in MySQL. These include strings, numbers, hexadecimal values, boolean values, and NULL. The section also covers the various nuances and ``gotchas'' that you may run into when dealing with these basic types in MySQL.

10.1.1 Strings

A string is a sequence of characters, surrounded by either single quote (`'') or double quote (`"') characters. Examples: 

'a string'
"another string"

If the server SQL mode has ANSI_QUOTES enabled, string literals can be quoted only with single quotes. A string quoted with double quotes will be interpreted as an identifier.

As of MySQL 4.1.1, string literals may have an optional character set introducer and COLLATE clause: 

[_charset_name]'string' [COLLATE collation_name]

Examples: 

SELECT _latin1'string';
SELECT _latin1'string' COLLATE latin1_danish_ci;

For more information about these forms of string syntax, see section 11.3.7 Character String Literal Character Set and Collation.

Within a string, certain sequences have special meaning. Each of these sequences begins with a backslash (`\'), known as the escape character. MySQL recognizes the following escape sequences:

\0 @tab An ASCII 0 (NUL) character.
\' @tab A single quote (`'') character.
\" @tab A double quote (`"') character.
\b @tab A backspace character.
\n @tab A newline (linefeed) character.
\r @tab A carriage return character.
\t @tab A tab character.
\Z @tab ASCII 26 (Control-Z). This character can be encoded as `\Z' to allow you to work around the problem that ASCII 26 stands for END-OF-FILE on Windows. (ASCII 26 will cause problems if you try to use mysql db_name < file_name.)
\\ @tab A backslash (`\') character.
\% @tab A `%' character. See note following table.
\_ @tab A `_' character. See note following table.

These sequences are case sensitive. For example, `\b' is interpreted as a backspace, but `\B' is interpreted as `B'.

The `\%' and `\_' sequences are used to search for literal instances of `%' and `_' in pattern-matching contexts where they would otherwise be interpreted as wildcard characters. See section 13.3.1 String Comparison Functions. Note that if you use `\%' or `\_' in other contexts, they return the strings `\%' and `\_' and not `%' and `_'.

In all other escape sequences, backslash is ignored. That is, the escaped character is interpreted as if it was not escaped.

There are several ways to include quotes within a string:

The following SELECT statements demonstrate how quoting and escaping work: 

mysql> SELECT 'hello', '"hello"', '""hello""', 'hel''lo', '\'hello';
+-------+---------+-----------+--------+--------+
| hello | "hello" | ""hello"" | hel'lo | 'hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT "hello", "'hello'", "''hello''", "hel""lo", "\"hello";
+-------+---------+-----------+--------+--------+
| hello | 'hello' | ''hello'' | hel"lo | "hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT 'This\nIs\nFour\nLines';
+--------------------+
| This
Is
Four
Lines |
+--------------------+

mysql> SELECT 'disappearing\ backslash';
+------------------------+
| disappearing backslash |
+------------------------+

If you want to insert binary data into a string column (such as a BLOB), the following characters must be represented by escape sequences:

NUL @tab NUL byte (ASCII 0). Represent this character by `\0' (a backslash followed by an ASCII `0' character).
\ @tab Backslash (ASCII 92). Represent this character by `\\'.
' @tab Single quote (ASCII 39). Represent this character by `\''.
" @tab Double quote (ASCII 34). Represent this character by `\"'.

When writing application programs, any string that might contain any of these special characters must be properly escaped before the string is used as a data value in an SQL statement that is sent to the MySQL server. You can do this in two ways:

10.1.2 Numbers

Integers are represented as a sequence of digits. Floats use `.' as a decimal separator. Either type of number may be preceded by `-' to indicate a negative value.

Examples of valid integers: 

1221
0
-32

Examples of valid floating-point numbers: 

294.42
-32032.6809e+10
148.00

An integer may be used in a floating-point context; it is interpreted as the equivalent floating-point number.

10.1.3 Hexadecimal Values

MySQL supports hexadecimal values. In numeric contexts, these act like integers (64-bit precision). In string contexts, these act like binary strings, where each pair of hex digits is converted to a character: 

mysql> SELECT x'4D7953514C';
        -> 'MySQL'
mysql> SELECT 0xa+0;
        -> 10
mysql> SELECT 0x5061756c;
        -> 'Paul'

In MySQL 4.1 (and in MySQL 4.0 when using the --new option), the default type of a hexadecimal value is a string. If you want to ensure that the value is treated as a number, you can use CAST(... AS UNSIGNED): 

mysql> SELECT 0x41, CAST(0x41 AS UNSIGNED);
        -> 'A', 65

The 0x syntax is based on ODBC. Hexadecimal strings are often used by ODBC to supply values for BLOB columns. The x'hexstring' syntax is new in 4.0 and is based on standard SQL.

Beginning with MySQL 4.0.1, you can convert a string or a number to a string in hexadecimal format with the HEX() function: 

mysql> SELECT HEX('cat');
        -> '636174'
mysql> SELECT 0x636174;
        -> 'cat'

10.1.4 Boolean Values

Beginning with MySQL 4.1, the constant TRUE evaluates to 1 and the constant FALSE evaluates to 0. The constant names can be written in any lettercase. 

mysql> SELECT TRUE, true, FALSE, false;
        -> 1, 1, 0, 0

10.1.5 NULL Values

The NULL value means ``no data.'' NULL can be written in any lettercase.

Be aware that the NULL value is different from values such as 0 for numeric types or the empty string for string types. See section A.5.3 Problems with NULL Values.

For text file import or export operations performed with LOAD DATA INFILE or SELECT ... INTO OUTFILE, NULL is represented by the \N sequence. See section 14.1.5 LOAD DATA INFILE Syntax.

10.2 Database, Table, Index, Column, and Alias Names

Database, table, index, column, and alias names are identifiers. This section describes the allowable syntax for identifiers in MySQL.

The following table describes the maximum length and allowable characters for each type of identifier.

Identifier Maximum Length (bytes) Allowed Characters
Database 64 Any character that is allowed in a directory name, except `/', `\', or `.'
Table 64 Any character that is allowed in a filename, except `/', `\', or `.'
Column 64 All characters
Index 64 All characters
Alias 255 All characters

In addition to the restrictions noted in the table, no identifier can contain ASCII 0 or a byte with a value of 255. Database, table, and column names should not end with space characters. Before MySQL 4.1, identifier quote characters should not be used in identifiers.

Beginning with MySQL 4.1, identifiers are stored using Unicode (UTF8). This applies to identifiers in table definitions that stored in `.frm' files and to identifiers stored in the grant tables in the mysql database. Although Unicode identifiers can include multi-byte characters, note that the maximum lengths shown in the table are byte counts. If an identifier does contain multi-byte characters, the number of characters allowed in the identifier is less than the value shown in the table.

An identifier may be quoted or unquoted. If an identifier is a reserved word or contains special characters, you must quote it whenever you refer to it. For a list of reserved words, see section 10.6 Treatment of Reserved Words in MySQL. Special characters are those outside the set of alphanumeric characters from the current character set, `_', and `$'.

The identifier quote character is the backtick (``'): 

mysql> SELECT * FROM `select` WHERE `select`.id > 100;

If the server SQL mode includes the ANSI_QUOTES mode option, it is also allowable to quote identifiers with double quotes: 

mysql> CREATE TABLE "test" (col INT);
ERROR 1064: You have an error in your SQL syntax. (...)
mysql> SET sql_mode='ANSI_QUOTES';
mysql> CREATE TABLE "test" (col INT);
Query OK, 0 rows affected (0.00 sec)

See section 5.2.2 The Server SQL Mode.

As of MySQL 4.1, identifier quote characters can be included within an identifier if you quote the identifier. If the character to be included within the identifier is the same as that used to quote the identifier itself, double the character. The following statement creates a table named a`b that contains a column named c"d: 

mysql> CREATE TABLE `a``b` (`c"d` INT);

Identifier quoting was introduced in MySQL 3.23.6 to allow use of identifiers that are reserved words or that contain special characters. Before 3.23.6, you cannot use identifiers that require quotes, so the rules for legal identifiers are more restrictive:

It is recommended that you do not use names like 1e, because an expression like 1e+1 is ambiguous. It might be interpreted as the expression 1e + 1 or as the number 1e+1, depending on context.

10.2.1 Identifier Qualifiers

MySQL allows names that consist of a single identifier or multiple identifiers. The components of a multiple-part name should be separated by period (`.') characters. The initial parts of a multiple-part name act as qualifiers that affect the context within which the final identifier is interpreted.

In MySQL you can refer to a column using any of the following forms:

Column Reference Meaning
col_name The column col_name from whichever table used in the query contains a column of that name.
tbl_name.col_name The column col_name from table tbl_name of the default database.
db_name.tbl_name.col_name The column col_name from table tbl_name of the database db_name. This syntax is unavailable before MySQL 3.22.

If any components of a multiple-part name require quoting, quote them individually rather than quoting the name as a whole. For example, `my-table`.`my-column` is legal, whereas `my-table.my-column` is not.

You need not specify a tbl_name or db_name.tbl_name prefix for a column reference in a statement unless the reference would be ambiguous. Suppose that tables t1 and t2 each contain a column c, and you retrieve c in a SELECT statement that uses both t1 and t2. In this case, c is ambiguous because it is not unique among the tables used in the statement. You must qualify it with a table name as t1.c or t2.c to indicate which table you mean. Similarly, to retrieve from a table t in database db1 and from a table t in database db2 in the same statement, you must refer to columns in those tables as db1.t.col_name and db2.t.col_name.

The syntax .tbl_name means the table tbl_name in the current database. This syntax is accepted for ODBC compatibility because some ODBC programs prefix table names with a `.' character.

10.2.2 Identifier Case Sensitivity

In MySQL, databases correspond to directories within the data directory. Tables within a database correspond to at least one file within the database directory (and possibly more, depending on the storage engine). Consequently, the case sensitivity of the underlying operating system determines the case sensitivity of database and table names. This means database and table names are not case sensitive in Windows, and case sensitive in most varieties of Unix. One notable exception is Mac OS X, which is Unix-based but uses a default filesystem type (HFS+) that is not case sensitive. However, Mac OS X also supports UFS volumes, which are case sensitive just as on any Unix. See section 1.8.4 MySQL Extensions to Standard SQL.

Note: Although database and table names are not case sensitive on some platforms, you should not refer to a given database or table using different cases within the same query. The following query would not work because it refers to a table both as my_table and as MY_TABLE: 

mysql> SELECT * FROM my_table WHERE MY_TABLE.col=1;

Column names, index names, and column aliases are not case sensitive on any platform.

Table aliases are case sensitive before MySQL 4.1.1. The following query would not work because it refers to the alias both as a and as A: 

mysql> SELECT col_name FROM tbl_name AS a
    -> WHERE a.col_name = 1 OR A.col_name = 2;

If you have trouble remembering the allowable lettercase for database and table names, adopt a consistent convention, such as always creating databases and tables using lowercase names.

How table and database names are stored on disk and used in MySQL is defined by the lower_case_table_names system variable, which you can set when starting @command{mysqld}. lower_case_table_names can take one of the following values:

Value Meaning
0 Table and database names are stored on disk using the lettercase specified in the CREATE TABLE or CREATE DATABASE statement. Name comparisons are case sensitive. This is the default on Unix systems. Note that if you force this to 0 with --lower-case-table-names=0 on a case-insensitive filesystem and access MyISAM tablenames using different lettercases, this may lead to index corruption.
1 Table names are stored in lowercase on disk and name comparisons are not case sensitive. MySQL converts all table names to lowercase on storage and lookup. This behavior also applies to database names as of MySQL 4.0.2, and to table aliases as of 4.1.1. This value is the default on Windows and Mac OS X systems.
2 Table and database names are stored on disk using the lettercase specified in the CREATE TABLE or CREATE DATABASE statement, but MySQL converts them to lowercase on lookup. Name comparisons are not case sensitive. Note: This works only on filesystems that are not case sensitive! InnoDB table names are stored in lowercase, as for lower_case_table_names=1. Setting lower_case_table_names to 2 can be done as of MySQL 4.0.18.

If you are using MySQL on only one platform, you don't normally have to change the lower_case_table_names variable. However, you may encounter difficulties if you want to transfer tables between platforms that differ in filesystem case sensitivity. For example, on Unix, you can have two different tables named my_table and MY_TABLE, but on Windows those names are considered the same. To avoid data transfer problems stemming from database or table name lettercase, you have two options:

Note that before setting lower_case_table_names to 1 on Unix, you must first convert your old database and table names to lowercase before restarting @command{mysqld}.

10.3 User Variables

MySQL supports user variables as of version 3.23.6. You can store a value in a user variable and refer to it later, which allows you to pass values from one statement to another. User variables are connection-specific. That is, a variable defined by one client cannot be seen or used by other clients. All variables for a client connection are automatically freed when the client exits.

User variables are written as @var_name, where the variable name var_name may consist of alphanumeric characters from the current character set, `.', `_', and `$'. The default character set is ISO-8859-1 (Latin1). This may be changed with the --default-character-set option to @command{mysqld}. See section 5.8.1 The Character Set Used for Data and Sorting. User variable names are not case sensitive beginning with MySQL 5.0. Before that, they are case sensitive.

One way to set a user variable is by issuing a SET statement: 

SET @var_name = expr [, @var_name = expr] ...

For SET, either = or := can be used as the assignment operator. The expr assigned to each variable can evaluate to an integer, real, string, or NULL value.

You can also assign a value to a user variable in statements other than SET. In this case, the assignment operator must be := and not = because = is treated as a comparison operator in non-SET statements: 

mysql> SET @t1=0, @t2=0, @t3=0;
mysql> SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
+----------------------+------+------+------+
| @t1:=(@t2:=1)+@t3:=4 | @t1  | @t2  | @t3  |
+----------------------+------+------+------+
|                    5 |    5 |    1 |    4 |
+----------------------+------+------+------+

User variables may be used where expressions are allowed. This does not currently include contexts that explicitly require a number, such as in the LIMIT clause of a SELECT statement, or the IGNORE number LINES clause of a LOAD DATA statement.

If you refer to a variable that has not been initialized, its value is NULL.

Note: In a SELECT statement, each expression is evaluated only when sent to the client. This means that in a HAVING, GROUP BY, or ORDER BY clause, you cannot refer to an expression that involves variables that are set in the SELECT list. For example, the following statement will not work as expected: 

mysql> SELECT (@aa:=id) AS a, (@aa+3) AS b FROM tbl_name HAVING b=5;

The reference to b in the HAVING clause refers to an alias for an expression in the SELECT list that uses @aa. This does not work as expected: @aa will not contain the value of the current row, but the value of id from the previous selected row.

The general rule is to never assign and use the same variable in the same statement.

Another issue with setting a variable and using it in the same statement is that the default result type of a variable is based on the type of the variable at the start of the statement. The following example illustrates this: 

mysql> SET @a='test';
mysql> SELECT @a,(@a:=20) FROM tbl_name;

For this SELECT statement, MySQL will report to the client that column one is a string and convert all accesses of @a to strings, even though @a is set to a number for the second row. After the SELECT statement executes, @a will be regarded as a number for the next statement.

To avoid problems with this behavior, either do not set and use the same variable within a single statement, or else set the variable to 0, 0.0, or '' to define its type before you use it.

An unassigned variable has a value of NULL with a type of string.

10.4 System Variables

Starting from MySQL 4.0.3, we provide better access to a lot of system and connection variables. Many variables can be changed dynamically while the server is running. This allows you to modify server operation without having to stop and restart it.

The @command{mysqld} server maintains two kinds of variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections.

When the server starts, it initializes all global variables to their default values. These defaults may be changed by options specified in option files or on the command line. After the server starts, those global variables that are dynamic can be changed by connecting to the server and issuing a SET GLOBAL var_name statement. To change a global variable, you must have the SUPER privilege.

The server also maintains a set of session variables for each client that connects. The client's session variables are initialized at connect time using the current values of the corresponding global variables. For those session variables that are dynamic, the client can change them by issuing a SET SESSION var_name statement. Setting a session variable requires no special privilege, but a client can change only its own session variables, not those of any other client.

A change to a global variable is visible to any client that accesses that global variable. However, it affects the corresponding session variable that is intialized from the global variable only for clients that connect after the change. It does not affect the session variable for any client that is already connected (not even that of the client that issues the SET GLOBAL statement).

Global or session variables may be set or retrieved using several syntax forms. The following examples use sort_buffer_size as a sample variable name.

To set the value of a GLOBAL variable, use one of the following syntaxes: 

mysql> SET GLOBAL sort_buffer_size=value;
mysql> SET @@global.sort_buffer_size=value;

To set the value of a SESSION variable, use one of the following syntaxes: 

mysql> SET SESSION sort_buffer_size=value;
mysql> SET @@session.sort_buffer_size=value;
mysql> SET sort_buffer_size=value;

LOCAL is a synonym for SESSION.

If you don't specify GLOBAL, SESSION, or LOCAL when setting a variable, SESSION is the default. See section 14.5.3.1 SET Syntax.

To retrieve the value of a GLOBAL variable, use one of the following statements: 

mysql> SELECT @@global.sort_buffer_size;
mysql> SHOW GLOBAL VARIABLES like 'sort_buffer_size';

To retrieve the value of a SESSION variable, use one of the following statements: 

mysql> SELECT @@sort_buffer_size;
mysql> SELECT @@session.sort_buffer_size;
mysql> SHOW SESSION VARIABLES like 'sort_buffer_size';

Here, too, LOCAL is a synonym for SESSION.

When you retrieve a variable with SELECT @@var_name (that is, you do not specify global., session., or local., MySQL returns the SESSION value if it exists and the GLOBAL value otherwise.

For SHOW VARIABLES, if you do not specify GLOBAL, SESSION, or LOCAL, MySQL returns the SESSION value.

The reason for requiring the GLOBAL keyword when setting GLOBAL-only variables but not when retrieving them is to prevent problems in the future. If we remove a SESSION variable with the same name as a GLOBAL variable, a client with the SUPER privilege might accidentally change the GLOBAL variable rather than just the SESSION variable for its own connection. If we add a SESSION variable with the same name as a GLOBAL variable, a client that intends to change the GLOBAL variable might find only its own SESSION variable changed.

Further information about system startup options and system variables can be found in section 5.2.1 @command{mysqld} Command-Line Options and section 5.2.3 Server System Variables. A list of the variables that can be set at runtime is given in section 5.2.3.1 Dynamic System Variables.

10.4.1 Structured System Variables

Structured system variables are supported beginning with MySQL 4.1.1. A structured variable differs from a regular system variable in two respects:

Currently, MySQL supports one structured variable type. It specifies parameters that govern the operation of key caches. A key cache structured variable has these components:

The purpose of this section is to describe the syntax for referring to structured variables. Key cache variables are used for syntax examples, but specific details about how key caches operate are found elsewhere, in section 7.4.6 The MyISAM Key Cache.

To refer to a component of a structured variable instance, you can use a compound name in instance_name.component_name format. Examples: 

hot_cache.key_buffer_size
hot_cache.key_cache_block_size
cold_cache.key_cache_block_size

For each structured system variable, an instance with the name of default is always predefined. If you refer to a component of a structured variable without any instance name, the default instance is used. Thus, default.key_buffer_size and key_buffer_size both refer to the same system variable.

The naming rules for structured variable instances and components are as follows:

At the moment, the first two rules have no possibility of being violated because the only structured variable type is the one for key caches. These rules will assume greater significance if some other type of structured variable is created in the future.

With one exception, it is allowable to refer to structured variable components using compound names in any context where simple variable names can occur. For example, you can assign a value to a structured variable using a command-line option: 

shell> mysqld --hot_cache.key_buffer_size=64K

In an option file, do this: 

[mysqld]
hot_cache.key_buffer_size=64K

If you start the server with such an option, it creates a key cache named hot_cache with a size of 64KB in addition to the default key cache that has a default size of 8MB.

Suppose that you start the server as follows: 

shell> mysqld --key_buffer_size=256K \
         --extra_cache.key_buffer_size=128K \
         --extra_cache.key_cache_block_size=2048

In this case, the server sets the size of the default key cache to 256KB. (You could also have written --default.key_buffer_size=256K.) In addition, the server creates a second key cache named extra_cache that has a size of 128KB, with the size of block buffers for caching table index blocks set to 2048 bytes.

The following example starts the server with three different key caches having sizes in a 3:1:1 ratio: 

shell> mysqld --key_buffer_size=6M \
         --hot_cache.key_buffer_size=2M \
         --cold_cache.key_buffer_size=2M

Structured variable values may be set and retrieved at runtime as well. For example, to set a key cache named hot_cache to a size of 10MB, use either of these statements: 

mysql> SET GLOBAL hot_cache.key_buffer_size = 10*1024*1024;
mysql> SET @@global.hot_cache.key_buffer_size = 10*1024*1024;

To retrieve the cache size, do this: 

mysql> SELECT @@global.hot_cache.key_buffer_size;

However, the following statement does not work. The variable is not interpreted as a compound name, but as a simple string for a LIKE pattern-matching operation: 

mysql> SHOW GLOBAL VARIABLES LIKE 'hot_cache.key_buffer_size';

This is the exception to being able to use structured variable names anywhere a simple variable name may occur.

10.5 Comment Syntax

The MySQL server supports three comment styles:

The following example demonstrates all three comment styles: 

mysql> SELECT 1+1;     # This comment continues to the end of line
mysql> SELECT 1+1;     -- This comment continues to the end of line
mysql> SELECT 1 /* this is an in-line comment */ + 1;
mysql> SELECT 1+
/*
this is a
multiple-line comment
*/
1;

The comment syntax just described applies to how the @command{mysqld} server parses SQL statements. The @command{mysql} client program also performs some parsing of statements before sending them to the server. (For example, it does this to determine statement boundaries within a multiple-statement input line.) However, there are some limitations on the way that @command{mysql} parses /* ... */ comments:

For affected versions of MySQL, these limitations apply both when you run @command{mysql} interactively and when you put commands in a file and use @command{mysql} in batch mode to process the file with @command{mysql < file_name}.

10.6 Treatment of Reserved Words in MySQL

A common problem stems from trying to use an identifier such as a table or column name that is the name of a built-in MySQL data type or function, such as TIMESTAMP or GROUP. You're allowed to do this (for example, ABS is allowed as a column name). However, by default, no whitespace is allowed in function invocations between the function name and the following `(' character. This requirement allows a function call to be distinguished from a reference to a column name.

A side effect of this behavior is that omitting a space in some contexts causes an identifier to be interpreted as a function name. For example, this statement is legal: 

mysql> CREATE TABLE abs (val INT);

But omitting the space after abs causes a syntax error because the statement then appears to invoke the ABS() function: 

mysql> CREATE TABLE abs(val INT);

If the server SQL mode includes the IGNORE_SPACE mode value, the server allows function invocations to have whitespace between a function name and the following `(' character. This causes function names to be treated as reserved words. As a result, identifiers that are the same as function names must be quoted as described in section 10.2 Database, Table, Index, Column, and Alias Names. The server SQL mode is controlled as described in section 5.2.2 The Server SQL Mode.

The words in the following table are explicitly reserved in MySQL. Most of them are forbidden by standard SQL as column and/or table names (for example, GROUP). A few are reserved because MySQL needs them and (currently) uses a @command{yacc} parser. A reserved word can be used as an identifier if you quote it.

Word Word Word
ADD ALL ALTER
ANALYZE AND AS
ASC ASENSITIVE BEFORE
BETWEEN BIGINT BINARY
BLOB BOTH BY
CALL CASCADE CASE
CHANGE CHAR CHARACTER
CHECK COLLATE COLUMN
COLUMNS CONDITION CONNECTION
CONSTRAINT CONTINUE CONVERT
CREATE CROSS CURRENT_DATE
CURRENT_TIME CURRENT_TIMESTAMP CURRENT_USER
CURSOR DATABASE DATABASES
DAY_HOUR DAY_MICROSECOND DAY_MINUTE
DAY_SECOND DEC DECIMAL
DECLARE DEFAULT DELAYED
DELETE DESC DESCRIBE
DETERMINISTIC DISTINCT DISTINCTROW
DIV DOUBLE DROP
DUAL EACH ELSE
ELSEIF ENCLOSED ESCAPED
EXISTS EXIT EXPLAIN
FALSE FETCH FIELDS
FLOAT FOR FORCE
FOREIGN FOUND FROM
FULLTEXT GOTO GRANT
GROUP HAVING HIGH_PRIORITY
HOUR_MICROSECOND HOUR_MINUTE HOUR_SECOND
IF IGNORE IN
INDEX INFILE INNER
INOUT INSENSITIVE INSERT
INT INTEGER INTERVAL
INTO IS ITERATE
JOIN KEY KEYS
KILL LEADING LEAVE
LEFT LIKE LIMIT
LINES LOAD LOCALTIME
LOCALTIMESTAMP LOCK LONG
LONGBLOB LONGTEXT LOOP
LOW_PRIORITY MATCH MEDIUMBLOB
MEDIUMINT MEDIUMTEXT MIDDLEINT
MINUTE_MICROSECOND MINUTE_SECOND MOD
MODIFIES NATURAL NOT
NO_WRITE_TO_BINLOG NULL NUMERIC
ON OPTIMIZE OPTION
OPTIONALLY OR ORDER
OUT OUTER OUTFILE
PRECISION PRIMARY PRIVILEGES
PROCEDURE PURGE READ
READS REAL REFERENCES
REGEXP RENAME REPEAT
REPLACE REQUIRE RESTRICT
RETURN REVOKE RIGHT
RLIKE SCHEMA SCHEMAS
SECOND_MICROSECOND SELECT SENSITIVE
SEPARATOR SET SHOW
SMALLINT SONAME SPATIAL
SPECIFIC SQL SQLEXCEPTION
SQLSTATE SQLWARNING SQL_BIG_RESULT
SQL_CALC_FOUND_ROWS SQL_SMALL_RESULT SSL
STARTING STRAIGHT_JOIN TABLE
TABLES TERMINATED THEN
TINYBLOB TINYINT TINYTEXT
TO TRAILING TRIGGER
TRUE UNDO UNION
UNIQUE UNLOCK UNSIGNED
UPDATE USAGE USE
USING UTC_DATE UTC_TIME
UTC_TIMESTAMP VALUES VARBINARY
VARCHAR VARCHARACTER VARYING
WHEN WHERE WHILE
WITH WRITE XOR
YEAR_MONTH ZEROFILL

MySQL allows some keywords to be used as unquoted identifiers because many people have already used them. Examples are those in the following list:

11 Character Set Support

Improved support for character set handling was added to MySQL in Version 4.1. The features described here are as implemented in MySQL 4.1.1. (MySQL 4.1.0 has some but not all of these features, and some of them are implemented differently.)

This chapter discusses the following topics:

Character set support currently is included in the MyISAM, MEMORY (HEAP), and (as of MySQL 4.1.2) InnoDB storage engines. The ISAM storage engine does not include character set support; there are no plans to change this, because ISAM is deprecated.

11.1 Character Sets and Collations in General

A character set is a set of symbols and encodings. A collation is a set of rules for comparing characters in a character set. Let's make the distinction clear with an example of an imaginary character set.

Suppose that we have an alphabet with four letters: `A', `B', `a', `b'. We give each letter a number: `A' = 0, `B' = 1, `a' = 2, `c' = 3. The letter `A' is a symbol, the number 0 is the encoding for `A', and the combination of all four letters and their encodings is a character set.

Now, suppose that we want to compare two string values, `A' and `B'. The simplest way to do this is to look at the encodings: 0 for `A' and 1 for `B'. Because 0 is less than 1, we say `A' is less than `B'. Now, what we've just done is apply a collation to our character set. The collation is a set of rules (only one rule in this case): ``compare the encodings.'' We call this simplest of all possible collations a binary collation.

But what if we want to say that the lowercase and uppercase letters are equivalent? Then we would have at least two rules: (1) treat the lowercase letters `a' and `b' as equivalent to `A' and `B'; (2) then compare the encodings. We call this a case-insensitive collation. It's a little more complex than a binary collation.

In real life, most character sets have many characters: not just `A' and `B' but whole alphabets, sometimes multiple alphabets or eastern writing systems with thousands of characters, along with many special symbols and punctuation marks. Also in real life, most collations have many rules: not just case insensitivity but also accent insensitivity (an ``accent'' is a mark attached to a character as in German `@"O') and multiple-character mappings (such as the rule that `@"O' = `OE' in one of the two German collations).

MySQL 4.1 can do these things for you:

In these respects, not only is MySQL 4.1 far more flexible than MySQL 4.0, it also is far ahead of other DBMSs. However, to use the new features effectively, you will need to learn what character sets and collations are available, how to change their defaults, and what the various string operators do with them.

11.2 Character Sets and Collations in MySQL

The MySQL server can support multiple character sets. To list the available character sets, use the SHOW CHARACTER SET statement: 

mysql> SHOW CHARACTER SET;
+----------+-----------------------------+---------------------+
| Charset  | Description                 | Default collation   |
+----------+-----------------------------+---------------------+
| big5     | Big5 Traditional Chinese    | big5_chinese_ci     |
| dec8     | DEC West European           | dec8_swedish_ci     |
| cp850    | DOS West European           | cp850_general_ci    |
| hp8      | HP West European            | hp8_english_ci      |
| koi8r    | KOI8-R Relcom Russian       | koi8r_general_ci    |
| latin1   | ISO 8859-1 West European    | latin1_swedish_ci   |
| latin2   | ISO 8859-2 Central European | latin2_general_ci   |
...

The output actually includes another column that is not shown so that the example fits better on the page.

Any given character set always has at least one collation. It may have several collations.

To list the collations for a character set, use the SHOW COLLATION statement. For example, to see the collations for the latin1 (``ISO-8859-1 West European'') character set, use this statement to find those collation names that begin with latin1: 

mysql> SHOW COLLATION LIKE 'latin1%';
+-------------------+---------+----+---------+----------+---------+
| Collation         | Charset | Id | Default | Compiled | Sortlen |
+-------------------+---------+----+---------+----------+---------+
| latin1_german1_ci | latin1  |  5 |         |          |       0 |
| latin1_swedish_ci | latin1  |  8 | Yes     | Yes      |       1 |
| latin1_danish_ci  | latin1  | 15 |         |          |       0 |
| latin1_german2_ci | latin1  | 31 |         | Yes      |       2 |
| latin1_bin        | latin1  | 47 |         | Yes      |       1 |
| latin1_general_ci | latin1  | 48 |         |          |       0 |
| latin1_general_cs | latin1  | 49 |         |          |       0 |
| latin1_spanish_ci | latin1  | 94 |         |          |       0 |
+-------------------+---------+----+---------+----------+---------+

The latin1 collations have the following meanings:

Collation Meaning
latin1_bin Binary according to latin1 encoding
latin1_danish_ci Danish/Norwegian
latin1_general_ci Multilingual
latin1_general_cs Multilingual, case sensitive
latin1_german1_ci German DIN-1
latin1_german2_ci German DIN-2
latin1_spanish_ci Modern Spanish
latin1_swedish_ci Swedish/Finnish

Collations have these general characteristics:

11.3 Determining the Default Character Set and Collation

There are default settings for character sets and collations at four levels: server, database, table, and connection. The following description may appear complex, but it has been found in practice that multiple-level defaulting leads to natural and obvious results.

11.3.1 Server Character Set and Collation

The MySQL Server has a server character set and a server collation, which may not be null.

MySQL determines the server character set and server collation thus:

At the server level, the decision is simple. The server character set and collation depend initially on the options that you use when you start @command{mysqld}. You can use --default-character-set for the character set, and along with it you can add --default-collation for the collation. If you don't specify a character set, that is the same as saying --default-character-set=latin1. If you specify only a character set (for example, latin1) but not a collation, that is the same as saying --default-charset=latin1 --default-collation=latin1_swedish_ci because latin1_swedish_ci is the default collation for latin1. Therefore, the following three commands all have the same effect: 

shell> mysqld
shell> mysqld --default-character-set=latin1
shell> mysqld --default-character-set=latin1 \
           --default-collation=latin1_swedish_ci

One way to change the settings is by recompiling. If you want to change the default server character set and collation when building from sources, use: --with-charset and --with-collation as arguments for @command{configure}. For example: 

shell> ./configure --with-charset=latin1

Or: 

shell> ./configure --with-charset=latin1 \
           --with-collation=latin1_german1_ci

Both @command{mysqld} and @command{configure} verify that the character set/collation combination is valid. If not, each program displays an error message and terminates.

The current server character set and collation are available as the values of the character_set_server and collation_server system variables. These variables can be changed at runtime.

11.3.2 Database Character Set and Collation

Every database has a database character set and a database collation, which may not be null. The CREATE DATABASE and ALTER DATABASE statements have optional clauses for specifying the database character set and collation: 

CREATE DATABASE db_name
    [[DEFAULT] CHARACTER SET charset_name]
    [[DEFAULT] COLLATE collation_name]

ALTER DATABASE db_name
    [[DEFAULT] CHARACTER SET charset_name]
    [[DEFAULT] COLLATE collation_name]

Example: 

CREATE DATABASE db_name
    DEFAULT CHARACTER SET latin1 COLLATE latin1_swedish_ci;

MySQL chooses the database character set and database collation thus:

MySQL's CREATE DATABASE ... DEFAULT CHARACTER SET ... syntax is analogous to the standard SQL CREATE SCHEMA ... CHARACTER SET ... syntax. Because of this, it is possible to create databases with different character sets and collations on the same MySQL server.

The database character set and collation are used as default values if the table character set and collation are not specified in CREATE TABLE statements. They have no other purpose.

The character set and collation for the default database are available as the values of the character_set_database and collation_database system variables. The server sets these variables whenever the default database changes. If there is no default database, the variables have the same value as the corresponding server-level variables, character_set_server and collation_server.

11.3.3 Table Character Set and Collation

Every table has a table character set and a table collation, which may not be null. The CREATE TABLE and ALTER TABLE statements have optional clauses for specifying the table character set and collation: 

CREATE TABLE tbl_name (column_list)
    [DEFAULT CHARACTER SET charset_name [COLLATE collation_name]]

ALTER TABLE tbl_name
    [DEFAULT CHARACTER SET charset_name] [COLLATE collation_name]

Example: 

CREATE TABLE t1 ( ... )
    DEFAULT CHARACTER SET latin1 COLLATE latin1_danish_ci;

MySQL chooses the table character set and collation thus:

The table character set and collation are used as default values if the column character set and collation are not specified in individual column definitions. The table character set and collation are MySQL extensions; there are no such things in standard SQL.

11.3.4 Column Character Set and Collation

Every ``character'' column (that is, a column of type CHAR, VARCHAR, or TEXT) has a column character set and a column collation, which may not be null. Column definition syntax has optional clauses for specifying the column character set and collation: 

col_name {CHAR | VARCHAR | TEXT} (col_length)
    [CHARACTER SET charset_name [COLLATE collation_name]]

Example: 

CREATE TABLE Table1
(
    column1 VARCHAR(5) CHARACTER SET latin1 COLLATE latin1_german1_ci
);

MySQL chooses the column character set and collation thus:

The CHARACTER SET and COLLATE clauses are standard SQL.

11.3.5 Examples of Character Set and Collation Assignment

The following examples show how MySQL determines default character set and collation values.

11.3.5.1 Example 1: Table + Column Definition

CREATE TABLE t1
(
    c1 CHAR(10) CHARACTER SET latin1 COLLATE latin1_german1_ci
) DEFAULT CHARACTER SET latin2 COLLATE latin2_bin;

Here we have a column with a latin1 character set and a latin1_german1_ci collation. The definition is explicit, so that's straightforward. Notice that there's no problem storing a latin1 column in a latin2 table.

11.3.5.2 Example 2: Table + Column Definition

CREATE TABLE t1
(
    c1 CHAR(10) CHARACTER SET latin1
) DEFAULT CHARACTER SET latin1 COLLATE latin1_danish_ci;

This time we have a column with a latin1 character set and a default collation. Now, although it might seem natural, the default collation is not taken from the table level. Instead, because the default collation for latin1 is always latin1_swedish_ci, column c1 will have a collation of latin1_swedish_ci (not latin1_danish_ci).

11.3.5.3 Example 3: Table + Column Definition

CREATE TABLE t1
(
    c1 CHAR(10)
) DEFAULT CHARACTER SET latin1 COLLATE latin1_danish_ci;

We have a column with a default character set and a default collation. In this circumstance, MySQL looks up to the table level for inspiration in determining the column character set and collation. So, the character set for column c1 is latin1 and its collation is latin1_danish_ci.

11.3.5.4 Example 4: Database + Table + Column Definition

CREATE DATABASE d1
    DEFAULT CHARACTER SET latin2 COLLATE latin2_czech_ci;
USE d1;
CREATE TABLE t1
(
    c1 CHAR(10)
);

We create a column without specifying its character set and collation. We're also not specifying a character set and a collation at the table level. In this circumstance, MySQL looks up to the database level for inspiration. (The database's settings become the table's settings, and thereafter become the column's setting.) So, the character set for column c1 is latin2 and its collation is latin2_czech_ci.

11.3.6 Connection Character Sets and Collations

Several character set and collation system variables relate to a client's interaction with the server. Some of these have already been mentioned in earlier sections:

Additional character set and collation variables are involved in handling traffic for the connection between a client and the server. Every client has connection-related character set and collation variables.

Consider what a ``connection'' is: It's what you make when you connect to the server. The client sends SQL statements, such as queries, over the connection to the server. The server sends responses, such as result sets, over the connection back to the client. This leads to several questions about character set and collation handling for client connections, each of which can be answered in terms of system variables:

You can fine-tune the settings for these variables, or you can depend on the defaults (in which case, you can skip this section).

There are two statements that affect the connection character sets: 

SET NAMES 'charset_name'
SET CHARACTER SET charset_name

SET NAMES indicates what is in the SQL statements that the client sends. Thus, SET NAMES 'cp1251' tells the server ``future incoming messages from this client will be in character set cp1251.'' It also specifies the character set for results that the server sends back to the client. (For example, it indicates what character set column values will have if you use a SELECT statement.)

A SET NAMES 'x' statement is equivalent to these three statements: 

mysql> SET character_set_client = x;
mysql> SET character_set_results = x;
mysql> SET character_set_connection = x;

Setting character_set_connection to x also sets collation_connection to the default collation for x.

SET CHARACTER SET is similar but sets the connection character set and collation to be those of the default database. A SET CHARACTER SET x statement is equivalent to these three statements: 

mysql> SET character_set_client = x;
mysql> SET character_set_results = x;
mysql> SET collation_connection = @@collation_database;

When a client connects, it sends to the server the name of the character set that it wants to use. The server sets the character_set_client, character_set_results, and character_set_connection variables to that character set. (In effect, the server performs a SET NAMES operation using the character set.)

With the @command{mysql} client, it is not necessary to execute SET NAMES every time you start up if you want to use a character set different from the default. You can add the --default-character-set option setting to your @command{mysql} statement line, or in your option file. For example, the following option file setting changes the three character set variables set to koi8r each time you run @command{mysql}: 

[mysql]
default-character-set=koi8r

Example: Suppose that column1 is defined as CHAR(5) CHARACTER SET latin2. If you do not say SET NAMES or SET CHARACTER SET, then for SELECT column1 FROM t, the server will send back all the values for column1 using the character set that the client specified when it connected. On the other hand, if you say SET NAMES 'latin1' or SET CHARACTER SET latin1, then just before sending results back, the server will convert the latin2 values to latin1. Conversion may be lossy if there are characters that are not in both character sets.

If you do not want the server to perform any conversion, set character_set_results to NULL: 

mysql> SET character_set_results = NULL;

11.3.7 Character String Literal Character Set and Collation

Every character string literal has a character set and a collation, which may not be null.

A character string literal may have an optional character set introducer and COLLATE clause: 

[_charset_name]'string' [COLLATE collation_name]

Examples: 

SELECT 'string';
SELECT _latin1'string';
SELECT _latin1'string' COLLATE latin1_danish_ci;

For the simple statement SELECT 'string', the string has the character set and collation defined by the character_set_connection and collation_connection system variables.

The _charset_name expression is formally called an introducer. It tells the parser, ``the string that is about to follow is in character set X.'' Because this has confused people in the past, we emphasize that an introducer does not cause any conversion, it is strictly a signal that does not change the string's value. An introducer is also legal before standard hex literal and numeric hex literal notation (x'literal' and 0xnnnn), and before ? (parameter substitution when using prepared statements within a programming language interface).

Examples: 

SELECT _latin1 x'AABBCC';
SELECT _latin1 0xAABBCC;
SELECT _latin1 ?;

MySQL determines a literal's character set and collation thus:

Examples:

Character set introducers and the COLLATE clause are implemented according to standard SQL specifications.

11.3.8 Using COLLATE in SQL Statements

With the COLLATE clause, you can override whatever the default collation is for a comparison. COLLATE may be used in various parts of SQL statements. Here are some examples:

11.3.9 COLLATE Clause Precedence

The COLLATE clause has high precedence (higher than ||), so the following two expressions are equivalent: 

x || y COLLATE z
x || (y COLLATE z)

11.3.10 BINARY Operator

The BINARY operator is a shorthand for a COLLATE clause. BINARY 'x' is equivalent to 'x' COLLATE y, where y is the name of the binary collation for the character set of 'x'. Every character set has a binary collation. For example, the binary collation for the latin1 character set is latin1_bin, so if the column a is of character set latin1, the following two statements have the same effect: 

SELECT * FROM t1 ORDER BY BINARY a;
SELECT * FROM t1 ORDER BY a COLLATE latin1_bin;

11.3.11 Some Special Cases Where the Collation Determination Is Tricky

In the great majority of queries, it is obvious what collation MySQL uses to resolve a comparison operation. For example, in the following cases, it should be clear that the collation will be ``the column collation of column x'': 

SELECT x FROM T ORDER BY x;
SELECT x FROM T WHERE x = x;
SELECT DISTINCT x FROM T;

However, when multiple operands are involved, there can be ambiguity. For example: 

SELECT x FROM T WHERE x = 'Y';

Should this query use the collation of the column x, or of the string literal 'Y'?

Standard SQL resolves such questions using what used to be called ``coercibility'' rules. The essence is: Because x and 'Y' both have collations, whose collation takes precedence? It's complex, but the following rules take care of most situations:

Those rules resolve ambiguities thus:

Examples:

column1 = 'A' Use collation of column1
column1 = 'A' COLLATE x Use collation of 'A'
column1 COLLATE x = 'A' COLLATE y Error

The COERCIBILITY() function can be used to determine the coercibility of a string expression: 

mysql> SELECT COERCIBILITY('A' COLLATE latin1_swedish_ci);
        -> 0
mysql> SELECT COERCIBILITY('A');
        -> 3

See section 13.8.3 Information Functions.

11.3.12 Collations Must Be for the Right Character Set

Recall that each character set has one or more collations, and each collation is associated with one and only one character set. Therefore, the following statement causes an error message because the latin2_bin collation is not legal with the latin1 character set: 

mysql> SELECT _latin1 'x' COLLATE latin2_bin;
ERROR 1251: COLLATION 'latin2_bin' is not valid
for CHARACTER SET 'latin1'

In some cases, expressions that worked before MySQL 4.1 fail as of MySQL 4.1 if you do not take character set and collation into account. For example, before 4.1, this statement works as is: 

mysql> SELECT SUBSTRING_INDEX(USER(),'@',1);
+-------------------------------+
| SUBSTRING_INDEX(USER(),'@',1) |
+-------------------------------+
| root                          |
+-------------------------------+

After an upgrade to MySQL 4.1, the statement fails: 

mysql> SELECT SUBSTRING_INDEX(USER(),'@',1);
ERROR 1267 (HY000): Illegal mix of collations
(utf8_general_ci,IMPLICIT) and (latin1_swedish_ci,COERCIBLE)
for operation 'substr_index'

The reason this occurs is that usernames are stored using UTF8 (see section 11.6 UTF8 for Metadata). As a result, the USER() function and the literal string '@' have different character sets (and thus different collations): 

mysql> SELECT COLLATION(USER()), COLLATION('@');
+-------------------+-------------------+
| COLLATION(USER()) | COLLATION('@')    |
+-------------------+-------------------+
| utf8_general_ci   | latin1_swedish_ci |
+-------------------+-------------------+

One way to deal with this is to tell MySQL to interpret the literal string as utf8: 

mysql> SELECT SUBSTRING_INDEX(USER(),_utf8'@',1);
+------------------------------------+
| SUBSTRING_INDEX(USER(),_utf8'@',1) |
+------------------------------------+
| root                               |
+------------------------------------+

Another way is to change the connection character set and collation to utf8. You can do that with SET NAMES 'utf8' or by setting the character_set_connection and collation_connection system variables directly.

11.3.13 An Example of the Effect of Collation

Suppose that column X in table T has these latin1 column values: 

Muffler
M@"uller
MX Systems
MySQL

And suppose that the column values are retrieved using the following statement: 

SELECT X FROM T ORDER BY X COLLATE collation_name;

The resulting order of the values for different collations is shown in this table:

latin1_swedish_ci latin1_german1_ci latin1_german2_ci
Muffler Muffler M@"uller
MX Systems M@"uller Muffler
M@"uller MX Systems MX Systems
MySQL MySQL MySQL

The table is an example that shows what the effect would be if we used different collations in an ORDER BY clause. The character that causes the different sort orders in this example is the U with two dots over it, which the Germans call U-umlaut, but we'll call it U-diaeresis.

Three different collations, three different results. That's what MySQL is here to handle. By using the appropriate collation, you can choose the sort order you want.

11.4 Operations Affected by Character Set Support

This section describes operations that take character set information into account as of MySQL 4.1.

11.4.1 Result Strings

MySQL has many operators and functions that return a string. This section answers the question: What is the character set and collation of such a string?

For simple functions that take string input and return a string result as output, the output's character set and collation are the same as those of the principal input value. For example, UPPER(X) returns a string whose character string and collation are the same as that of X. The same applies for INSTR(), LCASE(), LOWER(), LTRIM(), MID(), REPEAT(), REPLACE(), REVERSE(), RIGHT(), RPAD(), RTRIM(), SOUNDEX(), SUBSTRING(), TRIM(), UCASE(), and UPPER(). (Also note: The REPLACE() function, unlike all other functions, ignores the collation of the string input and performs a case-insensitive comparison every time.)

For operations that combine multiple string inputs and return a single string output, the ``aggregation rules'' of standard SQL apply:

For example, with CASE ... WHEN a THEN b WHEN b THEN c COLLATE X END, the resultant collation is X. The same applies for CASE, UNION, ||, CONCAT(), ELT(), GREATEST(), IF(), and LEAST().

For operations that convert to character data, the character set and collation of the strings that result from the operations are defined by the character_set_connection and collation_connection system variables. This applies for CAST(), CHAR(), CONV(), FORMAT(), HEX(), and SPACE().

11.4.2 CONVERT()

CONVERT() provides a way to convert data between different character sets. The syntax is: 

CONVERT(expr USING transcoding_name)

In MySQL, transcoding names are the same as the corresponding character set names.

Examples: 

SELECT CONVERT(_latin1'M@"uller' USING utf8);
INSERT INTO utf8table (utf8column)
    SELECT CONVERT(latin1field USING utf8) FROM latin1table;

CONVERT(... USING ...) is implemented according to the standard SQL specification.

11.4.3 CAST()

You may also use CAST() to convert a string to a different character set. The syntax is: 

CAST(character_string AS character_data_type CHARACTER SET charset_name)

Example: 

SELECT CAST(_latin1'test' AS CHAR CHARACTER SET utf8);

If you use CAST() without specifying CHARACTER SET, the resulting character set and collation are defined by the character_set_connection and collation_connection system variables. If you use CAST() with CHARACTER SET X, then the resulting character set and collation are X and the default collation of X.

You may not use a COLLATE clause inside a CAST(), but you may use it outside. That is, CAST(... COLLATE ...) is illegal, but CAST(...) COLLATE ... is legal.

Example: 

SELECT CAST(_latin1'test' AS CHAR CHARACTER SET utf8) COLLATE utf8_bin;

11.4.4 SHOW Statements

Several SHOW statements are new or modified in MySQL 4.1 to provide additional character set information. SHOW CHARACTER SET, SHOW COLLATION, and SHOW CREATE DATABASE are new. SHOW CREATE TABLE and SHOW COLUMNS are modified.

The SHOW CHARACTER SET command shows all available character sets. It takes an optional LIKE clause that indicates which character set names to match. For example: 

mysql> SHOW CHARACTER SET LIKE 'latin%';
+---------+-----------------------------+-------------------+--------+
| Charset | Description                 | Default collation | Maxlen |
+---------+-----------------------------+-------------------+--------+
| latin1  | ISO 8859-1 West European    | latin1_swedish_ci |      1 |
| latin2  | ISO 8859-2 Central European | latin2_general_ci |      1 |
| latin5  | ISO 8859-9 Turkish          | latin5_turkish_ci |      1 |
| latin7  | ISO 8859-13 Baltic          | latin7_general_ci |      1 |
+---------+-----------------------------+-------------------+--------+

See section 14.5.3.2 SHOW CHARACTER SET Syntax.

The output from SHOW COLLATION includes all available character sets. It takes an optional LIKE clause that indicates which collation names to match. For example: 

mysql> SHOW COLLATION LIKE 'latin1%';
+-------------------+---------+----+---------+----------+---------+
| Collation         | Charset | Id | Default | Compiled | Sortlen |
+-------------------+---------+----+---------+----------+---------+
| latin1_german1_ci | latin1  |  5 |         |          |       0 |
| latin1_swedish_ci | latin1  |  8 | Yes     | Yes      |       0 |
| latin1_danish_ci  | latin1  | 15 |         |          |       0 |
| latin1_german2_ci | latin1  | 31 |         | Yes      |       2 |
| latin1_bin        | latin1  | 47 |         | Yes      |       0 |
| latin1_general_ci | latin1  | 48 |         |          |       0 |
| latin1_general_cs | latin1  | 49 |         |          |       0 |
| latin1_spanish_ci | latin1  | 94 |         |          |       0 |
+-------------------+---------+----+---------+----------+---------+

See section 14.5.3.3 SHOW COLLATION Syntax.

SHOW CREATE DATABASE displays the CREATE DATABASE statement that will create a given database. The result includes all database options. DEFAULT CHARACTER SET and COLLATE are supported. All database options are stored in a text file named `db.opt' that can be found in the database directory. 

mysql> SHOW CREATE DATABASE a\G
*************************** 1. row ***************************
       Database: a
Create Database: CREATE DATABASE `a`
                 /*!40100 DEFAULT CHARACTER SET macce */

See section 14.5.3.5 SHOW CREATE DATABASE Syntax.

SHOW CREATE TABLE is similar, but displays the CREATE TABLE statement to create a given table. The column definitions now indicate any character set specifications, and the table options include character set information.

See section 14.5.3.6 SHOW CREATE TABLE Syntax.

The SHOW COLUMNS statement displays the collations of a table's columns when invoked as SHOW FULL COLUMNS. Columns with CHAR, VARCHAR, or TEXT data types have non-NULL collations. Numeric and other non-character types have NULL collations. For example: 

mysql> SHOW FULL COLUMNS FROM t;
+-------+---------+------------+------+-----+---------+-------+
| Field | Type    | Collation  | Null | Key | Default | Extra |
+-------+---------+------------+------+-----+---------+-------+
| a     | char(1) | latin1_bin | YES  |     | NULL    |       |
| b     | int(11) | NULL       | YES  |     | NULL    |       |
+-------+---------+------------+------+-----+---------+-------+

The character set is not part of the display. (The character set name is implied by the collation name.)

See section 14.5.3.4 SHOW COLUMNS Syntax.

11.5 Unicode Support

As of MySQL version 4.1, there are two new character sets for storing Unicode data:

In UCS-2 (binary Unicode representation), every character is represented by a two-byte Unicode code with the most significant byte first. For example: "LATIN CAPITAL LETTER A" has the code 0x0041 and it's stored as a two-byte sequence: 0x00 0x41. "CYRILLIC SMALL LETTER YERU" (Unicode 0x044B) is stored as a two-byte sequence: 0x04 0x4B. For Unicode characters and their codes, please refer to the Unicode Home Page.

A temporary restriction is that UCS-2 cannot yet be used as a client character set. That means that SET NAMES 'ucs2' will not work.

The UTF8 character set (transform Unicode representation) is an alternative way to store Unicode data. It is implemented according to RFC2279. The idea of the UTF8 character set is that various Unicode characters fit into byte sequences of different lengths:

Currently, MySQL UTF8 support does not include four-byte sequences.

Tip: To save space with UTF8, use VARCHAR instead of CHAR. Otherwise, MySQL has to reserve 30 bytes for a CHAR(10) CHARACTER SET utf8 column, because that's the maximum possible length.

11.6 UTF8 for Metadata

The metadata is the data about the data. Anything that describes the database, as opposed to being the contents of the database, is metadata. Thus column names, database names, usernames, version names, and most of the string results from SHOW are metadata.

Representation of metadata must satisfy these requirements:

In order to satisfy both requirements, MySQL stores metadata in a Unicode character set, namely UTF8. This will not cause any disruption if you never use accented characters. But if you do, you should be aware that metadata is in UTF8.

This means that the USER(), CURRENT_USER(), and VERSION() functions will have the UTF8 character set by default. So will any synonyms, such the SESSION_USER() and SYSTEM_USER() synonyms for USER().

The server sets the character_set_system system variable to the name of the metadata character set: 

mysql> SHOW VARIABLES LIKE 'character_set_system';
+----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| character_set_system | utf8  |
+----------------------+-------+

Storage of metadata using Unicode does not mean that the headers of columns and the results of DESCRIBE functions will be in the character_set_system character set by default. When you say SELECT column1 FROM t, the name column1 itself will be returned from the server to the client in the character set as determined by the SET NAMES statement. More specifically, the character set used is determined by the value of the character_set_results system variable. If this variable is set to NULL, no conversion is performed and the server returns metadata using its original character set (the set indicated by character_set_system).

If you want the server to pass metadata results back in a non-UTF8 character set, then use SET NAMES to force the server to perform character set conversion (see section 11.3.6 Connection Character Sets and Collations), or else set the client to do the conversion. It is always more efficient to set the client to do the conversion, but this option will not be available for many clients until late in the MySQL 4.x product cycle.

If you are just using, for example, the USER() function for comparison or assignment within a single statement, don't worry. MySQL will do some automatic conversion for you. 

SELECT * FROM Table1 WHERE USER() = latin1_column;

This will work because the contents of latin1_column are automatically converted to UTF8 before the comparison. 

INSERT INTO Table1 (latin1_column) SELECT USER();

This will work because the contents of USER() are automatically converted to latin1 before the assignment. Automatic conversion is not fully implemented yet, but should work correctly in a later version.

Although automatic conversion is not in the SQL standard, the SQL standard document does say that every character set is (in terms of supported characters) a ``subset'' of Unicode. Since it is a well-known principle that ``what applies to a superset can apply to a subset,'' we believe that a collation for Unicode can apply for comparisons with non-Unicode strings.

11.7 Compatibility with Other DBMSs

For MaxDB compatibility these two statements are the same: 

CREATE TABLE t1 (f1 CHAR(n) UNICODE);
CREATE TABLE t1 (f1 CHAR(n) CHARACTER SET ucs2);

11.8 New Character Set Configuration File Format

In MySQL 4.1, character set configuration is stored in XML files, one file per character set. In previous versions, this information was stored in `.conf' files.

11.9 National Character Set

Before MySQL 4.1, NCHAR and CHAR were synonymous. ANSI defines NCHAR or NATIONAL CHAR as a way to indicate that a CHAR column should use some predefined character set. MySQL 4.1 and up uses utf8 as that predefined character set. For example, these column type declarations are equivalent: 

CHAR(10) CHARACTER SET utf8
NATIONAL CHARACTER(10)
NCHAR(10)

As are these: 

VARCHAR(10) CHARACTER SET utf8
NATIONAL VARCHAR(10)
NCHAR VARCHAR(10)
NATIONAL CHARACTER VARYING(10)
NATIONAL CHAR VARYING(10)

You can use N'literal' to create a string in the national character set. These two statements are equivalent: 

SELECT N'some text';
SELECT _utf8'some text';

11.10 Upgrading Character Sets from MySQL 4.0

Now, what about upgrading from older versions of MySQL? MySQL 4.1 is almost upward compatible with MySQL 4.0 and earlier for the simple reason that almost all the features are new, so there's nothing in earlier versions to conflict with. However, there are some differences and a few things to be aware of.

Most important: The ``MySQL 4.0 character set'' has the properties of both ``MySQL 4.1 character sets'' and ``MySQL 4.1 collations.'' You will have to unlearn this. Henceforth, we will not bundle character set/collation properties in the same conglomerate object.

There is a special treatment of national character sets in MySQL 4.1. NCHAR is not the same as CHAR, and N'...' literals are not the same as '...' literals.

Finally, there is a different file format for storing information about character sets and collations. Make sure that you have reinstalled the `/share/mysql/charsets/' directory containing the new configuration files.

If you want to start @command{mysqld} from a 4.1.x distribution with data created by MySQL 4.0, you should start the server with the same character set and collation. In this case, you won't need to reindex your data.

There are two ways to do so: 

shell> ./configure --with-charset=... --with-collation=...
shell> ./mysqld --default-character-set=... --default-collation=...

If you used @command{mysqld} with, for example, the MySQL 4.0 danish character set, you should now use the latin1 character set and the latin1_danish_ci collation: 

shell> ./configure --with-charset=latin1 \
           --with-collation=latin1_danish_ci
shell> ./mysqld --default-character-set=latin1 \
           --default-collation=latin1_danish_ci

Use the table shown in section 11.10.1 4.0 Character Sets and Corresponding 4.1 Character Set/Collation Pairs to find old 4.0 character set names and their 4.1 character set/collation pair equivalents.

If you have non-latin1 data stored in a 4.0 latin1 table and want to convert the table column definitions to reflect the actual character set of the data, use the instructions in section 11.10.2 Converting 4.0 Character Columns to 4.1 Format.

11.10.1 4.0 Character Sets and Corresponding 4.1 Character Set/Collation Pairs

ID 4.0 Character Set 4.1 Character Set 4.1 Collation
1 big5 big5 big5_chinese_ci
2 czech latin2 latin2_czech_ci
3 dec8 dec8 dec8_swedish_ci
4 dos cp850 cp850_general_ci
5 german1 latin1 latin1_german1_ci
6 hp8 hp8 hp8_english_ci
7 koi8_ru koi8r koi8r_general_ci
8 latin1 latin1 latin1_swedish_ci
9 latin2 latin2 latin2_general_ci
10 swe7 swe7 swe7_swedish_ci
11 usa7 ascii ascii_general_ci
12 ujis ujis ujis_japanese_ci
13 sjis sjis sjis_japanese_ci
14 cp1251 cp1251 cp1251_bulgarian_ci
15 danish latin1 latin1_danish_ci
16 hebrew hebrew hebrew_general_ci
17 win1251 (removed) (removed)
18 tis620 tis620 tis620_thai_ci
19 euc_kr euckr euckr_korean_ci
20 estonia latin7 latin7_estonian_ci
21 hungarian latin2 latin2_hungarian_ci
22 koi8_ukr koi8u koi8u_ukrainian_ci
23 win1251ukr cp1251 cp1251_ukrainian_ci
24 gb2312 gb2312 gb2312_chinese_ci
25 greek greek greek_general_ci
26 win1250 cp1250 cp1250_general_ci
27 croat latin2 latin2_croatian_ci
28 gbk gbk gbk_chinese_ci
29 cp1257 cp1257 cp1257_lithuanian_ci
30 latin5 latin5 latin5_turkish_ci
31 latin1_de latin1 latin1_german2_ci

11.10.2 Converting 4.0 Character Columns to 4.1 Format

Normally, the server runs using the latin1 character set by default. If you have been storing column data that actually is in some other character set that the 4.1 server now supports directly, you can convert the column. However, you should avoid trying to convert directly from latin1 to the "real" character set. This may result in data loss. Instead, convert the column to a binary column type, and then from the binary type to a non-binary type with the desired character set. Conversion to and from binary involves no attempt at character value conversion and preserves your data intact. For example, suppose that you have a 4.0 table with three columns that are used to store values represented in latin1, latin2, and utf8: 

CREATE TABLE t
(
    latin1_col CHAR(50),
    latin2_col CHAR(100),
    utf8_col CHAR(150)
);

After upgrading to MySQL 4.1, you want to convert this table to leave latin1_col alone but change the latin2_col and utf8_col columns to have character sets of latin2 and utf8. First, back up your table, then convert the columns as follows: 

ALTER TABLE t MODIFY latin2_col BINARY(100);
ALTER TABLE t MODIFY utf8_col BINARY(150);
ALTER TABLE t MODIFY latin2_col CHAR(100) CHARACTER SET latin2;
ALTER TABLE t MODIFY utf8_col CHAR(150) CHARACTER SET utf8;

The first two statements ``remove'' the character set information from the latin2_col and utf8_col columns. The second two statements assign the proper character sets to the two columns.

If you like, you can combine the to-binary conversions and from-binary conversions into single statements: 

ALTER TABLE t
    MODIFY latin2_col BINARY(100),
    MODIFY utf8_col BINARY(150);
ALTER TABLE t
    MODIFY latin2_col CHAR(100) CHARACTER SET latin2,
    MODIFY utf8_col CHAR(150) CHARACTER SET utf8;

11.11 Character Sets and Collations That MySQL Supports

Here is an annotated list of character sets and collations that MySQL supports. Because options and installation settings differ, some sites might not have all items listed, and some sites might have items not listed.

MySQL supports 70+ collations for 30+ character sets. The character sets and their default collations are displayed by the SHOW CHARACTER SET STATEMENT. (The output actually includes another column that is not shown so that the example fits better on the page.) 

mysql> SHOW CHARACTER SET;
+----------+-----------------------------+---------------------+
| Charset  | Description                 | Default collation   |
+----------+-----------------------------+---------------------+
| big5     | Big5 Traditional Chinese    | big5_chinese_ci     |
| dec8     | DEC West European           | dec8_swedish_ci     |
| cp850    | DOS West European           | cp850_general_ci    |
| hp8      | HP West European            | hp8_english_ci      |
| koi8r    | KOI8-R Relcom Russian       | koi8r_general_ci    |
| latin1   | ISO 8859-1 West European    | latin1_swedish_ci   |
| latin2   | ISO 8859-2 Central European | latin2_general_ci   |
| swe7     | 7bit Swedish                | swe7_swedish_ci     |
| ascii    | US ASCII                    | ascii_general_ci    |
| ujis     | EUC-JP Japanese             | ujis_japanese_ci    |
| sjis     | Shift-JIS Japanese          | sjis_japanese_ci    |
| cp1251   | Windows Cyrillic            | cp1251_bulgarian_ci |
| hebrew   | ISO 8859-8 Hebrew           | hebrew_general_ci   |
| tis620   | TIS620 Thai                 | tis620_thai_ci      |
| euckr    | EUC-KR Korean               | euckr_korean_ci     |
| koi8u    | KOI8-U Ukrainian            | koi8u_general_ci    |
| gb2312   | GB2312 Simplified Chinese   | gb2312_chinese_ci   |
| greek    | ISO 8859-7 Greek            | greek_general_ci    |
| cp1250   | Windows Central European    | cp1250_general_ci   |
| gbk      | GBK Simplified Chinese      | gbk_chinese_ci      |
| latin5   | ISO 8859-9 Turkish          | latin5_turkish_ci   |
| armscii8 | ARMSCII-8 Armenian          | armscii8_general_ci |
| utf8     | UTF-8 Unicode               | utf8_general_ci     |
| ucs2     | UCS-2 Unicode               | ucs2_general_ci     |
| cp866    | DOS Russian                 | cp866_general_ci    |
| keybcs2  | DOS Kamenicky Czech-Slovak  | keybcs2_general_ci  |
| macce    | Mac Central European        | macce_general_ci    |
| macroman | Mac West European           | macroman_general_ci |
| cp852    | DOS Central European        | cp852_general_ci    |
| latin7   | ISO 8859-13 Baltic          | latin7_general_ci   |
| cp1256   | Windows Arabic              | cp1256_general_ci   |
| cp1257   | Windows Baltic              | cp1257_general_ci   |
| binary   | Binary pseudo charset       | binary              |
| geostd8  | GEOSTD8 Georgian            | geostd8_general_ci  |
+----------+-----------------------------+---------------------+

11.11.1 Unicode Character Sets

MySQL has two Unicode character sets. You can store texts in about 650 languages using these character sets. We have added several collations for these two new sets, with more to come.

11.11.2 West European Character Sets

West European Character Sets cover most West European languages, such as French, Spanish, Catalan, Basque, Portuguese, Italian, Albanian, Dutch, German, Danish, Swedish, Norwegian, Finnish, Faroese, Icelandic, Irish, Scottish, and English.

11.11.3 Central European Character Sets

We have some support for character sets used in the Czech Republic, Slovakia, Hungary, Romania, Slovenia, Croatia, and Poland.

11.11.4 South European and Middle East Character Sets

11.11.5 Baltic Character Sets

The Baltic character sets cover Estonian, Latvian, and Lithuanian languages. There are two Baltic character sets currently supported:

11.11.6 Cyrillic Character Sets

Here are the Cyrillic character sets and collations for use with Belarusian, Bulgarian, Russian, and Ukrainian languages.

11.11.7 Asian Character Sets

The Asian character sets that we support include Chinese, Japanese, Korean, and Thai. These can be complicated. For example, the Chinese sets must allow for thousands of different characters.

12 Column Types

MySQL supports a number of column types in several categories: numeric types, date and time types, and string (character) types. This chapter first gives an overview of these column types, and then provides a more detailed description of the properties of the types in each category, and a summary of the column type storage requirements. The overview is intentionally brief. The more detailed descriptions should be consulted for additional information about particular column types, such as the allowable formats in which you can specify values.

MySQL versions 4.1 and up support extensions for handing spatial data. Information about spatial types is provided in section 19 Spatial Extensions in MySQL.

Several of the column type descriptions use these conventions:

M
Indicates the maximum display width. The maximum legal display width is 255.
D
Applies to floating-point and fixed-point types and indicates the number of digits following the decimal point. The maximum possible value is 30, but should be no greater than M-2.
Square brackets (`[' and `]') indicate parts of type specifiers that are optional.

12.1 Column Type Overview

12.1.1 Overview of Numeric Types

A summary of the numeric column types follows. For additional information, see section 12.2 Numeric Types. Column storage requirements are given in section 12.5 Column Type Storage Requirements.

M indicates the maximum display width. The maximum legal display width is 255. Display width is unrelated to the storage size or range of values a type can contain, as described in section 12.2 Numeric Types.

If you specify ZEROFILL for a numeric column, MySQL automatically adds the UNSIGNED attribute to the column.

Warning: You should be aware that when you use subtraction between integer values where one is of type UNSIGNED, the result will be unsigned! See section 13.7 Cast Functions and Operators.

TINYINT[(M)] [UNSIGNED] [ZEROFILL]
A very small integer. The signed range is -128 to 127. The unsigned range is 0 to 255.
BIT
BOOL
BOOLEAN
These are synonyms for TINYINT(1). The BOOLEAN synonym was added in MySQL 4.1.0. A value of zero is considered false. Non-zero values are considered true. In the future, full boolean type handling will be introduced in accordance with standard SQL.
SMALLINT[(M)] [UNSIGNED] [ZEROFILL]
A small integer. The signed range is -32768 to 32767. The unsigned range is 0 to 65535.
MEDIUMINT[(M)] [UNSIGNED] [ZEROFILL]
A medium-size integer. The signed range is -8388608 to 8388607. The unsigned range is 0 to 16777215.
INT[(M)] [UNSIGNED] [ZEROFILL]
A normal-size integer. The signed range is -2147483648 to 2147483647. The unsigned range is 0 to 4294967295.
INTEGER[(M)] [UNSIGNED] [ZEROFILL]
This is a synonym for INT.
BIGINT[(M)] [UNSIGNED] [ZEROFILL]
A large integer. The signed range is -9223372036854775808 to 9223372036854775807. The unsigned range is 0 to 18446744073709551615. Some things you should be aware of with respect to BIGINT columns:
  • All arithmetic is done using signed BIGINT or DOUBLE values, so you shouldn't use unsigned big integers larger than 9223372036854775807 (63 bits) except with bit functions! If you do that, some of the last digits in the result may be wrong because of rounding errors when converting a BIGINT value to a DOUBLE. MySQL 4.0 can handle BIGINT in the following cases:
    • When using integers to store big unsigned values in a BIGINT column.
    • In MIN(col_name) or MAX(col_name), where col_name refers to a BIGINT column.
    • When using operators (+, -, *, and so on) where both operands are integers.
  • You can always store an exact integer value in a BIGINT column by storing it using a string. In this case, MySQL performs a string-to-number conversion that involves no intermediate double-precision representation.
  • The -, +, and * operators will use BIGINT arithmetic when both operands are integer values! This means that if you multiply two big integers (or results from functions that return integers), you may get unexpected results when the result is larger than 9223372036854775807.
FLOAT(p) [UNSIGNED] [ZEROFILL]
A floating-point number. p represents the precision. It can be from 0 to 24 for a single-precision floating-point number and from 25 to 53 for a double-precision floating-point number. These types are like the FLOAT and DOUBLE types described immediately following. FLOAT(p) has the same range as the corresponding FLOAT and DOUBLE types, but the display width and number of decimals are undefined. As of MySQL 3.23, this is a true floating-point value. In earlier MySQL versions, FLOAT(p) always has two decimals. This syntax is provided for ODBC compatibility. Using FLOAT might give you some unexpected problems because all calculations in MySQL are done with double precision. See section A.5.7 Solving Problems with No Matching Rows.
FLOAT[(M,D)] [UNSIGNED] [ZEROFILL]
A small (single-precision) floating-point number. Allowable values are -3.402823466E+38 to -1.175494351E-38, 0, and 1.175494351E-38 to 3.402823466E+38. If UNSIGNED is specified, negative values are disallowed. M is the display width and D is the number of decimals. FLOAT without arguments or FLOAT(p) (where p is in the range from 0 to 24) stands for a single-precision floating-point number.
DOUBLE[(M,D)] [UNSIGNED] [ZEROFILL]
A normal-size (double-precision) floating-point number. Allowable values are -1.7976931348623157E+308 to -2.2250738585072014E-308, 0, and 2.2250738585072014E-308 to 1.7976931348623157E+308. If UNSIGNED is specified, negative values are disallowed. M is the display width and D is the number of decimals. DOUBLE without arguments or FLOAT(p) (where p is in the range from 25 to 53) stands for a double-precision floating-point number.
DOUBLE PRECISION[(M,D)] [UNSIGNED] [ZEROFILL]
REAL[(M,D)] [UNSIGNED] [ZEROFILL]
These are synonyms for DOUBLE. Exception: If the server SQL mode includes the REAL_AS_FLOAT option, REAL is a synonym for FLOAT rather than DOUBLE.
DECIMAL[(M[,D])] [UNSIGNED] [ZEROFILL]
An unpacked fixed-point number. Behaves like a CHAR column; ``unpacked'' means the number is stored as a string, using one character for each digit of the value. M is the total number of digits and D is the number of decimals. The decimal point and (for negative numbers) the `-' sign are not counted in M, although space for them is reserved. If D is 0, values have no decimal point or fractional part. The maximum range of DECIMAL values is the same as for DOUBLE, but the actual range for a given DECIMAL column may be constrained by the choice of M and D. If UNSIGNED is specified, negative values are disallowed. If D is omitted, the default is 0. If M is omitted, the default is 10. Prior to MySQL 3.23, the M argument must be large enough to include the space needed for the sign and the decimal point.
DEC[(M[,D])] [UNSIGNED] [ZEROFILL]
NUMERIC[(M[,D])] [UNSIGNED] [ZEROFILL]
FIXED[(M[,D])] [UNSIGNED] [ZEROFILL]
These are synonyms for DECIMAL. The FIXED synonym was added in MySQL 4.1.0 for compatibility with other servers.

12.1.2 Overview of Date and Time Types

A summary of the temporal column types follows. For additional information, see section 12.3 Date and Time Types. Column storage requirements are given in section 12.5 Column Type Storage Requirements.

DATE
A date. The supported range is '1000-01-01' to '9999-12-31'. MySQL displays DATE values in 'YYYY-MM-DD' format, but allows you to assign values to DATE columns using either strings or numbers.
DATETIME
A date and time combination. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. MySQL displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format, but allows you to assign values to DATETIME columns using either strings or numbers.
TIMESTAMP[(M)]
A timestamp. The range is '1970-01-01 00:00:00' to partway through the year 2037. A TIMESTAMP column is useful for recording the date and time of an INSERT or UPDATE operation. The first TIMESTAMP column in a table is automatically set to the date and time of the most recent operation if you don't assign it a value yourself. You can also set any TIMESTAMP column to the current date and time by assigning it a NULL value. From MySQL 4.1 on, TIMESTAMP is returned as a string with the format 'YYYY-MM-DD HH:MM:SS'. If you want to obtain the value as a number, you should add +0 to the timestamp column. Different timestamp display widths are not supported. In MySQL 4.0 and earlier, TIMESTAMP values are displayed in YYYYMMDDHHMMSS, YYMMDDHHMMSS, YYYYMMDD, or YYMMDD format, depending on whether M is 14 (or missing), 12, 8, or 6, but allows you to assign values to TIMESTAMP columns using either strings or numbers. The M argument affects only how a TIMESTAMP column is displayed, not storage. Its values always are stored using four bytes each. From MySQL 4.0.12, the --new option can be used to make the server behave as in MySQL 4.1. Note that TIMESTAMP(M) columns where M is 8 or 14 are reported to be numbers, whereas other TIMESTAMP(M) columns are reported to be strings. This is just to ensure that you can reliably dump and restore the table with these types.
TIME
A time. The range is '-838:59:59' to '838:59:59'. MySQL displays TIME values in 'HH:MM:SS' format, but allows you to assign values to TIME columns using either strings or numbers.
YEAR[(2|4)]
A year in two-digit or four-digit format. The default is four-digit format. In four-digit format, the allowable values are 1901 to 2155, and 0000. In two-digit format, the allowable values are 70 to 69, representing years from 1970 to 2069. MySQL displays YEAR values in YYYY format, but allows you to assign values to YEAR columns using either strings or numbers. The YEAR type is unavailable prior to MySQL 3.22.

12.1.3 Overview of String Types

A summary of the string column types follows. For additional information, see section 12.4 String Types. Column storage requirements are given in section 12.5 Column Type Storage Requirements.

In some cases, MySQL may change a string column to a type different from that given in a CREATE TABLE or ALTER TABLE statement. See section 14.2.6.1 Silent Column Specification Changes.

A change that affects many string column types is that, as of MySQL 4.1, character column definitions can include a CHARACTER SET attribute to specify the character set and, optionally, a collation. This applies to CHAR, VARCHAR, the TEXT types, ENUM, and SET. For example: 

CREATE TABLE t
(
    c1 CHAR(20) CHARACTER SET utf8,
    c2 CHAR(20) CHARACTER SET latin1 COLLATE latin1_bin
);

This table definition creates a column named c1 that has a character set of utf8 with the default collation for that character set, and a column named c2 that has a character set of latin1 and the binary collation for the character set. The binary collation is not case sensitive.

Character column sorting and comparison are are based on the character set assigned to the column. Before MySQL 4.1, sorting and comparison are based on the collation of the server character set. For CHAR and VARCHAR columns, you can declare the column with the BINARY attribute to cause sorting and comparison to be case sensitive using the underlying character code values rather then a lexical ordering.

For more details, see section 11 Character Set Support.

Also as of 4.1, MySQL interprets length specifications in character column definitions in characters. (Earlier versions interpret them in bytes.)

[NATIONAL] CHAR(M) [BINARY | ASCII | UNICODE]
A fixed-length string that is always right-padded with spaces to the specified length when stored. M represents the column length. The range of M is 0 to 255 characters (1 to 255 prior to MySQL 3.23). Note: Trailing spaces are removed when CHAR values are retrieved. From MySQL 4.1.0, a CHAR column with a length specification greater than 255 is converted to the smallest TEXT type that can hold values of the given length. For example, CHAR(500) is converted to TEXT, and CHAR(200000) is converted to MEDIUMTEXT. This is a compatibility feature. However, this conversion causes the column to become a variable-length column, and also affects trailing-space removal. CHAR is shorthand for CHARACTER. NATIONAL CHAR (or its equivalent short form, NCHAR) is the standard SQL way to define that a CHAR column should use the default character set. This is the default in MySQL. The BINARY attribute causes sorting and comparisons to be case sensitive. From MySQL 4.1.0 on, the ASCII attribute can be specified. It assigns the latin1 character set to a CHAR column. From MySQL 4.1.1 on, the UNICODE attribute can be specified. It assigns the ucs2 character set to a CHAR column. MySQL allows you to create a column of type CHAR(0). This is mainly useful when you have to be compliant with some old applications that depend on the existence of a column but that do not actually use the value. This is also quite nice when you need a column that can take only two values: A CHAR(0) column that is not defined as NOT NULL occupies only one bit and can take only the values NULL and '' (the empty string).
CHAR
This is a synonym for CHAR(1).
[NATIONAL] VARCHAR(M) [BINARY]
A variable-length string. M represents the maximum column length. The range of M is 0 to 255 characters (1 to 255 prior to MySQL 4.0.2). Note: Trailing spaces are removed when VARCHAR values are stored, which differs from the standard SQL specification. From MySQL 4.1.0 on, a VARCHAR column with a length specification greater than 255 is converted to the smallest TEXT type that can hold values of the given length. For example, VARCHAR(500) is converted to TEXT, and VARCHAR(200000) is converted to MEDIUMTEXT. This is a compatibility feature. However, this conversion affects trailing-space removal. VARCHAR is shorthand for CHARACTER VARYING. The BINARY attribute causes sorting and comparisons to be case sensitive.
TINYBLOB
TINYTEXT
A BLOB or TEXT column with a maximum length of 255 (2^8 - 1) characters.
BLOB
TEXT
A BLOB or TEXT column with a maximum length of 65,535 (2^16 -1) characters.
MEDIUMBLOB
MEDIUMTEXT
A BLOB or TEXT column with a maximum length of 16,777,215 (2^24 - 1) characters.
LONGBLOB
LONGTEXT
A BLOB or TEXT column with a maximum length of 4,294,967,295 or 4GB (2^32 - 1) characters. Up to MySQL 3.23, the client/server protocol and MyISAM tables had a limit of 16MB per communication packet / table row. From MySQL 4.0, the maximum allowed length of LONGBLOB or LONGTEXT columns depends on the configured maximum packet size in the client/server protocol and available memory.
ENUM('value1','value2',...)
An enumeration. A string object that can have only one value, chosen from the list of values 'value1', 'value2', ..., NULL or the special '' error value. An ENUM column can have a maximum of 65,535 distinct values. ENUM values are represented internally as integers.
SET('value1','value2',...)
A set. A string object that can have zero or more values, each of which must be chosen from the list of values 'value1', 'value2', ... A SET column can have a maximum of 64 members. SET values are represented internally as integers.

12.2 Numeric Types

MySQL supports all of the standard SQL numeric data types. These types include the exact numeric data types (INTEGER, SMALLINT, DECIMAL, and NUMERIC), as well as the approximate numeric data types (FLOAT, REAL, and DOUBLE PRECISION). The keyword INT is a synonym for INTEGER, and the keyword DEC is a synonym for DECIMAL.

As an extension to the SQL standard, MySQL also supports the integer types TINYINT, MEDIUMINT, and BIGINT as listed in the following table.

Type Bytes Minimum Value Maximum Value
(Signed/Unsigned) (Signed/Unsigned)
TINYINT 1 -128 127
0 255
SMALLINT 2 -32768 32767
0 65535
MEDIUMINT 3 -8388608 8388607
0 16777215
INT 4 -2147483648 2147483647
0 4294967295
BIGINT 8 -9223372036854775808 9223372036854775807
0 18446744073709551615

Another extension is supported by MySQL for optionally specifying the display width of an integer value in parentheses following the base keyword for the type (for example, INT(4)). This optional display width specification is used to left-pad the display of values having a width less than the width specified for the column.

The display width does not constrain the range of values that can be stored in the column, nor the number of digits that will be displayed for values having a width exceeding that specified for the column.

When used in conjunction with the optional extension attribute ZEROFILL, the default padding of spaces is replaced with zeros. For example, for a column declared as INT(5) ZEROFILL, a value of 4 is retrieved as 00004. Note that if you store larger values than the display width in an integer column, you may experience problems when MySQL generates temporary tables for some complicated joins, because in these cases MySQL trusts that the data did fit into the original column width.

All integer types can have an optional (non-standard) attribute UNSIGNED. Unsigned values can be used when you want to allow only non-negative numbers in a column and you need a bigger upper numeric range for the column.

As of MySQL 4.0.2, floating-point and fixed-point types also can be UNSIGNED. As with integer types, this attribute prevents negative values from being stored in the column. However, unlike the integer types, the upper range of column values remains the same.

If you specify ZEROFILL for a numeric column, MySQL automatically adds the UNSIGNED attribute to the column.

The DECIMAL and NUMERIC types are implemented as the same type by MySQL. They are used to store values for which it is important to preserve exact precision, for example with monetary data. When declaring a column of one of these types, the precision and scale can be (and usually is) specified; for example: 

salary DECIMAL(5,2)

In this example, 5 is the precision and 2 is the scale. The precision represents the number of significant decimal digits that will be stored for values, and the scale represents the number of digits that will be stored following the decimal point.

MySQL stores DECIMAL and NUMERIC values as strings, rather than as binary floating-point numbers, in order to preserve the decimal precision of those values. One character is used for each digit of the value, the decimal point (if the scale is greater than 0), and the `-' sign (for negative numbers). If the scale is 0, DECIMAL and NUMERIC values contain no decimal point or fractional part.

Standard SQL requires that the salary column be able to store any value with five digits and two decimals. In this case, therefore, the range of values that can be stored in the salary column is from -999.99 to 999.99. MySQL varies from this in two ways:

In standard SQL, the syntax DECIMAL(M) is equivalent to DECIMAL(M,0). Similarly, the syntax DECIMAL is equivalent to DECIMAL(M,0), where the implementation is allowed to decide the value of M. As of MySQL 3.23.6, both of these variant forms of the DECIMAL and NUMERIC data types are supported. The default value of M is 10. Before 3.23.6, M and D both must be specified explicitly.

The maximum range of DECIMAL and NUMERIC values is the same as for DOUBLE, but the actual range for a given DECIMAL or NUMERIC column can be constrained by the precision or scale for a given column. When such a column is assigned a value with more digits following the decimal point than are allowed by the specified scale, the value is converted to that scale. (The precise behavior is operating system-specific, but generally the effect is truncation to the allowable number of digits.) When a DECIMAL or NUMERIC column is assigned a value that exceeds the range implied by the specified (or default) precision and scale, MySQL stores the value representing the corresponding end point of that range.

For floating-point column types, MySQL uses four bytes for single-precision values and eight bytes for double-precision values.

The FLOAT type is used to represent approximate numeric data types. The SQL standard allows an optional specification of the precision (but not the range of the exponent) in bits following the keyword FLOAT in parentheses. The MySQL implementation also supports this optional precision specification, but the precision value is used only to determine storage size. A precision from 0 to 23 results in four-byte single-precision FLOAT column. A precision from 24 to 53 results in eight-byte double-precision DOUBLE column.

When the keyword FLOAT is used for a column type without a precision specification, MySQL uses four bytes to store the values. MySQL also supports variant syntax with two numbers given in parentheses following the FLOAT keyword. The first number represents the display width and the second number specifies the number of digits to be stored and displayed following the decimal point (as with DECIMAL and NUMERIC). When MySQL is asked to store a number for such a column with more decimal digits following the decimal point than specified for the column, the value is rounded to eliminate the extra digits when the value is stored.

In standard SQL, the REAL and DOUBLE PRECISION types do not accept precision specifications. MySQL supports a variant syntax with two numbers given in parentheses following the type name. The first number represents the display width and the second number specifies the number of digits to be stored and displayed following the decimal point. As an extension to the SQL standard, MySQL recognizes DOUBLE as a synonym for the DOUBLE PRECISION type. In contrast with the standard's requirement that the precision for REAL be smaller than that used for DOUBLE PRECISION, MySQL implements both as eight-byte double-precision floating-point values (unless the server SQL mode includes the REAL_AS_FLOAT option).

For maximum portability, code requiring storage of approximate numeric data values should use FLOAT or DOUBLE PRECISION with no specification of precision or number of decimal points.

When asked to store a value in a numeric column that is outside the column type's allowable range, MySQL clips the value to the appropriate endpoint of the range and stores the resulting value instead.

For example, the range of an INT column is -2147483648 to 2147483647. If you try to insert -9999999999 into an INT column, MySQL clips the value to the lower endpoint of the range and stores -2147483648 instead. Similarly, if you try to insert 9999999999, MySQL clips the value to the upper endpoint of the range and stores 2147483647 instead.

If the INT column is UNSIGNED, the size of the column's range is the same but its endpoints shift up to 0 and 4294967295. If you try to store -9999999999 and 9999999999, the values stored in the column are 0 and 4294967296.

Conversions that occur due to clipping are reported as ``warnings'' for ALTER TABLE, LOAD DATA INFILE, UPDATE, and multiple-row INSERT statements.

12.3 Date and Time Types

The date and time types for representing temporal values are DATETIME, DATE, TIMESTAMP, TIME, and YEAR. Each temporal type has a range of legal values, as well as a ``zero'' value that is used when you specify an illegal value that MySQL cannot represent. The TIMESTAMP type has special automatic updating behavior, described later on.

Starting from MySQL 5.0.2, MySQL will give warnings/errors if you try to insert an illegal date. You can get MySQL to accept certain dates, such as '1999-11-31', by using the ALLOW_INVALID_DATES SQL mode. (Before 5.0.2, this mode was the default behavior for MySQL). This is useful when you want to store the ``possibly wrong'' value the user has specified (for example, in a web form) in the database for future processing. Under this mode, MySQL verifies only that the month is in the range from 0 to 12 and that the day is in the range from 0 to 31. These ranges are defined to include zero because MySQL allows you to store dates where the day or month and day are zero in a DATE or DATETIME column. This is extremely useful for applications that need to store a birthdate for which you don't know the exact date. In this case, you simply store the date as '1999-00-00' or '1999-01-00'. If you store dates such as these, you should not expect to get correct results for functions such as DATE_SUB() or DATE_ADD that require complete dates. (If you don't want to allow zero in dates, you can use the NO_ZERO_IN_DATE SQL mode).

MySQL also allows you to store '0000-00-00' as a ``dummy date'' (if you are not using the NO_ZERO_DATE SQL mode). This is in some cases is more convenient than using NULL values.

By setting the sql_mode system variable to the appropriate mode values, You can more exactly what kind of dates you want MySQL to support. See section 5.2.2 The Server SQL Mode.

Here are some general considerations to keep in mind when working with date and time types:

12.3.1 The DATETIME, DATE, and TIMESTAMP Types

The DATETIME, DATE, and TIMESTAMP types are related. This section describes their characteristics, how they are similar, and how they differ.

The DATETIME type is used when you need values that contain both date and time information. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. (``Supported'' means that although earlier values might work, there is no guarantee that they will.)

The DATE type is used when you need only a date value, without a time part. MySQL retrieves and displays DATE values in 'YYYY-MM-DD' format. The supported range is '1000-01-01' to '9999-12-31'.

The TIMESTAMP column type has varying properties, depending on the MySQL version and the SQL mode the server is running in. These properties are described later in this section.

You can specify DATETIME, DATE, and TIMESTAMP values using any of a common set of formats:

Illegal DATETIME, DATE, or TIMESTAMP values are converted to the ``zero'' value of the appropriate type ('0000-00-00 00:00:00', '0000-00-00', or 00000000000000).

For values specified as strings that include date part delimiters, it is not necessary to specify two digits for month or day values that are less than 10. '1979-6-9' is the same as '1979-06-09'. Similarly, for values specified as strings that include time part delimiters, it is not necessary to specify two digits for hour, minute, or second values that are less than 10. '1979-10-30 1:2:3' is the same as '1979-10-30 01:02:03'.

Values specified as numbers should be 6, 8, 12, or 14 digits long. If a number is 8 or 14 digits long, it is assumed to be in YYYYMMDD or YYYYMMDDHHMMSS format and that the year is given by the first 4 digits. If the number is 6 or 12 digits long, it is assumed to be in YYMMDD or YYMMDDHHMMSS format and that the year is given by the first 2 digits. Numbers that are not one of these lengths are interpreted as though padded with leading zeros to the closest length.

Values specified as non-delimited strings are interpreted using their length as given. If the string is 8 or 14 characters long, the year is assumed to be given by the first 4 characters. Otherwise, the year is assumed to be given by the first 2 characters. The string is interpreted from left to right to find year, month, day, hour, minute, and second values, for as many parts as are present in the string. This means you should not use strings that have fewer than 6 characters. For example, if you specify '9903', thinking that will represent March, 1999, you will find that MySQL inserts a ``zero'' date into your table. This is because the year and month values are 99 and 03, but the day part is completely missing, so the value is not a legal date. However, as of MySQL 3.23, you can explicitly specify a value of zero to represent missing month or day parts. For example, you can use '990300' to insert the value '1999-03-00'.

You can to some extent assign values of one date type to an object of a different date type. However, there may be some alteration of the value or loss of information:

Be aware of certain pitfalls when specifying date values:

12.3.1.1 TIMESTAMP Properties Prior to MySQL 4.1

The TIMESTAMP column type provides a type that you can use to automatically mark INSERT or UPDATE operations with the current date and time. If you have multiple TIMESTAMP columns in a table, only the first one is updated automatically. (From MySQL 4.1.2 on, you can specify which TIMESTAMP column updates; see section 12.3.1.2 TIMESTAMP Properties as of MySQL 4.1.)

Automatic updating of the first TIMESTAMP column in a table occurs under any of the following conditions:

TIMESTAMP columns other than the first can also be set to the current date and time. Just set the column to NULL or to any function that produces the current date and time (NOW(), CURRENT_TIMESTAMP).

You can set any TIMESTAMP column to a value different from the current date and time by setting it explicitly to the desired value. This is true even for the first TIMESTAMP column. You can use this property if, for example, you want a TIMESTAMP to be set to the current date and time when you create a row, but not to be changed whenever the row is updated later:

Another way to maintain a column that records row-creation time is to use a DATETIME column that you initialize to NOW() when the row is created and leave alone for subsequent updates.

TIMESTAMP values may range from the beginning of 1970 to partway through the year 2037, with a resolution of one second. Values are displayed as numbers. When you store a value in a TIMESTAMP column, it is assumed to be represented in the current time zone, and is converted to UTC for storage. When you retrieve the value, it is converted from UTC back to the local time zone for display. Before MySQL 4.1.3, the server has a single time zone. As of 4.1.3, clients can set their time zone on a per-connection basis, as described in Time zone support.

The format in which MySQL retrieves and displays TIMESTAMP values depends on the display size, as illustrated by the following table. The ``full'' TIMESTAMP format is 14 digits, but TIMESTAMP columns may be created with shorter display sizes:

Column Type Display Format
TIMESTAMP(14) YYYYMMDDHHMMSS
TIMESTAMP(12) YYMMDDHHMMSS
TIMESTAMP(10) YYMMDDHHMM
TIMESTAMP(8) YYYYMMDD
TIMESTAMP(6) YYMMDD
TIMESTAMP(4) YYMM
TIMESTAMP(2) YY

All TIMESTAMP columns have the same storage size, regardless of display size. The most common display sizes are 6, 8, 12, and 14. You can specify an arbitrary display size at table creation time, but values of 0 or greater than 14 are coerced to 14. Odd-valued sizes in the range from 1 to 13 are coerced to the next higher even number.

TIMESTAMP columns store legal values using the full precision with which the value was specified, regardless of the display size. This has several implications:

12.3.1.2 TIMESTAMP Properties as of MySQL 4.1

In MySQL 4.1 and up, the properties of the TIMESTAMP column type change in the ways described in this section.

From MySQL 4.1.0 on, TIMESTAMP display format differs from that of earlier MySQL releases:

Beginning with MySQL 4.1.1, the MySQL server can be run in MAXDB mode. When the server runs in this mode, TIMESTAMP is identical with DATETIME. That is, if the server is running in MAXDB mode at the time that a table is created, TIMESTAMP columns are created as DATETIME columns. As a result, such columns use DATETIME display format, have the same range of values, and there is no automatic initialization or updating to the current date and time.

To enable MAXDB mode, set the server SQL mode to MAXDB at startup using the --sql-mode=MAXDB server option or by setting the global sql_mode variable at runtime: 

mysql> SET GLOBAL sql_mode=MAXDB;

A client can cause the server to run in MAXDB mode for its own connection as follows: 

mysql> SET SESSION sql_mode=MAXDB;

As of MySQL 5.0.2, MySQL will not accept timestamp values that includes a zero in the day or month column or values that are not a valid date. (The execption is the special value '0000-00-00 00:00:00'.)

Beginning with MySQL 4.1.2, you have more flexible control over when automatic TIMESTAMP initialization and updating occur and which column should have those behaviors:

The following discussion describes the revised syntax and behavior. Note that this information applies only to TIMESTAMP columns for tables not created with MAXDB mode enabled. As noted earlier in this section, MAXDB mode causes columns to be created as DATETIME columns.

The following items summarize the pre-4.1.2 properties for TIMESTAMP initialization and updating:

The first TIMESTAMP column in table row automatically is set to the current timestamp when the record is created if the column is set to NULL or is not specified at all.

The first TIMESTAMP column in table row automatically is updated to the current timestamp when the value of any other column in the row is changed, unless the TIMESTAMP column explicitly is assigned a value other than NULL.

If a DEFAULT value is specified for the first TIMESTAMP column when the table is created, it is silently ignored.

Other TIMESTAMP columns in the table can be set to the current TIMESTAMP by assigning NULL to them, but they do not update automatically.

As of 4.1.2, you have more flexibility in deciding which TIMESTAMP column automatically is initialized and updated to the current timestamp. The rules are as follows:

If a DEFAULT value is specified for the first TIMESTAMP column in a table, it is not ignored. The default can be CURRENT_TIMESTAMP or a constant date and time value.

DEFAULT NULL is the same as DEFAULT CURRENT_TIMESTAMP for the first TIMESTAMP column. For any other TIMESTAMP column, DEFAULT NULL is treated as DEFAULT 0.

Any single TIMESTAMP column in a table can be set to be the one that is initialized to the current timestamp and/or updated automatically.

In a CREATE TABLE statement, the first TIMESTAMP column can be declared in any of the following ways:

In other words, you can use the current timestamp for both the initial value and the auto-update value, or either one, or neither. (For example, you can specify ON UPDATE to get auto-update without also having the column auto-initialized.)

Any of CURRENT_TIMESTAMP, CURRENT_TIMESTAMP(), or NOW() can be used in the DEFAULT and ON UPDATE clauses. They all have the same effect.

The order of the two attributes does not matter. If both DEFAULT and ON UPDATE are specified for a TIMESTAMP column, either can precede the other.

Example. These statements are equivalent: 

CREATE TABLE t (ts TIMESTAMP);
CREATE TABLE t (ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP
                             ON UPDATE CURRENT_TIMESTAMP);
CREATE TABLE t (ts TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
                             DEFAULT CURRENT_TIMESTAMP);

To specify automatic default or updating for a TIMESTAMP column other than the first one, you must suppress the automatic initialization and update behaviors for the first TIMESTAMP column by explicitly assigning it a constant DEFAULT value (for example, DEFAULT 0 or DEFAULT '2003-01-01 00:00:00'). Then for the other TIMESTAMP column, the rules are the same as for the first TIMESTAMP column, except that you cannot omit both of the DEFAULT and ON UPDATE clauses. If you do that, no automatic initialization or updating occurs.

Example. These statements are equivalent: 

CREATE TABLE t (
    ts1 TIMESTAMP DEFAULT 0,
    ts2 TIMESTAMP DEFAULT CURRENT_TIMESTAMP
                  ON UPDATE CURRENT_TIMESTAMP);
CREATE TABLE t (
    ts1 TIMESTAMP DEFAULT 0,
    ts2 TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
                  DEFAULT CURRENT_TIMESTAMP);

Beginning with MySQL 4.1.3, you can set the current time zone on a per-connection basis, as described in Time zone support. TIMESTAMP values still are stored in UTC, but are converted from the current time zone for storage, and converted back to the current time zone for retrieval. As long as the time zone setting remains the same, you will get back the same value you store. If you store a TIMESTAMP value, then change the time zone and retrieve the value, it will be different than the value you stored. This occurs because the same time zone is not used for conversion in both directions. The current time zone is available as the value of the time_zone system variable.

12.3.2 The TIME Type

MySQL retrieves and displays TIME values in 'HH:MM:SS' format (or 'HHH:MM:SS' format for large hours values). TIME values may range from '-838:59:59' to '838:59:59'. The reason the hours part may be so large is that the TIME type may be used not only to represent a time of day (which must be less than 24 hours), but also elapsed time or a time interval between two events (which may be much greater than 24 hours, or even negative).

You can specify TIME values in a variety of formats:

For TIME values specified as strings that include a time part delimiter, it is not necessary to specify two digits for hours, minutes, or seconds values that are less than 10. '8:3:2' is the same as '08:03:02'.

Be careful about assigning ``short'' TIME values to a TIME column. Without colons, MySQL interprets values using the assumption that the rightmost digits represent seconds. (MySQL interprets TIME values as elapsed time rather than as time of day.) For example, you might think of '1112' and 1112 as meaning '11:12:00' (12 minutes after 11 o'clock), but MySQL interprets them as '00:11:12' (11 minutes, 12 seconds). Similarly, '12' and 12 are interpreted as '00:00:12'. TIME values with colons, by contrast, are always treated as time of the day. That is '11:12' will mean '11:12:00', not '00:11:12'.

Values that lie outside the TIME range but are otherwise legal are clipped to the closest endpoint of the range. For example, '-850:00:00' and '850:00:00' are converted to '-838:59:59' and '838:59:59'.

Illegal TIME values are converted to '00:00:00'. Note that because '00:00:00' is itself a legal TIME value, there is no way to tell, from a value of '00:00:00' stored in a table, whether the original value was specified as '00:00:00' or whether it was illegal.

12.3.3 The YEAR Type

The YEAR type is a one-byte type used for representing years.

MySQL retrieves and displays YEAR values in YYYY format. The range is 1901 to 2155.

You can specify YEAR values in a variety of formats:

Illegal YEAR values are converted to 0000.

12.3.4 Y2K Issues and Date Types

MySQL itself is year 2000 (Y2K) safe (see section 1.2.5 Year 2000 Compliance), but input values presented to MySQL may not be. Any input containing two-digit year values is ambiguous, because the century is unknown. Such values must be interpreted into four-digit form because MySQL stores years internally using four digits.

For DATETIME, DATE, TIMESTAMP, and YEAR types, MySQL interprets dates with ambiguous year values using the following rules:

Remember that these rules provide only reasonable guesses as to what your data values mean. If the heuristics used by MySQL do not produce the correct values, you should provide unambiguous input containing four-digit year values.

ORDER BY properly sorts TIMESTAMP or YEAR values that have two-digit years.

Some functions like MIN() and MAX() will convert a TIMESTAMP or YEAR to a number. This means that a value with a two-digit year will not work properly with these functions. The fix in this case is to convert the TIMESTAMP or YEAR to four-digit year format or use something like MIN(DATE_ADD(timestamp,INTERVAL 0 DAYS)).

12.4 String Types

The string types are CHAR, VARCHAR, BLOB, TEXT, ENUM, and SET. This section describes how these types work and how to use them in your queries.

12.4.1 The CHAR and VARCHAR Types

The CHAR and VARCHAR types are similar, but differ in the way they are stored and retrieved.

The length of a CHAR column is fixed to the length that you declare when you create the table. The length can be any value from 0 to 255. (Before MySQL 3.23, the length of CHAR may be from 1 to 255.) When CHAR values are stored, they are right-padded with spaces to the specified length. When CHAR values are retrieved, trailing spaces are removed.

Values in VARCHAR columns are variable-length strings. You can declare a VARCHAR column to be any length from 0 to 255, just as for CHAR columns. (Before MySQL 4.0.2, the length of VARCHAR may be from 1 to 255.) However, in contrast to CHAR, VARCHAR values are stored using only as many characters as are needed, plus one byte to record the length. Values are not padded; instead, trailing spaces are removed when values are stored. This space removal differs from the standard SQL specification.

No lettercase conversion takes place during storage or retrieval.

If you assign a value to a CHAR or VARCHAR column that exceeds the column's maximum length, the value is truncated to fit.

If you need a column for which trailing spaces are not removed, consider using a BLOB or TEXT type. If you want to store binary values such as results from an encryption or compression function that might contain arbitrary byte values, use a BLOB column rather than a CHAR or VARCHAR column to avoid potential problems with trailing space removal that would change data values.

The following table illustrates the differences between the two types of columns by showing the result of storing various string values into CHAR(4) and VARCHAR(4) columns:

Value CHAR(4) Storage Required VARCHAR(4) Storage Required
'' ' ' 4 bytes '' 1 byte
'ab' 'ab ' 4 bytes 'ab' 3 bytes
'abcd' 'abcd' 4 bytes 'abcd' 5 bytes
'abcdefgh' 'abcd' 4 bytes 'abcd' 5 bytes

The values retrieved from the CHAR(4) and VARCHAR(4) columns are the same in each case, because trailing spaces are removed from CHAR columns upon retrieval.

As of MySQL 4.1, values in CHAR and VARCHAR columns are sorted and compared according to the collation of the character set assigned to the column. Before MySQL 4.1, sorting and comparison are based on the collation of the server character set; you can declare the column with the BINARY attribute to cause sorting and comparison to be case sensitive using the underlying character code values rather then a lexical ordering. BINARY doesn't affect how the column is stored or retrieved.

From MySQL 4.1.0 on, column type CHAR BYTE is an alias for CHAR BINARY. This is a compatibility feature.

The BINARY attribute is sticky. This means that if a column marked BINARY is used in an expression, the whole expression is treated as a BINARY value.

From MySQL 4.1.0 on, the ASCII attribute can be specified for CHAR. It assigns the latin1 character set.

From MySQL 4.1.1 on, the UNICODE attribute can be specified for CHAR. It assigns the ucs2 character set.

MySQL may silently change the type of a CHAR or VARCHAR column at table creation time. See section 14.2.6.1 Silent Column Specification Changes.

12.4.2 The BLOB and TEXT Types

A BLOB is a binary large object that can hold a variable amount of data. The four BLOB types, TINYBLOB, BLOB, MEDIUMBLOB, and LONGBLOB, differ only in the maximum length of the values they can hold. See section 12.5 Column Type Storage Requirements.

The four TEXT types, TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT, correspond to the four BLOB types and have the same maximum lengths and storage requirements.

BLOB columns are treated as binary strings, whereas TEXT columns are treated according to their character set. For BLOB columns, sorting and comparison is case sensitive. For TEXT columns, values are sorted and compared based on the collation of the character set assigned to the column as of MySQL 4.1. Before 4.1, TEXT sorting and comparison are based on the collation of the server character set.

No lettercase conversion takes place during storage or retrieval.

If you assign a value to a BLOB or TEXT column that exceeds the column type's maximum length, the value is truncated to fit.

In most respects, you can regard a TEXT column as a VARCHAR column that can be as big as you like. Similarly, you can regard a BLOB column as a VARCHAR BINARY column. The ways in which BLOB and TEXT differ from CHAR and VARCHAR are:

From MySQL 4.1.0 on, LONG and LONG VARCHAR map to the MEDIUMTEXT data type. This is a compatibility feature.

MySQL Connector/ODBC defines BLOB values as LONGVARBINARY and TEXT values as LONGVARCHAR.

Because BLOB and TEXT values may be extremely long, you may encounter some constraints in using them:

Each BLOB or TEXT value is represented internally by a separately allocated object. This is in contrast to all other column types, for which storage is allocated once per column when the table is opened.

12.4.3 The ENUM Type

An ENUM is a string object with a value chosen from a list of allowed values that are enumerated explicitly in the column specification at table creation time.

The value may also be the empty string ('') or NULL under certain circumstances:

Each enumeration value has an index:

For example, a column specified as ENUM('one', 'two', 'three') can have any of the values shown here. The index of each value is also shown:

Value Index
NULL NULL
'' 0
'one' 1
'two' 2
'three' 3

An enumeration can have a maximum of 65,535 elements.

Starting from MySQL 3.23.51, trailing spaces are automatically deleted from ENUM member values when the table is created.

Lettercase is irrelevant when you assign values to an ENUM column. However, values retrieved from the column later are displayed using the lettercase that was used in the column definition.

If you retrieve an ENUM value in a numeric context, the column value's index is returned. For example, you can retrieve numeric values from an ENUM column like this: 

mysql> SELECT enum_col+0 FROM tbl_name;

If you store a number into an ENUM column, the number is treated as an index, and the value stored is the enumeration member with that index. (However, this will not work with LOAD DATA, which treats all input as strings.) It's not advisable to define an ENUM column with enumeration values that look like numbers, because this can easily become confusing. For example, the following column has enumeration members with string values of '0', '1', and '2', but numeric index values of 1, 2, and 3: 

numbers ENUM('0','1','2')

ENUM values are sorted according to the order in which the enumeration members were listed in the column specification. (In other words, ENUM values are sorted according to their index numbers.) For example, 'a' sorts before 'b' for ENUM('a', 'b'), but 'b' sorts before 'a' for ENUM('b', 'a'). The empty string sorts before non-empty strings, and NULL values sort before all other enumeration values. To prevent unexpected results, specify the ENUM list in alphabetical order. You can also use GROUP BY CAST(col AS VARCHAR) or GROUP BY CONCAT(col) to make sure that the column is sorted lexically rather than by index number.

If you want to determine all possible values for an ENUM column, use SHOW COLUMNS FROM tbl_name LIKE enum_col and parse the ENUM definition in the second column of the output.

12.4.4 The SET Type

A SET is a string object that can have zero or more values, each of which must be chosen from a list of allowed values specified when the table is created. SET column values that consist of multiple set members are specified with members separated by commas (`,'). A consequence of this is that SET member values cannot themselves contain commas.

For example, a column specified as SET('one', 'two') NOT NULL can have any of these values: 

''
'one'
'two'
'one,two'

A SET can have a maximum of 64 different members.

Starting from MySQL 3.23.51, trailing spaces are automatically deleted from SET member values when the table is created.

MySQL stores SET values numerically, with the low-order bit of the stored value corresponding to the first set member. If you retrieve a SET value in a numeric context, the value retrieved has bits set corresponding to the set members that make up the column value. For example, you can retrieve numeric values from a SET column like this: 

mysql> SELECT set_col+0 FROM tbl_name;

If a number is stored into a SET column, the bits that are set in the binary representation of the number determine the set members in the column value. For a column specified as SET('a','b','c','d'), the members have the following decimal and binary values:

SET Member Decimal Value Binary Value
'a' 1 0001
'b' 2 0010
'c' 4 0100
'd' 8 1000

If you assign a value of 9 to this column, that is 1001 in binary, so the first and fourth SET value members 'a' and 'd' are selected and the resulting value is 'a,d'.

For a value containing more than one SET element, it does not matter what order the elements are listed in when you insert the value. It also does not matter how many times a given element is listed in the value. When the value is retrieved later, each element in the value will appear once, with elements listed according to the order in which they were specified at table creation time. If a column is specified as SET('a','b','c','d'), then 'a,d', 'd,a', and 'd,a,a,d,d' all will appear as 'a,d' when retrieved.

If you set a SET column to an unsupported value, the value will be ignored.

SET values are sorted numerically. NULL values sort before non-NULL SET values.

Normally, you search for SET values using the FIND_IN_SET() function or the LIKE operator: 

mysql> SELECT * FROM tbl_name WHERE FIND_IN_SET('value',set_col)>0;
mysql> SELECT * FROM tbl_name WHERE set_col LIKE '%value%';

The first statement finds rows where set_col contains the value set member. The second is similar, but not the same: It finds rows where set_col contains value anywhere, even as a substring of another set member.

The following statements also are legal: 

mysql> SELECT * FROM tbl_name WHERE set_col & 1;
mysql> SELECT * FROM tbl_name WHERE set_col = 'val1,val2';

The first of these statements looks for values containing the first set member. The second looks for an exact match. Be careful with comparisons of the second type. Comparing set values to 'val1,val2' will return different results than comparing values to 'val2,val1'. You should specify the values in the same order they are listed in the column definition.

If you want to determine all possible values for a SET column, use SHOW COLUMNS FROM tbl_name LIKE set_col and parse the SET definition in the second column of the output.

12.5 Column Type Storage Requirements

The storage requirements for each of the column types supported by MySQL are listed by category.

The maximum size of a row in a MyISAM table is 65,534 bytes. Each BLOB and TEXT column accounts for only five to nine bytes toward this size.

If a MyISAM or ISAM table includes any variable-length column types, the record format will also be variable length. When a table is created, MySQL may, under certain conditions, change a column from a variable-length type to a fixed-length type or vice versa. See section 14.2.6.1 Silent Column Specification Changes.

12.5.1 Storage Requirements for Numeric Types

Column Type Storage Required
TINYINT 1 byte
SMALLINT 2 bytes
MEDIUMINT 3 bytes
INT, INTEGER 4 bytes
BIGINT 8 bytes
FLOAT(p) 4 bytes if 0 <= p <= 24, 8 bytes if 25 <= p <= 53
FLOAT 4 bytes
DOUBLE [PRECISION], item REAL 8 bytes
DECIMAL(M,D), NUMERIC(M,D) M+2 bytes if D > 0, M+1 bytes if D = 0 (D+2, if M < D)

12.5.2 Storage Requirements for Date and Time Types

Column Type Storage Required
DATE 3 bytes
DATETIME 8 bytes
TIMESTAMP 4 bytes
TIME 3 bytes
YEAR 1 byte

12.5.3 Storage Requirements for String Types

Column Type Storage Required
CHAR(M) M bytes, 0 <= M <= 255
VARCHAR(M) L+1 bytes, where L <= M and 0 <= M <= 255
TINYBLOB, TINYTEXT L+1 bytes, where L < 2^8
BLOB, TEXT L+2 bytes, where L < 2^16
MEDIUMBLOB, MEDIUMTEXT L+3 bytes, where L < 2^24
LONGBLOB, LONGTEXT L+4 bytes, where L < 2^32
ENUM('value1','value2',...) 1 or 2 bytes, depending on the number of enumeration values (65,535 values maximum)
SET('value1','value2',...) 1, 2, 3, 4, or 8 bytes, depending on the number of set members (64 members maximum)

VARCHAR and the BLOB and TEXT types are variable-length types. For each, the storage requirements depend on the actual length of column values (represented by L in the preceding table), rather than on the type's maximum possible size. For example, a VARCHAR(10) column can hold a string with a maximum length of 10 characters. The actual storage required is the length of the string (L), plus 1 byte to record the length of the string. For the string 'abcd', L is 4 and the storage requirement is 5 bytes.

The BLOB and TEXT types require 1, 2, 3, or 4 bytes to record the length of the column value, depending on the maximum possible length of the type. See section 12.4.2 The BLOB and TEXT Types.

The size of an ENUM object is determined by the number of different enumeration values. One byte is used for enumerations with up to 255 possible values. Two bytes are used for enumerations with up to 65,535 values. See section 12.4.3 The ENUM Type.

The size of a SET object is determined by the number of different set members. If the set size is N, the object occupies (N+7)/8 bytes, rounded up to 1, 2, 3, 4, or 8 bytes. A SET can have a maximum of 64 members. See section 12.4.4 The SET Type.

12.6 Choosing the Right Type for a Column

For the most efficient use of storage, try to use the most precise type in all cases. For example, if an integer column will be used for values in the range from 1 to 99999, MEDIUMINT UNSIGNED is the best type. Of the types that represent all the required values, it uses the least amount of storage.

Accurate representation of monetary values is a common problem. In MySQL, you should use the DECIMAL type. This is stored as a string, so no loss of accuracy should occur. (Calculations on DECIMAL values may still be done using double-precision operations, however.) If accuracy is not too important, the DOUBLE type may also be good enough.

For high precision, you can always convert to a fixed-point type stored in a BIGINT. This allows you to do all calculations with integers and convert results back to floating-point values only when necessary.

12.7 Using Column Types from Other Database Engines

To make it easier to use code written for SQL implementations from other vendors, MySQL maps column types as shown in the following table. These mappings make it easier to import table definitions from other database engines into MySQL:

Other Vendor Type MySQL Type
BINARY(M) CHAR(M) BINARY
CHAR VARYING(M) VARCHAR(M)
FLOAT4 FLOAT
FLOAT8 DOUBLE
INT1 TINYINT
INT2 SMALLINT
INT3 MEDIUMINT
INT4 INT
INT8 BIGINT
LONG VARBINARY MEDIUMBLOB
LONG VARCHAR MEDIUMTEXT
LONG MEDIUMTEXT (MySQL 4.1.0 on)
MIDDLEINT MEDIUMINT
VARBINARY(M) VARCHAR(M) BINARY

Column type mapping occurs at table creation time, after which the original type specifications are discarded. If you create a table with types used by other vendors and then issue a DESCRIBE tbl_name statement, MySQL reports the table structure using the equivalent MySQL types.

13 Functions and Operators

Expressions can be used at several points in SQL statements, such as in the ORDER BY or HAVING clauses of SELECT statements, in the WHERE clause of a SELECT, DELETE, or UPDATE statement, or in SET statements. Expressions can be written using literal values, column values, NULL, functions, and operators. This chapter describes the functions and operators that are allowed for writing expressions in MySQL.

An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for a particular function or operator.

Note: By default, there must be no whitespace between a function name and the parenthesis following it. This helps the MySQL parser distinguish between function calls and references to tables or columns that happen to have the same name as a function. Spaces around function arguments are permitted, though.

You can tell the MySQL server to accept spaces after function names by starting it with the --sql-mode=IGNORE_SPACE option. Individual client programs can request this behavior by using the CLIENT_IGNORE_SPACE option for mysql_real_connect(). In either case, all function names will become reserved words. See section 5.2.2 The Server SQL Mode.

For the sake of brevity, most examples in this chapter display the output from the @command{mysql} program in abbreviated form. Instead of showing examples in this format: 

mysql> SELECT MOD(29,9);
+-----------+
| mod(29,9) |
+-----------+
|         2 |
+-----------+
1 rows in set (0.00 sec)

This format is used instead: 

mysql> SELECT MOD(29,9);
        -> 2

13.1 Operators

13.1.1 Operator Precedence

Operator precedences are shown in the following list, from lowest precedence to the highest. Operators that are shown together on a line have the same precedence. 

:=
||, OR, XOR
&&, AND
BETWEEN, CASE, WHEN, THEN, ELSE
=, <=>, >=, >, <=, <, <>, !=, IS, LIKE, REGEXP, IN
|
&
<<, >>
-, +
*, /, DIV, %, MOD
^
- (unary minus), ~ (unary bit inversion)
NOT, !
BINARY, COLLATE

13.1.2 Parentheses

( ... )
Use parentheses to force the order of evaluation in an expression. For example: 
mysql> SELECT 1+2*3;
        -> 7
mysql> SELECT (1+2)*3;
        -> 9

13.1.3 Comparison Functions and Operators

Comparison operations result in a value of 1 (TRUE), 0 (FALSE), or NULL. These operations work for both numbers and strings. Strings are automatically converted to numbers and numbers to strings as necessary.

Some of the functions in this section (such as LEAST() and GREATEST()) return values other than 1 (TRUE), 0 (FALSE), or NULL. However, the value they return is based on comparison operations performed as described by the following rules.

MySQL compares values using the following rules:

By default, string comparisons are not case sensitive and use the current character set (ISO-8859-1 Latin1 by default, which also works excellently for English).

To convert a value to a specific type for comparison purposes, you can use the CAST() function. String values can be converted to a different character set using CONVERT(). See section 13.7 Cast Functions and Operators.

The following examples illustrate conversion of strings to numbers for comparison operations: 

mysql> SELECT 1 > '6x';
        -> 0
mysql> SELECT 7 > '6x';
        -> 1
mysql> SELECT 0 > 'x6';
        -> 0
mysql> SELECT 0 = 'x6';
        -> 1

Note that when you are comparing a string column with a number, MySQL can't use an index on the column to quickly look up the value. If str_col is an indexed string column, the index cannot be used when performing the lookup in the following statement: 

SELECT * FROM tbl_name WHERE str_col=1;

The reason for this is that there are many different strings that may convert to the value 1: '1', ' 1', '1a', ...

=
Equal: 
mysql> SELECT 1 = 0;
        -> 0
mysql> SELECT '0' = 0;
        -> 1
mysql> SELECT '0.0' = 0;
        -> 1
mysql> SELECT '0.01' = 0;
        -> 0
mysql> SELECT '.01' = 0.01;
        -> 1
<=>
NULL-safe equal. This operator performs an equality comparison like the = operator, but returns 1 rather than NULL if both operands are NULL, and 0 rather than NULL if one operand is NULL. 
mysql> SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
        -> 1, 1, 0
mysql> SELECT 1 = 1, NULL = NULL, 1 = NULL;
        -> 1, NULL, NULL
<=> was added in MySQL 3.23.0.
<>
!=
Not equal: 
mysql> SELECT '.01' <> '0.01';
        -> 1
mysql> SELECT .01 <> '0.01';
        -> 0
mysql> SELECT 'zapp' <> 'zappp';
        -> 1
<=
Less than or equal: 
mysql> SELECT 0.1 <= 2;
        -> 1
<
Less than: 
mysql> SELECT 2 < 2;
        -> 0
>=
Greater than or equal: 
mysql> SELECT 2 >= 2;
        -> 1
>
Greater than: 
mysql> SELECT 2 > 2;
        -> 0
IS NULL
IS NOT NULL
Tests whether a value is or is not NULL. 
mysql> SELECT 1 IS NULL, 0 IS NULL, NULL IS NULL;
        -> 0, 0, 1
mysql> SELECT 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL;
        -> 1, 1, 0
To be able to work well with ODBC programs, MySQL supports the following extra features when using IS NULL:
  • You can find the row that contains the most recent AUTO_INCREMENT value by issuing a statement of the following form immediately after generating the value: 
    SELECT * FROM tbl_name WHERE auto_col IS NULL
    This behavior can be disabled by setting SQL_AUTO_IS_NULL=0. See section 14.5.3.1 SET Syntax.
  • For DATE and DATETIME columns that are declared as NOT NULL, you can find the special date '0000-00-00' by using a statement like this: 
    SELECT * FROM tbl_name WHERE date_column IS NULL
    This is needed to get some ODBC applications to work because ODBC doesn't support a '0000-00-00' date value.
expr BETWEEN min AND max
If expr is greater than or equal to min and expr is less than or equal to max, BETWEEN returns 1, otherwise it returns 0. This is equivalent to the expression (min <= expr AND expr <= max) if all the arguments are of the same type. Otherwise type conversion takes place according to the rules described at the beginning of this section, but applied to all the three arguments. Note: Before MySQL 4.0.5, arguments were converted to the type of expr instead. 
mysql> SELECT 1 BETWEEN 2 AND 3;
        -> 0
mysql> SELECT 'b' BETWEEN 'a' AND 'c';
        -> 1
mysql> SELECT 2 BETWEEN 2 AND '3';
        -> 1
mysql> SELECT 2 BETWEEN 2 AND 'x-3';
        -> 0
expr NOT BETWEEN min AND max
This is the same as NOT (expr BETWEEN min AND max).
COALESCE(value,...)
Returns the first non-NULL value in the list. 
mysql> SELECT COALESCE(NULL,1);
        -> 1
mysql> SELECT COALESCE(NULL,NULL,NULL);
        -> NULL
COALESCE() was added in MySQL 3.23.3.
GREATEST(value1,value2,...)
With two or more arguments, returns the largest (maximum-valued) argument. The arguments are compared using the same rules as for LEAST(). 
mysql> SELECT GREATEST(2,0);
        -> 2
mysql> SELECT GREATEST(34.0,3.0,5.0,767.0);
        -> 767.0
mysql> SELECT GREATEST('B','A','C');
        -> 'C'
Before MySQL 3.22.5, you can use MAX() instead of GREATEST().
expr IN (value,...)
Returns 1 if expr is any of the values in the IN list, else returns 0. If all values are constants, they are evaluated according to the type of expr and sorted. The search for the item then is done using a binary search. This means IN is very quick if the IN value list consists entirely of constants. If expr is a case-sensitive string expression, the string comparison is performed in case-sensitive fashion. 
mysql> SELECT 2 IN (0,3,5,'wefwf');
        -> 0
mysql> SELECT 'wefwf' IN (0,3,5,'wefwf');
        -> 1
The number of values in the IN list is only limited by the max_allowed_packet value. To comply with the SQL standard, from MySQL 4.1 on IN returns NULL not only if the expression on the left hand side is NULL, but also if no match is found in the list and one of the expressions in the list is NULL. From MySQL 4.1 on, IN() syntax also is used to write certain types of subqueries. See section 14.1.8.3 Subqueries with ANY, IN, and SOME.
expr NOT IN (value,...)
This is the same as NOT (expr IN (value,...)).
ISNULL(expr)
If expr is NULL, ISNULL() returns 1, otherwise it returns 0. 
mysql> SELECT ISNULL(1+1);
        -> 0
mysql> SELECT ISNULL(1/0);
        -> 1
Note that a comparison of NULL values using = will always be false!
INTERVAL(N,N1,N2,N3,...)
Returns 0 if N < N1, 1 if N < N2 and so on or -1 if N is NULL. All arguments are treated as integers. It is required that N1 < N2 < N3 < ... < Nn for this function to work correctly. This is because a binary search is used (very fast). 
mysql> SELECT INTERVAL(23, 1, 15, 17, 30, 44, 200);
        -> 3
mysql> SELECT INTERVAL(10, 1, 10, 100, 1000);
        -> 2
mysql> SELECT INTERVAL(22, 23, 30, 44, 200);
        -> 0
LEAST(value1,value2,...)
With two or more arguments, returns the smallest (minimum-valued) argument. The arguments are compared using the following rules.
  • If the return value is used in an INTEGER context or all arguments are integer-valued, they are compared as integers.
  • If the return value is used in a REAL context or all arguments are real-valued, they are compared as reals.
  • If any argument is a case-sensitive string, the arguments are compared as case-sensitive strings.
  • In other cases, the arguments are compared as case-insensitive strings. 
mysql> SELECT LEAST(2,0);
        -> 0
mysql> SELECT LEAST(34.0,3.0,5.0,767.0);
        -> 3.0
mysql> SELECT LEAST('B','A','C');
        -> 'A'
Before MySQL 3.22.5, you can use MIN() instead of LEAST(). Note that the preceding conversion rules can produce strange results in some borderline cases: 
mysql> SELECT CAST(LEAST(3600, 9223372036854775808.0) as SIGNED);
        -> -9223372036854775808
This happens because MySQL reads 9223372036854775808.0 in an integer context. The integer representation is not good enough to hold the value, so it wraps to a signed integer.

13.1.4 Logical Operators

In SQL, all logical operators evaluate to TRUE, FALSE, or NULL (UNKNOWN). In MySQL, these are implemented as 1 (TRUE), 0 (FALSE), and NULL. Most of this is common to different SQL database servers, although some servers may return any non-zero value for TRUE.

NOT
!
Logical NOT. Evaluates to 1 if the operand is 0, to 0 if the operand is non-zero, and NOT NULL returns NULL. 
mysql> SELECT NOT 10;
        -> 0
mysql> SELECT NOT 0;
        -> 1
mysql> SELECT NOT NULL;
        -> NULL
mysql> SELECT ! (1+1);
        -> 0
mysql> SELECT ! 1+1;
        -> 1
The last example produces 1 because the expression evaluates the same way as (!1)+1.
AND
&&
Logical AND. Evaluates to 1 if all operands are non-zero and not NULL, to 0 if one or more operands are 0, otherwise NULL is returned. 
mysql> SELECT 1 && 1;
        -> 1
mysql> SELECT 1 && 0;
        -> 0
mysql> SELECT 1 && NULL;
        -> NULL
mysql> SELECT 0 && NULL;
        -> 0
mysql> SELECT NULL && 0;
        -> 0
Please note that MySQL versions prior to 4.0.5 stop evaluation when a NULL is encountered, rather than continuing the process to check for possible 0 values. This means that in these versions, SELECT (NULL AND 0) returns NULL instead of 0. As of MySQL 4.0.5, the code has been re-engineered so that the result is always as prescribed by the SQL standards while still using the optimization wherever possible.
OR
||
Logical OR. Evaluates to 1 if any operand is non-zero, to NULL if any operand is NULL, otherwise 0 is returned. 
mysql> SELECT 1 || 1;
        -> 1
mysql> SELECT 1 || 0;
        -> 1
mysql> SELECT 0 || 0;
        -> 0
mysql> SELECT 0 || NULL;
        -> NULL
mysql> SELECT 1 || NULL;
        -> 1
XOR
Logical XOR. Returns NULL if either operand is NULL. For non-NULL operands, evaluates to 1 if an odd number of operands is non-zero, otherwise 0 is returned. 
mysql> SELECT 1 XOR 1;
        -> 0
mysql> SELECT 1 XOR 0;
        -> 1
mysql> SELECT 1 XOR NULL;
        -> NULL
mysql> SELECT 1 XOR 1 XOR 1;
        -> 1
a XOR b is mathematically equal to (a AND (NOT b)) OR ((NOT a) and b). XOR was added in MySQL 4.0.2.

13.2 Control Flow Functions

CASE value WHEN [compare-value] THEN result [WHEN [compare-value] THEN result ...] [ELSE result] END
CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END
The first version returns the result where value=compare-value. The second version returns the result for the first condition that is true. If there was no matching result value, the result after ELSE is returned, or NULL if there is no ELSE part. 
mysql> SELECT CASE 1 WHEN 1 THEN 'one'
    ->     WHEN 2 THEN 'two' ELSE 'more' END;
        -> 'one'
mysql> SELECT CASE WHEN 1>0 THEN 'true' ELSE 'false' END;
        -> 'true'
mysql> SELECT CASE BINARY 'B'
    ->     WHEN 'a' THEN 1 WHEN 'b' THEN 2 END;
        -> NULL
The type of the return value (INTEGER, DOUBLE, or STRING) is the same as the type of the first returned value (the expression after the first THEN). CASE was added in MySQL 3.23.3.
IF(expr1,expr2,expr3)
If expr1 is TRUE (expr1 <> 0 and expr1 <> NULL) then IF() returns expr2, else it returns expr3. IF() returns a numeric or string value, depending on the context in which it is used. 
mysql> SELECT IF(1>2,2,3);
        -> 3
mysql> SELECT IF(1<2,'yes','no');
        -> 'yes'
mysql> SELECT IF(STRCMP('test','test1'),'no','yes');
        -> 'no'
If only one of expr2 or expr3 is explicitly NULL, the result type of the IF() function is the type of non-NULL expression. (This behavior is new in MySQL 4.0.3.) expr1 is evaluated as an integer value, which means that if you are testing floating-point or string values, you should do so using a comparison operation. 
mysql> SELECT IF(0.1,1,0);
        -> 0
mysql> SELECT IF(0.1<>0,1,0);
        -> 1
In the first case shown, IF(0.1) returns 0 because 0.1 is converted to an integer value, resulting in a test of IF(0). This may not be what you expect. In the second case, the comparison tests the original floating-point value to see whether it is non-zero. The result of the comparison is used as an integer. The default return type of IF() (which may matter when it is stored into a temporary table) is calculated in MySQL 3.23 as follows:
Expression Return Value
expr2 or expr3 returns a string string
expr2 or expr3 returns a floating-point value floating-point
expr2 or expr3 returns an integer integer
If expr2 and expr3 are strings, the result is case sensitive if either string is case sensitive (starting from MySQL 3.23.51).
IFNULL(expr1,expr2)
If expr1 is not NULL, IFNULL() returns expr1, else it returns expr2. IFNULL() returns a numeric or string value, depending on the context in which it is used. 
mysql> SELECT IFNULL(1,0);
        -> 1
mysql> SELECT IFNULL(NULL,10);
        -> 10
mysql> SELECT IFNULL(1/0,10);
        -> 10
mysql> SELECT IFNULL(1/0,'yes');
        -> 'yes'
In MySQL 4.0.6 and above, the default result value of IFNULL(expr1,expr2) is the more ``general'' of the two expressions, in the order STRING, REAL, or INTEGER. The difference from earlier MySQL versions is mostly notable when you create a table based on expressions or MySQL has to internally store a value from IFNULL() in a temporary table. 
CREATE TABLE tmp SELECT IFNULL(1,'test') AS test;
As of MySQL 4.0.6, the type for the test column is CHAR(4), whereas in earlier versions the type would be BIGINT.
NULLIF(expr1,expr2)
Returns NULL if expr1 = expr2 is true, else returns expr1. This is the same as CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END. 
mysql> SELECT NULLIF(1,1);
        -> NULL
mysql> SELECT NULLIF(1,2);
        -> 1
Note that MySQL evaluates expr1 twice if the arguments are not equal. NULLIF() was added in MySQL 3.23.15.

13.3 String Functions

String-valued functions return NULL if the length of the result would be greater than the value of the max_allowed_packet system variable. See section 7.5.2 Tuning Server Parameters.

For functions that operate on string positions, the first position is numbered 1.

ASCII(str)
Returns the numeric value of the leftmost character of the string str. Returns 0 if str is the empty string. Returns NULL if str is NULL. ASCII() works for characters with numeric values from 0 to 255. 
mysql> SELECT ASCII('2');
        -> 50
mysql> SELECT ASCII(2);
        -> 50
mysql> SELECT ASCII('dx');
        -> 100
See also the ORD() function.
BIN(N)
Returns a string representation of the binary value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,2). Returns NULL if N is NULL. 
mysql> SELECT BIN(12);
        -> '1100'
BIT_LENGTH(str)
Returns the length of the string str in bits. 
mysql> SELECT BIT_LENGTH('text');
        -> 32
BIT_LENGTH() was added in MySQL 4.0.2.
CHAR(N,...)
CHAR() interprets the arguments as integers and returns a string consisting of the characters given by the code values of those integers. NULL values are skipped. 
mysql> SELECT CHAR(77,121,83,81,'76');
        -> 'MySQL'
mysql> SELECT CHAR(77,77.3,'77.3');
        -> 'MMM'
CHAR_LENGTH(str)
Returns the length of the string str, measured in characters. A multi-byte character counts as a single character. This means that for a string containing five two-byte characters, LENGTH() returns 10, whereas CHAR_LENGTH() returns 5.
CHARACTER_LENGTH(str)
CHARACTER_LENGTH() is a synonym for CHAR_LENGTH().
COMPRESS(string_to_compress)
Compresses a string. This function requires MySQL to have been compiled with a compression library such as zlib. Otherwise, the return value is always NULL. The compressed string can be uncompressed with UNCOMPRESS(). 
mysql> SELECT LENGTH(COMPRESS(REPEAT('a',1000)));
        -> 21
mysql> SELECT LENGTH(COMPRESS(''));
        -> 0
mysql> SELECT LENGTH(COMPRESS('a'));
        -> 13
mysql> SELECT LENGTH(COMPRESS(REPEAT('a',16)));
        -> 15
The compressed string contents are stored the following way:
  • Empty strings are stored as empty strings.
  • Non-empty strings are stored as a four-byte length of the uncompressed string (low byte first), followed by the compressed string. If the string ends with space, an extra `.' character is added to avoid problems with endspace trimming should the result be stored in a CHAR or VARCHAR column. (Use of CHAR or VARCHAR to store compressed strings is not recommended. It is better to use a BLOB column instead.)
COMPRESS() was added in MySQL 4.1.1.
CONCAT(str1,str2,...)
Returns the string that results from concatenating the arguments. Returns NULL if any argument is NULL. May have one or more arguments. A numeric argument is converted to its equivalent string form. 
mysql> SELECT CONCAT('My', 'S', 'QL');
        -> 'MySQL'
mysql> SELECT CONCAT('My', NULL, 'QL');
        -> NULL
mysql> SELECT CONCAT(14.3);
        -> '14.3'
CONCAT_WS(separator,str1,str2,...)
CONCAT_WS() stands for CONCAT With Separator and is a special form of CONCAT(). The first argument is the separator for the rest of the arguments. The separator is added between the strings to be concatenated. The separator can be a string as can the rest of the arguments. If the separator is NULL, the result is NULL. The function skips any NULL values after the separator argument. 
mysql> SELECT CONCAT_WS(',','First name','Second name','Last Name');
        -> 'First name,Second name,Last Name'
mysql> SELECT CONCAT_WS(',','First name',NULL,'Last Name');
        -> 'First name,Last Name'
Before MySQL 4.0.14, CONCAT_WS() skips empty strings as well as NULL values.
CONV(N,from_base,to_base)
Converts numbers between different number bases. Returns a string representation of the number N, converted from base from_base to base to_base. Returns NULL if any argument is NULL. The argument N is interpreted as an integer, but may be specified as an integer or a string. The minimum base is 2 and the maximum base is 36. If to_base is a negative number, N is regarded as a signed number. Otherwise, N is treated as unsigned. CONV() works with 64-bit precision. 
mysql> SELECT CONV('a',16,2);
        -> '1010'
mysql> SELECT CONV('6E',18,8);
        -> '172'
mysql> SELECT CONV(-17,10,-18);
        -> '-H'
mysql> SELECT CONV(10+'10'+'10'+0xa,10,10);
        -> '40'
ELT(N,str1,str2,str3,...)
Returns str1 if N = 1, str2 if N = 2, and so on. Returns NULL if N is less than 1 or greater than the number of arguments. ELT() is the complement of FIELD(). 
mysql> SELECT ELT(1, 'ej', 'Heja', 'hej', 'foo');
        -> 'ej'
mysql> SELECT ELT(4, 'ej', 'Heja', 'hej', 'foo');
        -> 'foo'
EXPORT_SET(bits,on,off[,separator[,number_of_bits]])
Returns a string in which for every bit set in the value bits, you get an on string and for every reset bit you get an off string. Bits in bits are examined from right to left (from low-order to high-order bits). Strings are added to the result from left to right, separated by the separator string (default `,'). The number of bits examined is given by number_of_bits (default 64). 
mysql> SELECT EXPORT_SET(5,'Y','N',',',4);
        -> 'Y,N,Y,N'
mysql> SELECT EXPORT_SET(6,'1','0',',',10);
        -> '0,1,1,0,0,0,0,0,0,0'
FIELD(str,str1,str2,str3,...)
Returns the index of str in the str1, str2, str3, ... list. Returns 0 if str is not found. FIELD() is the complement of ELT(). 
mysql> SELECT FIELD('ej', 'Hej', 'ej', 'Heja', 'hej', 'foo');
        -> 2
mysql> SELECT FIELD('fo', 'Hej', 'ej', 'Heja', 'hej', 'foo');
        -> 0
FIND_IN_SET(str,strlist)
Returns a value 1 to N if the string str is in the string list strlist consisting of N substrings. A string list is a string composed of substrings separated by `,' characters. If the first argument is a constant string and the second is a column of type SET, the FIND_IN_SET() function is optimized to use bit arithmetic. Returns 0 if str is not in strlist or if strlist is the empty string. Returns NULL if either argument is NULL. This function will not work properly if the first argument contains a comma (`,') character. 
mysql> SELECT FIND_IN_SET('b','a,b,c,d');
        -> 2
HEX(N_or_S)
If N_OR_S is a number, returns a string representation of the hexadecimal value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,16). From MySQL 4.0.1 and up, if N_OR_S is a string, returns a hexadecimal string of N_OR_S where each character in N_OR_S is converted to two hexadecimal digits. 
mysql> SELECT HEX(255);
        -> 'FF'
mysql> SELECT 0x616263;
        -> 'abc'
mysql> SELECT HEX('abc');
        -> 616263
INSERT(str,pos,len,newstr)
Returns the string str, with the substring beginning at position pos and len characters long replaced by the string newstr. 
mysql> SELECT INSERT('Quadratic', 3, 4, 'What');
        -> 'QuWhattic'
This function is multi-byte safe.
INSTR(str,substr)
Returns the position of the first occurrence of substring substr in string str. This is the same as the two-argument form of LOCATE(), except that the arguments are swapped. 
mysql> SELECT INSTR('foobarbar', 'bar');
        -> 4
mysql> SELECT INSTR('xbar', 'foobar');
        -> 0
This function is multi-byte safe. In MySQL 3.23, this function is case sensitive. For 4.0 on, it is case sensitive only if either argument is a binary string.
LCASE(str)
LCASE() is a synonym for LOWER().
LEFT(str,len)
Returns the leftmost len characters from the string str. 
mysql> SELECT LEFT('foobarbar', 5);
        -> 'fooba'
LENGTH(str)
Returns the length of the string str, measured in bytes. A multi-byte character counts as multiple bytes. This means that for a string containing five two-byte characters, LENGTH() returns 10, whereas CHAR_LENGTH() returns 5. 
mysql> SELECT LENGTH('text');
        -> 4
LOAD_FILE(file_name)
Reads the file and returns the file contents as a string. The file must be located on the server, you must specify the full pathname to the file, and you must have the FILE privilege. The file must be readable by all and be smaller than max_allowed_packet bytes. If the file doesn't exist or cannot be read because one of the preceding conditions is not satisfied, the function returns NULL. 
mysql> UPDATE tbl_name
           SET blob_column=LOAD_FILE('/tmp/picture')
           WHERE id=1;
Before MySQL 3.23, you must read the file inside your application and create an INSERT statement to update the database with the file contents. If you are using the MySQL++ library, one way to do this can be found in the MySQL++ manual, available at http://dev.mysql.com/doc/.
LOCATE(substr,str)
LOCATE(substr,str,pos)
The first syntax returns the position of the first occurrence of substring substr in string str. The second syntax returns the position of the first occurrence of substring substr in string str, starting at position pos. Returns 0 if substr is not in str. 
mysql> SELECT LOCATE('bar', 'foobarbar');
        -> 4
mysql> SELECT LOCATE('xbar', 'foobar');
        -> 0
mysql> SELECT LOCATE('bar', 'foobarbar',5);
        -> 7
This function is multi-byte safe. In MySQL 3.23, this function is case sensitive. For 4.0 on, it is case sensitive only if either argument is a binary string.
LOWER(str)
Returns the string str with all characters changed to lowercase according to the current character set mapping (the default is ISO-8859-1 Latin1). 
mysql> SELECT LOWER('QUADRATICALLY');
        -> 'quadratically'
This function is multi-byte safe.
LPAD(str,len,padstr)
Returns the string str, left-padded with the string padstr to a length of len characters. If str is longer than len, the return value is shortened to len characters. 
mysql> SELECT LPAD('hi',4,'??');
        -> '??hi'
mysql> SELECT LPAD('hi',1,'??');
        -> 'h'
LTRIM(str)
Returns the string str with leading space characters removed. 
mysql> SELECT LTRIM('  barbar');
        -> 'barbar'
This function is multi-byte safe.
MAKE_SET(bits,str1,str2,...)
Returns a set value (a string containing substrings separated by `,' characters) consisting of the strings that have the corresponding bit in bits set. str1 corresponds to bit 0, str2 to bit 1, and so on. NULL values in str1, str2, ... are not appended to the result. 
mysql> SELECT MAKE_SET(1,'a','b','c');
        -> 'a'
mysql> SELECT MAKE_SET(1 | 4,'hello','nice','world');
        -> 'hello,world'
mysql> SELECT MAKE_SET(1 | 4,'hello','nice',NULL,'world');
        -> 'hello'
mysql> SELECT MAKE_SET(0,'a','b','c');
        -> ''
MID(str,pos,len)
MID(str,pos,len) is a synonym for SUBSTRING(str,pos,len).
OCT(N)
Returns a string representation of the octal value of N, where N is a longlong (BIGINT)number. This is equivalent to CONV(N,10,8). Returns NULL if N is NULL. 
mysql> SELECT OCT(12);
        -> '14'
OCTET_LENGTH(str)
OCTET_LENGTH() is a synonym for LENGTH().
ORD(str)
If the leftmost character of the string str is a multi-byte character, returns the code for that character, calculated from the numeric values of its constituent bytes using this formula: 
  (1st byte code)
+ (2nd byte code * 256)
+ (3rd byte code * 256^2) ...
If the leftmost character is not a multi-byte character, ORD() returns the same value as the ASCII() function. 
mysql> SELECT ORD('2');
        -> 50
POSITION(substr IN str)
POSITION(substr IN str) is a synonym for LOCATE(substr,str).
QUOTE(str)
Quotes a string to produce a result that can be used as a properly escaped data value in an SQL statement. The string is returned surrounded by single quotes and with each instance of single quote (`''), backslash (`\'), ASCII NUL, and Control-Z preceded by a backslash. If the argument is NULL, the return value is the word ``NULL'' without surrounding single quotes. The QUOTE() function was added in MySQL 4.0.3. 
mysql> SELECT QUOTE('Don\'t!');
        -> 'Don\'t!'
mysql> SELECT QUOTE(NULL);
        -> NULL
REPEAT(str,count)
Returns a string consisting of the string str repeated count times. If count <= 0, returns an empty string. Returns NULL if str or count are NULL. 
mysql> SELECT REPEAT('MySQL', 3);
        -> 'MySQLMySQLMySQL'
REPLACE(str,from_str,to_str)
Returns the string str with all occurrences of the string from_str replaced by the string to_str. 
mysql> SELECT REPLACE('www.mysql.com', 'w', 'Ww');
        -> 'WwWwWw.mysql.com'
This function is multi-byte safe.
REVERSE(str)
Returns the string str with the order of the characters reversed. 
mysql> SELECT REVERSE('abc');
        -> 'cba'
This function is multi-byte safe.
RIGHT(str,len)
Returns the rightmost len characters from the string str. 
mysql> SELECT RIGHT('foobarbar', 4);
        -> 'rbar'
This function is multi-byte safe.
RPAD(str,len,padstr)
Returns the string str, right-padded with the string padstr to a length of len characters. If str is longer than len, the return value is shortened to len characters. 
mysql> SELECT RPAD('hi',5,'?');
        -> 'hi???'
mysql> SELECT RPAD('hi',1,'?');
        -> 'h'
This function is multi-byte safe.
RTRIM(str)
Returns the string str with trailing space characters removed. 
mysql> SELECT RTRIM('barbar   ');
        -> 'barbar'
This function is multi-byte safe.
SOUNDEX(str)
Returns a soundex string from str. Two strings that sound almost the same should have identical soundex strings. A standard soundex string is four characters long, but the SOUNDEX() function returns an arbitrarily long string. You can use SUBSTRING() on the result to get a standard soundex string. All non-alphabetic characters are ignored in the given string. All international alphabetic characters outside the A-Z range are treated as vowels. 
mysql> SELECT SOUNDEX('Hello');
        -> 'H400'
mysql> SELECT SOUNDEX('Quadratically');
        -> 'Q36324'
Note: This function implements the original Soundex algorithm, not the more popular enhanced version (also described by D. Knuth). The difference is that original version discards vowels first and then duplicates, whereas the enhanced version discards duplicates first and then vowels.
expr1 SOUNDS LIKE expr2
This is the same as SOUNDEX(expr1) = SOUNDEX(expr2). It is available only in MySQL 4.1 or later.
SPACE(N)
Returns a string consisting of N space characters. 
mysql> SELECT SPACE(6);
        -> '      '
SUBSTRING(str,pos)
SUBSTRING(str FROM pos)
SUBSTRING(str,pos,len)
SUBSTRING(str FROM pos FOR len)
The forms without a len argument return a substring from string str starting at position pos. The forms with a len argument return a substring len characters long from string str, starting at position pos. The forms that use FROM are standard SQL syntax. 
mysql> SELECT SUBSTRING('Quadratically',5);
        -> 'ratically'
mysql> SELECT SUBSTRING('foobarbar' FROM 4);
        -> 'barbar'
mysql> SELECT SUBSTRING('Quadratically',5,6);
        -> 'ratica'
This function is multi-byte safe.
SUBSTRING_INDEX(str,delim,count)
Returns the substring from string str before count occurrences of the delimiter delim. If count is positive, everything to the left of the final delimiter (counting from the left) is returned. If count is negative, everything to the right of the final delimiter (counting from the right) is returned. 
mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', 2);
        -> 'www.mysql'
mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', -2);
        -> 'mysql.com'
This function is multi-byte safe.
TRIM([{BOTH | LEADING | TRAILING} [remstr] FROM] str)
TRIM(remstr FROM] str)
Returns the string str with all remstr prefixes and/or suffixes removed. If none of the specifiers BOTH, LEADING, or TRAILING is given, BOTH is assumed. If remstr is optional and not specified, spaces are removed. 
mysql> SELECT TRIM('  bar   ');
        -> 'bar'
mysql> SELECT TRIM(LEADING 'x' FROM 'xxxbarxxx');
        -> 'barxxx'
mysql> SELECT TRIM(BOTH 'x' FROM 'xxxbarxxx');
        -> 'bar'
mysql> SELECT TRIM(TRAILING 'xyz' FROM 'barxxyz');
        -> 'barx'
This function is multi-byte safe.
UCASE(str)
UCASE() is a synonym for UPPER().
UNCOMPRESS(string_to_uncompress)
Uncompresses a string compressed by the COMPRESS() function. If the argument is not a compressed value, the result is NULL. This function requires MySQL to have been compiled with a compression library such as zlib. Otherwise, the return value is always NULL. 
mysql> SELECT UNCOMPRESS(COMPRESS('any string'));
        -> 'any string'
mysql> SELECT UNCOMPRESS('any string');
        -> NULL
UNCOMPRESS() was added in MySQL 4.1.1.
UNCOMPRESSED_LENGTH(compressed_string)
Returns the length of a compressed string before compression. 
mysql> SELECT UNCOMPRESSED_LENGTH(COMPRESS(REPEAT('a',30)));
        -> 30
UNCOMPRESSED_LENGTH() was added in MySQL 4.1.1.
UNHEX(str)
Does the opposite of HEX(str). That is, it interprets each pair of hexadecimal digits in the argument as a number and converts it to the character represented by the number. The resulting characters are returned as a binary string. 
mysql> SELECT UNHEX('4D7953514C');
        -> 'MySQL'
mysql> SELECT 0x4D7953514C;
        -> 'MySQL'
mysql> SELECT UNHEX(HEX('string'));
        -> 'string'
mysql> SELECT HEX(UNHEX('1267'));
        -> '1267'
UNHEX() was added in MySQL 4.1.2.
UPPER(str)
Returns the string str with all characters changed to uppercase according to the current character set mapping (the default is ISO-8859-1 Latin1). 
mysql> SELECT UPPER('Hej');
        -> 'HEJ'
This function is multi-byte safe.

13.3.1 String Comparison Functions

MySQL automatically converts numbers to strings as necessary, and vice versa. 

mysql> SELECT 1+'1';
        -> 2
mysql> SELECT CONCAT(2,' test');
        -> '2 test'

If you want to convert a number to a string explicitly, use the CAST() or CONCAT() function: 

mysql> SELECT 38.8, CAST(38.8 AS CHAR);
        -> 38.8, '38.8'
mysql> SELECT 38.8, CONCAT(38.8);
        -> 38.8, '38.8'

CAST() is preferable, but it is unavailable before MySQL 4.0.2.

If a string function is given a binary string as an argument, the resulting string is also a binary string. A number converted to a string is treated as a binary string. This affects only comparisons.

Normally, if any expression in a string comparison is case sensitive, the comparison is performed in case-sensitive fashion.

expr LIKE pat [ESCAPE 'escape-char']
Pattern matching using SQL simple regular expression comparison. Returns 1 (TRUE) or 0 (FALSE). If either expr or pat is NULL, the result is NULL. With LIKE you can use the following two wildcard characters in the pattern:
Character Description
% Matches any number of characters, even zero characters
_ Matches exactly one character 
mysql> SELECT 'David!' LIKE 'David_';
        -> 1
mysql> SELECT 'David!' LIKE '%D%v%';
        -> 1
To test for literal instances of a wildcard character, precede the character with the escape character. If you don't specify the ESCAPE character, `\' is assumed.
String Description
\% Matches one `%' character
\_ Matches one `_' character 
mysql> SELECT 'David!' LIKE 'David\_';
        -> 0
mysql> SELECT 'David_' LIKE 'David\_';
        -> 1
To specify a different escape character, use the ESCAPE clause: 
mysql> SELECT 'David_' LIKE 'David|_' ESCAPE '|';
        -> 1
The following two statements illustrate that string comparisons are not case sensitive unless one of the operands is a binary string: 
mysql> SELECT 'abc' LIKE 'ABC';
        -> 1
mysql> SELECT 'abc' LIKE BINARY 'ABC';
        -> 0
In MySQL, LIKE is allowed on numeric expressions. (This is an extension to the standard SQL LIKE.) 
mysql> SELECT 10 LIKE '1%';
        -> 1
Note: Because MySQL uses the C escape syntax in strings (for example, `\n' to represent newline), you must double any `\' that you use in your LIKE strings. For example, to search for `\n', specify it as `\\n'. To search for `\', specify it as `\\\\' (the backslashes are stripped once by the parser and another time when the pattern match is done, leaving a single backslash to be matched).
expr NOT LIKE pat [ESCAPE 'escape-char']
This is the same as NOT (expr LIKE pat [ESCAPE 'escape-char']).
expr NOT REGEXP pat
expr NOT RLIKE pat
This is the same as NOT (expr REGEXP pat).
expr REGEXP pat
expr RLIKE pat
Performs a pattern match of a string expression expr against a pattern pat. The pattern can be an extended regular expression. The syntax for regular expressions is discussed in section F MySQL Regular Expressions. Returns 1 if expr matches pat, otherwise returns 0. If either expr or pat is NULL, the result is NULL. RLIKE is a synonym for REGEXP, provided for mSQL compatibility. Note: Because MySQL uses the C escape syntax in strings (for example, `\n' to represent newline), you must double any `\' that you use in your REGEXP strings. As of MySQL 3.23.4, REGEXP is not case sensitive for normal (not binary) strings. 
mysql> SELECT 'Monty!' REGEXP 'm%y%%';
        -> 0
mysql> SELECT 'Monty!' REGEXP '.*';
        -> 1
mysql> SELECT 'new*\n*line' REGEXP 'new\\*.\\*line';
        -> 1
mysql> SELECT 'a' REGEXP 'A', 'a' REGEXP BINARY 'A';
        -> 1  0
mysql> SELECT 'a' REGEXP '^[a-d]';
        -> 1
REGEXP and RLIKE use the current character set (ISO-8859-1 Latin1 by default) when deciding the type of a character. However, these operators are not multi-byte safe.
STRCMP(expr1,expr2)
STRCMP() returns 0 if the strings are the same, -1 if the first argument is smaller than the second according to the current sort order, and 1 otherwise. 
mysql> SELECT STRCMP('text', 'text2');
        -> -1
mysql> SELECT STRCMP('text2', 'text');
        -> 1
mysql> SELECT STRCMP('text', 'text');
        -> 0
As of MySQL 4.0, STRCMP() uses the current character set when performing comparisons. This makes the default comparison behavior case insensitive unless one or both of the operands are binary strings. Before MySQL 4.0, STRCMP() is case sensitive.

13.4 Numeric Functions

13.4.1 Arithmetic Operators

The usual arithmetic operators are available. Note that in the case of -, +, and *, the result is calculated with BIGINT (64-bit) precision if both arguments are integers. If one of the argument is an unsigned integer, and the other argument is also an integer, the result will be an unsigned integer. See section 13.7 Cast Functions and Operators.

+
Addition: 
mysql> SELECT 3+5;
        -> 8
-
Subtraction: 
mysql> SELECT 3-5;
        -> -2
-
Unary minus. Changes the sign of the argument. 
mysql> SELECT - 2;
        -> -2
Note that if this operator is used with a BIGINT, the return value is a BIGINT! This means that you should avoid using - on integers that may have the value of -2^63!
*
Multiplication: 
mysql> SELECT 3*5;
        -> 15
mysql> SELECT 18014398509481984*18014398509481984.0;
        -> 324518553658426726783156020576256.0
mysql> SELECT 18014398509481984*18014398509481984;
        -> 0
The result of the last expression is incorrect because the result of the integer multiplication exceeds the 64-bit range of BIGINT calculations.
/
Division: 
mysql> SELECT 3/5;
        -> 0.60
Division by zero produces a NULL result: 
mysql> SELECT 102/(1-1);
        -> NULL
A division will be calculated with BIGINT arithmetic only if performed in a context where its result is converted to an integer!
DIV
Integer division. Similar to FLOOR() but safe with BIGINT values. 
mysql> SELECT 5 DIV 2;
        -> 2
DIV is new in MySQL 4.1.0.

13.4.2 Mathematical Functions

All mathematical functions return NULL in case of an error.

ABS(X)
Returns the absolute value of X. 
mysql> SELECT ABS(2);
        -> 2
mysql> SELECT ABS(-32);
        -> 32
This function is safe to use with BIGINT values.
ACOS(X)
Returns the arc cosine of X, that is, the value whose cosine is X. Returns NULL if X is not in the range -1 to 1. 
mysql> SELECT ACOS(1);
        -> 0.000000
mysql> SELECT ACOS(1.0001);
        -> NULL
mysql> SELECT ACOS(0);
        -> 1.570796
ASIN(X)
Returns the arc sine of X, that is, the value whose sine is X. Returns NULL if X is not in the range -1 to 1. 
mysql> SELECT ASIN(0.2);
        -> 0.201358
mysql> SELECT ASIN('foo');
        -> 0.000000
ATAN(X)
Returns the arc tangent of X, that is, the value whose tangent is X. 
mysql> SELECT ATAN(2);
        -> 1.107149
mysql> SELECT ATAN(-2);
        -> -1.107149
ATAN(Y,X)
ATAN2(Y,X)
Returns the arc tangent of the two variables X and Y. It is similar to calculating the arc tangent of Y / X, except that the signs of both arguments are used to determine the quadrant of the result. 
mysql> SELECT ATAN(-2,2);
        -> -0.785398
mysql> SELECT ATAN2(PI(),0);
        -> 1.570796
CEILING(X)
CEIL(X)
Returns the smallest integer value not less than X. 
mysql> SELECT CEILING(1.23);
        -> 2
mysql> SELECT CEIL(-1.23);
        -> -1
Note that the return value is converted to a BIGINT! The CEIL() alias was added in MySQL 4.0.6.
COS(X)
Returns the cosine of X, where X is given in radians. 
mysql> SELECT COS(PI());
        -> -1.000000
COT(X)
Returns the cotangent of X. 
mysql> SELECT COT(12);
        -> -1.57267341
mysql> SELECT COT(0);
        -> NULL
CRC32(expr)
Computes a cyclic redundancy check value and returns a 32-bit unsigned value. The result is NULL if the argument is NULL. The argument is expected be a string and will be treated as one if it is not. 
mysql> SELECT CRC32('MySQL');
        -> 3259397556
CRC32() is available as of MySQL 4.1.0.
DEGREES(X)
Returns the argument X, converted from radians to degrees. 
mysql> SELECT DEGREES(PI());
        -> 180.000000
EXP(X)
Returns the value of e (the base of natural logarithms) raised to the power of X. 
mysql> SELECT EXP(2);
        -> 7.389056
mysql> SELECT EXP(-2);
        -> 0.135335
FLOOR(X)
Returns the largest integer value not greater than X. 
mysql> SELECT FLOOR(1.23);
        -> 1
mysql> SELECT FLOOR(-1.23);
        -> -2
Note that the return value is converted to a BIGINT!
LN(X)
Returns the natural logarithm of X. 
mysql> SELECT LN(2);
        -> 0.693147
mysql> SELECT LN(-2);
        -> NULL
This function was added in MySQL 4.0.3. It is synonymous with LOG(X) in MySQL.
LOG(X)
LOG(B,X)
If called with one parameter, this function returns the natural logarithm of X. 
mysql> SELECT LOG(2);
        -> 0.693147
mysql> SELECT LOG(-2);
        -> NULL
If called with two parameters, this function returns the logarithm of X for an arbitrary base B. 
mysql> SELECT LOG(2,65536);
        -> 16.000000
mysql> SELECT LOG(1,100);
        -> NULL
The arbitrary base option was added in MySQL 4.0.3. LOG(B,X) is equivalent to LOG(X)/LOG(B).
LOG2(X)
Returns the base-2 logarithm of X. 
mysql> SELECT LOG2(65536);
        -> 16.000000
mysql> SELECT LOG2(-100);
        -> NULL
LOG2() is useful for finding out how many bits a number would require for storage. This function was added in MySQL 4.0.3. In earlier versions, you can use LOG(X)/LOG(2) instead.
LOG10(X)
Returns the base-10 logarithm of X. 
mysql> SELECT LOG10(2);
        -> 0.301030
mysql> SELECT LOG10(100);
        -> 2.000000
mysql> SELECT LOG10(-100);
        -> NULL
MOD(N,M)
N % M
N MOD M
Modulo (like the % operator in C). Returns the remainder of N divided by M. 
mysql> SELECT MOD(234, 10);
        -> 4
mysql> SELECT 253 % 7;
        -> 1
mysql> SELECT MOD(29,9);
        -> 2
mysql> SELECT 29 MOD 9;
        -> 2
This function is safe to use with BIGINT values. The N MOD M syntax works only as of MySQL 4.1.
PI()
Returns the value of PI. The default number of decimals displayed is five, but MySQL internally uses the full double-precision value for PI. 
mysql> SELECT PI();
        -> 3.141593
mysql> SELECT PI()+0.000000000000000000;
        -> 3.141592653589793116
POW(X,Y)
POWER(X,Y)
Returns the value of X raised to the power of Y. 
mysql> SELECT POW(2,2);
        -> 4.000000
mysql> SELECT POW(2,-2);
        -> 0.250000
RADIANS(X)
Returns the argument X, converted from degrees to radians. 
mysql> SELECT RADIANS(90);
        -> 1.570796
RAND()
RAND(N)
Returns a random floating-point value in the range from 0 to 1.0. If an integer argument N is specified, it is used as the seed value (producing a repeatable sequence). 
mysql> SELECT RAND();
        -> 0.9233482386203
mysql> SELECT RAND(20);
        -> 0.15888261251047
mysql> SELECT RAND(20);
        -> 0.15888261251047
mysql> SELECT RAND();
        -> 0.63553050033332
mysql> SELECT RAND();
        -> 0.70100469486881
You can't use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times. As of MySQL 3.23, you can retrieve rows in random order like this: 
mysql> SELECT * FROM tbl_name ORDER BY RAND();
ORDER BY RAND() combined with LIMIT is useful for selecting a random sample of a set of rows: 
mysql> SELECT * FROM table1, table2 WHERE a=b AND c<d
    -> ORDER BY RAND() LIMIT 1000;
Note that RAND() in a WHERE clause is re-evaluated every time the WHERE is executed. RAND() is not meant to be a perfect random generator, but instead a fast way to generate ad hoc random numbers that will be portable between platforms for the same MySQL version.
ROUND(X)
ROUND(X,D)
Returns the argument X, rounded to the nearest integer. With two arguments, returns X rounded to D decimals. If D is negative, the integer part of the number is zeroed out. 
mysql> SELECT ROUND(-1.23);
        -> -1
mysql> SELECT ROUND(-1.58);
        -> -2
mysql> SELECT ROUND(1.58);
        -> 2
mysql> SELECT ROUND(1.298, 1);
        -> 1.3
mysql> SELECT ROUND(1.298, 0);
        -> 1
mysql> SELECT ROUND(23.298, -1);
        -> 20
Note that the behavior of ROUND() when the argument is halfway between two integers depends on the C library implementation. Different implementations round to the nearest even number, always up, always down, or always toward zero. If you need one kind of rounding, you should use a well-defined function such as TRUNCATE() or FLOOR() instead.
SIGN(X)
Returns the sign of the argument as -1, 0, or 1, depending on whether X is negative, zero, or positive. 
mysql> SELECT SIGN(-32);
        -> -1
mysql> SELECT SIGN(0);
        -> 0
mysql> SELECT SIGN(234);
        -> 1
SIN(X)
Returns the sine of X, where X is given in radians. 
mysql> SELECT SIN(PI());
        -> 0.000000
SQRT(X)
Returns the non-negative square root of X. 
mysql> SELECT SQRT(4);
        -> 2.000000
mysql> SELECT SQRT(20);
        -> 4.472136
TAN(X)
Returns the tangent of X, where X is given in radians. 
mysql> SELECT TAN(PI()+1);
        -> 1.557408
TRUNCATE(X,D)
Returns the number X, truncated to D decimals. If D is 0, the result will have no decimal point or fractional part. If D is negative, the integer part of the number is zeroed out. 
mysql> SELECT TRUNCATE(1.223,1);
        -> 1.2
mysql> SELECT TRUNCATE(1.999,1);
        -> 1.9
mysql> SELECT TRUNCATE(1.999,0);
        -> 1
mysql> SELECT TRUNCATE(-1.999,1);
        -> -1.9
mysql> SELECT TRUNCATE(122,-2);
       -> 100
Starting from MySQL 3.23.51, all numbers are rounded toward zero. Note that decimal numbers are normally not stored as exact numbers in computers, but as double-precision values, so you may be surprised by the following result: 
mysql> SELECT TRUNCATE(10.28*100,0);
       -> 1027
This happens because 10.28 is actually stored as something like 10.2799999999999999.

13.5 Date and Time Functions

This section describes the functions that can be used to manipulate temporal values. See section 12.3 Date and Time Types for a description of the range of values each date and time type has and the valid formats in which values may be specified.

Here is an example that uses date functions. The following query selects all records with a date_col value from within the last 30 days: 

mysql> SELECT something FROM tbl_name
    -> WHERE DATE_SUB(CURDATE(),INTERVAL 30 DAY) <= date_col;

Note that the query also will select records with dates that lie in the future.

Functions that expect date values usually will accept datetime values and ignore the time part. Functions that expect time values usually will accept datetime values and ignore the date part.

Functions that return the current date or time each are evaluated only once per query at the start of query execution. This means that multiple references to a function such as NOW() within a single query will always produce the same result. This principle also applies to CURDATE(), CURTIME(), UTC_DATE(), UTC_TIME(), UTC_TIMESTAMP(), and to any of their synonyms.

Beginning with MySQL 4.1.3, the CURRENT_TIMESTAMP(), CURRENT_TIME(), CURRENT_DATE(), and FROM_UNIXTIME() functions return values in the connection's current time zone, which is available as the value of the time_zone system variable. Also, UNIX_TIMESTAMP() assumes that its argument is a datetime value in the current time zone. See section 5.8.8 MySQL Server Time Zone Support.

The return value ranges in the following function descriptions apply for complete dates. If a date is a ``zero'' value or an incomplete date such as '2001-11-00', functions that extract a part of a date may return 0. For example, DAYOFMONTH('2001-11-00') returns 0.

ADDDATE(date,INTERVAL expr type)
ADDDATE(expr,days)
When invoked with the INTERVAL form of the second argument, ADDDATE() is a synonym for DATE_ADD(). The related function SUBDATE() is a synonym for DATE_SUB(). For information on the INTERVAL argument, see the discussion for DATE_ADD(). 
mysql> SELECT DATE_ADD('1998-01-02', INTERVAL 31 DAY);
        -> '1998-02-02'
mysql> SELECT ADDDATE('1998-01-02', INTERVAL 31 DAY);
        -> '1998-02-02'
As of MySQL 4.1.1, the second syntax is allowed, where expr is a date or datetime expression and days is the number of days to be added to expr. 
mysql> SELECT ADDDATE('1998-01-02', 31);
        -> '1998-02-02'
ADDTIME(expr,expr2)
ADDTIME() adds expr2 to expr and returns the result. expr is a time or datetime expression, and expr2 is a time expression. 
mysql> SELECT ADDTIME('1997-12-31 23:59:59.999999',
    ->                '1 1:1:1.000002');
        -> '1998-01-02 01:01:01.000001'
mysql> SELECT ADDTIME('01:00:00.999999', '02:00:00.999998');
        -> '03:00:01.999997'
ADDTIME() was added in MySQL 4.1.1.
CONVERT_TZ(dt,from_tz,to_tz)
CONVERT_TZ() converts a datetime value dt from time zone given by from_tz to the time zone given by to_tz and returns the resulting value. Time zones may be specified as described in section 5.8.8 MySQL Server Time Zone Support. This function returns NULL if the arguments are invalid. If the value falls out of the supported range of the TIMESTAMP type when converted fom from_tz to UTC, no conversion occurs. The TIMESTAMP range is described at section 12.1.2 Overview of Date and Time Types. 
mysql> SELECT CONVERT_TZ('2004-01-01 12:00:00','GMT','MET');
        -> '2004-01-01 13:00:00'
mysql> SELECT CONVERT_TZ('2004-01-01 12:00:00','+00:00','-07:00');
        -> '2004-01-01 05:00:00'
To use named time zones such as 'MET' or 'Europe/Moscow', the time zone tables must be properly set up. See section 5.8.8 MySQL Server Time Zone Support for instructions. CONVERT_TZ() was added in MySQL 4.1.3.
CURDATE()
Returns the current date as a value in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context. 
mysql> SELECT CURDATE();
        -> '1997-12-15'
mysql> SELECT CURDATE() + 0;
        -> 19971215
CURRENT_DATE
CURRENT_DATE()
CURRENT_DATE and CURRENT_DATE() are synonyms for CURDATE().
CURTIME()
Returns the current time as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context. 
mysql> SELECT CURTIME();
        -> '23:50:26'
mysql> SELECT CURTIME() + 0;
        -> 235026
CURRENT_TIME
CURRENT_TIME()
CURRENT_TIME and CURRENT_TIME() are synonyms for CURTIME().
CURRENT_TIMESTAMP
CURRENT_TIMESTAMP()
CURRENT_TIMESTAMP and CURRENT_TIMESTAMP() are synonyms for NOW().
DATE(expr)
Extracts the date part of the date or datetime expression expr. 
mysql> SELECT DATE('2003-12-31 01:02:03');
        -> '2003-12-31'
DATE() is available as of MySQL 4.1.1.
DATEDIFF(expr,expr2)
DATEDIFF() returns the number of days between the start date expr and the end date expr2. expr and expr2 are date or date-and-time expressions. Only the date parts of the values are used in the calculation. 
mysql> SELECT DATEDIFF('1997-12-31 23:59:59','1997-12-30');
        -> 1
mysql> SELECT DATEDIFF('1997-11-30 23:59:59','1997-12-31');
        -> -31
DATEDIFF() was added in MySQL 4.1.1.
DATE_ADD(date,INTERVAL expr type)
DATE_SUB(date,INTERVAL expr type)
These functions perform date arithmetic. date is a DATETIME or DATE value specifying the starting date. expr is an expression specifying the interval value to be added or subtracted from the starting date. expr is a string; it may start with a `-' for negative intervals. type is a keyword indicating how the expression should be interpreted. The INTERVAL keyword and the type specifier are not case sensitive. The following table shows how the type and expr arguments are related:
type Value Expected expr Format
MICROSECOND MICROSECONDS
SECOND SECONDS
MINUTE MINUTES
HOUR HOURS
DAY DAYS
WEEK WEEKS
MONTH MONTHS
QUARTER QUARTERS
YEAR YEARS
SECOND_MICROSECOND 'SECONDS.MICROSECONDS'
MINUTE_MICROSECOND 'MINUTES.MICROSECONDS'
MINUTE_SECOND 'MINUTES:SECONDS'
HOUR_MICROSECOND 'HOURS.MICROSECONDS'
HOUR_SECOND 'HOURS:MINUTES:SECONDS'
HOUR_MINUTE 'HOURS:MINUTES'
DAY_MICROSECOND 'DAYS.MICROSECONDS'
DAY_SECOND 'DAYS HOURS:MINUTES:SECONDS'
DAY_MINUTE 'DAYS HOURS:MINUTES'
DAY_HOUR 'DAYS HOURS'
YEAR_MONTH 'YEARS-MONTHS'
The type values DAY_MICROSECOND, HOUR_MICROSECOND, MINUTE_MICROSECOND, SECOND_MICROSECOND, and MICROSECOND are allowed as of MySQL 4.1.1. The values QUARTER and WEEK are allowed as of MySQL 5.0.0. MySQL allows any punctuation delimiter in the expr format. Those shown in the table are the suggested delimiters. If the date argument is a DATE value and your calculations involve only YEAR, MONTH, and DAY parts (that is, no time parts), the result is a DATE value. Otherwise, the result is a DATETIME value. As of MySQL 3.23, INTERVAL expr type is allowed on either side of the + operator if the expression on the other side is a date or datetime value. For the - operator, INTERVAL expr type is allowed only on the right side, because it makes no sense to subtract a date or datetime value from an interval. (See examples below.) 
mysql> SELECT '1997-12-31 23:59:59' + INTERVAL 1 SECOND;
        -> '1998-01-01 00:00:00'
mysql> SELECT INTERVAL 1 DAY + '1997-12-31';
        -> '1998-01-01'
mysql> SELECT '1998-01-01' - INTERVAL 1 SECOND;
        -> '1997-12-31 23:59:59'
mysql> SELECT DATE_ADD('1997-12-31 23:59:59',
    ->                 INTERVAL 1 SECOND);
        -> '1998-01-01 00:00:00'
mysql> SELECT DATE_ADD('1997-12-31 23:59:59',
    ->                 INTERVAL 1 DAY);
        -> '1998-01-01 23:59:59'
mysql> SELECT DATE_ADD('1997-12-31 23:59:59',
    ->                 INTERVAL '1:1' MINUTE_SECOND);
        -> '1998-01-01 00:01:00'
mysql> SELECT DATE_SUB('1998-01-01 00:00:00',
    ->                 INTERVAL '1 1:1:1' DAY_SECOND);
        -> '1997-12-30 22:58:59'
mysql> SELECT DATE_ADD('1998-01-01 00:00:00',
    ->                 INTERVAL '-1 10' DAY_HOUR);
        -> '1997-12-30 14:00:00'
mysql> SELECT DATE_SUB('1998-01-02', INTERVAL 31 DAY);
        -> '1997-12-02'
mysql> SELECT DATE_ADD('1992-12-31 23:59:59.000002',
    ->            INTERVAL '1.999999' SECOND_MICROSECOND);
        -> '1993-01-01 00:00:01.000001'
If you specify an interval value that is too short (does not include all the interval parts that would be expected from the type keyword), MySQL assumes that you have left out the leftmost parts of the interval value. For example, if you specify a type of DAY_SECOND, the value of expr is expected to have days, hours, minutes, and seconds parts. If you specify a value like '1:10', MySQL assumes that the days and hours parts are missing and the value represents minutes and seconds. In other words, '1:10' DAY_SECOND is interpreted in such a way that it is equivalent to '1:10' MINUTE_SECOND. This is analogous to the way that MySQL interprets TIME values as representing elapsed time rather than as time of day. If you add to or subtract from a date value something that contains a time part, the result is automatically converted to a datetime value: 
mysql> SELECT DATE_ADD('1999-01-01', INTERVAL 1 DAY);
        -> '1999-01-02'
mysql> SELECT DATE_ADD('1999-01-01', INTERVAL 1 HOUR);
        -> '1999-01-01 01:00:00'
If you use really malformed dates, the result is NULL. If you add MONTH, YEAR_MONTH, or YEAR and the resulting date has a day that is larger than the maximum day for the new month, the day is adjusted to the maximum days in the new month: 
mysql> SELECT DATE_ADD('1998-01-30', INTERVAL 1 MONTH);
        -> '1998-02-28'
DATE_FORMAT(date,format)
Formats the date value according to the format string. The following specifiers may be used in the format string:
Specifier Description
%a Abbreviated weekday name (Sun..Sat)
%b Abbreviated month name (Jan..Dec)
%c Month, numeric (0..12)
%D Day of the month with English suffix (0th, 1st, 2nd, 3rd, ...)
%d Day of the month, numeric (00..31)
%e Day of the month, numeric (0..31)
%f Microseconds (000000..999999)
%H Hour (00..23)
%h Hour (01..12)
%I Hour (01..12)
%i Minutes, numeric (00..59)
%j Day of year (001..366)
%k Hour (0..23)
%l Hour (1..12)
%M Month name (January..December)
%m Month, numeric (00..12)
%p AM or PM
%r Time, 12-hour (hh:mm:ss followed by AM or PM)
%S Seconds (00..59)
%s Seconds (00..59)
%T Time, 24-hour (hh:mm:ss)
%U Week (00..53), where Sunday is the first day of the week
%u Week (00..53), where Monday is the first day of the week
%V Week (01..53), where Sunday is the first day of the week; used with %X
%v Week (01..53), where Monday is the first day of the week; used with %x
%W Weekday name (Sunday..Saturday)
%w Day of the week (0=Sunday..6=Saturday)
%X Year for the week where Sunday is the first day of the week, numeric, four digits; used with %V
%x Year for the week, where Monday is the first day of the week, numeric, four digits; used with %v
%Y Year, numeric, four digits
%y Year, numeric, two digits
%% A literal `%'.
All other characters are copied to the result without interpretation. The %v, %V, %x, and %X format specifiers are available as of MySQL 3.23.8. %f is available as of MySQL 4.1.1. As of MySQL 3.23, the `%' character is required before format specifier characters. In earlier versions of MySQL, `%' was optional. The reason the ranges for the month and day specifiers begin with zero is that MySQL allows incomplete dates such as '2004-00-00' to be stored as of MySQL 3.23. 
mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y');
        -> 'Saturday October 1997'
mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00', '%H:%i:%s');
        -> '22:23:00'
mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00',
                          '%D %y %a %d %m %b %j');
        -> '4th 97 Sat 04 10 Oct 277'
mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00',
                          '%H %k %I %r %T %S %w');
        -> '22 22 10 10:23:00 PM 22:23:00 00 6'
mysql> SELECT DATE_FORMAT('1999-01-01', '%X %V');
        -> '1998 52'
DAY(date)
DAY() is a synonym for DAYOFMONTH(). It is available as of MySQL 4.1.1.
DAYNAME(date)
Returns the name of the weekday for date. 
mysql> SELECT DAYNAME('1998-02-05');
        -> 'Thursday'
DAYOFMONTH(date)
Returns the day of the month for date, in the range 1 to 31. 
mysql> SELECT DAYOFMONTH('1998-02-03');
        -> 3
DAYOFWEEK(date)
Returns the weekday index for date (1 = Sunday, 2 = Monday, ..., 7 = Saturday). These index values correspond to the ODBC standard. 
mysql> SELECT DAYOFWEEK('1998-02-03');
        -> 3
DAYOFYEAR(date)
Returns the day of the year for date, in the range 1 to 366. 
mysql> SELECT DAYOFYEAR('1998-02-03');
        -> 34
EXTRACT(type FROM date)
The EXTRACT() function uses the same kinds of interval type specifiers as DATE_ADD() or DATE_SUB(), but extracts parts from the date rather than performing date arithmetic. 
mysql> SELECT EXTRACT(YEAR FROM '1999-07-02');
       -> 1999
mysql> SELECT EXTRACT(YEAR_MONTH FROM '1999-07-02 01:02:03');
       -> 199907
mysql> SELECT EXTRACT(DAY_MINUTE FROM '1999-07-02 01:02:03');
       -> 20102
mysql> SELECT EXTRACT(MICROSECOND
    ->                FROM '2003-01-02 10:30:00.00123');
        -> 123
EXTRACT() was added in MySQL 3.23.0.
FROM_DAYS(N)
Given a daynumber N, returns a DATE value. 
mysql> SELECT FROM_DAYS(729669);
        -> '1997-10-07'
FROM_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582), because it does not take into account the days that were lost when the calendar was changed.
FROM_UNIXTIME(unix_timestamp)
FROM_UNIXTIME(unix_timestamp,format)
Returns a representation of the unix_timestamp argument as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context. 
mysql> SELECT FROM_UNIXTIME(875996580);
        -> '1997-10-04 22:23:00'
mysql> SELECT FROM_UNIXTIME(875996580) + 0;
        -> 19971004222300
If format is given, the result is formatted according to the format string. format may contain the same specifiers as those listed in the entry for the DATE_FORMAT() function. 
mysql> SELECT FROM_UNIXTIME(UNIX_TIMESTAMP(),
    ->                      '%Y %D %M %h:%i:%s %x');
        -> '2003 6th August 06:22:58 2003'
GET_FORMAT(DATE|TIME|DATETIME, 'EUR'|'USA'|'JIS'|'ISO'|'INTERNAL')
Returns a format string. This function is useful in combination with the DATE_FORMAT() and the STR_TO_DATE() functions. The three possible values for the first argument and the five possible values for the second argument result in 15 possible format strings (for the specifiers used, see the table in the DATE_FORMAT() function description).
Function Call Result
GET_FORMAT(DATE,'USA') '%m.%d.%Y'
GET_FORMAT(DATE,'JIS') '%Y-%m-%d'
GET_FORMAT(DATE,'ISO') '%Y-%m-%d'
GET_FORMAT(DATE,'EUR') '%d.%m.%Y'
GET_FORMAT(DATE,'INTERNAL') '%Y%m%d'
GET_FORMAT(DATETIME,'USA') '%Y-%m-%d-%H.%i.%s'
GET_FORMAT(DATETIME,'JIS') '%Y-%m-%d %H:%i:%s'
GET_FORMAT(DATETIME,'ISO') '%Y-%m-%d %H:%i:%s'
GET_FORMAT(DATETIME,'EUR') '%Y-%m-%d-%H.%i.%s'
GET_FORMAT(DATETIME,'INTERNAL') '%Y%m%d%H%i%s'
GET_FORMAT(TIME,'USA') '%h:%i:%s %p'
GET_FORMAT(TIME,'JIS') '%H:%i:%s'
GET_FORMAT(TIME,'ISO') '%H:%i:%s'
GET_FORMAT(TIME,'EUR') '%H.%i.%S'
GET_FORMAT(TIME,'INTERNAL') '%H%i%s'
ISO format is ISO 9075, not ISO 8601. As of MySQL 4.1.4, TIMESTAMP can also be used; GET_FORMAT() returns the same values as for DATETIME. 
mysql> SELECT DATE_FORMAT('2003-10-03',GET_FORMAT(DATE,'EUR'));
        -> '03.10.2003'
mysql> SELECT STR_TO_DATE('10.31.2003',GET_FORMAT(DATE,'USA'));
        -> 2003-10-31
GET_FORMAT() is available as of MySQL 4.1.1. See section 14.5.3.1 SET Syntax.
HOUR(time)
Returns the hour for time. The range of the return value will be 0 to 23 for time-of-day values. 
mysql> SELECT HOUR('10:05:03');
        -> 10
However, the range of TIME values actually is much larger, so HOUR can return values greater than 23. 

mysql> SELECT HOUR('272:59:59');
        -> 272
LAST_DAY(date)
Takes a date or datetime value and returns the corresponding value for the last day of the month. Returns NULL if the argument is invalid. 
mysql> SELECT LAST_DAY('2003-02-05');
        -> '2003-02-28'
mysql> SELECT LAST_DAY('2004-02-05');
        -> '2004-02-29'
mysql> SELECT LAST_DAY('2004-01-01 01:01:01');
        -> '2004-01-31'
mysql> SELECT LAST_DAY('2003-03-32');
        -> NULL
LAST_DAY() is available as of MySQL 4.1.1.
LOCALTIME
LOCALTIME()
LOCALTIME and LOCALTIME() are synonyms for NOW(). They were added in MySQL 4.0.6.
LOCALTIMESTAMP
LOCALTIMESTAMP()
LOCALTIMESTAMP and LOCALTIMESTAMP() are synonyms for NOW(). They were added in MySQL 4.0.6.
MAKEDATE(year,dayofyear)
Returns a date, given year and day-of-year values. dayofyear must be greater than 0 or the result will be NULL. 
mysql> SELECT MAKEDATE(2001,31), MAKEDATE(2001,32);
        -> '2001-01-31', '2001-02-01'
mysql> SELECT MAKEDATE(2001,365), MAKEDATE(2004,365);
        -> '2001-12-31', '2004-12-30'
mysql> SELECT MAKEDATE(2001,0);
        -> NULL
MAKEDATE() is available as of MySQL 4.1.1.
MAKETIME(hour,minute,second)
Returns a time value calculated from the hour, minute, and second arguments. 
mysql> SELECT MAKETIME(12,15,30);
        -> '12:15:30'
MAKETIME() is available as of MySQL 4.1.1.
MICROSECOND(expr)
Returns the microseconds from the time or datetime expression expr as a number in the range from 0 to 999999. 
mysql> SELECT MICROSECOND('12:00:00.123456');
        -> 123456
mysql> SELECT MICROSECOND('1997-12-31 23:59:59.000010');
        -> 10
MICROSECOND() is available as of MySQL 4.1.1.
MINUTE(time)
Returns the minute for time, in the range 0 to 59. 
mysql> SELECT MINUTE('98-02-03 10:05:03');
        -> 5
MONTH(date)
Returns the month for date, in the range 1 to 12. 
mysql> SELECT MONTH('1998-02-03');
        -> 2
MONTHNAME(date)
Returns the full name of the month for date. 
mysql> SELECT MONTHNAME('1998-02-05');
        -> 'February'
NOW()
Returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context. 
mysql> SELECT NOW();
        -> '1997-12-15 23:50:26'
mysql> SELECT NOW() + 0;
        -> 19971215235026
PERIOD_ADD(P,N)
Adds N months to period P (in the format YYMM or YYYYMM). Returns a value in the format YYYYMM. Note that the period argument P is not a date value. 
mysql> SELECT PERIOD_ADD(9801,2);
        -> 199803
PERIOD_DIFF(P1,P2)
Returns the number of months between periods P1 and P2. P1 and P2 should be in the format YYMM or YYYYMM. Note that the period arguments P1 and P2 are not date values. 
mysql> SELECT PERIOD_DIFF(9802,199703);
        -> 11
QUARTER(date)
Returns the quarter of the year for date, in the range 1 to 4. 
mysql> SELECT QUARTER('98-04-01');
        -> 2
SECOND(time)
Returns the second for time, in the range 0 to 59. 
mysql> SELECT SECOND('10:05:03');
        -> 3
SEC_TO_TIME(seconds)
Returns the seconds argument, converted to hours, minutes, and seconds, as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context. 
mysql> SELECT SEC_TO_TIME(2378);
        -> '00:39:38'
mysql> SELECT SEC_TO_TIME(2378) + 0;
        -> 3938
STR_TO_DATE(str,format)
This is the reverse function of the DATE_FORMAT() function. It takes a string str and a format string format. STR_TO_DATE() returns a DATETIME value if the format string contains both date and time parts, or a DATE or TIME value if the string contains only date or time parts. The date, time, or datetime values contained in str should be given in the format indicated by format. For the specifiers that can be used in format, see the table in the DATE_FORMAT() function description. All other characters are just taken verbatim, thus not being interpreted. If str contains an illegal date, time, or datetime value, STR_TO_DATE() returns NULL. 
mysql> SELECT STR_TO_DATE('03.10.2003 09.20',
    ->                    '%d.%m.%Y %H.%i');
        -> '2003-10-03 09:20:00'
mysql> SELECT STR_TO_DATE('10arp', '%carp');
        -> '0000-10-00 00:00:00'
mysql> SELECT STR_TO_DATE('2003-15-10 00:00:00',
    ->                    '%Y-%m-%d %H:%i:%s');
        -> NULL
Range checking on the parts of date values is as described in section 12.3.1 The DATETIME, DATE, and TIMESTAMP Types. This means, for example, that a date with a day part larger than the number of days in a month is allowable as long as the day part is in the range from 1 to 31. Also, ``zero'' dates or dates with part values of 0 are allowed. 
mysql> SELECT STR_TO_DATE('00/00/0000', '%m/%d/%Y');
        -> '0000-00-00'
mysql> SELECT STR_TO_DATE('04/31/2004', '%m/%d/%Y');
        -> '2004-04-31'
STR_TO_DATE() is available as of MySQL 4.1.1.
SUBDATE(date,INTERVAL expr type)
SUBDATE(expr,days)
When invoked with the INTERVAL form of the second argument, SUBDATE() is a synonym for DATE_SUB(). For information on the INTERVAL argument, see the discussion for DATE_ADD(). 
mysql> SELECT DATE_SUB('1998-01-02', INTERVAL 31 DAY);
        -> '1997-12-02'
mysql> SELECT SUBDATE('1998-01-02', INTERVAL 31 DAY);
        -> '1997-12-02'
As of MySQL 4.1.1, the second syntax is allowed, where expr is a date or datetime expression and days is the number of days to be subtracted from expr. 
mysql> SELECT SUBDATE('1998-01-02 12:00:00', 31);
        -> '1997-12-02 12:00:00'
SUBTIME(expr,expr2)
SUBTIME() subtracts expr2 from expr and returns the result. expr is a time or datetime expression, and expr2 is a time expression. 
mysql> SELECT SUBTIME('1997-12-31 23:59:59.999999',
    ->                '1 1:1:1.000002');
        -> '1997-12-30 22:58:58.999997'
mysql> SELECT SUBTIME('01:00:00.999999', '02:00:00.999998');
        -> '-00:59:59.999999'
SUBTIME() was added in MySQL 4.1.1.
SYSDATE()
SYSDATE() is a synonym for NOW().
TIME(expr)
Extracts the time part of the time or datetime expression expr. 
mysql> SELECT TIME('2003-12-31 01:02:03');
        -> '01:02:03'
mysql> SELECT TIME('2003-12-31 01:02:03.000123');
        -> '01:02:03.000123'
TIME() is available as of MySQL 4.1.1.
TIMEDIFF(expr,expr2)
TIMEDIFF() returns the time between the start time expr and the end time expr2. expr and expr2 are time or date-and-time expressions, but both must be of the same type. 
mysql> SELECT TIMEDIFF('2000:01:01 00:00:00',
    ->                 '2000:01:01 00:00:00.000001');
        -> '-00:00:00.000001'
mysql> SELECT TIMEDIFF('1997-12-31 23:59:59.000001',
    ->                 '1997-12-30 01:01:01.000002');
        -> '46:58:57.999999'
TIMEDIFF() was added in MySQL 4.1.1.
TIMESTAMP(expr)
TIMESTAMP(expr,expr2)
With one argument, returns the date or datetime expression expr as a datetime value. With two arguments, adds the time expression expr2 to the date or datetime expression expr and returns a datetime value. 
mysql> SELECT TIMESTAMP('2003-12-31');
        -> '2003-12-31 00:00:00'
mysql> SELECT TIMESTAMP('2003-12-31 12:00:00','12:00:00');
        -> '2004-01-01 00:00:00'
TIMESTAMP() is available as of MySQL 4.1.1.
TIMESTAMPADD(interval,int_expr,datetime_expr)
Adds the integer expression int_expr to the date or datetime expression datetime_expr. The unit for int_expr is given by the interval argument, which should be one of the following values: FRAC_SECOND, SECOND, MINUTE, HOUR, DAY, WEEK, MONTH, QUARTER, or YEAR. The interval value may be specified using one of keywords as shown, or with a prefix of SQL_TSI_. For example, DAY or SQL_TSI_DAY both are legal. 
mysql> SELECT TIMESTAMPADD(MINUTE,1,'2003-01-02');
        -> '2003-01-02 00:01:00'
mysql> SELECT TIMESTAMPADD(WEEK,1,'2003-01-02');
        -> '2003-01-09'
TIMESTAMPADD() is available as of MySQL 5.0.0.
TIMESTAMPDIFF(interval,datetime_expr1,datetime_expr2)
Returns the integer difference between the date or datetime expressions datetime_expr1 and datetime_expr2. The unit for the result is given by the interval argument. The legal values for interval are the same as those listed in the description of the TIMESTAMPADD() function. 
mysql> SELECT TIMESTAMPDIFF(MONTH,'2003-02-01','2003-05-01');
        -> 3
mysql> SELECT TIMESTAMPDIFF(YEAR,'2002-05-01','2001-01-01');
        -> -1
TIMESTAMPDIFF() is available as of MySQL 5.0.0.
TIME_FORMAT(time,format)
This is used like the DATE_FORMAT() function, but the format string may contain only those format specifiers that handle hours, minutes, and seconds. Other specifiers produce a NULL value or 0. If the time value contains an hour part that is greater than 23, the %H and %k hour format specifiers produce a value larger than the usual range of 0..23. The other hour format specifiers produce the hour value modulo 12. 
mysql> SELECT TIME_FORMAT('100:00:00', '%H %k %h %I %l');
        -> '100 100 04 04 4'
TIME_TO_SEC(time)
Returns the time argument, converted to seconds. 
mysql> SELECT TIME_TO_SEC('22:23:00');
        -> 80580
mysql> SELECT TIME_TO_SEC('00:39:38');
        -> 2378
TO_DAYS(date)
Given a date date, returns a daynumber (the number of days since year 0). 
mysql> SELECT TO_DAYS(950501);
        -> 728779
mysql> SELECT TO_DAYS('1997-10-07');
        -> 729669
TO_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582), because it does not take into account the days that were lost when the calendar was changed. Remember that MySQL converts two-digit year values in dates to four-digit form using the rules in section 12.3 Date and Time Types. For example, '1997-10-07' and '97-10-07' are seen as identical dates: 
mysql> SELECT TO_DAYS('1997-10-07'), TO_DAYS('97-10-07');
        -> 729669, 729669
For other dates before 1582, results from this function are undefined.
UNIX_TIMESTAMP()
UNIX_TIMESTAMP(date)
If called with no argument, returns a Unix timestamp (seconds since '1970-01-01 00:00:00' GMT) as an unsigned integer. If UNIX_TIMESTAMP() is called with a date argument, it returns the value of the argument as seconds since '1970-01-01 00:00:00' GMT. date may be a DATE string, a DATETIME string, a TIMESTAMP, or a number in the format YYMMDD or YYYYMMDD in local time. 
mysql> SELECT UNIX_TIMESTAMP();
        -> 882226357
mysql> SELECT UNIX_TIMESTAMP('1997-10-04 22:23:00');
        -> 875996580
When UNIX_TIMESTAMP is used on a TIMESTAMP column, the function returns the internal timestamp value directly, with no implicit ``string-to-Unix-timestamp'' conversion. If you pass an out-of-range date to UNIX_TIMESTAMP(), it returns 0, but please note that only basic range checking is performed (year from 1970 to 2037, month from 01 to 12, day from 01 from 31). If you want to subtract UNIX_TIMESTAMP() columns, you might want to cast the result to signed integers. See section 13.7 Cast Functions and Operators.
UTC_DATE
UTC_DATE()
Returns the current UTC date as a value in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context. 
mysql> SELECT UTC_DATE(), UTC_DATE() + 0;
        -> '2003-08-14', 20030814
UTC_DATE() is available as of MySQL 4.1.1.
UTC_TIME
UTC_TIME()
Returns the current UTC time as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context. 
mysql> SELECT UTC_TIME(), UTC_TIME() + 0;
        -> '18:07:53', 180753
UTC_TIME() is available as of MySQL 4.1.1.
UTC_TIMESTAMP
UTC_TIMESTAMP()
Returns the current UTC date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context. 
mysql> SELECT UTC_TIMESTAMP(), UTC_TIMESTAMP() + 0;
        -> '2003-08-14 18:08:04', 20030814180804
UTC_TIMESTAMP() is available as of MySQL 4.1.1.
WEEK(date[,mode])
The function returns the week number for date. The two-argument form of WEEK() allows you to specify whether the week starts on Sunday or Monday and whether the return value should be in the range from 0 to 53 or from 1 to 52. If the mode argument is omitted, the value of the default_week_format system variable is used (or 0 before MySQL 4.0.14). See section 5.2.3 Server System Variables. The following table describes how the mode argument works:
Value Meaning
0 Week starts on Sunday; return value range is 0 to 53; week 1 is the first week that starts in this year
1 Week starts on Monday; return value range is 0 to 53; week 1 is the first week that has more than three days in this year
2 Week starts on Sunday; return value range is 1 to 53; week 1 is the first week that starts in this year
3 Week starts on Monday; return value range is 1 to 53; week 1 is the first week that has more than three days in this year
4 Week starts on Sunday; return value range is 0 to 53; week 1 is the first week that has more than three days in this year
5 Week starts on Monday; return value range is 0 to 53; week 1 is the first week that starts in this year
6 Week starts on Sunday; return value range is 1 to 53; week 1 is the first week that has more than three days in this year
7 Week starts on Monday; return value range is 1 to 53; week 1 is the first week that starts in this year
The mode value of 3 can be used as of MySQL 4.0.5. Values of 4 and above can be used as of MySQL 4.0.17. 
mysql> SELECT WEEK('1998-02-20');
        -> 7
mysql> SELECT WEEK('1998-02-20',0);
        -> 7
mysql> SELECT WEEK('1998-02-20',1);
        -> 8
mysql> SELECT WEEK('1998-12-31',1);
        -> 53
Note: In MySQL 4.0, WEEK(date,0) was changed to match the calendar in the USA. Before that, WEEK() was calculated incorrectly for dates in the USA. (In effect, WEEK(date) and WEEK(date,0) were incorrect for all cases.) Note that if a date falls in the last week of the previous year, MySQL returns 0 if you don't use 2, 3, 6, or 7 as the optional mode argument: 
mysql> SELECT YEAR('2000-01-01'), WEEK('2000-01-01',0);
        -> 2000, 0
One might argue that MySQL should return 52 for the WEEK() function, because the given date actually occurs in the 52nd week of 1999. We decided to return 0 instead because we want the function to return ``the week number in the given year.'' This makes use of the WEEK() function reliable when combined with other functions that extract a date part from a date. If you would prefer the result to be evaluated with respect to the year that contains the first day of the week for the given date, you should use 2, 3, 6, or 7 as the optional mode argument. 
mysql> SELECT WEEK('2000-01-01',2);
        -> 52
Alternatively, use the YEARWEEK() function: 
mysql> SELECT YEARWEEK('2000-01-01');
        -> 199952
mysql> SELECT MID(YEARWEEK('2000-01-01'),5,2);
        -> '52'
WEEKDAY(date)
Returns the weekday index for date (0 = Monday, 1 = Tuesday, ... 6 = Sunday). 
mysql> SELECT WEEKDAY('1998-02-03 22:23:00');
        -> 1
mysql> SELECT WEEKDAY('1997-11-05');
        -> 2
WEEKOFYEAR(date)
Returns the calendar week of the date as a number in the range from 1 to 53. 
mysql> SELECT WEEKOFYEAR('1998-02-20');
        -> 8
WEEKOFYEAR() is available as of MySQL 4.1.1.
YEAR(date)
Returns the year for date, in the range 1000 to 9999. 
mysql> SELECT YEAR('98-02-03');
        -> 1998
YEARWEEK(date)
YEARWEEK(date,start)
Returns year and week for a date. The start argument works exactly like the start argument to WEEK(). The year in the result may be different from the year in the date argument for the first and the last week of the year. 
mysql> SELECT YEARWEEK('1987-01-01');
        -> 198653
Note that the week number is different from what the WEEK() function would return (0) for optional arguments 0 or 1, as WEEK() then returns the week in the context of the given year. YEARWEEK() was added in MySQL 3.23.8.

13.6 Full-Text Search Functions

MATCH (col1,col2,...) AGAINST (expr [IN BOOLEAN MODE | WITH QUERY EXPANSION])
As of MySQL 3.23.23, MySQL has support for full-text indexing and searching. A full-text index in MySQL is an index of type FULLTEXT. FULLTEXT indexes are used with MyISAM tables only and can be created from CHAR, VARCHAR, or TEXT columns at CREATE TABLE time or added later with ALTER TABLE or CREATE INDEX. For large datasets, it will be much faster to load your data into a table that has no FULLTEXT index, then create the index with ALTER TABLE (or CREATE INDEX). Loading data into a table that already has a FULLTEXT index could be significantly slower. Constraints on full-text searching are listed in section 13.6.3 Full-Text Restrictions.

Full-text searching is performed with the MATCH() function. 

mysql> CREATE TABLE articles (
    ->   id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,
    ->   title VARCHAR(200),
    ->   body TEXT,
    ->   FULLTEXT (title,body)
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO articles (title,body) VALUES
    -> ('MySQL Tutorial','DBMS stands for DataBase ...'),
    -> ('How To Use MySQL Well','After you went through a ...'),
    -> ('Optimizing MySQL','In this tutorial we will show ...'),
    -> ('1001 MySQL Tricks','1. Never run mysqld as root. 2. ...'),
    -> ('MySQL vs. YourSQL','In the following database comparison ...'),
    -> ('MySQL Security','When configured properly, MySQL ...');
Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM articles
    -> WHERE MATCH (title,body) AGAINST ('database');
+----+-------------------+------------------------------------------+
| id | title             | body                                     |
+----+-------------------+------------------------------------------+
|  5 | MySQL vs. YourSQL | In the following database comparison ... |
|  1 | MySQL Tutorial    | DBMS stands for DataBase ...             |
+----+-------------------+------------------------------------------+
2 rows in set (0.00 sec)

The MATCH() function performs a natural language search for a string against a text collection. A collection is a set of one or more columns included in a FULLTEXT index. The search string is given as the argument to AGAINST(). For every row in the table, MATCH() returns a relevance value, that is, a similarity measure between the search string and the text in that row in the columns named in the MATCH() list.

By default, the search is performed in case-insensitive fashion. In MySQL 4.1 and up, you can make a full-text search by using a binary collation for the indexed columns. For example, a column that has a character set of latin1 can be assigned a collation of latin1_bin to make it case sensitive for full-text searches.

When MATCH() is used in a WHERE clause, as in the preceding example, the rows returned are automatically sorted with the highest relevance first. Relevance values are non-negative floating-point numbers. Zero relevance means no similarity. Relevance is computed based on the number of words in the row, the number of unique words in that row, the total number of words in the collection, and the number of documents (rows) that contain a particular word.

For natural-language full-text searches, it is a requirement that the columns named in the MATCH() function be the same columns included in some FULLTEXT index in your table. For the preceding query, note that the columns named in the MATCH() function (title and body) are the same as those named in the definition of the article table's FULLTEXT index. If you wanted to search the title or body separately, you would need to create FULLTEXT indexes for each column.

It is also possible to perform a boolean search or a search with query expansion. These search types are described in section 13.6.1 Boolean Full-Text Searches and section 13.6.2 Full-Text Searches with Query Expansion.

The preceding example is a basic illustration showing how to use the MATCH() function where rows are returned in order of decreasing relevance. The next example shows how to retrieve the relevance values explicitly. Returned rows are not ordered because the SELECT statement includes neither WHERE nor ORDER BY clauses: 

mysql> SELECT id, MATCH (title,body) AGAINST ('Tutorial')
    -> FROM articles;
+----+-----------------------------------------+
| id | MATCH (title,body) AGAINST ('Tutorial') |
+----+-----------------------------------------+
|  1 |                        0.65545833110809 |
|  2 |                                       0 |
|  3 |                        0.66266459226608 |
|  4 |                                       0 |
|  5 |                                       0 |
|  6 |                                       0 |
+----+-----------------------------------------+
6 rows in set (0.00 sec)

The following example is more complex. The query returns the relevance values and it also sorts the rows in order of decreasing relevance. To achieve this result, you should specify MATCH() twice: once in the SELECT list and once in the WHERE clause. This causes no additional overhead, because the MySQL optimizer notices that the two MATCH() calls are identical and invokes the full-text search code only once. 

mysql> SELECT id, body, MATCH (title,body) AGAINST
    -> ('Security implications of running MySQL as root') AS score
    -> FROM articles WHERE MATCH (title,body) AGAINST
    -> ('Security implications of running MySQL as root');
+----+-------------------------------------+-----------------+
| id | body                                | score           |
+----+-------------------------------------+-----------------+
|  4 | 1. Never run mysqld as root. 2. ... | 1.5219271183014 |
|  6 | When configured properly, MySQL ... | 1.3114095926285 |
+----+-------------------------------------+-----------------+
2 rows in set (0.00 sec)

MySQL uses a very simple parser to split text into words. A ``word'' is any sequence of characters consisting of letters, digits, `'', or `_'. Some words are ignored in full-text searches:

The default minimum word length and stopword list can be changed as described in section 13.6.4 Fine-Tuning MySQL Full-Text Search.

Every correct word in the collection and in the query is weighted according to its significance in the collection or query. This way, a word that is present in many documents has a lower weight (and may even have a zero weight), because it has lower semantic value in this particular collection. Conversely, if the word is rare, it receives a higher weight. The weights of the words are then combined to compute the relevance of the row.

Such a technique works best with large collections (in fact, it was carefully tuned this way). For very small tables, word distribution does not adequately reflect their semantic value, and this model may sometimes produce bizarre results. For example, although the word ``MySQL'' is present in every row of the articles table, a search for the word produces no results: 

mysql> SELECT * FROM articles
    -> WHERE MATCH (title,body) AGAINST ('MySQL');
Empty set (0.00 sec)

The search result is empty because the word ``MySQL'' is present in at least 50% of the rows. As such, it is effectively treated as a stopword. For large datasets, this is the most desirable behavior--a natural language query should not return every second row from a 1GB table. For small datasets, it may be less desirable.

A word that matches half of rows in a table is less likely to locate relevant documents. In fact, it will most likely find plenty of irrelevant documents. We all know this happens far too often when we are trying to find something on the Internet with a search engine. It is with this reasoning that rows containing the word are assigned a low semantic value for the particular dataset in which they occur. A given word may exceed the 50% threshold in one dataset but not another.

The 50% threshold has a significant implication when you first try full-text searching to see how it works: If you create a table and insert only one or two rows of text into it, every word in the text occurs in at least 50% of the rows. As a result, no search returns any results. Be sure to insert at least three rows, and preferably many more.

13.6.1 Boolean Full-Text Searches

As of Version 4.0.1, MySQL can also perform boolean full-text searches using the IN BOOLEAN MODE modifier. 

mysql> SELECT * FROM articles WHERE MATCH (title,body)
    -> AGAINST ('+MySQL -YourSQL' IN BOOLEAN MODE);
+----+-----------------------+-------------------------------------+
| id | title                 | body                                |
+----+-----------------------+-------------------------------------+
|  1 | MySQL Tutorial        | DBMS stands for DataBase ...        |
|  2 | How To Use MySQL Well | After you went through a ...        |
|  3 | Optimizing MySQL      | In this tutorial we will show ...   |
|  4 | 1001 MySQL Tricks     | 1. Never run mysqld as root. 2. ... |
|  6 | MySQL Security        | When configured properly, MySQL ... |
+----+-----------------------+-------------------------------------+

This query retrieves all the rows that contain the word ``MySQL'' but that do not contain the word ``YourSQL''.

Boolean full-text searches have these characteristics:

The boolean full-text search capability supports the following operators:

+
A leading plus sign indicates that this word must be present in every row returned.
-
A leading minus sign indicates that this word must not be present in any row returned.
(no operator)
By default (when neither + nor - is specified) the word is optional, but the rows that contain it will be rated higher. This mimics the behavior of MATCH() ... AGAINST() without the IN BOOLEAN MODE modifier.
> <
These two operators are used to change a word's contribution to the relevance value that is assigned to a row. The > operator increases the contribution and the < operator decreases it. See the example below.
( )
Parentheses are used to group words into subexpressions. Parenthesized groups can be nested.
~
A leading tilde acts as a negation operator, causing the word's contribution to the row relevance to be negative. It's useful for marking noise words. A row that contains such a word will be rated lower than others, but will not be excluded altogether, as it would be with the - operator.
*
An asterisk is the truncation operator. Unlike the other operators, it should be appended to the word.
"
A phrase that is enclosed within double quote (`"') characters matches only rows that contain the phrase literally, as it was typed.

The following examples demonstrate some search strings that use boolean full-text operators:

'apple banana'
Find rows that contain at least one of the two words.
'+apple +juice'
Find rows that contain both words.
'+apple macintosh'
Find rows that contain the word ``apple'', but rank rows higher if they also contain ``macintosh''.
'+apple -macintosh'
Find rows that contain the word ``apple'' but not ``macintosh''.
'+apple +(>turnover <strudel)'
Find rows that contain the words ``apple'' and ``turnover'', or ``apple'' and ``strudel'' (in any order), but rank ``apple turnover'' higher than ``apple strudel''.
'apple*'
Find rows that contain words such as ``apple'', ``apples'', ``applesauce'', or ``applet''.
'"some words"'
Find rows that contain the exact phrase ``some words'' (for example, rows that contain ``some words of wisdom'' but not ``some noise words''). Note that the `"' characters that surround the phrase are operator characters that delimit the phrase. They are not the quotes that surround the search string itself.

13.6.2 Full-Text Searches with Query Expansion

As of MySQL 4.1.1, full-text search supports query expansion (in particular, its variant ``blind query expansion''). This is generally useful when a search phrase is too short, which often means that the user is relying on implied knowledge that the full-text search engine usually lacks. For example, a user searching for ``database'' may really mean that ``MySQL'', ``Oracle'', ``DB2'', and ``RDBMS'' all are phrases that should match ``databases'' and should be returned, too. This is implied knowledge.

Blind query expansion (also known as automatic relevance feedback) is enabled by adding WITH QUERY EXPANSION following the search phrase. It works by performing the search twice, where the search phrase for the second search is the original search phrase concatenated with the few top found documents from the first search. Thus, if one of these documents contains the word ``databases'' and the word ``MySQL'', the second search will find the documents that contain the word ``MySQL'' even if they do not contain the word ``database''. The following example shows this difference: 

mysql> SELECT * FROM articles
    -> WHERE MATCH (title,body) AGAINST ('database');
+----+-------------------+------------------------------------------+
| id | title             | body                                     |
+----+-------------------+------------------------------------------+
|  5 | MySQL vs. YourSQL | In the following database comparison ... |
|  1 | MySQL Tutorial    | DBMS stands for DataBase ...             |
+----+-------------------+------------------------------------------+
2 rows in set (0.00 sec)

mysql> SELECT * FROM articles
    -> WHERE MATCH (title,body)
    -> AGAINST ('database' WITH QUERY EXPANSION);
+----+-------------------+------------------------------------------+
| id | title             | body                                     |
+----+-------------------+------------------------------------------+
|  1 | MySQL Tutorial    | DBMS stands for DataBase ...             |
|  5 | MySQL vs. YourSQL | In the following database comparison ... |
|  3 | Optimizing MySQL  | In this tutorial we will show ...        |
+----+-------------------+------------------------------------------+
3 rows in set (0.00 sec)

Another example could be searching for books by Georges Simenon about Maigret, when a user is not sure how to spell ``Maigret''. A search for ``Megre and the reluctant witnesses'' will find only ``Maigret and the Reluctant Witnesses'' without query expansion. A search with query expansion will find all books with the word ``Maigret'' on the second pass.

Note: Because blind query expansion tends to increase noise significantly by returning non-relevant documents, it's only meaningful to use when a search phrase is rather short.

13.6.3 Full-Text Restrictions

13.6.4 Fine-Tuning MySQL Full-Text Search

The MySQL full-text search capability has few user-tunable parameters yet, although adding more is very high on the TODO. You can exert more control over full-text searching behavior if you have a MySQL source distribution because some changes require source code modfications. See section 2.3 MySQL Installation Using a Source Distribution.

Note that full-text search was carefully tuned for the best searching effectiveness. Modifying the default behavior will, in most cases, make the search results worse. Do not alter the MySQL sources unless you know what you are doing!

Most full-text variables described in the following items must be set at server startup time. For these variables, a server restart is required to change them and you cannot modify them dynamically while the server is running.

Some variable changes require that you rebuild the FULLTEXT indexes in your tables. Instructions for doing this are given at the end of this section.

If you modify full-text variables that affect indexing (ft_min_word_len, ft_max_word_len, or ft_stopword_file), you must rebuild your FULLTEXT indexes after making the changes and restarting the server. To rebuild the indexes in this case, it's sufficient to do a QUICK repair operation: 

mysql> REPAIR TABLE tbl_name QUICK;

With regard specifically to using the IN BOOLEAN MODE capability, if you upgrade from MySQL 3.23 to 4.0 or later, it's necessary to replace the index header as well. To do this, do a USE_FRM repair operation: 

mysql> REPAIR TABLE tbl_name USE_FRM;

This is necessary because boolean full-text searches require a flag in the index header that was not present in MySQL 3.23, and that is not added if you do only a QUICK repair. If you attempt a boolean full-text search without rebuilding the indexes this way, the search will return incorrect results.

Note that if you use @command{myisamchk} to perform an operation that modifies table indexes (such as repair or analyze), the FULLTEXT indexes are rebuilt using the default full-text parameter values for minimum and maximum word length and the stopword file unless you specify otherwise. This can result in queries failing.

The problem occurs because these parameters are known only by the server. They are not stored in MyISAM index files. To avoid the problem if you have modified the minimum or maximum word length or the stopword file in the server, specify the same ft_min_word_len, ft_max_word_len, and ft_stopword_file values to @command{myisamchk} that you use for @command{mysqld}. For example, if you have set the minimum word length to 3, you can repair a table with @command{myisamchk} like this: 

shell> myisamchk --recover --ft_min_word_len=3 tbl_name.MYI

To ensure that @command{myisamchk} and the server use the same values for full-text parameters, you can place each one in both the [mysqld] and [myisamchk] sections of an option file: 

[mysqld]
ft_min_word_len=3

[myisamchk]
ft_min_word_len=3

An alternative to using @command{myisamchk} is to use the REPAIR TABLE, ANALYZE TABLE, OPTIMIZE TABLE, or ALTER TABLE. These statements are performed by the server, which knows the proper full-text parameter values to use.

13.6.5 Full-Text Search TODO

13.7 Cast Functions and Operators

BINARY
The BINARY operator casts the string following it to a binary string. This is an easy way to force a column comparison to be done byte by byte rather than character by character. This causes the comparison to be case sensitive even if the column isn't defined as BINARY or BLOB. BINARY also causes trailing spaces to be significant. 
mysql> SELECT 'a' = 'A';
        -> 1
mysql> SELECT BINARY 'a' = 'A';
        -> 0
mysql> SELECT 'a' = 'a ';
        -> 1
mysql> SELECT BINARY 'a' = 'a ';
        -> 0
BINARY affects the entire comparison; it can be given before either operand with the same result. BINARY was added in MySQL 3.23.0. As of MySQL 4.0.2, BINARY str is a shorthand for CAST(str AS BINARY). Note that in some contexts, if you cast an indexed column to BINARY, MySQL will not be able to use the index efficiently. If you want to compare a BLOB value in case-insensitive fashion, you can do so as follows:
  • Before MySQL 4.1.1, use the UPPER() function to convert the BLOB value to uppercase before performing the comparison: 
    SELECT 'A' LIKE UPPER(blob_col) FROM tbl_name;
    If the comparison value is lowercase, convert the BLOB value using LOWER() instead.
  • For MySQL 4.1.1 and up, BLOB columns have a character set of binary, which has no concept of lettercase. To perform a case-insensitive comparison, use the CONVERT() function to convert the BLOB value to a character set that is not case sensitive. The result is a non-binary string, so the LIKE operation is not case sensitive: 
    SELECT 'A' LIKE CONVERT(blob_col USING latin1) FROM tbl_name;
    To use a different character set, substitute its name for latin1 in the preceding statement.
CONVERT() can be used more generally for comparing strings that are represented in different character sets.
CAST(expr AS type)
CONVERT(expr,type)
CONVERT(expr USING transcoding_name)
The CAST() and CONVERT() functions can be used to take a value of one type and produce a value of another type. The type can be one of the following values:
  • BINARY
  • CHAR
  • DATE
  • DATETIME
  • SIGNED [INTEGER]
  • TIME
  • UNSIGNED [INTEGER]
BINARY produces a binary string. See the entry for the BINARY operator in this section for a description of how this affects comparisons. CAST() and CONVERT() are available as of MySQL 4.0.2. The CHAR conversion type is available as of 4.0.6. The USING form of CONVERT() is available as of 4.1.0. CAST() and CONVERT(... USING ...) are standard SQL syntax. The non-USING form of CONVERT() is ODBC syntax. CONVERT() with USING is used to convert data between different character sets. In MySQL, transcoding names are the same as the corresponding character set names. For example, this statement converts the string 'abc' in the server's default character set to the corresponding string in the utf8 character set: 
SELECT CONVERT('abc' USING utf8);

The cast functions are useful when you want to create a column with a specific type in a CREATE ... SELECT statement: 

CREATE TABLE new_table SELECT CAST('2000-01-01' AS DATE);

The functions also can be useful for sorting ENUM columns in lexical order. Normally sorting of ENUM columns occurs using the internal numeric values. Casting the values to CHAR results in a lexical sort: 

SELECT enum_col FROM tbl_name ORDER BY CAST(enum_col AS CHAR);

CAST(str AS BINARY) is the same thing as BINARY str. CAST(expr AS CHAR) treats the expression as a string with the default character set.

Note: In MysQL 4.0, a CAST() to DATE, DATETIME, or TIME only marks the column to be a specific type but doesn't change the value of the column.

As of MySQL 4.1.0, the value is converted to the correct column type when it's sent to the user (this is a feature of how the new protocol in 4.1 sends date information to the client): 

mysql> SELECT CAST(NOW() AS DATE);
       -> 2003-05-26

As of MySQL 4.1.1, CAST() also changes the result if you use it as part of a more complex expression such as CONCAT('Date: ',CAST(NOW() AS DATE)).

You should not use CAST() to extract data in different formats but instead use string functions like LEFT() or EXTRACT(). See section 13.5 Date and Time Functions.

To cast a string to a numeric value in numeric context, you don't normally have to do anything. Just use the string value as though it were a number: 

mysql> SELECT 1+'1';
       -> 2

If you use a number in string context, the number automatically is converted to a BINARY string. 

mysql> SELECT CONCAT('hello you ',2);
        -> 'hello you 2'

MySQL supports arithmetic with both signed and unsigned 64-bit values. If you are using numeric operators (such as +) and one of the operands is an unsigned integer, the result is unsigned. You can override this by using the SIGNED and UNSIGNED cast operators to cast the operation to a signed or unsigned 64-bit integer, respectively. 

mysql> SELECT CAST(1-2 AS UNSIGNED)
        -> 18446744073709551615
mysql> SELECT CAST(CAST(1-2 AS UNSIGNED) AS SIGNED);
        -> -1

Note that if either operand is a floating-point value, the result is a floating-point value and is not affected by the preceding rule. (In this context, DECIMAL column values are regarded as floating-point values.) 

mysql> SELECT CAST(1 AS UNSIGNED) - 2.0;
        -> -1.0

If you are using a string in an arithmetic operation, this is converted to a floating-point number.

The handing of unsigned values was changed in MySQL 4.0 to be able to support BIGINT values properly. If you have some code that you want to run in both MySQL 4.0 and 3.23, you probably can't use the CAST() function. You can use the following technique to get a signed result when subtracting two unsigned integer columns ucol1 and ucol2: 

mysql> SELECT (ucol1+0.0)-(ucol2+0.0) FROM ...;

The idea is that the columns are converted to floating-point values before the subtraction occurs.

If you have a problem with UNSIGNED columns in old MySQL applications when porting them to MySQL 4.0, you can use the --sql-mode=NO_UNSIGNED_SUBTRACTION option when starting @command{mysqld}. However, as long as you use this option, you will not be able to make efficient use of the BIGINT UNSIGNED column type.

13.8 Other Functions

13.8.1 Bit Functions

MySQL uses BIGINT (64-bit) arithmetic for bit operations, so these operators have a maximum range of 64 bits.

|
Bitwise OR: 
mysql> SELECT 29 | 15;
        -> 31
The result is an unsigned 64-bit integer.
&
Bitwise AND: 
mysql> SELECT 29 & 15;
        -> 13
The result is an unsigned 64-bit integer.
^
Bitwise XOR: 
mysql> SELECT 1 ^ 1;
        -> 0
mysql> SELECT 1 ^ 0;
        -> 1
mysql> SELECT 11 ^ 3;
        -> 8
The result is an unsigned 64-bit integer. Bitwise XOR was added in MySQL 4.0.2.
<<
Shifts a longlong (BIGINT) number to the left. 
mysql> SELECT 1 << 2;
        -> 4
The result is an unsigned 64-bit integer.
>>
Shifts a longlong (BIGINT) number to the right. 
mysql> SELECT 4 >> 2;
        -> 1
The result is an unsigned 64-bit integer.
~
Invert all bits. 
mysql> SELECT 5 & ~1;
        -> 4
The result is an unsigned 64-bit integer.
BIT_COUNT(N)
Returns the number of bits that are set in the argument N. 
mysql> SELECT BIT_COUNT(29);
        -> 4

13.8.2 Encryption Functions

The functions in this section encrypt and decrypt data values. If you want to store results from an encryption function that might contain arbitrary byte values, use a BLOB column rather than a CHAR or VARCHAR column to avoid potential problems with trailing space removal that would change data values.

AES_ENCRYPT(str,key_str)
AES_DECRYPT(crypt_str,key_str)
These functions allow encryption and decryption of data using the official AES (Advanced Encryption Standard) algorithm, previously known as "Rijndael." Encoding with a 128-bit key length is used, but you can extend it up to 256 bits by modifying the source. We chose 128 bits because it is much faster and it is usually secure enough. The input arguments may be any length. If either argument is NULL, the result of this function is also NULL. Because AES is a block-level algorithm, padding is used to encode uneven length strings and so the result string length may be calculated as 16*(trunc(string_length/16)+1). If AES_DECRYPT() detects invalid data or incorrect padding, it returns NULL. However, it is possible for AES_DECRYPT() to return a non-NULL value (possibly garbage) if the input data or the key is invalid. You can use the AES functions to store data in an encrypted form by modifying your queries: 
INSERT INTO t VALUES (1,AES_ENCRYPT('text','password'));
You can get even more security by not transferring the key over the connection for each query, which can be accomplished by storing it in a server-side variable at connection time. For example: 
SELECT @password:='my password';
INSERT INTO t VALUES (1,AES_ENCRYPT('text',@password));
AES_ENCRYPT() and AES_DECRYPT() were added in MySQL 4.0.2, and can be considered the most cryptographically secure encryption functions currently available in MySQL.
DECODE(crypt_str,pass_str)
Decrypts the encrypted string crypt_str using pass_str as the password. crypt_str should be a string returned from ENCODE().
ENCODE(str,pass_str)
Encrypt str using pass_str as the password. To decrypt the result, use DECODE(). The result is a binary string of the same length as str. If you want to save it in a column, use a BLOB column type.
DES_DECRYPT(crypt_str[,key_str])
Decrypts a string encrypted with DES_ENCRYPT(). On error, this function returns NULL. Note that this function works only if MySQL has been configured with SSL support. See section 5.6.7 Using Secure Connections. If no key_str argument is given, DES_DECRYPT() examines the first byte of the encrypted string to determine the DES key number that was used to encrypt the original string, and then reads the key from the DES key file to decrypt the message. For this to work, the user must have the SUPER privilege. The key file can be specified with the --des-key-file server option. If you pass this function a key_str argument, that string is used as the key for decrypting the message. If the crypt_str argument doesn't look like an encrypted string, MySQL will return the given crypt_str. DES_DECRYPT() was added in MySQL 4.0.1.
DES_ENCRYPT(str[,(key_num|key_str)])
Encrypts the string with the given key using the Triple-DES algorithm. On error, this function returns NULL. Note that this function works only if MySQL has been configured with SSL support. See section 5.6.7 Using Secure Connections. The encryption key to use is chosen based on the second argument to DES_ENCRYPT(), if one was given:
Argument Description
No argument The first key from the DES key file is used.
key_num The given key number (0-9) from the DES key file is used.
key_str The given key string is used to encrypt str.
The key file can be specified with the --des-key-file server option. The return string is a binary string where the first character is CHAR(128 | key_num). The 128 is added to make it easier to recognize an encrypted key. If you use a string key, key_num will be 127. The string length for the result will be new_len = orig_len + (8-(orig_len % 8))+1. Each line in the DES key file has the following format: 
key_num des_key_str
Each key_num must be a number in the range from 0 to 9. Lines in the file may be in any order. des_key_str is the string that will be used to encrypt the message. Between the number and the key there should be at least one space. The first key is the default key that is used if you don't specify any key argument to DES_ENCRYPT() You can tell MySQL to read new key values from the key file with the FLUSH DES_KEY_FILE statement. This requires the RELOAD privilege. One benefit of having a set of default keys is that it gives applications a way to check for the existence of encrypted column values, without giving the end user the right to decrypt those values. 
mysql> SELECT customer_address FROM customer_table WHERE
       crypted_credit_card = DES_ENCRYPT('credit_card_number');
DES_ENCRYPT() was added in MySQL 4.0.1.
ENCRYPT(str[,salt])
Encrypt str using the Unix crypt() system call. The salt argument should be a string with two characters. (As of MySQL 3.22.16, salt may be longer than two characters.) If no salt argument is given, a random value is used. 
mysql> SELECT ENCRYPT('hello');
        -> 'VxuFAJXVARROc'
ENCRYPT() ignores all but the first eight characters of str, at least on some systems. This behavior is determined by the implementation of the underlying crypt() system call. If crypt() is not available on your system, ENCRYPT() always returns NULL. Because of this, we recommend that you use MD5() or SHA1() instead, because those two functions exist on all platforms.
MD5(str)
Calculates an MD5 128-bit checksum for the string. The value is returned as a string of 32 hex digits, or NULL if the argument was NULL. The return value can, for example, be used as a hash key. 
mysql> SELECT MD5('testing');
        -> 'ae2b1fca515949e5d54fb22b8ed95575'
This is the "RSA Data Security, Inc. MD5 Message-Digest Algorithm." MD5() was added in MySQL 3.23.2.
OLD_PASSWORD(str)
OLD_PASSWORD() is available as of MySQL 4.1, when the implementation of PASSWORD() was changed to improve security. OLD_PASSWORD() returns the value of the pre-4.1 implementation of PASSWORD(). section 5.5.9 Password Hashing in MySQL 4.1.
PASSWORD(str)
Calculates and returns a password string from the plaintext password str, or NULL if the argument was NULL. This is the function that is used for encrypting MySQL passwords for storage in the Password column of the user grant table. 
mysql> SELECT PASSWORD('badpwd');
        -> '7f84554057dd964b'
PASSWORD() encryption is one-way (not reversible). PASSWORD() does not perform password encryption in the same way that Unix passwords are encrypted. See ENCRYPT(). Note: The PASSWORD() function is used by the authentication system in MySQL Server, you should not use it in your own applications. For that purpose, use MD5() or SHA1() instead. Also see RFC 2195 for more information about handling passwords and authentication securely in your application.
SHA1(str)
SHA(str)
Calculates an SHA1 160-bit checksum for the string, as described in RFC 3174 (Secure Hash Algorithm). The value is returned as a string of 40 hex digits, or NULL if the argument was NULL. One of the possible uses for this function is as a hash key. You can also use it as a cryptographically safe function for storing passwords. 
mysql> SELECT SHA1('abc');
        -> 'a9993e364706816aba3e25717850c26c9cd0d89d'
SHA1() was added in MySQL 4.0.2, and can be considered a cryptographically more secure equivalent of MD5(). SHA() is synonym for SHA1().

13.8.3 Information Functions

BENCHMARK(count,expr)
The BENCHMARK() function executes the expression expr repeatedly count times. It may be used to time how fast MySQL processes the expression. The result value is always 0. The intended use is from within the @command{mysql} client, which reports query execution times: 
mysql> SELECT BENCHMARK(1000000,ENCODE('hello','goodbye'));
+----------------------------------------------+
| BENCHMARK(1000000,ENCODE('hello','goodbye')) |
+----------------------------------------------+
|                                            0 |
+----------------------------------------------+
1 row in set (4.74 sec)
The time reported is elapsed time on the client end, not CPU time on the server end. It is advisable to execute BENCHMARK() several times, and to interpret the result with regard to how heavily loaded the server machine is.
CHARSET(str)
Returns the character set of the string argument. 
mysql> SELECT CHARSET('abc');
        -> 'latin1'
mysql> SELECT CHARSET(CONVERT('abc' USING utf8));
        -> 'utf8'
mysql> SELECT CHARSET(USER());
        -> 'utf8'
CHARSET() was added in MySQL 4.1.0.
COERCIBILITY(str)
Returns the collation coercibility value of the string argument. 
mysql> SELECT COERCIBILITY('abc' COLLATE latin1_swedish_ci);
        -> 0
mysql> SELECT COERCIBILITY('abc');
        -> 3
mysql> SELECT COERCIBILITY(USER());
        -> 2
The return values have the following meanings:
Coercibility Meaning
0 Explicit collation
1 No collation
2 Implicit collation
3 Coercible
Lower values have higher precedence. COERCIBILITY() was added in MySQL 4.1.1.
COLLATION(str)
Returns the collation for the character set of the string argument. 
mysql> SELECT COLLATION('abc');
        -> 'latin1_swedish_ci'
mysql> SELECT COLLATION(_utf8'abc');
        -> 'utf8_general_ci'
COLLATION() was added in MySQL 4.1.0.
CONNECTION_ID()
Returns the connection ID (thread ID) for the connection. Every connection has its own unique ID. 
mysql> SELECT CONNECTION_ID();
        -> 23786
CONNECTION_ID() was added in MySQL 3.23.14.
CURRENT_USER()
Returns the username and hostname combination that the current session was authenticated as. This value corresponds to the MySQL account that determines your access privileges. It can be different from the value of USER(). 
mysql> SELECT USER();
        -> 'davida@localhost'
mysql> SELECT * FROM mysql.user;
ERROR 1044: Access denied for user ''@'localhost' to
database 'mysql'
mysql> SELECT CURRENT_USER();
        -> '@localhost'
The example illustrates that although the client specified a username of davida (as indicated by the value of the USER() function), the server authenticated the client using an anonymous user account (as seen by the empty username part of the CURRENT_USER() value). One way this might occur is that there is no account listed in the grant tables for davida. CURRENT_USER() was added in MySQL 4.0.6.
DATABASE()
Returns the default (current) database name. 
mysql> SELECT DATABASE();
        -> 'test'
If there is no default database, DATABASE() returns NULL as of MySQL 4.1.1, and the empty string before that.
FOUND_ROWS()
A SELECT statement may include a LIMIT clause to restrict the number of rows the server returns to the client. In some cases, it is desirable to know how many rows the statement would have returned without the LIMIT, but without running the statement again. To get this row count, include a SQL_CALC_FOUND_ROWS option in the SELECT statement, then invoke FOUND_ROWS() afterward: 
mysql> SELECT SQL_CALC_FOUND_ROWS * FROM tbl_name
    -> WHERE id > 100 LIMIT 10;
mysql> SELECT FOUND_ROWS();
The second SELECT will return a number indicating how many rows the first SELECT would have returned had it been written without the LIMIT clause. (If the preceding SELECT statement does not include the SQL_CALC_FOUND_ROWS option, then FOUND_ROWS() may return a different result when LIMIT is used than when it is not.) Note that if you are using SELECT SQL_CALC_FOUND_ROWS, MySQL must calculate how many rows are in the full result set. However, this is faster than running the query again without LIMIT, because the result set need not be sent to the client. SQL_CALC_FOUND_ROWS and FOUND_ROWS() can be useful in situations when you want to restrict the number of rows that a query returns, but also determine the number of rows in the full result set without running the query again. An example is a Web script that presents a paged display containing links to the pages that show other sections of a search result. Using FOUND_ROWS() allows you to determine how many other pages are needed for the rest of the result. The use of SQL_CALC_FOUND_ROWS and FOUND_ROWS() is more complex for UNION queries than for simple SELECT statements, because LIMIT may occur at multiple places in a UNION. It may be applied to individual SELECT statements in the UNION, or global to the UNION result as a whole. The intent of SQL_CALC_FOUND_ROWS for UNION is that it should return the row count that would be returned without a global LIMIT. The conditions for use of SQL_CALC_FOUND_ROWS with UNION are:
  • The SQL_CALC_FOUND_ROWS keyword must appear in the first SELECT of the UNION.
  • The value of FOUND_ROWS() is exact only if UNION ALL is used. If UNION without ALL is used, duplicate removal occurs and the value of FOUND_ROWS() is only approximate.
  • If no LIMIT is present in the UNION, SQL_CALC_FOUND_ROWS is ignored and returns the number of rows in the temporary table that is created to process the UNION.
SQL_CALC_FOUND_ROWS and FOUND_ROWS() are available starting at MySQL 4.0.0.
LAST_INSERT_ID()
LAST_INSERT_ID(expr)
Returns the last automatically generated value that was inserted into an AUTO_INCREMENT column. 
mysql> SELECT LAST_INSERT_ID();
        -> 195
The last ID that was generated is maintained in the server on a per-connection basis. This means the value the function returns to a given client is the most recent AUTO_INCREMENT value generated by that client. The value cannot be affected by other clients, even if they generate AUTO_INCREMENT values of their own. This behavior ensures that you can retrieve your own ID without concern for the activity of other clients, and without the need for locks or transactions. The value of LAST_INSERT_ID() is not changed if you update the AUTO_INCREMENT column of a row with a non-magic value (that is, a value that is not NULL and not 0). If you insert many rows at the same time with an insert statement, LAST_INSERT_ID() returns the value for the first inserted row. The reason for this is to make it possible to easily reproduce the same INSERT statement against some other server. If you use INSERT IGNORE and the record is ignored, the AUTO_INCREMENT counter still is incremented and LAST_INSERT_ID() returns the new value. If expr is given as an argument to LAST_INSERT_ID(), the value of the argument is returned by the function and is remembered as the next value to be returned by LAST_INSERT_ID(). This can be used to simulate sequences:
  • Create a table to hold the sequence counter and initialize it: 
    mysql> CREATE TABLE sequence (id INT NOT NULL);
mysql> INSERT INTO sequence VALUES (0);
  • Use the table to generate sequence numbers like this: 
    mysql> UPDATE sequence SET id=LAST_INSERT_ID(id+1);
mysql> SELECT LAST_INSERT_ID();
    The UPDATE statement increments the sequence counter and causes the next call to LAST_INSERT_ID() to return the updated value. The SELECT statement retrieves that value. The mysql_insert_id() C API function can also be used to get the value. See section 21.2.3.134 mysql_insert_id().
You can generate sequences without calling LAST_INSERT_ID(), but the utility of using the function this way is that the ID value is maintained in the server as the last automatically generated value. It is multi-user safe because multiple clients can issue the UPDATE statement and get their own sequence value with the SELECT statement (or mysql_insert_id()), without affecting or being affected by other clients that generate their own sequence values. Note that mysql_insert_id() is only updated after INSERT and UPDATE statements, so you cannot use the C API function to retrieve the value for LAST_INSERT_ID(expr) after executing other SQL statements like SELECT or SET.
ROW_COUNT()
ROW_COUNT() returns the number of rows updated, inserted, or deleted by the preceding statement. This is the same as the row count that the @command{mysql} client displays and the value from the mysql_affected_rows() C API function. 
mysql> INSERT INTO t VALUES(1),(2),(3);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT ROW_COUNT();
+-------------+
| ROW_COUNT() |
+-------------+
|           3 |
+-------------+
1 row in set (0.00 sec)

mysql> DELETE FROM t WHERE i IN(1,2);
Query OK, 2 rows affected (0.00 sec)

mysql> SELECT ROW_COUNT();
+-------------+
| ROW_COUNT() |
+-------------+
|           2 |
+-------------+
1 row in set (0.00 sec)
ROW_COUNT() was added in MySQL 5.0.1.
SESSION_USER()
SESSION_USER() is a synonym for USER().
SYSTEM_USER()
SYSTEM_USER() is a synonym for USER().
USER()
Returns the current MySQL username and hostname. 
mysql> SELECT USER();
        -> 'davida@localhost'
The value indicates the username you specified when connecting to the server, and the client host from which you connected. The value can be different than that of CURRENT_USER(). Prior to MySQL 3.22.11, the function value does not include the client hostname. You can extract just the username part, regardless of whether the value includes a hostname part, like this: 
mysql> SELECT SUBSTRING_INDEX(USER(),'@',1);
        -> 'davida'
As of MySQL 4.1, USER() returns a value in the utf8 character set, so you should also make sure that the '@' string literal is interpreted in that character set: 
mysql> SELECT SUBSTRING_INDEX(USER(),_utf8'@',1);
        -> 'davida'
VERSION()
Returns a string that indicates the MySQL server version. 
mysql> SELECT VERSION();
        -> '4.1.3-beta-log'
Note that if your version string ends with -log this means that logging is enabled.

13.8.4 Miscellaneous Functions

DEFAULT(col_name)
Returns the default value for a table column. Starting from MySQL 5.0.2, you will get an error if the column doesn't have a default value. 
mysql> UPDATE t SET i = DEFAULT(i)+1 WHERE id < 100;
DEFAULT() was added in MySQL 4.1.0.
FORMAT(X,D)
Formats the number X to a format like '#,###,###.##', rounded to D decimals, and returns the result as a string. If D is 0, the result will have no decimal point or fractional part. 
mysql> SELECT FORMAT(12332.123456, 4);
        -> '12,332.1235'
mysql> SELECT FORMAT(12332.1,4);
        -> '12,332.1000'
mysql> SELECT FORMAT(12332.2,0);
        -> '12,332'
GET_LOCK(str,timeout)
Tries to obtain a lock with a name given by the string str, with a timeout of timeout seconds. Returns 1 if the lock was obtained successfully, 0 if the attempt timed out (for example, because another client has already locked the name), or NULL if an error occurred (such as running out of memory or the thread was killed with @command{mysqladmin kill}). If you have a lock obtained with GET_LOCK(), it is released when you execute RELEASE_LOCK(), execute a new GET_LOCK(), or your connection terminates (either normally or abnormally). This function can be used to implement application locks or to simulate record locks. Names are locked on a server-wide basis. If a name has been locked by one client, GET_LOCK() blocks any request by another client for a lock with the same name. This allows clients that agree on a given lock name to use the name to perform cooperative advisory locking. 
mysql> SELECT GET_LOCK('lock1',10);
        -> 1
mysql> SELECT IS_FREE_LOCK('lock2');
        -> 1
mysql> SELECT GET_LOCK('lock2',10);
        -> 1
mysql> SELECT RELEASE_LOCK('lock2');
        -> 1
mysql> SELECT RELEASE_LOCK('lock1');
        -> NULL
Note that the second RELEASE_LOCK() call returns NULL because the lock 'lock1' was automatically released by the second GET_LOCK() call.
INET_ATON(expr)
Given the dotted-quad representation of a network address as a string, returns an integer that represents the numeric value of the address. Addresses may be 4- or 8-byte addresses. 
mysql> SELECT INET_ATON('209.207.224.40');
        -> 3520061480
The generated number is always in network byte order. For the example just shown, the number is calculated as 209*256^3 + 207*256^2 + 224*256 + 40. As of MySQL 4.1.2, INET_ATON() also understands short-form IP addresses: 
mysql> SELECT INET_ATON('127.0.0.1'), INET_ATON('127.1');
        -> 2130706433, 2130706433
INET_ATON() was added in MySQL 3.23.15.
INET_NTOA(expr)
Given a numeric network address (4 or 8 byte), returns the dotted-quad representation of the address as a string. 
mysql> SELECT INET_NTOA(3520061480);
        -> '209.207.224.40'
INET_NTOA() was added in MySQL 3.23.15.
IS_FREE_LOCK(str)
Checks whether the lock named str is free to use (that is, not locked). Returns 1 if the lock is free (no one is using the lock), 0 if the lock is in use, and NULL on errors (such as incorrect arguments). IS_FREE_LOCK() was added in MySQL 4.0.2.
IS_USED_LOCK(str)
Checks whether the lock named str is in use (that is, locked). If so, it returns the connection identifier of the client that holds the lock. Otherwise, it returns NULL. IS_USED_LOCK() was added in MySQL 4.1.0.
MASTER_POS_WAIT(log_name,log_pos[,timeout])
This function is useful for control of master/slave synchronization. It blocks until the slave has read and applied all updates up to the specified position in the master log. The return value is the number of log events it had to wait for to get to the specified position. The function returns NULL if the slave SQL thread is not started, the slave's master information is not initialized, the arguments are incorrect, or an error occurs. It returns -1 if the timeout has been exceeded. If the slave SQL thread stops while MASTER_POS_WAIT() is waiting, the function returns NULL. If the slave is already past the specified position, the function returns immediately. If a timeout value is specified, MASTER_POS_WAIT() stops waiting when timeout seconds have elapsed. timeout must be greater than 0; a zero or negative timeout means no timeout. MASTER_POS_WAIT() was added in MySQL 3.23.32. The timeout argument was added in 4.0.10.
RELEASE_LOCK(str)
Releases the lock named by the string str that was obtained with GET_LOCK(). Returns 1 if the lock was released, 0 if the lock wasn't locked by this thread (in which case the lock is not released), and NULL if the named lock didn't exist. The lock will not exist if it was never obtained by a call to GET_LOCK() or if it already has been released. The DO statement is convenient to use with RELEASE_LOCK(). See section 14.1.2 DO Syntax.
UUID()
Returns a Universal Unique Identifier (UUID) generated according to ``DCE 1.1: Remote Procedure Call'' (Appendix A) CAE (Common Applications Environment) Specifications published by The Open Group in October 1997 (Document Number C706). A UUID is designed as a number that is globally unique in space and time. Two calls to UUID() are expected to generate two different values, even if these calls are performed on two separate computers that are not connected to each other. A UUID is a 128-bit number represented by a string of five hexadecimal numbers in aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee format:
  • The first three numbers are generated from a timestamp.
  • The fourth number preserves temporal uniqueness in case the timestamp value loses monotonicity (for example, due to daylight saving time).
  • The fifth number is an IEEE 802 node number that provides spatial uniqueness. A random number is substituted if the latter is not available (for example, because the host computer has no Ethernet card, or we do not know how to find the hardware address of an interface on your operating system). In this case, spatial uniqueness cannot be guaranteed. Nevertheless, a collision should have very low probability. Currently, the MAC address of an interface is taken into account only on FreeBSD and Linux. On other operating systems, MySQL uses a randomly generated 48-bit number. 
mysql> SELECT UUID();
        -> '6ccd780c-baba-1026-9564-0040f4311e29'
Note that UUID() does not yet work with replication. UUID() was added in MySQL 4.1.2.

13.9 Functions and Modifiers for Use with GROUP BY Clauses

13.9.1 GROUP BY (Aggregate) Functions

If you use a group function in a statement containing no GROUP BY clause, it is equivalent to grouping on all rows.

AVG(expr)
Returns the average value of expr. 
mysql> SELECT student_name, AVG(test_score)
    ->        FROM student
    ->        GROUP BY student_name;
BIT_AND(expr)
Returns the bitwise AND of all bits in expr. The calculation is performed with 64-bit (BIGINT) precision. As of MySQL 4.0.17, this function returns 18446744073709551615 if there were no matching rows. (This is an unsigned BIGINT value with all bits set to 1.) Before 4.0.17, the function returns -1 if there were no matching rows.
BIT_OR(expr)
Returns the bitwise OR of all bits in expr. The calculation is performed with 64-bit (BIGINT) precision. This function returns 0 if there were no matching rows.
BIT_XOR(expr)
Returns the bitwise XOR of all bits in expr. The calculation is performed with 64-bit (BIGINT) precision. This function returns 0 if there were no matching rows. This function is available as of MySQL 4.1.1.
COUNT(expr)
Returns a count of the number of non-NULL values in the rows retrieved by a SELECT statement. 
mysql> SELECT student.student_name,COUNT(*)
    ->        FROM student,course
    ->        WHERE student.student_id=course.student_id
    ->        GROUP BY student_name;
COUNT(*) is somewhat different in that it returns a count of the number of rows retrieved, whether or not they contain NULL values. COUNT(*) is optimized to return very quickly if the SELECT retrieves from one table, no other columns are retrieved, and there is no WHERE clause. For example: 
mysql> SELECT COUNT(*) FROM student;
This optimization applies only to MyISAM and ISAM tables only, because an exact record count is stored for these table types and can be accessed very quickly. For transactional storage engines (InnoDB, BDB), storing an exact row count is more problematic because multiple transactions may be occurring, each of which may affect the count.
COUNT(DISTINCT expr,[expr...])
Returns a count of the number of different non-NULL values. 
mysql> SELECT COUNT(DISTINCT results) FROM student;
In MySQL, you can get the number of distinct expression combinations that don't contain NULL by giving a list of expressions. In standard SQL, you would have to do a concatenation of all expressions inside COUNT(DISTINCT ...). COUNT(DISTINCT ...) was added in MySQL 3.23.2.
GROUP_CONCAT(expr)
This function returns a string result with the concatenated values from a group. The full syntax is as follows: 
GROUP_CONCAT([DISTINCT] expr [,expr ...]
             [ORDER BY {unsigned_integer | col_name | expr}
                 [ASC | DESC] [,col_name ...]]
             [SEPARATOR str_val])

mysql> SELECT student_name,
    ->     GROUP_CONCAT(test_score)
    ->     FROM student 
    ->     GROUP BY student_name;
Or: 
mysql> SELECT student_name,
    ->     GROUP_CONCAT(DISTINCT test_score
    ->               ORDER BY test_score DESC SEPARATOR ' ')
    ->     FROM student
    ->     GROUP BY student_name;
In MySQL, you can get the concatenated values of expression combinations. You can eliminate duplicate values by using DISTINCT. If you want to sort values in the result, you should use ORDER BY clause. To sort in reverse order, add the DESC (descending) keyword to the name of the column you are sorting by in the ORDER BY clause. The default is ascending order; this may be specified explicitly using the ASC keyword. SEPARATOR is followed by the string value that should be inserted between values of result. The default is a comma (`,'). You can remove the separator altogether by specifying SEPARATOR ''. You can set a maximum allowed length with the group_concat_max_len system variable. The syntax to do this at runtime is as follows, where val is an unsigned integer: 
SET [SESSION | GLOBAL] group_concat_max_len = val;
If a maximum length has been set, the result is truncated to this maximum length. GROUP_CONCAT() was added in MySQL 4.1. Note: Before MySQL 4.1.6, there are some small limitations with GROUP_CONCAT() for BLOB and TEXT values when it comes to using DISTINCT together with ORDER BY. To work around this limitation, use MID(expr,1,255) instead.
MIN(expr)
MAX(expr)
Returns the minimum or maximum value of expr. MIN() and MAX() may take a string argument; in such cases they return the minimum or maximum string value. See section 7.4.5 How MySQL Uses Indexes. 
mysql> SELECT student_name, MIN(test_score), MAX(test_score)
    ->        FROM student
    ->        GROUP BY student_name;
For MIN(), MAX(), and other aggregate functions, MySQL currently compares ENUM and SET columns by their string value rather than by the string's relative position in the set. This differs from how ORDER BY compares them. This will be rectified.
STD(expr)
STDDEV(expr)
Returns the standard deviation of expr (the square root of VARIANCE()). This is an extension to standard SQL. The STDDEV() form of this function is provided for Oracle compatibility.
SUM(expr)
Returns the sum of expr. If the return set has no rows, SUM() returns NULL.
VARIANCE(expr)
Returns the standard variance of expr (considering rows as the whole population, not as a sample; so it has the number of rows as denominator). This is an extension to standard SQL, available only in MySQL 4.1 or later.

13.9.2 GROUP BY Modifiers

As of MySQL 4.1.1, the GROUP BY clause allows a WITH ROLLUP modifier that causes extra rows to be added to the summary output. These rows represent higher-level (or super-aggregate) summary operations. ROLLUP thus allows you to answer questions at multiple levels of analysis with a single query. It can be used, for example, to provide support for OLAP (Online Analytical Processing) operations.

As an illustration, suppose that a table named sales has year, country, product, and profit columns for recording sales profitability: 

CREATE TABLE sales
(
    year    INT NOT NULL,
    country VARCHAR(20) NOT NULL,
    product VARCHAR(32) NOT NULL,
    profit  INT
);

The table's contents can be summarized per year with a simple GROUP BY like this: 

mysql> SELECT year, SUM(profit) FROM sales GROUP BY year;
+------+-------------+
| year | SUM(profit) |
+------+-------------+
| 2000 |        4525 |
| 2001 |        3010 |
+------+-------------+

This output shows the total profit for each year, but if you also want to determine the total profit summed over all years, you must add up the individual values yourself or run an additional query.

Or you can use ROLLUP, which provides both levels of analysis with a single query. Adding a WITH ROLLUP modifier to the GROUP BY clause causes the query to produce another row that shows the grand total over all year values: 

mysql> SELECT year, SUM(profit) FROM sales GROUP BY year WITH ROLLUP;
+------+-------------+
| year | SUM(profit) |
+------+-------------+
| 2000 |        4525 |
| 2001 |        3010 |
| NULL |        7535 |
+------+-------------+

The grand total super-aggregate line is identified by the value NULL in the year column.

ROLLUP has a more complex effect when there are multiple GROUP BY columns. In this case, each time there is a ``break'' (change in value) in any but the last grouping column, the query produces an extra super-aggregate summary row.

For example, without ROLLUP, a summary on the sales table based on year, country, and product might look like this: 

mysql> SELECT year, country, product, SUM(profit)
    -> FROM sales
    -> GROUP BY year, country, product;
+------+---------+------------+-------------+
| year | country | product    | SUM(profit) |
+------+---------+------------+-------------+
| 2000 | Finland | Computer   |        1500 |
| 2000 | Finland | Phone      |         100 |
| 2000 | India   | Calculator |         150 |
| 2000 | India   | Computer   |        1200 |
| 2000 | USA     | Calculator |          75 |
| 2000 | USA     | Computer   |        1500 |
| 2001 | Finland | Phone      |          10 |
| 2001 | USA     | Calculator |          50 |
| 2001 | USA     | Computer   |        2700 |
| 2001 | USA     | TV         |         250 |
+------+---------+------------+-------------+

The output indicates summary values only at the year/country/product level of analysis. When ROLLUP is added, the query produces several extra rows: 

mysql> SELECT year, country, product, SUM(profit)
    -> FROM sales
    -> GROUP BY year, country, product WITH ROLLUP;
+------+---------+------------+-------------+
| year | country | product    | SUM(profit) |
+------+---------+------------+-------------+
| 2000 | Finland | Computer   |        1500 |
| 2000 | Finland | Phone      |         100 |
| 2000 | Finland | NULL       |        1600 |
| 2000 | India   | Calculator |         150 |
| 2000 | India   | Computer   |        1200 |
| 2000 | India   | NULL       |        1350 |
| 2000 | USA     | Calculator |          75 |
| 2000 | USA     | Computer   |        1500 |
| 2000 | USA     | NULL       |        1575 |
| 2000 | NULL    | NULL       |        4525 |
| 2001 | Finland | Phone      |          10 |
| 2001 | Finland | NULL       |          10 |
| 2001 | USA     | Calculator |          50 |
| 2001 | USA     | Computer   |        2700 |
| 2001 | USA     | TV         |         250 |
| 2001 | USA     | NULL       |        3000 |
| 2001 | NULL    | NULL       |        3010 |
| NULL | NULL    | NULL       |        7535 |
+------+---------+------------+-------------+

For this query, adding ROLLUP causes the output to include summary information at four levels of analysis, not just one. Here's how to interpret the ROLLUP output:

Other Considerations When using ROLLUP

The following items list some behaviors specific to the MySQL implementation of ROLLUP:

When you use ROLLUP, you cannot also use an ORDER BY clause to sort the results. In other words, ROLLUP and ORDER BY are mutually exclusive. However, you still have some control over sort order. GROUP BY in MySQL sorts results, and you can use explicit ASC and DESC keywords with columns named in the GROUP BY list to specify sort order for individual columns. (The higher-level summary rows added by ROLLUP still appear after the rows from which they are calculated, regardless of the sort order.)

LIMIT can be used to restrict the number of rows returned to the client. LIMIT is applied after ROLLUP, so the limit applies against the extra rows added by ROLLUP. For example: 

mysql> SELECT year, country, product, SUM(profit)
    -> FROM sales
    -> GROUP BY year, country, product WITH ROLLUP
    -> LIMIT 5;
+------+---------+------------+-------------+
| year | country | product    | SUM(profit) |
+------+---------+------------+-------------+
| 2000 | Finland | Computer   |        1500 |
| 2000 | Finland | Phone      |         100 |
| 2000 | Finland | NULL       |        1600 |
| 2000 | India   | Calculator |         150 |
| 2000 | India   | Computer   |        1200 |
+------+---------+------------+-------------+

Using LIMIT with ROLLUP may produce results that are more difficult to interpret, because you have less context for understanding the super-aggregate rows.

The NULL indicators in each super-aggregate row are produced when the row is sent to the client. The server looks at the columns named in the GROUP BY clause following the leftmost one that has changed value. For any column in the result set with a name that is a lexical match to any of those names, its value is set to NULL. (If you specify grouping columns by column number, the server identifies which columns to set to NULL by number.)

Because the NULL values in the super-aggregate rows are placed into the result set at such a late stage in query processing, you cannot test them as NULL values within the query itself. For example, you cannot add HAVING product IS NULL to the query to eliminate from the output all but the super-aggregate rows.

On the other hand, the NULL values do appear as NULL on the client side and can be tested as such using any MySQL client programming interface.

13.9.3 GROUP BY with Hidden Fields

MySQL extends the use of GROUP BY so that you can use columns or calculations in the SELECT list that don't appear in the GROUP BY clause. This stands for any possible value for this group. You can use this to get better performance by avoiding sorting and grouping on unnecessary items. For example, you don't need to group on customer.name in the following query: 

mysql> SELECT order.custid, customer.name, MAX(payments)
    ->        FROM order,customer
    ->        WHERE order.custid = customer.custid
    ->        GROUP BY order.custid;

In standard SQL, you would have to add customer.name to the GROUP BY clause. In MySQL, the name is redundant if you don't run in ANSI mode.

Do not use this feature if the columns you omit from the GROUP BY part are not unique in the group! You will get unpredictable results.

In some cases, you can use MIN() and MAX() to obtain a specific column value even if it isn't unique. The following gives the value of column from the row containing the smallest value in the sort column: 

SUBSTR(MIN(CONCAT(RPAD(sort,6,' '),column)),7)

See section 3.6.4 The Rows Holding the Group-wise Maximum of a Certain Field.

Note that if you are using MySQL 3.22 (or earlier) or if you are trying to follow standard SQL, you can't use expressions in GROUP BY or ORDER BY clauses. You can work around this limitation by using an alias for the expression: 

mysql> SELECT id,FLOOR(value/100) AS val FROM tbl_name
    ->     GROUP BY id, val ORDER BY val;

In MySQL 3.23 and up, aliases are unnecessary. You can use expressions in GROUP BY and ORDER BY clauses. For example: 

mysql> SELECT id, FLOOR(value/100) FROM tbl_name ORDER BY RAND();

14 SQL Statement Syntax

This chapter describes the syntax for the SQL statements supported in MySQL.

14.1 Data Manipulation Statements

14.1.1 DELETE Syntax

Single-table syntax: 

DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM tbl_name
       [WHERE where_definition]
       [ORDER BY ...]
       [LIMIT row_count]

Multiple-table syntax: 

DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
       tbl_name[.*] [, tbl_name[.*] ...]
       FROM table_references
       [WHERE where_definition]

Or: 

DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
       FROM tbl_name[.*] [, tbl_name[.*] ...]
       USING table_references
       [WHERE where_definition]

DELETE deletes rows from tbl_name that satisfy the condition given by where_definition, and returns the number of records deleted.

If you issue a DELETE statement with no WHERE clause, all rows are deleted. A faster way to do this, when you don't want to know the number of deleted rows, is to use TRUNCATE TABLE. See section 14.1.9 TRUNCATE Syntax.

In MySQL 3.23, DELETE without a WHERE clause returns zero as the number of affected records.

In MySQL 3.23, if you really want to know how many records are deleted when you are deleting all rows, and are willing to suffer a speed penalty, you can use a DELETE statement that includes a WHERE clause with an expression that is true for every row. For example: 

mysql> DELETE FROM tbl_name WHERE 1>0;

This is much slower than TRUNCATE tbl_name, because it deletes rows one at a time.

If you delete the row containing the maximum value for an AUTO_INCREMENT column, the value will be reused for an ISAM or BDB table, but not for a MyISAM or InnoDB table. If you delete all rows in the table with DELETE FROM tbl_name (without a WHERE) in AUTOCOMMIT mode, the sequence starts over for all table types except for InnoDB and (as of MySQL 4.0) MyISAM. There are some exceptions to this behavior for InnoDB tables, discussed in section 16.7.3 How an AUTO_INCREMENT Column Works in InnoDB.

For MyISAM and BDB tables, you can specify an AUTO_INCREMENT secondary column in a multiple-column key. In this case, reuse of values deleted from the top of the sequence occurs even for MyISAM tables. See section 3.6.9 Using AUTO_INCREMENT.

The DELETE statement supports the following modifiers:

The speed of delete operations may also be affected by factors discussed in section 7.2.16 Speed of DELETE Statements.

In MyISAM tables, deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. To reclaim unused space and reduce file sizes, use the OPTIMIZE TABLE statement or the @command{myisamchk} utility to reorganize tables. OPTIMIZE TABLE is easier, but @command{myisamchk} is faster. See section 14.5.2.5 OPTIMIZE TABLE Syntax and section 5.7.2.10 Table Optimization.

The MySQL-specific LIMIT row_count option to DELETE tells the server the maximum number of rows to be deleted before control is returned to the client. This can be used to ensure that a specific DELETE statement doesn't take too much time. You can simply repeat the DELETE statement until the number of affected rows is less than the LIMIT value.

If the DELETE statement includes an ORDER BY clause, the rows are deleted in the order specified by the clause. This is really useful only in conjunction with LIMIT. For example, the following statement finds rows matching the WHERE clause, sorts them in timestamp order, and deletes the first (oldest) one: 

DELETE FROM somelog
WHERE user = 'jcole'
ORDER BY timestamp
LIMIT 1

ORDER BY can be used with DELETE beginning with MySQL 4.0.0.

From MySQL 4.0, you can specify multiple tables in the DELETE statement to delete rows from one or more tables depending on a particular condition in multiple tables. However, you cannot use ORDER BY or LIMIT in a multiple-table DELETE.

The first multiple-table DELETE syntax is supported starting from MySQL 4.0.0. The second is supported starting from MySQL 4.0.2. The table_references part lists the tables involved in the join. Its syntax is described in section 14.1.7.1 JOIN Syntax.

For the first syntax, only matching rows from the tables listed before the FROM clause are deleted. For the second syntax, only matching rows from the tables listed in the FROM clause (before the USING clause) are deleted. The effect is that you can delete rows from many tables at the same time and also have additional tables that are used for searching: 

DELETE t1, t2 FROM t1, t2, t3 WHERE t1.id=t2.id AND t2.id=t3.id;

Or: 

DELETE FROM t1, t2 USING t1, t2, t3 WHERE t1.id=t2.id AND t2.id=t3.id;

These statements use all three files when searching for rows to delete, but delete matching rows only from tables t1 and t2.

The examples show inner joins using the comma operator, but multiple-table DELETE statements can use any type of join allowed in SELECT statements, such as LEFT JOIN.

The syntax allows .* after the table names for compatibility with Access.

If you use a multiple-table DELETE statement involving InnoDB tables for which there are foreign key constraints, the MySQL optimizer might process tables in an order that differs from that of their parent/child relationship. In this case, the statement fails and rolls back. Instead, delete from a single table and rely on the ON DELETE capabilities that InnoDB provides to cause the other tables to be modified accordingly.

Note: In MySQL 4.0, you should refer to the table names to be deleted with the true table name. In MySQL 4.1, you must use the alias (if one was given) when referring to a table name:

In MySQL 4.0: 

DELETE test FROM test AS t1, test2 WHERE ...

In MySQL 4.1: 

DELETE t1 FROM test AS t1, test2 WHERE ...

The reason we didn't make this change in 4.0 is that we didn't want to break any old 4.0 applications that were using the old syntax.

Currently, you cannot delete from a table and select from the same table in a subquery.

14.1.2 DO Syntax

DO expr [, expr] ...

DO executes the expressions but doesn't return any results. This is shorthand for SELECT expr, ..., but has the advantage that it's slightly faster when you don't care about the result.

DO is useful mainly with functions that have side effects, such as RELEASE_LOCK().

DO was added in MySQL 3.23.47.

14.1.3 HANDLER Syntax

HANDLER tbl_name OPEN [ AS alias ]
HANDLER tbl_name READ index_name { = | >= | <= | < } (value1,value2,...)
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name READ index_name { FIRST | NEXT | PREV | LAST }
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name READ { FIRST | NEXT }
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name CLOSE

The HANDLER statement provides direct access to table storage engine interfaces. It is available for MyISAM tables as MySQL 4.0.0 and InnoDB tables as of MySQL 4.0.3.

The HANDLER ... OPEN statement opens a table, making it accessible via subsequent HANDLER ... READ statements. This table object is not shared by other threads and is not closed until the thread calls HANDLER ... CLOSE or the thread terminates. If you open the table using an alias, further references to the table with other HANDLER statements must use the alias rather than the table name.

The first HANDLER ... READ syntax fetches a row where the index specified satisfies the given values and the WHERE condition is met. If you have a multiple-column index, specify the index column values as a comma-separated list. Either specify values for all the columns in the index, or specify values for a leftmost prefix of the index columns. Suppose that an index includes three columns named col_a, col_b, and col_c, in that order. The HANDLER statement can specify values for all three columns in the index, or for the columns in a leftmost prefix. For example: 

HANDLER ... index_name = (col_a_val,col_b_val,col_c_val) ...
HANDLER ... index_name = (col_a_val,col_b_val) ...
HANDLER ... index_name = (col_a_val) ...

The second HANDLER ... READ syntax fetches a row from the table in index order that that matches WHERE condition.

The third HANDLER ... READ syntax fetches a row from the table in natural row order that matches the WHERE condition. It is faster than HANDLER tbl_name READ index_name when a full table scan is desired. Natural row order is the order in which rows are stored in a MyISAM table data file. This statement works for InnoDB tables as well, but there is no such concept because there is no separate data file.

Without a LIMIT clause, all forms of HANDLER ... READ fetch a single row if one is available. To return a specific number of rows, include a LIMIT clause. It has the same syntax as for the SELECT statement. See section 14.1.7 SELECT Syntax.

HANDLER ... CLOSE closes a table that was opened with HANDLER ... OPEN.

Note: To use the HANDLER interface to refer to a table's PRIMARY KEY, use the quoted identifier `PRIMARY`: 

HANDLER tbl_name READ `PRIMARY` > (...);

HANDLER is a somewhat low-level statement. For example, it does not provide consistency. That is, HANDLER ... OPEN does not take a snapshot of the table, and does not lock the table. This means that after a HANDLER ... OPEN statement is issued, table data can be modified (by this or any other thread) and these modifications might appear only partially in HANDLER ... NEXT or HANDLER ... PREV scans.

There are several reasons to use the HANDLER interface instead of normal SELECT statements:

14.1.4 INSERT Syntax

INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name [(col_name,...)]
    VALUES ({expr | DEFAULT},...),(...),...
    [ ON DUPLICATE KEY UPDATE col_name=expr, ... ]

Or: 

INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name
    SET col_name={expr | DEFAULT}, ...
    [ ON DUPLICATE KEY UPDATE col_name=expr, ... ]

Or: 

INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name [(col_name,...)]
    SELECT ...

INSERT inserts new rows into an existing table. The INSERT ... VALUES and INSERT ... SET forms of the statement insert rows based on explicitly specified values. The INSERT ... SELECT form inserts rows selected from another table or tables. The INSERT ... VALUES form with multiple value lists is supported in MySQL 3.22.5 or later. The INSERT ... SET syntax is supported in MySQL 3.22.10 or later. INSERT ... SELECT is discussed further in See section 14.1.4.1 INSERT ... SELECT Syntax.

tbl_name is the table into which rows should be inserted. The columns for which the statement provides values can be specified as follows:

Column values can be given in several ways:

The INSERT statement supports the following modifiers:

If you specify the ON DUPLICATE KEY UPDATE clause (new in MySQL 4.1.0), and a row is inserted that would cause a duplicate value in a UNIQUE index or PRIMARY KEY, an UPDATE of the old row is performed. For example, if column a is declared as UNIQUE and already contains the value 1, the following two statements have identical effect: 

mysql> INSERT INTO table (a,b,c) VALUES (1,2,3)
    -> ON DUPLICATE KEY UPDATE c=c+1;

mysql> UPDATE table SET c=c+1 WHERE a=1;

The rows-affected value is 1 if the row is inserted as a new record and 2 if an existing record is updated.

Note: If column b is unique too, the INSERT would be equivalent to this UPDATE statement instead: 

mysql> UPDATE table SET c=c+1 WHERE a=1 OR b=2 LIMIT 1;

If a=1 OR b=2 matches several rows, only one row is updated! In general, you should try to avoid using the ON DUPLICATE KEY clause on tables with multiple UNIQUE keys.

As of MySQL 4.1.1, you can use the VALUES(col_name) function in the UPDATE clause to refer to column values from the INSERT part of the INSERT ... UPDATE statement. In other words, VALUES(col_name) in the UPDATE clause refers to the value of col_name that would be inserted if no duplicate-key conflict occurred. This function is especially useful in multiple-row inserts. The VALUES() function is meaningful only in INSERT ... UPDATE statements and returns NULL otherwise.

Example: 

mysql> INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)
    -> ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);

That statement is identical to the following two statements: 

mysql> INSERT INTO table (a,b,c) VALUES (1,2,3)
    -> ON DUPLICATE KEY UPDATE c=3;
mysql> INSERT INTO table (a,b,c) VALUES (4,5,6)
    -> ON DUPLICATE KEY UPDATE c=9;

When you use ON DUPLICATE KEY UPDATE, the DELAYED option is ignored.

You can find the value used for an AUTO_INCREMENT column by using the LAST_INSERT_ID() function. From within the C API, use the mysql_insert_id() function. However, note that the two functions do not behave quite identically under all circumstances. The behavior of INSERT statements with respect to AUTO_INCREMENT columns is discussed further in section 13.8.3 Information Functions and section 21.2.3.134 mysql_insert_id().

If you use an INSERT ... VALUES statement with multiple value lists or INSERT ... SELECT, the statement returns an information string in this format: 

Records: 100 Duplicates: 0 Warnings: 0

Records indicates the number of rows processed by the statement. (This is not necessarily the number of rows actually inserted. Duplicates can be non-zero.) Duplicates indicates the number of rows that couldn't be inserted because they would duplicate some existing unique index value. Warnings indicates the number of attempts to insert column values that were problematic in some way. Warnings can occur under any of the following conditions:

If you are using the C API, the information string can be obtained by invoking the mysql_info() function. See section 21.2.3.126 mysql_info().

14.1.4.1 INSERT ... SELECT Syntax

INSERT [LOW_PRIORITY] [IGNORE] [INTO] tbl_name [(column_list)]
    SELECT ...

With INSERT ... SELECT, you can quickly insert many rows into a table from one or many tables.

For example: 

INSERT INTO tbl_temp2 (fld_id)
    SELECT tbl_temp1.fld_order_id
    FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;

The following conditions hold for an INSERT ... SELECT statement:

You can use REPLACE instead of INSERT to overwrite old rows. REPLACE is the counterpart to INSERT IGNORE in the treatment of new rows that contain unique key values that duplicate old rows: The new rows are used to replace the old rows rather than being discarded.

14.1.4.2 INSERT DELAYED Syntax

INSERT DELAYED ...

The DELAYED option for the INSERT statement is a MySQL extension to standard SQL that is very useful if you have clients that can't wait for the INSERT to complete. This is a common problem when you use MySQL for logging and you also periodically run SELECT and UPDATE statements that take a long time to complete. DELAYED was introduced in MySQL 3.22.15.

When a client uses INSERT DELAYED, it gets an okay from the server at once, and the row is queued to be inserted when the table is not in use by any other thread.

Another major benefit of using INSERT DELAYED is that inserts from many clients are bundled together and written in one block. This is much faster than doing many separate inserts.

There are some constraints on the use of DELAYED:

Note that currently the queued rows are held only in memory until they are inserted into the table. This means that if you terminate @command{mysqld} forcibly (for example, with kill -9) or if @command{mysqld} dies unexpectedly, any queued rows that have not been written to disk are lost!

The following describes in detail what happens when you use the DELAYED option to INSERT or REPLACE. In this description, the ``thread'' is the thread that received an INSERT DELAYED statement and ``handler'' is the thread that handles all INSERT DELAYED statements for a particular table.

Note that INSERT DELAYED is slower than a normal INSERT if the table is not in use. There is also the additional overhead for the server to handle a separate thread for each table for which there are delayed rows. This means that you should use INSERT DELAYED only when you are really sure that you need it!

14.1.5 LOAD DATA INFILE Syntax

LOAD DATA [LOW_PRIORITY | CONCURRENT] [LOCAL] INFILE 'file_name.txt'
    [REPLACE | IGNORE]
    INTO TABLE tbl_name
    [FIELDS
        [TERMINATED BY '\t']
        [[OPTIONALLY] ENCLOSED BY '']
        [ESCAPED BY '\\' ]
    ]
    [LINES 
        [STARTING BY '']    
        [TERMINATED BY '\n']
    ]
    [IGNORE number LINES]
    [(col_name,...)]

The LOAD DATA INFILE statement reads rows from a text file into a table at a very high speed. For more information about the efficiency of INSERT versus LOAD DATA INFILE and speeding up LOAD DATA INFILE, section 7.2.14 Speed of INSERT Statements.

You can also load data files by using the @command{mysqlimport} utility; it operates by sending a LOAD DATA INFILE statement to the server. The --local option causes @command{mysqlimport} to read data files from the client host. You can specify the --compress option to get better performance over slow networks if the client and server support the compressed protocol. @xref{mysqlimport, , @command{mysqlimport}}.

If you specify the LOW_PRIORITY keyword, execution of the LOAD DATA statement is delayed until no other clients are reading from the table.

If you specify the CONCURRENT keyword with a MyISAM table that satisfies the condition for concurrent inserts (that is, it contains no free blocks in the middle), then other threads can retrieve data from the table while LOAD DATA is executing. Using this option affects the performance of LOAD DATA a bit, even if no other thread is using the table at the same time.

If the LOCAL keyword is specified, it is interpreted with respect to the client end of the connection:

LOCAL is available in MySQL 3.22.6 or later.

For security reasons, when reading text files located on the server, the files must either reside in the database directory or be readable by all. Also, to use LOAD DATA INFILE on server files, you must have the FILE privilege. See section 5.5.3 Privileges Provided by MySQL.

Using LOCAL is a bit slower than letting the server access the files directly, because the contents of the file must be sent over the connection by the client to the server. On the other hand, you do not need the FILE privilege to load local files.

As of MySQL 3.23.49 and MySQL 4.0.2 (4.0.13 on Windows), LOCAL works only if your server and your client both have been enabled to allow it. For example, if @command{mysqld} was started with --local-infile=0, LOCAL will not work. See section 5.4.4 Security Issues with LOAD DATA LOCAL.

If you need LOAD DATA to read from a pipe, you can use the following technique (here we load the listing of the '/' directory into a table): 

mkfifo /mysql/db/x/x
chmod 666 /mysql/db/x/x
find / -ls > /mysql/db/x/x
mysql -e "LOAD DATA INFILE 'x' INTO TABLE x" x

If you are using a version of MySQL older than 3.23.25, you can use this technique only with LOAD DATA LOCAL INFILE.

If you are using MySQL before Version 3.23.24, you can't read from a FIFO with LOAD DATA INFILE. If you need to read from a FIFO (for example, the output from gunzip), use LOAD DATA LOCAL INFILE instead.

When locating files on the server host, the server uses the following rules:

Note that these rules mean that a file named as `./myfile.txt' is read from the server's data directory, whereas the same file named as `myfile.txt' is read from the database directory of the default database. For example, the following LOAD DATA statement reads the file `data.txt' from the database directory for db1 because db1 is the current database, even though the statement explicitly loads the file into a table in the db2 database: 

mysql> USE db1;
mysql> LOAD DATA INFILE 'data.txt' INTO TABLE db2.my_table;

The REPLACE and IGNORE keywords control handling of input records that duplicate existing records on unique key values.

If you specify REPLACE, input rows replace existing rows (in other words, rows that have the same value for a primary or unique index as an existing row). See section 14.1.6 REPLACE Syntax.

If you specify IGNORE, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, the behavior depends on whether or not the LOCAL keyword is specified. Without LOCAL, an error occurs when a duplicate key value is found, and the rest of the text file is ignored. With LOCAL, the default behavior is the same as if IGNORE is specified; this is because the server has no way to stop transmission of the file in the middle of the operation.

If you want to ignore foreign key constraints during the load operation, you can issue a SET FOREIGN_KEY_CHECKS=0 statement before executing LOAD DATA.

If you use LOAD DATA INFILE on an empty MyISAM table, all non-unique indexes are created in a separate batch (as for REPAIR TABLE). This normally makes LOAD DATA INFILE much faster when you have many indexes. Normally this is very fast, but in some extreme cases, you can create the indexes even faster by turning them off with ALTER TABLE .. DISABLE KEYS before loading the file into the table and using ALTER TABLE .. ENABLE KEYS to re-create the indexes after loading the file. See section 7.2.14 Speed of INSERT Statements.

LOAD DATA INFILE is the complement of SELECT ... INTO OUTFILE. See section 14.1.7 SELECT Syntax. To write data from a table to a file, use SELECT ... INTO OUTFILE. To read the file back into a table, use LOAD DATA INFILE. The syntax of the FIELDS and LINES clauses is the same for both statements. Both clauses are optional, but FIELDS must precede LINES if both are specified.

If you specify a FIELDS clause, each of its subclauses (TERMINATED BY, [OPTIONALLY] ENCLOSED BY, and ESCAPED BY) is also optional, except that you must specify at least one of them.

If you don't specify a FIELDS clause, the defaults are the same as if you had written this: 

FIELDS TERMINATED BY '\t' ENCLOSED BY '' ESCAPED BY '\\'

If you don't specify a LINES clause, the default is the same as if you had written this: 

LINES TERMINATED BY '\n' STARTING BY ''

In other words, the defaults cause LOAD DATA INFILE to act as follows when reading input:

Conversely, the defaults cause SELECT ... INTO OUTFILE to act as follows when writing output:

Note that to write FIELDS ESCAPED BY '\\', you must specify two backslashes for the value to be read as a single backslash.

Note: If you have generated the text file on a Windows system, you might have to use LINES TERMINATED BY '\r\n' to read the file properly, because Windows programs typically use two characters as a line terminator. Some programs, such as WordPad, might use \r as a line terminator when writing files. To read such files, use LINES TERMINATED BY '\r'.

If all the lines you want to read in have a common prefix that you want to ignore, you can use LINES STARTING BY 'prefix_string' to skip over the prefix (and anything before it). If a line doesn't include the prefix, the entire line is skipped. Note that prefix_string may be in the middle of the line!

Example: 

mysql> LOAD DATA INFILE '/tmp/test.txt'
    -> INTO TABLE test LINES STARTING BY "xxx";

With this you can read in a file that contains something like: 

xxx"Row",1
something xxx"Row",2

And just get the data ("row",1) and ("row",2).

The IGNORE number LINES option can be used to ignore lines at the start of the file. For example, you can use IGNORE 1 LINES to skip over an initial header line containing column names: 

mysql> LOAD DATA INFILE '/tmp/test.txt'
    -> INTO TABLE test IGNORE 1 LINES;

When you use SELECT ... INTO OUTFILE in tandem with LOAD DATA INFILE to write data from a database into a file and then read the file back into the database later, the field- and line-handling options for both statements must match. Otherwise, LOAD DATA INFILE will not interpret the contents of the file properly. Suppose that you use SELECT ... INTO OUTFILE to write a file with fields delimited by commas: 

mysql> SELECT * INTO OUTFILE 'data.txt'
    ->          FIELDS TERMINATED BY ','
    ->          FROM table2;

To read the comma-delimited file back in, the correct statement would be: 

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
    ->           FIELDS TERMINATED BY ',';

If instead you tried to read in the file with the statement shown here, it wouldn't work because it instructs LOAD DATA INFILE to look for tabs between fields: 

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
    ->           FIELDS TERMINATED BY '\t';

The likely result is that each input line would be interpreted as a single field.

LOAD DATA INFILE can be used to read files obtained from external sources, too. For example, a file in dBASE format will have fields separated by commas and enclosed within double quotes. If lines in the file are terminated by newlines, the statement shown here illustrates the field- and line-handling options you would use to load the file: 

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE tbl_name
    ->           FIELDS TERMINATED BY ',' ENCLOSED BY '"'
    ->           LINES TERMINATED BY '\n';

Any of the field- or line-handling options can specify an empty string (''). If not empty, the FIELDS [OPTIONALLY] ENCLOSED BY and FIELDS ESCAPED BY values must be a single character. The FIELDS TERMINATED BY, LINES STARTING BY, and LINES TERMINATED BY values can be more than one character. For example, to write lines that are terminated by carriage return/linefeed pairs, or to read a file containing such lines, specify a LINES TERMINATED BY '\r\n' clause.

To read a file containing jokes that are separated by lines consisting of of %%, you can do this 

mysql> CREATE TABLE jokes
    ->     (a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    ->     joke TEXT NOT NULL);
mysql> LOAD DATA INFILE '/tmp/jokes.txt' INTO TABLE jokes
    ->     FIELDS TERMINATED BY ''
    ->     LINES TERMINATED BY '\n%%\n' (joke);

FIELDS [OPTIONALLY] ENCLOSED BY controls quoting of fields. For output (SELECT ... INTO OUTFILE), if you omit the word OPTIONALLY, all fields are enclosed by the ENCLOSED BY character. An example of such output (using a comma as the field delimiter) is shown here: 

"1","a string","100.20"
"2","a string containing a , comma","102.20"
"3","a string containing a \" quote","102.20"
"4","a string containing a \", quote and comma","102.20"

If you specify OPTIONALLY, the ENCLOSED BY character is used only to enclose CHAR and VARCHAR fields: 

1,"a string",100.20
2,"a string containing a , comma",102.20
3,"a string containing a \" quote",102.20
4,"a string containing a \", quote and comma",102.20

Note that occurrences of the ENCLOSED BY character within a field value are escaped by prefixing them with the ESCAPED BY character. Also note that if you specify an empty ESCAPED BY value, it is possible to generate output that cannot be read properly by LOAD DATA INFILE. For example, the preceding output just shown would appear as follows if the escape character is empty. Observe that the second field in the fourth line contains a comma following the quote, which (erroneously) appears to terminate the field: 

1,"a string",100.20
2,"a string containing a , comma",102.20
3,"a string containing a " quote",102.20
4,"a string containing a ", quote and comma",102.20

For input, the ENCLOSED BY character, if present, is stripped from the ends of field values. (This is true whether or not OPTIONALLY is specified; OPTIONALLY has no effect on input interpretation.) Occurrences of the ENCLOSED BY character preceded by the ESCAPED BY character are interpreted as part of the current field value.

If the field begins with the ENCLOSED BY character, instances of that character are recognized as terminating a field value only if followed by the field or line TERMINATED BY sequence. To avoid ambiguity, occurrences of the ENCLOSED BY character within a field value can be doubled and will be interpreted as a single instance of the character. For example, if ENCLOSED BY '"' is specified, quotes are handled as shown here: 

"The ""BIG"" boss"  -> The "BIG" boss
The "BIG" boss      -> The "BIG" boss
The ""BIG"" boss    -> The ""BIG"" boss

FIELDS ESCAPED BY controls how to write or read special characters. If the FIELDS ESCAPED BY character is not empty, it is used to prefix the following characters on output:

If the FIELDS ESCAPED BY character is empty, no characters are escaped and NULL is output as NULL, not \N. It is probably not a good idea to specify an empty escape character, particularly if field values in your data contain any of the characters in the list just given.

For input, if the FIELDS ESCAPED BY character is not empty, occurrences of that character are stripped and the following character is taken literally as part of a field value. The exceptions are an escaped `0' or `N' (for example, \0 or \N if the escape character is `\'). These sequences are interpreted as ASCII NUL (a zero-valued byte) and NULL. The rules for NULL handling are described later in this section.

For more information about `\'-escape syntax, see section 10.1 Literal Values.

In certain cases, field- and line-handling options interact:

Handling of NULL values varies according to the FIELDS and LINES options in use:

Some cases are not supported by LOAD DATA INFILE:

The following example loads all columns of the persondata table: 

mysql> LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata;

By default, when no column list is provided at the end of the LOAD DATA INFILE statement, input lines are expected to contain a field for each table column. If you want to load only some of a table's columns, specify a column list: 

mysql> LOAD DATA INFILE 'persondata.txt'
    ->           INTO TABLE persondata (col1,col2,...);

You must also specify a column list if the order of the fields in the input file differs from the order of the columns in the table. Otherwise, MySQL cannot tell how to match up input fields with table columns.

If an input line has too many fields, the extra fields are ignored and the number of warnings is incremented.

If an input line has too few fields, the table columns for which input fields are missing are set to their default values. Default value assignment is described in section 14.2.6 CREATE TABLE Syntax.

An empty field value is interpreted differently than if the field value is missing:

These are the same values that result if you assign an empty string explicitly to a string, numeric, or date or time type explicitly in an INSERT or UPDATE statement.

TIMESTAMP columns are set to the current date and time only if there is a NULL value for the column (that is, \N), or (for the first TIMESTAMP column only) if the TIMESTAMP column is omitted from the field list when a field list is specified.

LOAD DATA INFILE regards all input as strings, so you can't use numeric values for ENUM or SET columns the way you can with INSERT statements. All ENUM and SET values must be specified as strings!

When the LOAD DATA INFILE statement finishes, it returns an information string in the following format: 

Records: 1  Deleted: 0  Skipped: 0  Warnings: 0

If you are using the C API, you can get information about the statement by calling the mysql_info() function. See section 21.2.3.126 mysql_info().

Warnings occur under the same circumstances as when values are inserted via the INSERT statement (see section 14.1.4 INSERT Syntax), except that LOAD DATA INFILE also generates warnings when there are too few or too many fields in the input row. The warnings are not stored anywhere; the number of warnings can be used only as an indication of whether everything went well.

From MySQL 4.1.1 on, you can use SHOW WARNINGS to get a list of the first max_error_count warnings as information about what went wrong. See section 14.5.3.21 SHOW WARNINGS Syntax.

Before MySQL 4.1.1, only a warning count is available to indicate that something went wrong. If you get warnings and want to know exactly why you got them, one way to do this is to dump the table into another file using SELECT ... INTO OUTFILE and compare the file to your original input file.

14.1.6 REPLACE Syntax

REPLACE [LOW_PRIORITY | DELAYED]
    [INTO] tbl_name [(col_name,...)]
    VALUES ({expr | DEFAULT},...),(...),...

Or: 

REPLACE [LOW_PRIORITY | DELAYED]
    [INTO] tbl_name
    SET col_name={expr | DEFAULT}, ...

Or: 

REPLACE [LOW_PRIORITY | DELAYED]
    [INTO] tbl_name [(col_name,...)]
    SELECT ...

REPLACE works exactly like INSERT, except that if an old record in the table has the same value as a new record for a PRIMARY KEY or a UNIQUE index, the old record is deleted before the new record is inserted. See section 14.1.4 INSERT Syntax.

Note that unless the table has a PRIMARY KEY or UNIQUE index, using a REPLACE statement makes no sense. It becomes equivalent to INSERT, because there is no index to be used to determine whether a new row duplicates another.

Values for all columns are taken from the values specified in the REPLACE statement. Any missing columns are set to their default values, just as happens for INSERT. You can't refer to values from the old row and use them in the new row. It appeared that you could do this in some old MySQL versions, but that was a bug that has been corrected.

To be able to use REPLACE, you must have INSERT and DELETE privileges for the table.

The REPLACE statement returns a count to indicate the number of rows affected. This is the sum of the rows deleted and inserted. If the count is 1 for a single-row REPLACE, a row was inserted and no rows were deleted. If the count is greater than 1, one or more old rows were deleted before the new row was inserted. It is possible for a single row to replace more than one old row if the table contains multiple unique indexes and the new row duplicates values for different old rows in different unique indexes.

The affected-rows count makes it easy to determine whether REPLACE only added a row or whether it also replaced any rows: Check whether the count is 1 (added) or greater (replaced).

If you are using the C API, the affected-rows count can be obtained using the mysql_affected_rows() function.

Currently, you cannot replace into a table and select from the same table in a subquery.

Here follows in more detail the algorithm that is used (it is also used with LOAD DATA ... REPLACE):

  1. Try to insert the new row into the table
  2. While the insertion fails because a duplicate-key error occurs for a primary or unique key:
    1. Delete from the table the conflicting row that has the duplicate key value
    2. Try again to insert the new row into the table

14.1.7 SELECT Syntax

SELECT
    [ALL | DISTINCT | DISTINCTROW ]
      [HIGH_PRIORITY]
      [STRAIGHT_JOIN]
      [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
      [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr, ...
    [INTO OUTFILE 'file_name' export_options
      | INTO DUMPFILE 'file_name']
    [FROM table_references
      [WHERE where_definition]
      [GROUP BY {col_name | expr | position}
        [ASC | DESC], ... [WITH ROLLUP]]
      [HAVING where_definition]
      [ORDER BY {col_name | expr | position}
        [ASC | DESC] , ...]
      [LIMIT {[offset,] row_count | row_count OFFSET offset}]
      [PROCEDURE procedure_name(argument_list)]
      [FOR UPDATE | LOCK IN SHARE MODE]]

SELECT is used to retrieve rows selected from one or more tables. Support for UNION statements and subqueries is available as of MySQL 4.0 and 4.1, respectively. See section 14.1.7.2 UNION Syntax and section 14.1.8 Subquery Syntax.

SELECT can also be used to retrieve rows computed without reference to any table. For example: 

mysql> SELECT 1 + 1;
        -> 2

All clauses used must be given in exactly the order shown in the syntax description. For example, a HAVING clause must come after any GROUP BY clause and before any ORDER BY clause.

Following the SELECT keyword, you can give a number of options that affect the operation of the statement.

The ALL, DISTINCT, and DISTINCTROW options specify whether duplicate rows should be returned. If none of these options are given, the default is ALL (all matching rows are returned). DISTINCT and DISTINCTROW are synonyms and specify that duplicate rows in the result set should be removed.

HIGH_PRIORITY, STRAIGHT_JOIN, and options beginning with SQL_ are MySQL extensions to standard SQL.

14.1.7.1 JOIN Syntax

MySQL supports the following JOIN syntaxes for the table_references part of SELECT statements and multiple-table DELETE and UPDATE statements: 

table_reference, table_reference
table_reference [INNER | CROSS] JOIN table_reference [join_condition]
table_reference STRAIGHT_JOIN table_reference
table_reference LEFT [OUTER] JOIN table_reference [join_condition]
table_reference NATURAL [LEFT [OUTER]] JOIN table_reference
{ OJ table_reference LEFT OUTER JOIN table_reference
    ON conditional_expr }
table_reference RIGHT [OUTER] JOIN table_reference [join_condition]
table_reference NATURAL [RIGHT [OUTER]] JOIN table_reference

table_reference is defined as: 

tbl_name [[AS] alias]
    [[USE INDEX (key_list)]
      | [IGNORE INDEX (key_list)]
      | [FORCE INDEX (key_list)]]

join_condition is defined as: 

ON conditional_expr | USING (column_list)

You should generally not have any conditions in the ON part that are used to restrict which rows you want in the result set, but rather specify these conditions in the WHERE clause. There are exceptions to this rule.

Note that INNER JOIN syntax allows a join_condition only from MySQL 3.23.17 on. The same is true for JOIN and CROSS JOIN only as of MySQL 4.0.11.

The { OJ ... LEFT OUTER JOIN ...} syntax shown in the preceding list exists only for compatibility with ODBC.

As of MySQL 3.23.12, you can give hints about which index MySQL should use when retrieving information from a table. By specifying USE INDEX (key_list), you can tell MySQL to use only one of the possible indexes to find rows in the table. The alternative syntax IGNORE INDEX (key_list) can be used to tell MySQL to not use some particular index. These hints are useful if EXPLAIN shows that MySQL is using the wrong index from the list of possible indexes.

From MySQL 4.0.9 on, you can also use FORCE INDEX. This acts likes USE INDEX (key_list) but with the addition that a table scan is assumed to be very expensive. In other words, a table scan will only be used if there is no way to use one of the given indexes to find rows in the table.

USE KEY, IGNORE KEY, and FORCE KEY are synonyms for USE INDEX, IGNORE INDEX, and FORCE INDEX.

Note: USE INDEX, IGNORE INDEX, and FORCE INDEX only affect which indexes are used when MySQL decides how to find rows in the table and how to do the join. They do not affect whether an index will be used when resolving an ORDER BY or GROUP BY.

Some join examples: 

mysql> SELECT * FROM table1,table2 WHERE table1.id=table2.id;
mysql> SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id;
mysql> SELECT * FROM table1 LEFT JOIN table2 USING (id);
mysql> SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id
    ->          LEFT JOIN table3 ON table2.id=table3.id;
mysql> SELECT * FROM table1 USE INDEX (key1,key2)
    ->          WHERE key1=1 AND key2=2 AND key3=3;
mysql> SELECT * FROM table1 IGNORE INDEX (key3)
    ->          WHERE key1=1 AND key2=2 AND key3=3;

See section 7.2.9 How MySQL Optimizes LEFT JOIN and RIGHT JOIN.

14.1.7.2 UNION Syntax

SELECT ...
UNION [ALL | DISTINCT]
SELECT ...
  [UNION [ALL | DISTINCT]
   SELECT ...]

UNION is used to combine the result from many SELECT statements into one result set. UNION is available from MySQL 4.0.0 on.

Selected columns listed in corresponding positions of each SELECT statement should have the same type. (For example, the first column selected by the first statement should have the same type as the first column selected by the other statements.) The column names used in the first SELECT statement are used as the column names for the results returned.

The SELECT statements are normal select statements, but with the following restrictions:

If you don't use the keyword ALL for the UNION, all returned rows will be unique, as if you had done a DISTINCT for the total result set. If you specify ALL, you will get all matching rows from all the used SELECT statements.

The DISTINCT keyword is an optional word (introduced in MySQL 4.0.17). It does nothing, but is allowed in the syntax as required by the SQL standard.

Before MySQL 4.1.2, you cannot mix UNION ALL and UNION DISTINCT in the same query. If you use ALL for one UNION, it is used for all of them. As of MySQL 4.1.2, mixed UNION types are treated such that a DISTINCT union overrides any ALL union to its left. A DISTINCT union can be produced explicitly by using UNION DISTINCT or implicitly by using UNION with no following DISTINCT or ALL keyword.

If you want to use an ORDER BY to sort the entire UNION result, you should use parentheses: 

(SELECT a FROM tbl_name WHERE a=10 AND B=1 ORDER BY a LIMIT 10)
UNION
(SELECT a FROM tbl_name WHERE a=11 AND B=2 ORDER BY a LIMIT 10)
ORDER BY a;

This kind of ORDER BY cannot use column references that include a table name (that is, names in tbl_name.col_name format). Instead, provide a column alias in the first SELECT statement and refer to the alias in the ORDER BY, or else refer to the column in the ORDER BY using its column position. (An alias is preferable because use of column positions is deprecated.)

The types and lengths of the columns in the result set of a UNION take into account the values retrieved by all the SELECT statements. Before MySQL 4.1.1, a limitation of UNION is that only the values from the first SELECT are used to determine result column types and lengths. This could result in value truncation if, for example, the first SELECT retrieves shorter values than the second SELECT: 

mysql> SELECT REPEAT('a',1) UNION SELECT REPEAT('b',10);
+---------------+
| REPEAT('a',1) |
+---------------+
| a             |
| b             |
+---------------+

That limitation has been removed as of MySQL 4.1.1: 

mysql> SELECT REPEAT('a',1) UNION SELECT REPEAT('b',10);
+---------------+
| REPEAT('a',1) |
+---------------+
| a             |
| bbbbbbbbbb    |
+---------------+

14.1.8 Subquery Syntax

A subquery is a SELECT statement inside another statement.

Starting with MySQL 4.1, all subquery forms and operations that the SQL standard requires are supported, as well as a few features that are MySQL-specific.

With earlier MySQL versions, it was necessary to work around or avoid the use of subqueries, but people starting to write code now will find that subqueries are a very useful part of the MySQL toolkit.

For MySQL versions prior to 4.1, most subqueries can be successfully rewritten using joins and other methods. See section 14.1.8.11 Rewriting Subqueries as Joins for Earlier MySQL Versions.

Here is an example of a subquery: 

SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);

In this example, SELECT * FROM t1 ... is the outer query (or outer statement), and (SELECT column1 FROM t2) is the subquery. We say that the subquery is nested in the outer query, and in fact it's possible to nest subqueries within other subqueries, to a great depth. A subquery must always appear within parentheses.

The main advantages of subqueries are:

Here is an example statement that shows the major points about subquery syntax as specified by the SQL standard and supported in MySQL: 

DELETE FROM t1
WHERE s11 > ANY
 (SELECT COUNT(*) /* no hint */ FROM t2
 WHERE NOT EXISTS
  (SELECT * FROM t3
   WHERE ROW(5*t2.s1,77)=
    (SELECT 50,11*s1 FROM t4 UNION SELECT 50,77 FROM
     (SELECT * FROM t5) AS t5)));

Currently, you cannot modify a table and select from the same table in a subquery. This applies to statements such as DELETE, INSERT, REPLACE, and UPDATE.

14.1.8.1 The Subquery as Scalar Operand

In its simplest form (the scalar subquery as opposed to the row or table subqueries that are discussed later), a subquery is a simple operand. Thus, you can use it wherever a column value or literal is legal, and you can expect it to have those characteristics that all operands have: a data type, a length, an indication whether it can be NULL, and so on. For example: 

CREATE TABLE t1 (s1 INT, s2 CHAR(5) NOT NULL);
SELECT (SELECT s2 FROM t1);

The subquery in this SELECT has a data type of CHAR, a length of 5, a character set and collation equal to the defaults in effect at CREATE TABLE time, and an indication that the value in the column can be NULL. In fact, almost all subqueries can be NULL, because if the table is empty, as in the example, the value of the subquery will be NULL. There are few restrictions.

So, when you see examples in the following sections that contain the rather spartan construct (SELECT column1 FROM t1), imagine that your own code will contain much more diverse and complex constructions.

For example, suppose that we make two tables: 

CREATE TABLE t1 (s1 INT);
INSERT INTO t1 VALUES (1);
CREATE TABLE t2 (s1 INT);
INSERT INTO t2 VALUES (2);

Then perform a SELECT: 

SELECT (SELECT s1 FROM t2) FROM t1;

The result will be 2 because there is a row in t2 containing a column s1 that has a value of 2.

The subquery can be part of an expression. If it is an operand for a function, don't forget the parentheses. For example: 

SELECT UPPER((SELECT s1 FROM t1)) FROM t2;

14.1.8.2 Comparisons Using Subqueries

The most common use of a subquery is in the form: 

non_subquery_operand comparison_operator (subquery)

Where comparison_operator is one of: 

=  >  <  >=  <=  <>

For example: 

... 'a' = (SELECT column1 FROM t1)

At one time the only legal place for a subquery was on the right side of a comparison, and you might still find some old DBMSs that insist on this.

Here is an example of a common-form subquery comparison that you cannot do with a join. It finds all the values in table t1 that are equal to a maximum value in table t2: 

SELECT column1 FROM t1
       WHERE column1 = (SELECT MAX(column2) FROM t2);

Here is another example, which again is impossible with a join because it involves aggregating for one of the tables. It finds all rows in table t1 containing a value that occurs twice in a given column: 

SELECT * FROM t1 AS t
    WHERE 2 = (SELECT COUNT(*) FROM t1 WHERE t1.id = t.id);

14.1.8.3 Subqueries with ANY, IN, and SOME

Syntax: 

operand comparison_operator ANY (subquery)
operand IN (subquery)
operand comparison_operator SOME (subquery)

The ANY keyword, which must follow a comparison operator, means ``return TRUE if the comparison is TRUE for ANY of the rows that the subquery returns.'' For example: 

SELECT s1 FROM t1 WHERE s1 > ANY (SELECT s1 FROM t2);

Suppose that there is a row in table t1 containing (10). The expression is TRUE if table t2 contains (21,14,7) because there is a value 7 in t2 that is less than 10. The expression is FALSE if table t2 contains (20,10), or if table t2 is empty. The expression is UNKNOWN if table t2 contains (NULL,NULL,NULL).

The word IN is an alias for = ANY. Thus these two statements are the same: 

SELECT s1 FROM t1 WHERE s1 = ANY (SELECT s1 FROM t2);
SELECT s1 FROM t1 WHERE s1 IN    (SELECT s1 FROM t2);

However, NOT IN is not an alias for <> ANY, but for <> ALL. See section 14.1.8.4 Subqueries with ALL.

The word SOME is an alias for ANY. Thus these two statements are the same: 

SELECT s1 FROM t1 WHERE s1 <> ANY  (SELECT s1 FROM t2);
SELECT s1 FROM t1 WHERE s1 <> SOME (SELECT s1 FROM t2);

Use of the word SOME is rare, but this example shows why it might be useful. To most people's ears, the English phrase ``a is not equal to any b'' means ``there is no b which is equal to a,'' but that isn't what is meant by the SQL syntax. Using <> SOME instead helps ensure that everyone understands the true meaning of the query.

14.1.8.4 Subqueries with ALL

Syntax: 

operand comparison_operator ALL (subquery)

The word ALL, which must follow a comparison operator, means ``return TRUE if the comparison is TRUE for ALL of the rows that the subquery returns.'' For example: 

SELECT s1 FROM t1 WHERE s1 > ALL (SELECT s1 FROM t2);

Suppose that there is a row in table t1 containing (10). The expression is TRUE if table t2 contains (-5,0,+5) because 10 is greater than all three values in t2. The expression is FALSE if table t2 contains (12,6,NULL,-100) because there is a single value 12 in table t2 that is greater than 10. The expression is UNKNOWN if table t2 contains (0,NULL,1).

Finally, if table t2 is empty, the result is TRUE. You might think the result should be UNKNOWN, but sorry, it's TRUE. So, rather oddly, the following statement is TRUE when table t2 is empty: 

SELECT * FROM t1 WHERE 1 > ALL (SELECT s1 FROM t2);

But this statement is UNKNOWN when table t2 is empty: 

SELECT * FROM t1 WHERE 1 > (SELECT s1 FROM t2);

In addition, the following statement is UNKNOWN when table t2 is empty: 

SELECT * FROM t1 WHERE 1 > ALL (SELECT MAX(s1) FROM t2);

In general, tables with NULL values and empty tables are edge cases. When writing subquery code, always consider whether you have taken those two possibilities into account.

NOT IN is an alias for <> ALL. Thus these two statements are the same: 

SELECT s1 FROM t1 WHERE s1 <> ALL (SELECT s1 FROM t2);
SELECT s1 FROM t1 WHERE s1 NOT IN (SELECT s1 FROM t2);

14.1.8.5 Correlated Subqueries

A correlated subquery is a subquery that contains a reference to a table that also appears in the outer query. For example: 

SELECT * FROM t1 WHERE column1 = ANY
       (SELECT column1 FROM t2 WHERE t2.column2 = t1.column2);

Notice that the subquery contains a reference to a column of t1, even though the subquery's FROM clause doesn't mention a table t1. So, MySQL looks outside the subquery, and finds t1 in the outer query.

Suppose that table t1 contains a row where column1 = 5 and column2 = 6; meanwhile, table t2 contains a row where column1 = 5 and column2 = 7. The simple expression ... WHERE column1 = ANY (SELECT column1 FROM t2) would be TRUE, but in this example, the WHERE clause within the subquery is FALSE (because (5,6) is not equal to (5,7)), so the subquery as a whole is FALSE.

Scoping rule: MySQL evaluates from inside to outside. For example: 

SELECT column1 FROM t1 AS x
  WHERE x.column1 = (SELECT column1 FROM t2 AS x
    WHERE x.column1 = (SELECT column1 FROM t3
      WHERE x.column2 = t3.column1));

In this statement, x.column2 must be a column in table t2 because SELECT column1 FROM t2 AS x ... renames t2. It is not a column in table t1 because SELECT column1 FROM t1 ... is an outer query that is farther out.

For subqueries in HAVING or ORDER BY clauses, MySQL also looks for column names in the outer select list.

For certain cases, a correlated subquery is optimized. For example: 

val IN (SELECT key_val FROM tbl_name WHERE correlated_condition)

Otherwise, they are inefficient and likely to be slow. Rewriting the query as a join might improve performance.

14.1.8.6 EXISTS and NOT EXISTS

If a subquery returns any values at all, then EXISTS subquery is TRUE, and NOT EXISTS subquery is FALSE. For example: 

SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);

Traditionally, an EXISTS subquery starts with SELECT *, but it could begin with SELECT 5 or SELECT column1 or anything at all. MySQL ignores the SELECT list in such a subquery, so it doesn't matter.

For the preceding example, if t2 contains any rows, even rows with nothing but NULL values, then the EXISTS condition is TRUE. This is actually an unlikely example, since almost always a [NOT] EXISTS subquery will contain correlations. Here are some more realistic examples:

The last example is a double-nested NOT EXISTS query. That is, it has a NOT EXISTS clause within a NOT EXISTS clause. Formally, it answers the question ``does a city exist with a store that is not in Stores?'' But it's easier to say that a nested NOT EXISTS answers the question ``is x TRUE for all y?''

14.1.8.7 Row Subqueries

The discussion to this point has been of column (or scalar) subqueries: subqueries that return a single column value. A row subquery is a subquery variant that returns a single row value and can thus return more than one column value. Here are two examples: 

SELECT * FROM t1 WHERE (1,2) = (SELECT column1, column2 FROM t2);
SELECT * FROM t1 WHERE ROW(1,2) = (SELECT column1, column2 FROM t2);

The queries here are both TRUE if table t2 has a row where column1 = 1 and column2 = 2.

The expressions (1,2) and ROW(1,2) are sometimes called row constructors. The two are equivalent. They are legal in other contexts, too. For example, the following two statements are semantically equivalent (although currently only the second one can be optimized): 

SELECT * FROM t1 WHERE (column1,column2) = (1,1);
SELECT * FROM t1 WHERE column1 = 1 AND column2 = 1;

The normal use of row constructors, though, is for comparisons with subqueries that return two or more columns. For example, the following query answers the request, ``find all rows in table t1 that also exist in table t2'': 

SELECT column1,column2,column3
       FROM t1
       WHERE (column1,column2,column3) IN
             (SELECT column1,column2,column3 FROM t2);

14.1.8.8 Subqueries in the FROM clause

Subqueries are legal in a SELECT statement's FROM clause. The syntax that you'll actually see is: 

SELECT ... FROM (subquery) AS name ...

The AS name clause is mandatory, because every table in a FROM clause must have a name. Any columns in the subquery select list must have unique names. You can find this syntax described elsewhere in this manual, where the term used is ``derived tables.''

For illustration, assume that you have this table: 

CREATE TABLE t1 (s1 INT, s2 CHAR(5), s3 FLOAT);

Here's how to use a subquery in the FROM clause, using the example table: 

INSERT INTO t1 VALUES (1,'1',1.0);
INSERT INTO t1 VALUES (2,'2',2.0);
SELECT sb1,sb2,sb3
       FROM (SELECT s1 AS sb1, s2 AS sb2, s3*2 AS sb3 FROM t1) AS sb
       WHERE sb1 > 1;

Result: 2, '2', 4.0.

Here's another example: Suppose that you want to know the average of a set of sums for a grouped table. This won't work: 

SELECT AVG(SUM(column1)) FROM t1 GROUP BY column1;

But this query will provide the desired information: 

SELECT AVG(sum_column1)
       FROM (SELECT SUM(column1) AS sum_column1
             FROM t1 GROUP BY column1) AS t1;

Notice that the column name used within the subquery (sum_column1) is recognized in the outer query.

At the moment, subqueries in the FROM clause cannot be correlated subqueries.

Subquery in the FROM clause will be executed (that is, derived temporary tables will be built) even for the EXPLAIN statement, because upper level queries need information about all tables during optimization phase.

14.1.8.9 Subquery Errors

There are some new error returns that apply only to subqueries. This section groups them together because reviewing them will help remind you of some points.

Usually, failure of a subquery causes the entire statement to fail.

14.1.8.10 Optimizing Subqueries

Development is ongoing, so no optimization tip is reliable for the long term. Some interesting tricks that you might want to play with are:

These tricks might cause programs to go faster or slower. Using MySQL facilities like the BENCHMARK() function, you can get an idea about what helps in your own situation. Don't worry too much about transforming to joins except for compatibility with older versions of MySQL before 4.1 that do not support subqueries.

Some optimizations that MySQL itself makes are:

There is a chapter titled ``How MySQL Transforms Subqueries'' in the MySQL Internals Manual. You can obtain this document by downloading the MySQL source package and looking for a file named `internals.texi' in the `Docs' directory.

14.1.8.11 Rewriting Subqueries as Joins for Earlier MySQL Versions

Before MySQL 4.1, only nested queries of the form INSERT ... SELECT ... and REPLACE ... SELECT ... are supported. The IN() construct can be used in other contexts to test membership in a set of values.

It is often possible to rewrite a query without a subquery: 

SELECT * FROM t1 WHERE id IN (SELECT id FROM t2);

This can be rewritten as: 

SELECT DISTINCT t1.* FROM t1, t2 WHERE t1.id=t2.id;

The queries: 

SELECT * FROM t1 WHERE id NOT IN (SELECT id FROM t2);
SELECT * FROM t1 WHERE NOT EXISTS (SELECT id FROM t2 WHERE t1.id=t2.id);

Can be rewritten as: 

SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id
                                       WHERE table2.id IS NULL;

A LEFT [OUTER] JOIN can be faster than an equivalent subquery because the server might be able to optimize it better--a fact that is not specific to MySQL Server alone. Prior to SQL-92, outer joins did not exist, so subqueries were the only way to do certain things in those bygone days. Today, MySQL Server and many other modern database systems offer a whole range of outer join types.

For more complicated subqueries, you can often create temporary tables to hold the subquery. In some cases, however, this option will not work. The most frequently encountered of these cases arises with DELETE statements, for which standard SQL does not support joins (except in subqueries). For this situation, there are three options available:

MySQL Server 4.0 supports multiple-table DELETE statements that can be used to efficiently delete rows based on information from one table or even from many tables at the same time. Multiple-table UPDATE statements are also supported as of MySQL 4.0.

14.1.9 TRUNCATE Syntax

TRUNCATE TABLE tbl_name

TRUNCATE TABLE empties a table completely. Logically, this is equivalent to a DELETE statement that deletes all rows, but there are practical differences under some circumstances.

For InnoDB, TRUNCATE TABLE is mapped to DELETE, so there is no difference. For other storage engines, TRUNCATE TABLE differs from DELETE FROM ... in the following ways from MySQL 4.0 and up:

In MySQL 3.23, TRUNCATE TABLE is mapped to COMMIT; DELETE FROM tbl_name, so it behaves like DELETE. See section 14.1.1 DELETE Syntax.

TRUNCATE TABLE is an Oracle SQL extension. This statement was added in MySQL 3.23.28, although from 3.23.28 to 3.23.32, the keyword TABLE must be omitted.

14.1.10 UPDATE Syntax

Single-table syntax: 

UPDATE [LOW_PRIORITY] [IGNORE] tbl_name
    SET col_name1=expr1 [, col_name2=expr2 ...]
    [WHERE where_definition]
    [ORDER BY ...]
    [LIMIT row_count]

Multiple-table syntax: 

UPDATE [LOW_PRIORITY] [IGNORE] tbl_name [, tbl_name ...]
    SET col_name1=expr1 [, col_name2=expr2 ...]
    [WHERE where_definition]

The UPDATE statement updates columns in existing table rows with new values. The SET clause indicates which columns to modify and the values they should be given. The WHERE clause, if given, specifies which rows should be updated. Otherwise, all rows are updated. If the ORDER BY clause is specified, the rows will be updated in the order that is specified. The LIMIT clause places a limit on the number of rows that can be updated.

The UPDATE statement supports the following modifiers:

If you access a column from tbl_name in an expression, UPDATE uses the current value of the column. For example, the following statement sets the age column to one more than its current value: 

mysql> UPDATE persondata SET age=age+1;

UPDATE assignments are evaluated from left to right. For example, the following statement doubles the age column, then increments it: 

mysql> UPDATE persondata SET age=age*2, age=age+1;

If you set a column to the value it currently has, MySQL notices this and doesn't update it.

If you update a column that has been declared NOT NULL by setting to NULL, the column is set to the default value appropriate for the column type and the warning count is incremented. The default value is 0 for numeric types, the empty string ('') for string types, and the ``zero'' value for date and time types.

UPDATE returns the number of rows that were actually changed. In MySQL 3.22 or later, the mysql_info() C API function returns the number of rows that were matched and updated and the number of warnings that occurred during the UPDATE.

Starting from MySQL 3.23, you can use LIMIT row_count to restrict the scope of the UPDATE. A LIMIT clause works as follows:

If an UPDATE statement includes an ORDER BY clause, the rows are updated in the order specified by the clause. ORDER BY can be used from MySQL 4.0.0.

Starting with MySQL 4.0.4, you can also perform UPDATE operations that cover multiple tables: 

UPDATE items,month SET items.price=month.price
WHERE items.id=month.id;

The example shows an inner join using the comma operator, but multiple-table UPDATE statements can use any type of join allowed in SELECT statements, such as LEFT JOIN.

Note: You cannot use ORDER BY or LIMIT with multiple-table UPDATE.

Before MySQL 4.0.18, you need the UPDATE privilege for all tables used in a multiple-table UPDATE, even if they were not updated. As of MySQL 4.0.18, you need only the SELECT privilege for any columns that are read but not modified.

If you use a multiple-table UPDATE statement involving InnoDB tables for which there are foreign key constraints, the MySQL optimizer might process tables in an order that differs from that of their parent/child relationship. In this case, the statement will fail and roll back. Instead, update a single table and rely on the ON UPDATE capabilities that InnoDB provides to cause the other tables to be modified accordingly.

Currently, you cannot update a table and select from the same table in a subquery.

14.2 Data Definition Statements

14.2.1 ALTER DATABASE Syntax

ALTER {DATABASE | SCHEMA} db_name
    alter_specification [, alter_specification] ...

alter_specification:
    [DEFAULT] CHARACTER SET charset_name
  | [DEFAULT] COLLATE collation_name

ALTER DATABASE allows you to change the overall characteristics of a database. These characteristics are stored in the `db.opt' file in the database directory. To use ALTER DATABASE, you need the ALTER privilege on the database.

The CHARACTER SET clause changes the default database character set. The COLLATE clause changes the default database collation. Character set and collation names are discussed in section 11 Character Set Support.

ALTER DATABASE was added in MySQL 4.1.1. ALTER SCHEMA can be used as of MySQL 5.0.2.

14.2.2 ALTER TABLE Syntax

ALTER [IGNORE] TABLE tbl_name
    alter_specification [, alter_specification] ...

alter_specification:
    ADD [COLUMN] column_definition [FIRST | AFTER col_name ]
  | ADD [COLUMN] (column_definition,...)
  | ADD INDEX [index_name] [index_type] (index_col_name,...)
  | ADD [CONSTRAINT [symbol]]
        PRIMARY KEY [index_type] (index_col_name,...)
  | ADD [CONSTRAINT [symbol]]
        UNIQUE [index_name] [index_type] (index_col_name,...)
  | ADD [FULLTEXT|SPATIAL] [index_name] (index_col_name,...)
  | ADD [CONSTRAINT [symbol]]
        FOREIGN KEY [index_name] (index_col_name,...)
        [reference_definition]
  | ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  | CHANGE [COLUMN] old_col_name column_definition
        [FIRST|AFTER col_name]
  | MODIFY [COLUMN] column_definition [FIRST | AFTER col_name]
  | DROP [COLUMN] col_name
  | DROP PRIMARY KEY
  | DROP INDEX index_name
  | DROP FOREIGN KEY fk_symbol
  | DISABLE KEYS
  | ENABLE KEYS
  | RENAME [TO] new_tbl_name
  | ORDER BY col_name
  | CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]
  | [DEFAULT] CHARACTER SET charset_name [COLLATE collation_name]
  | DISCARD TABLESPACE
  | IMPORT TABLESPACE
  | table_options

ALTER TABLE allows you to change the structure of an existing table. For example, you can add or delete columns, create or destroy indexes, change the type of existing columns, or rename columns or the table itself. You can also change the comment for the table and type of the table.

The syntax for many of the allowable alterations is similar to clauses of the CREATE TABLE statement. See section 14.2.6 CREATE TABLE Syntax.

Some operations may result in warnings if attempted on a table for which the storage engine does not support the operation. In MySQL 4.1 and up, these warnings can be displayed with SHOW WARNINGS. See section 14.5.3.21 SHOW WARNINGS Syntax.

If you use ALTER TABLE to change a column specification but DESCRIBE tbl_name indicates that your column was not changed, it is possible that MySQL ignored your modification for one of the reasons described in section 14.2.6.1 Silent Column Specification Changes. For example, if you try to change a VARCHAR column to CHAR, MySQL will still use VARCHAR if the table contains other variable-length columns.

ALTER TABLE works by making a temporary copy of the original table. The alteration is performed on the copy, then the original table is deleted and the new one is renamed. While ALTER TABLE is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready, then are automatically redirected to the new table without any failed updates.

Note that if you use any other option to ALTER TABLE than RENAME, MySQL always creates a temporary table, even if the data wouldn't strictly need to be copied (such as when you change the name of a column). We plan to fix this in the future, but because ALTER TABLE is not a statement that is normally used frequently, this isn't high on our TODO list. For MyISAM tables, you can speed up the index re-creation operation (which is the slowest part of the alteration process) by setting the myisam_sort_buffer_size system variable to a high value.

Here are some examples that show uses of ALTER TABLE. Begin with a table t1 that is created as shown here: 

mysql> CREATE TABLE t1 (a INTEGER,b CHAR(10));

To rename the table from t1 to t2: 

mysql> ALTER TABLE t1 RENAME t2;

To change column a from INTEGER to TINYINT NOT NULL (leaving the name the same), and to change column b from CHAR(10) to CHAR(20) as well as renaming it from b to c: 

mysql> ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20);

To add a new TIMESTAMP column named d: 

mysql> ALTER TABLE t2 ADD d TIMESTAMP;

To add indexes on column d and on column a: 

mysql> ALTER TABLE t2 ADD INDEX (d), ADD INDEX (a);

To remove column c: 

mysql> ALTER TABLE t2 DROP COLUMN c;

To add a new AUTO_INCREMENT integer column named c: 

mysql> ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT,
    ->     ADD PRIMARY KEY (c);

Note that we indexed c (as a PRIMARY KEY), because AUTO_INCREMENT columns must be indexed, and also that we declare c as NOT NULL, because primary key columns cannot be NULL.

When you add an AUTO_INCREMENT column, column values are filled in with sequence numbers for you automatically. For MyISAM tables, you can set the first sequence number by executing SET INSERT_ID=value before ALTER TABLE or by using the AUTO_INCREMENT=value table option. See section 14.5.3.1 SET Syntax.

With MyISAM tables, if you don't change the AUTO_INCREMENT column, the sequence number will not be affected. If you drop an AUTO_INCREMENT column and then add another AUTO_INCREMENT column, the numbers are resequenced beginning with 1.

See section A.7.1 Problems with ALTER TABLE.

14.2.3 ALTER VIEW Syntax

ALTER VIEW view_name [(column_list)] AS select_statement

This statement changes the definition of an existing view. select_statement is as for CREATE VIEW. See section 14.2.7 CREATE VIEW Syntax.

This statement was added in MySQL 5.0.1.

14.2.4 CREATE DATABASE Syntax

CREATE {DATABASE | SCHEMA} [IF NOT EXISTS] db_name
    [create_specification [, create_specification] ...]

create_specification:
    [DEFAULT] CHARACTER SET charset_name
  | [DEFAULT] COLLATE collation_name

CREATE DATABASE creates a database with the given name. To use CREATE DATABASE, you need the CREATE privilege on the database.

Rules for allowable database names are given in section 10.2 Database, Table, Index, Column, and Alias Names. An error occurs if the database already exists and you didn't specify IF NOT EXISTS.

As of MySQL 4.1.1, create_specification options can be given to specify database characteristics. Database characteristics are stored in the `db.opt' file in the database directory. The CHARACTER SET clause specifies the default database character set. The COLLATE clause specifies the default database collation. Character set and collation names are discussed in section 11 Character Set Support.

Databases in MySQL are implemented as directories containing files that correspond to tables in the database. Because there are no tables in a database when it is initially created, the CREATE DATABASE statement only creates a directory under the MySQL data directory (and the `db.opt' file, for MySQL 4.1.1 and up).

CREATE SCHEMA can be used as of MySQL 5.0.2.

You can also use the @command{mysqladmin} program to create databases. @xref{mysqladmin, , @command{mysqladmin}}.

14.2.5 CREATE INDEX Syntax

CREATE [UNIQUE|FULLTEXT|SPATIAL] INDEX index_name [index_type]
    ON tbl_name (index_col_name,...)

index_col_name:
    col_name [(length)] [ASC | DESC]

In MySQL 3.22 or later, CREATE INDEX is mapped to an ALTER TABLE statement to create indexes. See section 14.2.2 ALTER TABLE Syntax. The CREATE INDEX statement doesn't do anything prior to MySQL 3.22.

Normally, you create all indexes on a table at the time the table itself is created with CREATE TABLE. See section 14.2.6 CREATE TABLE Syntax. CREATE INDEX allows you to add indexes to existing tables.

A column list of the form (col1,col2,...) creates a multiple-column index. Index values are formed by concatenating the values of the given columns.

For CHAR and VARCHAR columns, indexes can be created that use only part of a column, using col_name(length) syntax to index a prefix consisting of the first length characters of each column value. BLOB and TEXT columns also can be indexed, but a prefix length must be given.

The statement shown here creates an index using the first 10 characters of the name column: 

CREATE INDEX part_of_name ON customer (name(10));

Because most names usually differ in the first 10 characters, this index should not be much slower than an index created from the entire name column. Also, using partial columns for indexes can make the index file much smaller, which could save a lot of disk space and might also speed up INSERT operations!

Prefixes can be up to 255 bytes long (or 1000 bytes for MyISAM and InnoDB tables as of MySQL 4.1.2). Note that prefix limits are measured in bytes, whereas the prefix length in CREATE INDEX statements is interpreted as number of characters. Take this into account when specifying a prefix length for a column that uses a multi-byte character set.

Note that you can add an index on a column that can have NULL values only if you are using MySQL 3.23.2 or newer and are using the MyISAM, InnoDB, or BDB table type. You can only add an index on a BLOB or TEXT column if you are using MySQL 3.23.2 or newer and are using the MyISAM or BDB table type, or MySQL 4.0.14 or newer and the InnoDB table type.

An index_col_name specification can end with ASC or DESC. These keywords are allowed for future extensions for specifying ascending or descending index value storage. Currently they are parsed but ignored; index values are always stored in ascending order.

For more information about how MySQL uses indexes, see section 7.4.5 How MySQL Uses Indexes.

FULLTEXT indexes can index only CHAR, VARCHAR, and TEXT columns, and only in MyISAM tables. FULLTEXT indexes are available in MySQL 3.23.23 or later. section 13.6 Full-Text Search Functions.

SPATIAL indexes can index only spatial columns, and only in MyISAM tables. SPATIAL indexes are available in MySQL 4.1 or later. Spatial column types are described in section 19 Spatial Extensions in MySQL.

14.2.6 CREATE TABLE Syntax

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    [(create_definition,...)]
    [table_options] [select_statement]

Or: 

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    [(] LIKE old_tbl_name [)];

create_definition:
    column_definition
  | [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...)
  | KEY [index_name] [index_type] (index_col_name,...)
  | INDEX [index_name] [index_type] (index_col_name,...)
  | [CONSTRAINT [symbol]] UNIQUE [INDEX]
        [index_name] [index_type] (index_col_name,...)
  | [FULLTEXT|SPATIAL] [INDEX] [index_name] (index_col_name,...)
  | [CONSTRAINT [symbol]] FOREIGN KEY
        [index_name] (index_col_name,...) [reference_definition]
  | CHECK (expr)

column_definition:
    col_name type [NOT NULL | NULL] [DEFAULT default_value]
        [AUTO_INCREMENT] [[PRIMARY] KEY] [COMMENT 'string']
        [reference_definition]

type:
    TINYINT[(length)] [UNSIGNED] [ZEROFILL]
  | SMALLINT[(length)] [UNSIGNED] [ZEROFILL]
  | MEDIUMINT[(length)] [UNSIGNED] [ZEROFILL]
  | INT[(length)] [UNSIGNED] [ZEROFILL]
  | INTEGER[(length)] [UNSIGNED] [ZEROFILL]
  | BIGINT[(length)] [UNSIGNED] [ZEROFILL]
  | REAL[(length,decimals)] [UNSIGNED] [ZEROFILL]
  | DOUBLE[(length,decimals)] [UNSIGNED] [ZEROFILL]
  | FLOAT[(length,decimals)] [UNSIGNED] [ZEROFILL]
  | DECIMAL(length,decimals) [UNSIGNED] [ZEROFILL]
  | NUMERIC(length,decimals) [UNSIGNED] [ZEROFILL]
  | DATE
  | TIME
  | TIMESTAMP
  | DATETIME
  | CHAR(length) [BINARY | ASCII | UNICODE]
  | VARCHAR(length) [BINARY]
  | TINYBLOB
  | BLOB
  | MEDIUMBLOB
  | LONGBLOB
  | TINYTEXT
  | TEXT
  | MEDIUMTEXT
  | LONGTEXT
  | ENUM(value1,value2,value3,...)
  | SET(value1,value2,value3,...)
  | spatial_type

index_col_name:
    col_name [(length)] [ASC | DESC]

reference_definition:
    REFERENCES tbl_name [(index_col_name,...)]
               [MATCH FULL | MATCH PARTIAL]
               [ON DELETE reference_option]
               [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

table_options: table_option [table_option] ...

table_option:
    {ENGINE|TYPE} = {BDB|HEAP|ISAM|InnoDB|MERGE|MRG_MYISAM|MYISAM}
  | AUTO_INCREMENT = value
  | AVG_ROW_LENGTH = value
  | CHECKSUM = {0 | 1}
  | COMMENT = 'string'
  | MAX_ROWS = value
  | MIN_ROWS = value
  | PACK_KEYS = {0 | 1 | DEFAULT}
  | PASSWORD = 'string'
  | DELAY_KEY_WRITE = {0 | 1}
  | ROW_FORMAT = { DEFAULT | DYNAMIC | FIXED | COMPRESSED }
  | RAID_TYPE = { 1 | STRIPED | RAID0 }
        RAID_CHUNKS = value
        RAID_CHUNKSIZE = value
  | UNION = (tbl_name[,tbl_name]...)
  | INSERT_METHOD = { NO | FIRST | LAST }
  | DATA DIRECTORY = 'absolute path to directory'
  | INDEX DIRECTORY = 'absolute path to directory'
  | [DEFAULT] CHARACTER SET charset_name [COLLATE collation_name]

select_statement:
    [IGNORE | REPLACE] [AS] SELECT ...   (Some legal select statement)

CREATE TABLE creates a table with the given name. You must have the CREATE privilege for the table.

Rules for allowable table names are given in section 10.2 Database, Table, Index, Column, and Alias Names. By default, the table is created in the current database. An error occurs if the table already exists, if there is no current database, or if the database does not exist.

In MySQL 3.22 or later, the table name can be specified as db_name.tbl_name to create the table in a specific database. This works whether or not there is a current database. If you use quoted identifiers, quote the database and table names separately. For example, `mydb`.`mytbl` is legal, but `mydb.mytbl` is not.

From MySQL 3.23 on, you can use the TEMPORARY keyword when creating a table. A TEMPORARY table is visible only to the current connection, and is dropped automatically when the connection is closed. This means that two different connections can use the same temporary table name without conflicting with each other or with an existing non-TEMPORARY table of the same name. (The existing table is hidden until the temporary table is dropped.) From MySQL 4.0.2 on, you must have the CREATE TEMPORARY TABLES privilege to be able to create temporary tables.

In MySQL 3.23 or later, you can use the keywords IF NOT EXISTS so that an error does not occur if the table already exists. Note that there is no verification that the existing table has a structure identical to that indicated by the CREATE TABLE statement.

MySQL represents each table by an `.frm' table format (definition) file in the database directory. The storage engine for the table might create other files as well. In the case of MyISAM tables, the storage engine creates three files for a table named tbl_name:

File Purpose
tbl_name.frm Table format (definition) file
tbl_name.MYD Data file
tbl_name.MYI Index file

The files created by each storage engine to represent tables are described in section 15 MySQL Storage Engines and Table Types.

For general information on the properties of the various column types, see section 12 Column Types. For information about spatial column types, see section 19 Spatial Extensions in MySQL.

The table_options part of the CREATE TABLE syntax can be used in MySQL 3.23 and above.

The ENGINE and TYPE options specify the storage engine for the table. ENGINE was added in MySQL 4.0.18 (for 4.0) and 4.1.2 (for 4.1). It is the preferred option name as of those versions, and TYPE has become deprecated. TYPE will be supported throughout the 4.x series, but likely will be removed in MySQL 5.1.

The ENGINE and TYPE options take the following values:

Storage Engine Description
BDB Transaction-safe tables with page locking. See section 15.4 The BDB (BerkeleyDB) Storage Engine.
BerkeleyDB An alias for BDB.
HEAP The data for this table is stored only in memory. See section 15.3 The MEMORY (HEAP) Storage Engine.
ISAM The original MySQL storage engine. See section 15.5 The ISAM Storage Engine.
InnoDB Transaction-safe tables with row locking and foreign keys. See section 16 The InnoDB Storage Engine.
MEMORY An alias for HEAP. (Actually, as of MySQL 4.1, MEMORY is the preferred term.)
MERGE A collection of MyISAM tables used as one table. See section 15.2 The MERGE Storage Engine.
MRG_MyISAM An alias for MERGE.
MyISAM The binary portable storage engine that is the improved replacement for ISAM. See section 15.1 The MyISAM Storage Engine.

See section 15 MySQL Storage Engines and Table Types.

If a storage engine is specified that is not available, MySQL uses MyISAM instead. For example, if a table definition includes the ENGINE=BDB option but the MySQL server does not support BDB tables, the table is created as a MyISAM table. This makes it possible to have a replication setup where you have transactional tables on the master but tables created on the slave are non-transactional (to get more speed). In MySQL 4.1.1, a warning occurs if the storage engine specification is not honored.

The other table options are used to optimize the behavior of the table. In most cases, you don't have to specify any of them. The options work for all storage engines unless otherwise indicated:

AUTO_INCREMENT
The initial AUTO_INCREMENT value for the table. This works for MyISAM only. To set the first auto-increment value for an InnoDB table, insert a dummy row with a value one less than the desired value after creating the table, and then delete the dummy row.
AVG_ROW_LENGTH
An approximation of the average row length for your table. You need to set this only for large tables with variable-size records. When you create a MyISAM table, MySQL uses the product of the MAX_ROWS and AVG_ROW_LENGTH options to decide how big the resulting table will be. If you don't specify either option, the maximum size for a table will be 4GB (or 2GB if your operating system only supports 2GB tables). The reason for this is just to keep down the pointer sizes to make the index smaller and faster if you don't really need big files. If you want all your tables to be able to grow above the 4GB limit and are willing to have your smaller tables slightly slower and larger than necessary, you may increase the default pointer size by setting the myisam_data_pointer_size system variable, which was added in MySQL 4.1.2. See section 5.2.3 Server System Variables.
CHECKSUM
Set this to 1 if you want MySQL to maintain a live checksum for all rows (that is, a checksum that MySQL updates automatically as the table changes). This makes the table a little slower to update, but also makes it easier to find corrupted tables. The CHECKSUM TABLE statement reports the checksum. (MyISAM only.)
COMMENT
A comment for your table, up to 60 characters long.
MAX_ROWS
The maximum number of rows you plan to store in the table.
MIN_ROWS
The minimum number of rows you plan to store in the table.
PACK_KEYS
Set this option to 1 if you want to have smaller indexes. This usually makes updates slower and reads faster. Setting the option to 0 disables all packing of keys. Setting it to DEFAULT (MySQL 4.0) tells the storage engine to only pack long CHAR/VARCHAR columns. (MyISAM and ISAM only.) If you don't use PACK_KEYS, the default is to only pack strings, not numbers. If you use PACK_KEYS=1, numbers will be packed as well. When packing binary number keys, MySQL uses prefix compression:
  • Every key needs one extra byte to indicate how many bytes of the previous key are the same for the next key.
  • The pointer to the row is stored in high-byte-first order directly after the key, to improve compression.
This means that if you have many equal keys on two consecutive rows, all following ``same'' keys will usually only take two bytes (including the pointer to the row). Compare this to the ordinary case where the following keys will take storage_size_for_key + pointer_size (where the pointer size is usually 4). Conversely, you will get a big benefit from prefix compression only if you have many numbers that are the same. If all keys are totally different, you will use one byte more per key, if the key isn't a key that can have NULL values. (In this case, the packed key length will be stored in the same byte that is used to mark if a key is NULL.)
PASSWORD
Encrypt the `.frm' file with a password. This option doesn't do anything in the standard MySQL version.
DELAY_KEY_WRITE
Set this to 1 if you want to delay key updates for the table until the table is closed. (MyISAM only.)
ROW_FORMAT
Defines how the rows should be stored. Currently this option works only with MyISAM tables. The option value can FIXED or DYNAMIC for static or variable-length row format. @command{myisampack} sets the type to COMPRESSED. See section 15.1.3 MyISAM Table Storage Formats.
RAID_TYPE
The RAID_TYPE option can help you to exceed the 2GB/4GB limit for the MyISAM data file (not the index file) on operating systems that don't support big files. This option is unnecessary and not recommended for filesystems that support big files. You can get more speed from the I/O bottleneck by putting RAID directories on different physical disks. For now, the only allowed RAID_TYPE is STRIPED. 1 and RAID0 are aliases for STRIPED. If you specify the RAID_TYPE option for a MyISAM table, specify the RAID_CHUNKS and RAID_CHUNKSIZE options as well. The maximum RAID_CHUNKS value is 255. MyISAM will create RAID_CHUNKS subdirectories named `00', `01', `02', ... `09', `0a', `0b', ... in the database directory. In each of these directories, MyISAM will create a file `tbl_name.MYD'. When writing data to the data file, the RAID handler maps the first RAID_CHUNKSIZE*1024 bytes to the first file, the next RAID_CHUNKSIZE*1024 bytes to the next file, and so on. RAID_TYPE works on any operating system, as long as you have built MySQL with the --with-raid option to @command{configure}. To determine whether a server supports RAID tables, use SHOW VARIABLES LIKE 'have_raid' to see whether the variable value is YES.
UNION
UNION is used when you want to use a collection of identical tables as one. This works only with MERGE tables. See section 15.2 The MERGE Storage Engine. For the moment, you must have SELECT, UPDATE, and DELETE privileges for the tables you map to a MERGE table. Originally, all used tables had to be in the same database as the MERGE table itself. This restriction has been lifted as of MySQL 4.1.1.
INSERT_METHOD
If you want to insert data in a MERGE table, you have to specify with INSERT_METHOD into which table the row should be inserted. INSERT_METHOD is an option useful for MERGE tables only. This option was introduced in MySQL 4.0.0. See section 15.2 The MERGE Storage Engine.
DATA DIRECTORY
INDEX DIRECTORY
By using DATA DIRECTORY='directory' or INDEX DIRECTORY='directory' you can specify where the MyISAM storage engine should put a table's data file and index file. Note that the directory should be a full path to the directory (not a relative path). These options work only for MyISAM tables from MySQL 4.0 on, when you are not using the --skip-symbolic-links option. Your operating system must also have a working, thread-safe realpath() call. See section 7.6.1.2 Using Symbolic Links for Tables on Unix.

As of MySQL 3.23, you can create one table from another by adding a SELECT statement at the end of the CREATE TABLE statement: 

CREATE TABLE new_tbl SELECT * FROM orig_tbl;

MySQL will create new column for all elements in the SELECT. For example: 

mysql> CREATE TABLE test (a INT NOT NULL AUTO_INCREMENT,
    ->        PRIMARY KEY (a), KEY(b))
    ->        TYPE=MyISAM SELECT b,c FROM test2;

This creates a MyISAM table with three columns, a, b, and c. Notice that the columns from the SELECT statement are appended to the right side of the table, not overlapped onto it. Take the following example: 

mysql> SELECT * FROM foo;
+---+
| n |
+---+
| 1 |
+---+

mysql> CREATE TABLE bar (m INT) SELECT n FROM foo;
Query OK, 1 row affected (0.02 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM bar;
+------+---+
| m    | n |
+------+---+
| NULL | 1 |
+------+---+
1 row in set (0.00 sec)

For each row in table foo, a row is inserted in bar with the values from foo and default values for the new columns.

If any errors occur while copying the data to the table, it is automatically dropped and not created.

CREATE TABLE ... SELECT will not automatically create any indexes for you. This is done intentionally to make the statement as flexible as possible. If you want to have indexes in the created table, you should specify these before the SELECT statement: 

mysql> CREATE TABLE bar (UNIQUE (n)) SELECT n FROM foo;

Some conversion of column types might occur. For example, the AUTO_INCREMENT attribute is not preserved, and VARCHAR columns can become CHAR columns.

When creating a table with CREATE ... SELECT, make sure to alias any function calls or expressions in the query. If you do not, the CREATE statement might fail or result in undesirable column names. 

CREATE TABLE artists_and_works
SELECT artist.name, COUNT(work.artist_id) AS number_of_works
FROM artist LEFT JOIN work ON artist.id = work.artist_id
GROUP BY artist.id;

As of MySQL 4.1, you can explicitly specify the type for a generated column: 

CREATE TABLE foo (a TINYINT NOT NULL) SELECT b+1 AS a FROM bar;

In MySQL 4.1, you can also use LIKE to create an empty table based on the definition of another table, including any column attributes and indexes the original table has: 

CREATE TABLE new_tbl LIKE orig_tbl;

CREATE TABLE ... LIKE does not copy any DATA DIRECTORY or INDEX DIRECTORY table options that were specified for the original table, or any foreign key definitions.

You can precede the SELECT by IGNORE or REPLACE to indicate how to handle records that duplicate unique key values. With IGNORE, new records that duplicate an existing record on a unique key value are discarded. With REPLACE, new records replace records that have the same unique key value. If neither IGNORE nor REPLACE is specified, duplicate unique key values result in an error.

To ensure that the update log/binary log can be used to re-create the original tables, MySQL will not allow concurrent inserts during CREATE TABLE ... SELECT.

14.2.6.1 Silent Column Specification Changes

In some cases, MySQL silently changes column specifications from those given in a CREATE TABLE or ALTER TABLE statement:

To see whether MySQL used a column type other than the one you specified, issue a DESCRIBE or SHOW CREATE TABLE statement after creating or altering your table.

Certain other column type changes can occur if you compress a table using @command{myisampack}. See section 15.1.3.3 Compressed Table Characteristics.

14.2.7 CREATE VIEW Syntax

CREATE [OR REPLACE] [ALGORITHM = {MERGE | TEMPTABLE}] VIEW view_name [(column_list)] AS select_statement [WITH [CASCADED | LOCAL] CHECK OPTION] 





This statement creates a new view, or replaces an existing one if the
OR REPLACE clause is given. The select_statement is a
SELECT statement that provides the definition of the view.
The optional column list can be given to define explicit names for the
view columns.





WITH CHECK OPTION, if given, is parsed and ignored.





A view can be created from many kinds of SELECT statements.
For example, the SELECT can refer to a single table, a join of
multiple tables, or a UNION.  The SELECT need not even
refer to any tables. The following example defines a view that
selects two columns from another table, as well as an expression calculated
from those columns:





mysql> CREATE TABLE t (qty INT, price INT);
mysql> INSERT INTO t VALUES(3, 50);
mysql> CREATE VIEW v AS SELECT qty, price, qty*price AS value FROM t;
mysql> SELECT * FROM v;
+------+-------+-------+
| qty  | price | value |
+------+-------+-------+
|    3 |    50 |   150 |
+------+-------+-------+





By default, the view is in the current database. To create
the view explicitly in a given database, specify the name as
db_name.view_name when you create it.





mysql> CREATE VIEW test.v AS SELECT * FROM t;





This statement was added in MySQL 5.0.1.







14.2.8  DROP DATABASE Syntax















DROP {DATABASE | SCHEMA} [IF EXISTS] db_name





DROP DATABASE drops all tables in the database and deletes the
database.  Be very careful with this statement!
To use DROP DATABASE, you need the DROP privilege on the
database.





In MySQL 3.22 or later, you can use the keywords IF EXISTS
to prevent an error from occurring if the database doesn't exist.





DROP SCHEMA can be used as of MySQL 5.0.2.





If you use DROP DATABASE on a symbolically linked
database, both the link and the original database are deleted.





As of MySQL 4.1.2, DROP DATABASE returns the number of tables that
were removed.  This corresponds to the number of `.frm' files removed.





The DROP DATABASE statement removes from the given database directory
those files and directories that MySQL itself may create during normal
operation:








If other files or directories remain in the database directory after MySQL
removes those just listed, the database directory cannot be removed. In this
case, you must remove any remaining files or directories manually and issue
the DROP DATABASE statement again.






You can also drop databases with @command{mysqladmin}.
@xref{mysqladmin,  , @command{mysqladmin}}.







14.2.9  DROP INDEX Syntax












DROP INDEX index_name ON tbl_name





DROP INDEX drops the index named index_name from the table
tbl_name.  In MySQL 3.22 or later, DROP INDEX is mapped to an
ALTER TABLE statement to drop the index.  See section 14.2.2  ALTER TABLE Syntax.  DROP INDEX doesn't do anything prior to MySQL
3.22.







14.2.10  DROP TABLE Syntax












DROP [TEMPORARY] TABLE [IF EXISTS]
    tbl_name [, tbl_name] ...
    [RESTRICT | CASCADE]





DROP TABLE removes one or more tables. You must have the DROP
privilege for each table. All table data and the table
definition are removed, so be careful with this statement!





In MySQL 3.22 or later, you can use the keywords IF EXISTS
to prevent an error from occurring for tables that don't exist.  As of
MySQL 4.1, a NOTE is generated for each non-existent table when
using IF EXISTS.
See section 14.5.3.21  SHOW WARNINGS Syntax.





RESTRICT and CASCADE are allowed to make porting easier.
For the moment, they do nothing.





Note: DROP TABLE automatically commits the current
active transaction, unless you are using MySQL 4.1 or higher and the
TEMPORARY keyword.





The TEMPORARY keyword is ignored in MySQL 4.0.  As of 4.1, it has the
following effect:








Using TEMPORARY is a good way to ensure that you don't accidentally
drop a non-TEMPORARY table.







14.2.11  DROP VIEW Syntax










DROP VIEW [IF EXISTS]
    view_name [, view_name] ...
    [RESTRICT | CASCADE]





DROP VIEW removes one or more views. You must have the DROP
privilege for each view.





You can use the keywords IF EXISTS to prevent an error from occurring
for views that don't exist.  When this clause is given, a NOTE is
generated for each non-existent view.
See section 14.5.3.21  SHOW WARNINGS Syntax.





RESTRICT and CASCADE, if given, are parsed and ignored.





This statement was added in MySQL 5.0.1.







14.2.12  RENAME TABLE Syntax










RENAME TABLE tbl_name TO new_tbl_name
    [, tbl_name2 TO new_tbl_name2] ...





This statement renames one or more tables.  It was added in MySQL 3.23.23.





The rename operation is done atomically, which means that no other thread
can access any of the tables while the rename is running. For example,
if you have an existing table old_table, you can create another
table new_table that has the same structure but is empty, and then
replace the existing table with the empty one as follows:





CREATE TABLE new_table (...);
RENAME TABLE old_table TO backup_table, new_table TO old_table;





If the statement renames more than one table, renaming operations are done
from left to right. If you want to swap two table names, you can do so
like this (assuming that no table named tmp_table currently exists):





RENAME TABLE old_table TO tmp_table,
             new_table TO old_table,
             tmp_table TO new_table;





As long as two databases are on the same filesystem you can also rename
a table to move it from one database to another:





RENAME TABLE current_db.tbl_name TO other_db.tbl_name;





When you execute RENAME, you can't have any locked tables or
active transactions.  You must also have the ALTER and DROP
privileges on the original table, and the CREATE and INSERT
privileges on the new table.





If MySQL encounters any errors in a multiple-table rename, it
will do a reverse rename for all renamed tables to get everything back
to the original state.






14.3  MySQL Utility Statements





14.3.1  DESCRIBE Syntax (Get Information About Columns)











{DESCRIBE | DESC} tbl_name [col_name | wild]





DESCRIBE provides information about a table's columns.  It is a
shortcut for SHOW COLUMNS FROM.
See section 14.5.3.4  SHOW COLUMNS Syntax.





col_name can be a column name, or a string containing the SQL
`%' and `_' wildcard characters to obtain output only for the
columns with names matching the string. There is no need to enclose the
string in quotes unless it contains spaces or other special characters.





mysql> DESCRIBE city;
+------------+----------+------+-----+---------+----------------+
| Field      | Type     | Null | Key | Default | Extra          |
+------------+----------+------+-----+---------+----------------+
| Id         | int(11)  |      | PRI | NULL    | auto_increment |
| Name       | char(35) |      |     |         |                |
| Country    | char(3)  |      | UNI |         |                |
| District   | char(20) | YES  | MUL |         |                |
| Population | int(11)  |      |     | 0       |                |
+------------+----------+------+-----+---------+----------------+
5 rows in set (0.00 sec)





The Null column indicates whether NULL values can be stored, 
with YES displayed when NULL values are allowed. 





The Key column indicates whether the field is indexed. A value of PRI
indicates that the field is part of the table's primary key. UNI indicates that
the field is part of a a UNIQUE index. The MUL value 
indicates that multiple occurences of a given value allowed within the field. 





A field can be designated as MUL even if a UNIQUE index is used if 
NULL values are allowed, as multiple rows in a UNIQUE index can hold 
a NULL value if the column is not declared NOT NULL. Another cause 
for MUL on a UNIQUE index is when two columns form a composite 
UNIQUE index; while the combination of the columns will be unique, each 
column can still hold multiple occurences of a given value. Note that in a composite index only the leftmost field of the index will have an entry in the Key column.





The Default column indicates the default value that is assigned to the field.





The Extra column contains any additional information that is available about
a given field. In our example the Extra column indicates that our Id column
was created with the AUTO_INCREMENT keyword. 





If the column types are different from what you expect them to be based on a
CREATE TABLE statement, note that MySQL sometimes
changes column types.  See section 14.2.6.1  Silent Column Specification Changes.







The DESCRIBE statement is provided for Oracle compatibility.





The SHOW CREATE TABLE and SHOW TABLE STATUS statements also
provide information about tables.
See section 14.5.3  SET and SHOW Syntax.







14.3.2  USE Syntax










USE db_name





The USE db_name statement tells MySQL to use the db_name
database as the default (current) database for subsequent statements.
The database remains the default until the end of the session or until
another USE statement is issued:





mysql> USE db1;
mysql> SELECT COUNT(*) FROM mytable;   # selects from db1.mytable
mysql> USE db2;
mysql> SELECT COUNT(*) FROM mytable;   # selects from db2.mytable





Making a particular database current by means of the USE statement
does not preclude you from accessing tables in other databases.  The following example
accesses the author table from the db1 database and the
editor table from the db2 database:





mysql> USE db1;
mysql> SELECT author_name,editor_name FROM author,db2.editor
    ->        WHERE author.editor_id = db2.editor.editor_id;







The USE statement is provided for Sybase compatibility.







14.4  MySQL Transactional and Locking Statements





14.4.1  START TRANSACTION, COMMIT, and ROLLBACK Syntax













By default, MySQL runs with autocommit mode enabled. This means that
as soon as you execute a statement that updates (modifies) a table,
MySQL stores the update on disk.





If you are using transaction-safe tables (like InnoDB or BDB),
you can disable autocommit mode with the following statement:





SET AUTOCOMMIT=0;





After disabling autocommit mode by setting the AUTOCOMMIT variable to
zero, you must use COMMIT to store your changes to disk or
ROLLBACK if you want to ignore the changes you have made since
the beginning of your transaction.





If you want to disable autocommit mode for a single series of
statements, you can use the START TRANSACTION statement:





START TRANSACTION;
SELECT @A:=SUM(salary) FROM table1 WHERE type=1;
UPDATE table2 SET summary=@A WHERE type=1;
COMMIT;





With START TRANSACTION, autocommit remains disabled until you end the
transaction with COMMIT or ROLLBACK. The autocommit mode then
reverts to its previous state.





BEGIN and BEGIN WORK can be used instead of START
TRANSACTION to initiate a transaction.  START TRANSACTION was
added in MySQL 4.0.11.  This is standard SQL syntax and is the recommended way
to start an ad-hoc transaction.  BEGIN and BEGIN WORK are
available from MySQL 3.23.17 and 3.23.19, respectively.





Beginning a transaction causes an implicit UNLOCK TABLES to be
performed.





Note that if you are not using transaction-safe tables, any changes are
stored at once, regardless of the status of autocommit mode.





If you issue a ROLLBACK statement after updating a non-transactional
table within a transaction, an ER_WARNING_NOT_COMPLETE_ROLLBACK
warning occurs.  Changes to transaction-safe tables will be rolled back, but
not changes to non-transaction-safe tables.





If you are using START TRANSACTION or SET AUTOCOMMIT=0, you
should use the MySQL binary log for backups instead of the
older update log.  Transactions are stored in the binary log
in one chunk, upon COMMIT. Transactions that are
rolled back are not logged.
(Exception: Modifications to non-transactional tables cannot be rolled
back.  If a transaction that is rolled back includes modifications
to non-transactional tables, the entire transaction is logged with a
ROLLBACK statement at the end to ensure that the modifications to
those tables are replicated. This is true as of MySQL 4.0.15.)
See section 5.9.4  The Binary Log.





You can change the isolation level for transactions with
SET TRANSACTION ISOLATION LEVEL.
See section 14.4.6  SET TRANSACTION Syntax.







14.4.2  Statements That Cannot Be Rolled Back




Some statements cannot be rolled back.  In general, these include data
definition language (DDL) statements, such as those that create or drop
databases, or those that create, drop, or alter tables.





You should design your transactions not to include such statements.
If you issue a statement early in a transaction that cannot be rolled back,
and then another statement later fails, the full effect of the transaction
cannot be rolled back by issuing a ROLLBACK statement.







14.4.3  Statements That Cause an Implicit Commit




Each of the following statements (and any synonyms for them) implicitly end
a transaction, as if you had done a COMMIT before executing the
statement:






ALTER TABLE  BEGIN  CREATE INDEX



DROP DATABASE  DROP INDEX  DROP TABLE



LOAD MASTER DATA  LOCK TABLES  RENAME TABLE



SET AUTOCOMMIT=1  START TRANSACTION  TRUNCATE TABLE







UNLOCK TABLES also ends a transaction if any tables currently are
locked.  Prior to MySQL 4.0.13, CREATE TABLE ends a transaction if
the binary update log is enabled.





Transactions cannot be nested. This is a consequence of the implicit
COMMIT performed for any current transaction when you issue a
START TRANSACTION statement or one of its synonyms.







14.4.4  SAVEPOINT and ROLLBACK TO SAVEPOINT Syntax











SAVEPOINT identifier
ROLLBACK TO SAVEPOINT identifier





Starting from MySQL 4.0.14 and 4.1.1, InnoDB supports the SQL statements
SAVEPOINT and ROLLBACK TO SAVEPOINT.





The SAVEPOINT statement sets a named transaction savepoint with a name
of identifier.  If the current transaction already has a savepoint
with the same name, the old savepoint is deleted and a new one is set.





The ROLLBACK TO SAVEPOINT statement rolls back a transaction to the
named savepoint.  Modifications that the current transaction made to rows
after the savepoint was set are undone in the rollback, but InnoDB
does not release the row locks that were stored in memory after
the savepoint.  (Note that for a new inserted row, the lock information is
carried by the transaction ID stored in the row; the lock is not separately
stored in memory. In this case, the row lock is released in the undo.)
Savepoints that were set at a later time than the named savepoint are
deleted.





If the statement returns the following error, it means that
no savepoint with the specified name exists:





ERROR 1181: Got error 153 during ROLLBACK





All savepoints of the current transaction are deleted if you execute a
COMMIT, or a ROLLBACK that does not name a savepoint.







14.4.5  LOCK TABLES and UNLOCK TABLES Syntax











LOCK TABLES
    tbl_name [AS alias] {READ [LOCAL] | [LOW_PRIORITY] WRITE}
    [, tbl_name [AS alias] {READ [LOCAL] | [LOW_PRIORITY] WRITE}] ...
UNLOCK TABLES





LOCK TABLES locks tables for the current thread.  UNLOCK
TABLES releases any locks held by the current thread.  All tables that
are locked by the current thread are implicitly unlocked when the
thread issues another LOCK TABLES, or when the connection to the
server is closed.





Note: LOCK TABLES is not transaction-safe and implicitly
commits any active transactions before attempting to lock the tables.
Also, beginning a transaction (for example, with START TRANSACTION)
implicitly performs an UNLOCK TABLES.





As of MySQL 4.0.2, to use LOCK TABLES you must have the
LOCK TABLES privilege and a SELECT privilege for the
involved tables. In MySQL 3.23, you must have SELECT,
INSERT, DELETE, and UPDATE privileges for the
tables.





The main reasons to use LOCK TABLES are for emulating transactions
or to get more speed when updating tables.  This is explained in more
detail later.





If a thread obtains a READ lock on a table, that thread (and all other
threads) can only read from the table. If a thread obtains a WRITE
lock on a table, only the thread holding the lock can read from
or write to the table.  Other threads are blocked.





The difference between READ LOCAL and READ is that READ
LOCAL allows non-conflicting INSERT statements (concurrent inserts)
to execute while the lock is held.  However, this can't be used if you are
going to manipulate the database files outside MySQL while you hold the
lock.





When you use LOCK TABLES, you must lock all tables that you are going to
use in your queries.  While the locks obtained with a LOCK TABLES
statement are in effect, you cannot access any tables that were not locked by
the statement.  Also, you cannot use a locked table multiple times in one query
- use aliases for that.  Note that in that case you must get a lock for each
alias separately.





mysql> LOCK TABLE t WRITE, t AS t1 WRITE;
mysql> INSERT INTO t SELECT * FROM t;
ERROR 1100: Table 't' was not locked with LOCK TABLES
mysql> INSERT INTO t SELECT * FROM t AS t1;





If your queries refer to a table using an alias, then you must lock the
table using that same alias.  It will not work to lock the table without
specifying the alias:





mysql> LOCK TABLE t READ;
mysql> SELECT * FROM t AS myalias;
ERROR 1100: Table 'myalias' was not locked with LOCK TABLES





Conversely, if you lock a table using an alias, you must refer to it in
your queries using that alias:





mysql> LOCK TABLE t AS myalias READ;
mysql> SELECT * FROM t;
ERROR 1100: Table 't' was not locked with LOCK TABLES
mysql> SELECT * FROM t AS myalias;





WRITE locks normally have higher priority than READ locks to
ensure that updates are processed as soon as possible. This means that if one
thread obtains a READ lock and then another thread requests a
WRITE lock, subsequent READ lock requests will wait until the
WRITE thread has gotten the lock and released it.  You can use
LOW_PRIORITY WRITE locks to allow other threads to obtain READ
locks while the thread is waiting for the WRITE lock. You should
use LOW_PRIORITY WRITE locks only if you are sure that there will
eventually be a time when no threads will have a READ lock.





LOCK TABLES works as follows:





    

  1. 

Sort all tables to be locked in an internally defined order. From the
user standpoint, this order is undefined.

  2. 

If a table is locked with a read and a write lock, put the write lock
before the read lock.

  3. 

Lock one table at a time until the thread gets all locks.





This policy ensures that table locking is deadlock free. There are,
however, other things you need to be aware of about this policy:





If you are using a LOW_PRIORITY WRITE lock for a table, it
means only that MySQL will wait for this particular lock until
there are no threads that want a READ lock.  When the thread has
gotten the WRITE lock and is waiting to get the lock for the next
table in the lock table list, all other threads will wait for the
WRITE lock to be released.  If this becomes a serious problem
with your application, you should consider converting some of your
tables to transaction-safe tables.





You can safely use KILL to terminate a thread that is waiting for a
table lock.
See section 14.5.4.3  KILL Syntax.





Note that you should not lock any tables that you are using with
INSERT DELAYED because in that case the INSERT is done by a
separate thread.





Normally, you don't have to lock tables, because all single UPDATE statements
are atomic; no other thread can interfere with any other currently executing
SQL statement. There are a few cases when you would like to lock tables
anyway:








You can avoid using LOCK TABLES in many cases
by using relative updates (UPDATE customer SET
value=value+new_value) or the LAST_INSERT_ID() function,
See section 1.8.5.3  Transactions and Atomic Operations.





You can also avoid locking tables in some cases by using the user-level
advisory lock functions GET_LOCK() and RELEASE_LOCK().
These locks are saved in a hash table in the server and implemented with
pthread_mutex_lock() and pthread_mutex_unlock() for high speed.
See section 13.8.4  Miscellaneous Functions.





See section 7.3.1  Locking Methods, for more information on locking policy.





You can lock all tables in all databases with read locks with the FLUSH
TABLES WITH READ LOCK statement. See section 14.5.4.2  FLUSH Syntax. This
is a very convenient way to get backups if you have a filesystem such as
Veritas that can take snapshots in time.





Note: If you use ALTER TABLE on a locked table, it may become
unlocked. See section A.7.1  Problems with ALTER TABLE.







14.4.6  SET TRANSACTION Syntax











SET [GLOBAL | SESSION] TRANSACTION ISOLATION LEVEL
{ READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE }





This statement
sets the transaction isolation level for the next transaction, globally, or
for the current session.





The default behavior of SET TRANSACTION is to set the isolation level
for the next (not yet
started) transaction.  If you use the GLOBAL keyword, the statement
sets the default transaction level globally for all new connections
created from that point on. Existing connections are unaffected.
You need the SUPER
privilege to do this.  Using the SESSION keyword sets the
default transaction level for all future transactions performed on the
current connection.





For descriptions of each InnoDB transaction isolation level, see
section 16.11.2  InnoDB and TRANSACTION ISOLATION LEVEL.
InnoDB supports each of these levels
from MySQL 4.0.5 on. The default level is REPEATABLE READ.





You can set the initial default global isolation level for @command{mysqld} with
the --transaction-isolation option.
See section 5.2.1  @command{mysqld} Command-Line Options.







14.5  Database Administration Statements





14.5.1  Account Management Statements





14.5.1.1  DROP USER Syntax























DROP USER user





The DROP USER statement deletes a MySQL account that
doesn't have any privileges.  It serves to remove the account record from
the user table. The account is named using the same format as for
GRANT or REVOKE; for example, 'jeffrey'@'localhost'.
The user and host parts of the account name correspond to the User and
Host column values of the user table record for the account.





To remove a MySQL user account, you should use the following procedure,
performing the steps in the order shown:





    

  1. 

Use SHOW GRANTS to determine what privileges the account has.
See section 14.5.3.11  SHOW GRANTS Syntax.

  2. 

Use REVOKE to revoke the privileges displayed by SHOW GRANTS.
This removes records for the account from all the grant tables except the
user table, and revokes any global privileges listed in the user
table.
See section 14.5.1.2  GRANT and REVOKE Syntax.

  3. 

Delete the account by using DROP USER to remove the user table
record.





The DROP USER statement was added in MySQL 4.1.1.  Before 4.1.1,
you should first revoke the account privileges as just described. Then
delete the user table record and flush the grant tables like this:





mysql> DELETE FROM mysql.user
    -> WHERE User='user_name' and Host='host_name';
mysql> FLUSH PRIVILEGES;






14.5.1.2  GRANT and REVOKE Syntax





















GRANT priv_type [(column_list)] [, priv_type [(column_list)]] ...
    ON {tbl_name | * | *.* | db_name.*}
    TO user [IDENTIFIED BY [PASSWORD] 'password']
        [, user [IDENTIFIED BY [PASSWORD] 'password']] ...
    [REQUIRE
        NONE |
        [{SSL| X509}]
        [CIPHER 'cipher' [AND]]
        [ISSUER 'issuer' [AND]]
        [SUBJECT 'subject']]
    [WITH [GRANT OPTION | MAX_QUERIES_PER_HOUR count |
                          MAX_UPDATES_PER_HOUR count |
                          MAX_CONNECTIONS_PER_HOUR count]]





REVOKE priv_type [(column_list)] [, priv_type [(column_list)]] ...
    ON {tbl_name | * | *.* | db_name.*}
    FROM user [, user] ...

REVOKE ALL PRIVILEGES, GRANT OPTION FROM user [, user] ...





The GRANT and REVOKE statements allow system administrators to
create MySQL user accounts and to grant rights to and revoke them from
accounts.  GRANT and REVOKE are implemented in MySQL 3.22.11
or later. For earlier MySQL versions, these statements do nothing.





MySQL account information is stored in the tables of the mysql
database. This database and the access control system are discussed
extensively in section 5  Database Administration, which you should consult
for additional details.





Privileges can be granted at four levels:







Global level


Global privileges apply to all databases on a given server. These privileges
are stored in the mysql.user table.
GRANT ALL ON *.* and
REVOKE ALL ON *.* grant and revoke only global privileges.


Database level


Database privileges apply to all tables in a given database. These privileges
are stored in the mysql.db and mysql.host tables.
GRANT ALL ON db_name.* and
REVOKE ALL ON db_name.* grant and revoke only database privileges.


Table level


Table privileges apply to all columns in a given table. These privileges are
stored in the mysql.tables_priv table.
GRANT ALL ON db_name.tbl_name and
REVOKE ALL ON db_name.tbl_name grant and revoke only table privileges.


Column level


Column privileges apply to single columns in a given table. These privileges are
stored in the mysql.columns_priv table.
When using REVOKE, you must specify the same columns that were granted.





To make it easy to revoke all privileges, MySQL 4.1.2 has added the
following syntax, which drops all database-, table-, and column-level
privileges for the named users:





REVOKE ALL PRIVILEGES, GRANT OPTION FROM user [, user] ...





Before MySQL 4.1.2, all privileges cannot be dropped at once.
Two statements are necessary:





REVOKE ALL PRIVILEGES ON *.* FROM user [, user] ...
REVOKE GRANT OPTION ON *.* FROM user [, user] ...





For the GRANT and REVOKE statements, priv_type can be
specified as any of the following:






Privilege  Meaning



ALL [PRIVILEGES]  Sets all simple privileges except GRANT OPTION



ALTER  Allows use of ALTER TABLE



CREATE  Allows use of CREATE TABLE



CREATE TEMPORARY TABLES  Allows use of CREATE TEMPORARY TABLE



CREATE VIEW  Allows use of CREATE VIEW



DELETE  Allows use of DELETE



DROP  Allows use of DROP TABLE



EXECUTE  Allows the user to run stored procedures (MySQL 5.0)



FILE  Allows use of SELECT ... INTO OUTFILE and LOAD DATA INFILE



INDEX  Allows use of CREATE INDEX and DROP INDEX



INSERT  Allows use of INSERT



LOCK TABLES  Allows use of LOCK TABLES on tables for which you have the SELECT privilege



PROCESS  Allows use of SHOW FULL PROCESSLIST



REFERENCES  Not yet implemented



RELOAD  Allows use of FLUSH



REPLICATION CLIENT  Allows the user to ask where the slave or master servers are



REPLICATION SLAVE  Needed for replication slaves (to read binary log events from the master)



SELECT  Allows use of SELECT



SHOW DATABASES  SHOW DATABASES shows all databases



SHOW VIEW  Allows use of SHOW CREATE VIEW



SHUTDOWN  Allows use of @command{mysqladmin shutdown}



SUPER  Allows use of CHANGE MASTER, KILL,
PURGE MASTER LOGS, and SET GLOBAL statements, the @command{mysqladmin debug} command; allows you to connect (once) even if max_connections is reached



UPDATE  Allows use of UPDATE



USAGE  Synonym for ``no privileges''



GRANT OPTION  Allows privileges to be granted







USAGE can be used when you want to create a user that has no privileges.





Use SHOW GRANTS to determine what privileges the account has.
See section 14.5.3.11  SHOW GRANTS Syntax.





The CREATE TEMPORARY TABLES, EXECUTE,
LOCK TABLES, REPLICATION ..., SHOW DATABASES and
SUPER privileges are new in MySQL 4.0.2.  To use them
after upgrading to 4.0.2 or later, you must upgrade your grant tables.
Similarly,
the CREATE VIEW and SHOW VIEW
privileges are new in MySQL 5.0.1.  To use them
after upgrading to 5.0.1 or later, you must upgrade your grant tables.
See section 2.5.8  Upgrading the Grant Tables.





In older MySQL versions that do not have the SUPER privilege, the
PROCESS privilege can be used instead.





You can assign global privileges by using ON *.* syntax or
database privileges by using ON db_name.* syntax. If you specify
ON * and you have a current database, the privileges will be granted in
that database.  (Warning: If you specify ON * and you
don't have a current database, the privileges granted will be global!)





The EXECUTION, FILE, PROCESS, RELOAD,
REPLICATION CLIENT, REPLICATION SLAVE, SHOW DATABASES,
SHUTDOWN, and SUPER privileges are administrative privileges
that can only be granted globally (using ON *.* syntax).





Other privileges can be granted globally or at more specific levels.





The only priv_type values you can specify for a table are SELECT,
INSERT, UPDATE, DELETE, CREATE, DROP,
GRANT OPTION, INDEX, and ALTER.





The only priv_type values you can specify for a column (that is, when
you use a column_list clause) are SELECT, INSERT, and
UPDATE.





For the global, database, and table levels,
GRANT ALL assigns only the privileges that exist at the level you are
granting. For example, if you use GRANT ALL ON db_name.*, that is a
database-level statement, so none of the global-only privileges such as
FILE will be granted.





For column-level privileges (that is, when you specify column_list),
you must explicitly name the privileges to be granted. You cannot use
ALL as a privilege specifier.





MySQL allows you to create database-level privileges even if the database
doesn't exist, to make it easy to prepare for database use.  However, MySQL
currently does not allow you to create table-level privileges if the table
doesn't exist.





MySQL does not automatically revoke any privileges even if you drop a table
or drop a database.





Note: the `_' and `%' wildcards are allowed when
specifying database names in GRANT statements that grant privileges at
the global or database levels. This means, for example, that if you
want to use a `_' character as part of a database name,
you should specify it as `\_' in the GRANT statement, to prevent
the user from being able to access additional databases matching the
wildcard pattern; for example, GRANT ... ON `foo\_bar`.* TO ....





In order to accommodate granting rights to users from arbitrary hosts,
MySQL supports specifying the user value in the form
user_name@host_name.  If you want to specify a user_name string
containing special characters (such as `-'), or a host_name string
containing special characters or wildcard characters (such as `%'), you
can quote the username or hostname (for example,
'test-user'@'test-hostname'). Quote the username and hostname
separately.





You can specify wildcards in the hostname.  For example,
user_name@'%.loc.gov' applies to user_name for any host in the
loc.gov domain, and user_name@'144.155.166.%' applies to
user_name for any host in the 144.155.166 class C subnet.





The simple form user_name is a synonym for user_name@'%'.





MySQL doesn't support wildcards in usernames.  Anonymous users are
defined by inserting entries with User='' into the
mysql.user table or creating a user with an empty name with the
GRANT statement:





mysql> GRANT ALL ON test.* TO ''@'localhost' ...





When specifying quoted values, quote database, table, or column names as
identifiers, using backticks (``').
Quote hostnames, usernames, or passwords as strings, using apostrophes
(`'').





Warning: If you allow anonymous users to connect to the MySQL
server, you should also grant privileges to all local users as
user_name@localhost. Otherwise, the anonymous-user account for
the local host in the mysql.user table will be used when named users
try to log in to the MySQL server from the local machine!
(This anonymous-user account is created during MySQL installation.)





You can determine whether this applies to you by executing the following query:





mysql> SELECT Host, User FROM mysql.user WHERE User='';





If you want to delete the local anonymous-user account to avoid the problem
just described, use these statements:





mysql> DELETE FROM mysql.user WHERE Host='localhost' AND User='';
mysql> FLUSH PRIVILEGES;





For the moment, GRANT only supports host, table, database, and
column names up to 60 characters long. A username can be up to 16
characters.





The privileges for a table or column are formed additively from the
logical OR of the privileges at each of the four privilege
levels.  For example, if the mysql.user table specifies that a
user has a global SELECT privilege, the privilege cannot be denied by an
entry at the database, table, or column level.





The privileges for a column can be calculated as follows:





global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges





In most cases, you grant rights to a user at only one of the privilege
levels, so life isn't normally this complicated. The details of the
privilege-checking procedure are presented in
section 5.5  The MySQL Access Privilege System.





If you grant privileges for a username/hostname combination that does not exist
in the mysql.user table, an entry is added and remains there until
deleted with a DELETE statement.  In other words, GRANT may
create user table entries, but REVOKE will not remove them;
you must do that explicitly using DROP USER or DELETE.






In MySQL 3.22.12 or later,
if a new user is created or if you have global grant privileges, the user's
password is set to the password specified by the IDENTIFIED BY
clause, if one is given.  If the user already had a password, it is replaced
by the new one.





Warning: If you create a new user but do not specify an
IDENTIFIED BY clause, the user has no password.  This is insecure.





Passwords can also be set with the SET PASSWORD statement.
See section 14.5.1.3  SET PASSWORD Syntax.





In the IDENTIFIED BY clause, the password should be given as the
literal password value.  It is unnecessary to use the PASSWORD()
function as it is for the SET PASSWORD statement. For example:





GRANT ... IDENTIFIED BY 'mypass';





If you don't want to send the password in clear text and you know the hashed
value that PASSWORD() would return for the password, you can specify
the hashed value preceded by the keyword PASSWORD:





GRANT ... IDENTIFIED BY PASSWORD '*6C8989366EAF75BB670AD8EA7A7FC1176A95CEF4';





In a C program, you can get the hashed value by using the
make_scrambled_password() C API function.





If you grant privileges for a database, an entry in the mysql.db
table is created if needed. If all privileges for the database are
removed with REVOKE, this entry is deleted.





If a user has no privileges for a table, the table name is not displayed
when the user requests a list of tables (for example, with a SHOW TABLES
statement). If a user has no privileges for a database, the database name
is not displayed by SHOW DATABASES unless the user has the SHOW
DATABASES privilege.





The WITH GRANT OPTION clause gives the user the ability to give
to other users any privileges the user has at the specified privilege level.
You should be careful to whom you give the GRANT OPTION privilege,
because two users with different privileges may be able to join privileges!





You cannot grant another user a privilege you don't have yourself; the
GRANT OPTION privilege allows you to give away only those privileges
you possess.





Be aware that when you grant a user the GRANT OPTION privilege at a
particular privilege level, any privileges the user already possesses (or
is given in the future!) at that level are also grantable by that user.
Suppose that you grant a user the INSERT privilege on a database.  If
you then grant the SELECT privilege on the database and specify
WITH GRANT OPTION, the user can give away not only the SELECT
privilege, but also INSERT.  If you then grant the UPDATE
privilege to the user on the database, the user can give away
INSERT, SELECT, and UPDATE.





You should not grant ALTER privileges to a normal user.  If you
do that, the user can try to subvert the privilege system by renaming
tables!





The MAX_QUERIES_PER_HOUR count, MAX_UPDATES_PER_HOUR count,
and MAX_CONNECTIONS_PER_HOUR count options are new in MySQL 4.0.2.
They limit the number of queries, updates, and logins a user can perform
during one hour. If count is 0 (the default), this means there is no
limitation for that user. See section 5.6.4  Limiting Account Resources.  Note: To specify any of
these options for an existing user without affecting existing privileges,
use GRANT USAGE ON *.* ... WITH MAX_....












MySQL can check X509 certificate attributes in addition to the usual
authentication that is based on the username and password.  To specify
SSL-related options for a MySQL account, use the REQUIRE clause of
the GRANT statement.
(For background on the use of SSL with MySQL, see section 5.6.7  Using Secure Connections.)





There are different possibilities for limiting connection types for an account:








The SUBJECT, ISSUER, and CIPHER options can be
combined in the REQUIRE clause like this:





mysql> GRANT ALL PRIVILEGES ON test.* TO 'root'@'localhost'
    -> IDENTIFIED BY 'goodsecret'
    -> REQUIRE SUBJECT '/C=EE/ST=Some-State/L=Tallinn/
       O=MySQL demo client certificate/
       CN=Tonu Samuel/[email protected]'
    -> AND ISSUER '/C=FI/ST=Some-State/L=Helsinki/
       O=MySQL Finland AB/CN=Tonu Samuel/[email protected]'
    -> AND CIPHER 'EDH-RSA-DES-CBC3-SHA';





Note that the SUBJECT and ISSUER values each should be entered
as a single string.





Starting from MySQL 4.0.4, the AND keyword is optional between
REQUIRE options.





The order of the options does not matter, but no option can be specified
twice.





When @command{mysqld} starts, all privileges are read into memory.
Database, table, and column privileges take effect at once, and
user-level privileges take effect the next time the user connects.
Modifications to the grant tables that you perform using GRANT or
REVOKE are noticed by the server immediately.
If you modify the grant tables manually (using INSERT, UPDATE,
and so on), you should execute a FLUSH PRIVILEGES statement or run
@command{mysqladmin flush-privileges} to tell the server to reload the grant
tables.
See section 5.5.7  When Privilege Changes Take Effect.





Note that if you are using table or column privileges for even one user, the
server examines table and column privileges for all users and this slows
down MySQL a bit.  Similarly, if you limit the number of queries, updates,
or connections for any users, the server must monitor these values.






The biggest differences between the standard SQL and MySQL versions of
GRANT are:









14.5.1.3  SET PASSWORD Syntax

















SET PASSWORD = PASSWORD('some password')
SET PASSWORD FOR user = PASSWORD('some password')





The SET PASSWORD statement assigns a password to an existing
MySQL user account.





The first syntax sets the password for the current user. Any client that has
connected to the server using a non-anonymous account can change the password
for that account.






The second syntax sets the password for a specific account on the current
server host. Only clients with access to the mysql database can do
this.  The user value should be given in user_name@host_name
format, where user_name and host_name are exactly as they are
listed in the User and Host columns of the mysql.user
table entry.  For example, if you had an entry with User and
Host column values of 'bob' and '%.loc.gov', you would
write the statement like this:





mysql> SET PASSWORD FOR 'bob'@'%.loc.gov' = PASSWORD('newpass');





That is equivalent to the following statements:





mysql> UPDATE mysql.user SET Password=PASSWORD('newpass')
    -> WHERE User='bob' AND Host='%.loc.gov';
mysql> FLUSH PRIVILEGES;





Note: If you are connecting to a MySQL 4.1 or later server using
a pre-4.1 client program, do not use this statement without reading
section 5.5.9  Password Hashing in MySQL 4.1 first. The password format changed in MySQL 4.1,
and under certain circumstances it is possible that if you change your
password, you might not be able to connect to the server afterward.







14.5.2  Table Maintenance Statements





14.5.2.1  ANALYZE TABLE Syntax










ANALYZE [LOCAL | NO_WRITE_TO_BINLOG] TABLE tbl_name [, tbl_name] ...





This statement analyzes and stores the key distribution for a table.
During the analysis, the table is locked with a read lock.  This works on
MyISAM and BDB tables and (as of MySQL 4.0.13) InnoDB
tables. For MyISAM tables, this
statement is equivalent to using @command{myisamchk -a}.





MySQL uses the stored key distribution to decide the order in which
tables should be joined when you perform a join on something other than a
constant.





The statement returns a table with the following columns:






Column  Value



Table  The table name



Op  Always analyze



Msg_type  One of status, error, info, or warning



Msg_text  The message







You can check the stored key distribution with the SHOW INDEX statement.
See section 14.5.3.12  SHOW INDEX Syntax.





If the table hasn't changed since the last ANALYZE TABLE statement,
the table will not be analyzed again.





Before MySQL 4.1.1, ANALYZE TABLE statements are not written
to the binary log. As of MySQL 4.1.1, they are written to the binary
log unless the optional NO_WRITE_TO_BINLOG keyword 
(or its alias LOCAL) is used.







14.5.2.2  BACKUP TABLE Syntax
















BACKUP TABLE tbl_name [, tbl_name] ... TO '/path/to/backup/directory'





Note: This statement is deprecated. We are working on a better
replacement for it that will provide online backup capabilities.
In the meantime, the @command{mysqlhotcopy} script can be used instead.





BACKUP TABLE copies to the backup directory the minimum number of
table files needed to restore the table, after flushing any buffered changes
to disk. The statement works only for MyISAM tables.  It copies the
`.frm' definition  and `.MYD' data files. The `.MYI'
index file can be rebuilt from those two files.
The directory should be specified as a full pathname.





Before using this statement, please see section 5.7.1  Database Backups.





During the backup, a read lock is held for each table, one at time,
as they are being backed up. If you want to back up several tables as
a snapshot (preventing any of them from being changed during the backup
operation), you must first issue a LOCK TABLES statement to obtain a read
lock for every table in the group.





The statement returns a table with the following columns:






Column  Value



Table  The table name



Op  Always backup



Msg_type  One of status, error, info, or warning



Msg_text  The message







BACKUP TABLE is available in MySQL 3.23.25 and later.







14.5.2.3  CHECK TABLE Syntax










CHECK TABLE tbl_name [, tbl_name] ... [option] ...

option = {QUICK | FAST | MEDIUM | EXTENDED | CHANGED}





CHECK TABLE works only on MyISAM and InnoDB tables. On
MyISAM tables, This is the same thing as running @command{myisamchk
--medium-check tbl_name} on the table.





If you don't specify any option, MEDIUM is used.





Checks the table or tables for errors. For MyISAM tables, the key statistics
are updated. The statement returns a table with the following columns:






Column  Value



Table  The table name



Op  Always check



Msg_type  One of status, error, info, or warning



Msg_text  The message







Note that the statement might produce many rows of information for each
checked table.  The last row will have a Msg_type value of
status and the Msg_text normally should be OK.  If you
don't get OK, or Table is already up to date you should
normally run a repair of the table. See section 5.7.2  Table Maintenance and Crash Recovery. Table is
already up to date means that the storage engine for the table indicated
that there was no need to check the table.





The different check options are as follows:






Type  Meaning



QUICK  Don't scan the rows to check for incorrect links.



FAST  Only check tables that haven't been closed properly.



CHANGED  Only check tables that have been changed since the last check or haven't been closed properly.



MEDIUM  Scan rows to verify that deleted links are okay. This also calculates a key checksum for the rows and verifies this with a calculated checksum for the keys.



EXTENDED  Do a full key lookup for all keys for each row.  This ensures that the table is 100% consistent, but will take a long time!







These options apply only to checking MyISAM tables and are ignored for
InnoDB tables.





If none of the options QUICK, MEDIUM, or EXTENDED
are specified, the default check type for dynamic-format MyISAM
tables is MEDIUM.  The default check type also is MEDIUM for
static-format MyISAM tables, unless CHANGED or FAST
is specified.  In that case, the default is QUICK. The row scan
is skipped for CHANGED and FAST because the rows are very
seldom corrupted.





You can combine check options, as in the following example, which does
a quick check on the table to see whether it was
closed properly:





CHECK TABLE test_table FAST QUICK;





Note: In some cases, CHECK TABLE will change the
table!  This happens if the table is marked as ``corrupted'' or ``not
closed properly'' but CHECK TABLE doesn't find any problems in the
table.  In this case, CHECK TABLE marks the table as okay.





If a table is corrupted, it's most likely that the problem is in
the indexes and not in the data part.  All of the preceding check types
check the indexes thoroughly and should thus find most errors.





If you just want to check a table that you assume is okay, you should use
no check options or the QUICK option. The latter should be used
when you are in a hurry and can take the very small risk that
QUICK doesn't find an error in the data file. (In most cases,
MySQL should find, under normal usage, any error in the data file.
If this happens, the table is marked as ``corrupted'' and cannot
be used until it's repaired.)





FAST and CHANGED are mostly intended to be used from a
script (for example, to be executed from @command{cron}) if you want to check your
table from time to time. In most cases, FAST is to be preferred
over CHANGED.  (The only case when it isn't preferred is when you suspect
that you have found a bug in the MyISAM code.)





EXTENDED is to be used only after you have run a normal check but
still get strange errors from a table when MySQL tries to
update a row or find a row by key. (This is very unlikely if a
normal check has succeeded!)





Some problems reported by CHECK TABLE can't be corrected automatically:









14.5.2.4  CHECKSUM TABLE Syntax









CHECKSUM TABLE tbl_name [, tbl_name] ... [ QUICK | EXTENDED ]





Reports a table checksum.





If QUICK is specified, the live table checksum is reported if it is
available, or NULL otherwise.  This is very fast.  A live checksum
is enabled by specifying the CHECKSUM=1 table option, currently
supported only for MyISAM tables.
See section 14.2.6  CREATE TABLE Syntax.





In EXTENDED mode the whole table is read row by row and the checksum
is calculated. This can be very slow for large tables.





By default, if neither QUICK nor EXTENDED is specified, MySQL
returns a live checksum if the table storage engine supports it and scans
the table otherwise.





This statement is implemented in MySQL 4.1.1.







14.5.2.5  OPTIMIZE TABLE Syntax

















OPTIMIZE [LOCAL | NO_WRITE_TO_BINLOG] TABLE tbl_name [, tbl_name] ...





OPTIMIZE TABLE should be used if you have deleted a large part of a
table or if you have made many changes to a table with variable-length rows
(tables that have VARCHAR, BLOB, or TEXT columns).
Deleted records are maintained in a linked list and subsequent INSERT
operations reuse old record positions. You can use OPTIMIZE TABLE to
reclaim the unused space and to defragment the data file.





In most setups, you need not run OPTIMIZE TABLE at all.  Even
if you do a lot of updates to variable-length rows, it's not likely that
you need to do this more than once a week or month and only on certain
tables.





For the moment, OPTIMIZE TABLE works only on MyISAM,
BDB and InnoDB tables. For BDB tables, OPTIMIZE
TABLE is currently mapped to ANALYZE TABLE. It was also the case for
InnoDB tables before MySQL 4.1.3; starting from this version it is
mapped to ALTER TABLE.
See section 14.5.2.1  ANALYZE TABLE Syntax.





You can get OPTIMIZE TABLE to work on other table types by starting
@command{mysqld} with the --skip-new or --safe-mode option;
in this case, OPTIMIZE TABLE is just mapped to ALTER TABLE.





OPTIMIZE TABLE works as follows:


    

  1. 

If the table has deleted or split rows, repair the table.

  2. 

If the index pages are not sorted, sort them.

  3. 

If the statistics are not up to date (and the repair couldn't be done
by sorting the index), update them.





Note that MySQL locks the table during the time OPTIMIZE TABLE is
running.





Before MySQL 4.1.1, OPTIMIZE TABLE statements are not written
to the binary log. As of MySQL 4.1.1, they are written to the binary
log unless the optional NO_WRITE_TO_BINLOG keyword 
(or its alias LOCAL) is used.







14.5.2.6  REPAIR TABLE Syntax










REPAIR [LOCAL | NO_WRITE_TO_BINLOG] TABLE
    tbl_name [, tbl_name] ... [QUICK] [EXTENDED] [USE_FRM]





REPAIR TABLE repairs a possibly corrupted table.
By default,
it has the same effect as @command{myisamchk --recover tbl_name}.
REPAIR TABLE works only on MyISAM tables.





Normally you should never have to run this statement. However, if disaster
strikes, REPAIR TABLE is very likely to get back all your data from
a MyISAM table.  If your tables become corrupted often, you should try
to find the reason for it, to eliminate the need to use REPAIR TABLE.
See section A.4.2  What to Do If MySQL Keeps Crashing.  See section 15.1.4  MyISAM Table Problems.





The statement returns a table with the following columns:






Column  Value



Table  The table name



Op  Always repair



Msg_type  One of status, error, info, or warning



Msg_text  The message







The REPAIR TABLE statement might produce many rows of information
for each repaired table.  The last row will have a Msg_type value of
status and Msg_test normally should be OK.  If you
don't get OK, you should try repairing the table with @command{myisamchk
--safe-recover}, because REPAIR TABLE does not yet implement all the
options of @command{myisamchk}. We plan to make it more flexible in the future.





If QUICK is given, REPAIR TABLE tries to repair
only the index tree.  This type of repair is like that done by
@command{myisamchk --recover --quick}.





If you use EXTENDED, MySQL creates the index row by row instead of
creating one index at a time with sorting.  (Before MySQL 4.1, this might
be better than sorting on fixed-length keys if you have long CHAR
keys that compress very well.)  This type of repair is like that done by
@command{myisamchk --safe-recover}.





As of MySQL 4.0.2, there is a USE_FRM mode for REPAIR TABLE.
Use it if the `.MYI' index file is missing or if its header is
corrupted.  In this mode, MySQL will re-create the `.MYI' file using
information from the `.frm' file. This kind of repair cannot be done
with @command{myisamchk}. Note: Use this mode only if you cannot
use regular REPAIR modes. `.MYI' header contains important
table metadata (in particular, current AUTO_INCREMENT value
and Delete link) that will be lost in REPAIR ... USE_FRM.
Don't use USE_FRM if the table is compressed, as this information
is also stored in `.MYI' file.





Before MySQL 4.1.1, REPAIR TABLE statements are not written
to the binary log. As of MySQL 4.1.1, they are written to the binary
log unless the optional NO_WRITE_TO_BINLOG keyword 
(or its alias LOCAL) is used.





Warning: If the server dies during a REPAIR TABLE operation,
it's essential after restarting it that you immediately execute another
REPAIR TABLE statement for the table before performing any other
operations on it. (It's always good to start by making a backup.)  In the
worst case, you might have a new clean index file without information about
the data file, and then the next operation you perform could overwrite the
data file.  This is an unlikely, but possible scenario.






14.5.2.7  RESTORE TABLE Syntax











RESTORE TABLE tbl_name [, tbl_name] ... FROM '/path/to/backup/directory'





Restores the table or tables from a backup that was made with BACKUP
TABLE. Existing tables will not be overwritten; if you try to restore over
an existing table, you will get an error.  Just as BACKUP TABLE,
RESTORE TABLE currently works only for MyISAM tables.
The directory should be specified as a full pathname.





The backup for each table consists of its `.frm' format file and
`.MYD' data file. The restore operation restores those files, then
uses them to rebuild the `.MYI' index file.  Restoring takes longer
than backing up due to the need to rebuild the indexes. The more indexes the
table has, the longer it will take.





The statement returns a table with the following columns:






Column  Value



Table  The table name



Op  Always restore



Msg_type  One of status, error, info, or warning



Msg_text  The message








14.5.3  SET and SHOW Syntax


































SET allows you to set variables and options.





SHOW has many forms that provide information about databases,
tables, columns, or status information about the server.
This section describes those following:





SHOW [FULL] COLUMNS FROM tbl_name [FROM db_name] [LIKE 'pattern']
SHOW CREATE DATABASE db_name
SHOW CREATE TABLE tbl_name
SHOW DATABASES [LIKE 'pattern']
SHOW [STORAGE] ENGINES
SHOW ERRORS [LIMIT [offset,] row_count]
SHOW GRANTS FOR user
SHOW INDEX FROM tbl_name [FROM db_name]
SHOW INNODB STATUS
SHOW [BDB] LOGS
SHOW PRIVILEGES
SHOW [FULL] PROCESSLIST
SHOW STATUS [LIKE 'pattern']
SHOW TABLE STATUS [FROM db_name] [LIKE 'pattern']
SHOW [OPEN] TABLES [FROM db_name] [LIKE 'pattern']
SHOW [GLOBAL | SESSION] VARIABLES [LIKE 'pattern']
SHOW WARNINGS [LIMIT [offset,] row_count]





If the syntax for a given SHOW statement includes a LIKE
'pattern' part, 'pattern' is a string that can contain the SQL `%'
and `_' wildcard characters.
The pattern is useful for restricting statement output to matching values.





Note that there are other forms of these statements described elsewhere:









14.5.3.1  SET Syntax












SET variable_assignment [, variable_assignment] ...

variable_assignment:
      user_var_name = expr
    | [GLOBAL | SESSION] system_var_name = expr
    | @@[global. | session.]system_var_name = expr





SET sets different types of variables that affect the operation of the
server or your client. It can be used to assign values to user variables or
system variables.





In MySQL 4.0.3, we added the GLOBAL and SESSION options
and allowed most important system variables to be changed dynamically at
runtime.
The system variables that you can set at runtime are described in
section 5.2.3.1  Dynamic System Variables.





In older versions of MySQL, SET OPTION is used instead of SET,
but this is now deprecated; just leave out the word OPTION.





The following example show the different syntaxes you can use to set
variables.





A user variable is written as @var_name and can be set as follows:





SET @var_name = expr;





Further information about user variables is given in section 10.3  User Variables.





System variables can be referred to in SET statements as
var_name. The name optionally can be preceded by GLOBAL or
@@global. to indicate explicitly that the variable is a global
variable, or by SESSION, @@session., or @@ to
indicate that it is a session variable.  LOCAL and @@local.
are synonyms for SESSION and @@session.. If no modifier is
present, SET sets the session variable.





The @@var_name syntax for system variables is supported to make
MySQL syntax compatible with some other database systems.





If you set several system variables in the same statement, the last used
GLOBAL or SESSION option is used for variables that have no mode
specified.





SET sort_buffer_size=10000;
SET @@local.sort_buffer_size=10000;
SET GLOBAL sort_buffer_size=1000000, SESSION sort_buffer_size=1000000;
SET @@sort_buffer_size=1000000;
SET @@global.sort_buffer_size=1000000, @@local.sort_buffer_size=1000000;





If you set a system variable using SESSION (the default), the value
remains in effect until the current session ends or until you set the
variable to a different value. If you set a system variable using
GLOBAL, which requires the SUPER privilege, the value is
remembered and used for new connections until the server restarts. If you
want to make a variable setting permanent, you should put it in an option
file.  See section 4.3.2  Using Option Files.





To prevent incorrect usage, MySQL produces an error if you use SET
GLOBAL with a variable that can only be used with SET SESSION or if
you do not specify GLOBAL when setting a global variable.





If you want to set a SESSION variable to the GLOBAL value or a
GLOBAL value to the compiled-in MySQL default value, you can set it to
DEFAULT. For example, the following two statements are identical in
setting the session value of max_join_size to the global value:





SET max_join_size=DEFAULT;
SET @@session.max_join_size=@@global.max_join_size;





You can get a list of most system variables with SHOW VARIABLES.
See section 14.5.3.20  SHOW VARIABLES Syntax.
To get a specific variable name or list of names that match a pattern, use a
LIKE clause:





SHOW VARIABLES LIKE 'max_join_size';
SHOW GLOBAL VARIABLES LIKE 'max_join_size';





You can also get the value for a specific value by using the
@@[global.|local.]var_name syntax with SELECT:





SELECT @@max_join_size, @@global.max_join_size;





When you retrieve a variable with SELECT @@var_name (that is, you
do not specify global., session., or local.), MySQL returns
the SESSION value if it exists and the GLOBAL value otherwise.





The following list describes variables that have non-standard syntax or that are
not described in the list of system variables that is found in
section 5.2.3  Server System Variables.
Although these variables are not displayed by SHOW VARIABLES, you can
obtain their values with SELECT (with the exception of CHARACTER
SET and SET NAMES). For example:





mysql> SELECT @@AUTOCOMMIT;
+--------------+
| @@autocommit |
+--------------+
|            1 |
+--------------+







AUTOCOMMIT = {0 | 1}


Set the autocommit mode.
If set to 1, all changes to a table take effect immediately.
If set to 0, you have to use COMMIT to accept a transaction or
ROLLBACK to cancel it.  If you change AUTOCOMMIT
mode from 0 to 1, MySQL performs an automatic COMMIT
of any open transaction.  Another way to begin a transaction is to use a
START TRANSACTION or BEGIN statement.
See section 14.4.1  START TRANSACTION, COMMIT, and ROLLBACK Syntax.


BIG_TABLES = {0 | 1}



If set to 1, all temporary tables are stored on disk rather than in
memory.  This is a little slower, but the error The table tbl_name is
full will not occur for SELECT operations that require a large
temporary table.  The default value for a new connection is
0 (use in-memory temporary tables).  As of MySQL 4.0, you
should normally never need to set this variable, because MySQL automatically
converts in-memory tables to disk-based tables as necessary.  This variable
previously was named SQL_BIG_TABLES.


CHARACTER SET {charset_name | DEFAULT}


This maps all strings from and to the client with the given mapping.
Before MySQL 4.1, the only allowable value for charset_name
is cp1251_koi8, but you can add new mappings by editing the
`sql/convert.cc' file in the MySQL source distribution.  As of MySQL
4.1.1, SET CHARACTER SET sets three session system variables:
character_set_client and character_set_results are set to
the given character set, and character_set_connection to the value
of character_set_database.

The default mapping can be restored by using a value of DEFAULT.

Note that the syntax for SET CHARACTER SET differs
from that for setting most other options.


FOREIGN_KEY_CHECKS = {0 | 1}


If set to 1 (the default), foreign key constraints for InnoDB
tables are checked.  If set to 0, they are ignored.  Disabling foreign
key checking can be useful for reloading InnoDB tables in an order
different than that required by their parent/child relationships.
This variable was added in MySQL 3.23.52.
See section 16.7.4  FOREIGN KEY Constraints.


IDENTITY = value


The variable is a synonym for the LAST_INSERT_ID variable.
It exists for compatibility with other databases.
As of MySQL 3.23.25, you can read its value with SELECT @@IDENTITY.
As of MySQL 4.0.3, you can also set its value with SET IDENTITY.


INSERT_ID = value


Set the value to be used by the following INSERT or ALTER TABLE
statement when inserting an AUTO_INCREMENT value.  This is mainly used
with the binary log.


LAST_INSERT_ID = value


Set the value to be returned from LAST_INSERT_ID(). This is stored in
the binary log when you use LAST_INSERT_ID() in a statement that updates
a table. Setting this variable does not update theh value returned by the
mysql_insert_id() C API function.


NAMES {'charset_name' | DEFAULT}


SET NAMES sets the three session system variables
character_set_client, character_set_connection, and
character_set_results to the given character set.

The default mapping can be restored by using a value of DEFAULT.

Note that the syntax for SET NAMES differs
from that for setting most other options.
This statement is available as of MySQL 4.1.0.


SQL_AUTO_IS_NULL = {0 | 1}


If set to 1 (the default), you can find the last inserted row for a table
that contains an AUTO_INCREMENT column by using the following construct:



WHERE auto_increment_column IS NULL



This behavior is used by some ODBC programs, such as Access.
SQL_AUTO_IS_NULL was added in MySQL 3.23.52.


SQL_BIG_SELECTS = {0 | 1}


If set to 0, MySQL aborts SELECT statements
that probably will take a very long time (that is, statements for which
the optimizer estimates that the number of examined rows will
exceed the value of max_join_size).
This is useful when an inadvisable WHERE statement has been
issued.  The default value for a new connection is 1, which allows
all SELECT statements.

If you set the max_join_size system variable to a value other than
DEFAULT, SQL_BIG_SELECTS will be set to 0.


SQL_BUFFER_RESULT = {0 | 1}


SQL_BUFFER_RESULT forces results from SELECT statements
to be put into temporary tables. This helps MySQL free the
table locks early and can be beneficial in cases where it takes a long time to
send results to the client.
This variable was added in MySQL 3.23.13.


SQL_LOG_BIN = {0 | 1}


If set to 0, no logging is done to the binary log for the client.
The client must have the SUPER privilege to set this option.
This variable was added in MySQL 3.23.16.


SQL_LOG_OFF = {0 | 1}


If set to 1, no logging is done to the general query log for this
client.  The client must have the SUPER privilege to set this option.


SQL_LOG_UPDATE = {0 | 1}


If set to 0, no logging is done to the update log for the client.
The client must have the SUPER privilege to set this option.
This variable was added in MySQL 3.22.5.
Starting from MySQL 5.0.0, it is deprecated and is mapped to
SQL_LOG_BIN (see section C.1.3  Changes in release 5.0.0 (22 Dec 2003: Alpha)).


SQL_QUOTE_SHOW_CREATE = {0 | 1}


If set to 1, SHOW CREATE TABLE quotes table and column
names. If set to 0, quoting is disabled.  This option is enabled
by default so that replication will work for tables with table and column
names that require quoting.
This variable was added in MySQL 3.23.26.
section 14.5.3.6  SHOW CREATE TABLE Syntax.


SQL_SAFE_UPDATES = {0 | 1}


If set to 1, MySQL aborts UPDATE or DELETE statements
that do not use a key in the WHERE clause or a LIMIT clause.
This makes it possible to catch UPDATE or DELETE statements
where keys are not used properly and that would probably change or delete a
large number of rows.
This variable was added in MySQL 3.22.32.


SQL_SELECT_LIMIT = {value | DEFAULT}


The maximum number of records to return from SELECT statements.  The
default value for a new connection is ``unlimited.'' If you have changed the
limit, the default value can be restored by using a SQL_SELECT_LIMIT
value of DEFAULT.

If a SELECT has a LIMIT clause, the LIMIT takes
precedence over the value of SQL_SELECT_LIMIT.


SQL_WARNINGS = {0 | 1}


This variable controls whether single-row INSERT statements produce
an information string if warnings occur.  The default is 0.  Set the value
to 1 to produce an information string.
This variable was added in MySQL 3.22.11.


TIMESTAMP = {timestamp_value | DEFAULT}


Set the time for this client.  This is used to get the original timestamp if
you use the binary log to restore rows.  timestamp_value should be a
Unix epoch timestamp, not a MySQL timestamp.


UNIQUE_CHECKS = {0 | 1}


If set to 1 (the default), uniqueness checks for secondary indexes in
InnoDB tables are performed.  If set to 0, no uniqueness
checks are done.  This variable was added in MySQL 3.23.52.
See section 16.7.4  FOREIGN KEY Constraints.







14.5.3.2  SHOW CHARACTER SET Syntax










SHOW CHARACTER SET [LIKE 'pattern']





The SHOW CHARACTER SET statement shows all available character sets.
It takes an optional LIKE clause that indicates which character set
names to match. For example:





mysql> SHOW CHARACTER SET LIKE 'latin%';
+---------+-----------------------------+-------------------+--------+
| Charset | Description                 | Default collation | Maxlen |
+---------+-----------------------------+-------------------+--------+
| latin1  | ISO 8859-1 West European    | latin1_swedish_ci |      1 |
| latin2  | ISO 8859-2 Central European | latin2_general_ci |      1 |
| latin5  | ISO 8859-9 Turkish          | latin5_turkish_ci |      1 |
| latin7  | ISO 8859-13 Baltic          | latin7_general_ci |      1 |
+---------+-----------------------------+-------------------+--------+





The Maxlen column shows the maximum number of bytes used to
store one character.





SHOW CHARACTER SET is available as of MySQL 4.1.0.







14.5.3.3  SHOW COLLATION Syntax










SHOW COLLATION [LIKE 'pattern']





The output from SHOW COLLATION includes all available character
sets.
It takes an optional LIKE clause that indicates which collation
names to match. For example:





mysql> SHOW COLLATION LIKE 'latin1%';
+-------------------+---------+----+---------+----------+---------+
| Collation         | Charset | Id | Default | Compiled | Sortlen |
+-------------------+---------+----+---------+----------+---------+
| latin1_german1_ci | latin1  |  5 |         |          |       0 |
| latin1_swedish_ci | latin1  |  8 | Yes     | Yes      |       0 |
| latin1_danish_ci  | latin1  | 15 |         |          |       0 |
| latin1_german2_ci | latin1  | 31 |         | Yes      |       2 |
| latin1_bin        | latin1  | 47 |         | Yes      |       0 |
| latin1_general_ci | latin1  | 48 |         |          |       0 |
| latin1_general_cs | latin1  | 49 |         |          |       0 |
| latin1_spanish_ci | latin1  | 94 |         |          |       0 |
+-------------------+---------+----+---------+----------+---------+





The Default column indicates whether a collation is the
default for its character set.
Compiled indicates whether the character set is
compiled into the server.
Sortlen is related to the amount of memory required to sort strings
expressed in the character set.





SHOW COLLATION is available as of MySQL 4.1.0.







14.5.3.4  SHOW COLUMNS Syntax











SHOW [FULL] COLUMNS FROM tbl_name [FROM db_name] [LIKE 'pattern']





SHOW COLUMNS lists the columns in a given table.  If the column types
differ from what you expect them to be based on your CREATE TABLE
statement, note that MySQL sometimes changes column types when you create or
alter a table.  The conditions for which this occurs are described in
section 14.2.6.1  Silent Column Specification Changes.





The FULL keyword can be used from MySQL 3.23.32 on. It causes the
output to include the privileges you have for each column.  As of MySQL 4.1,
FULL also causes any per-column comments to be displayed.





You can use db_name.tbl_name as an alternative to the tbl_name
FROM db_name syntax.  These two statements are equivalent:





mysql> SHOW COLUMNS FROM mytable FROM mydb;
mysql> SHOW COLUMNS FROM mydb.mytable;





SHOW FIELDS is a synonym for SHOW COLUMNS.  You can also
list a table's columns with the @command{mysqlshow db_name tbl_name} command.





The DESCRIBE statement provides information similar to
SHOW COLUMNS.
See section 14.3.1  DESCRIBE Syntax (Get Information About Columns).







14.5.3.5  SHOW CREATE DATABASE Syntax











SHOW CREATE {DATABASE | SCHEMA} db_name





Shows a CREATE DATABASE statement that will create the given
database.  It was added in MySQL 4.1.  SHOW CREATE SCHEMA can be used
as of MySQL 5.0.2.





mysql> SHOW CREATE DATABASE test\G
*************************** 1. row ***************************
       Database: test
Create Database: CREATE DATABASE `test`
                 /*!40100 DEFAULT CHARACTER SET latin1 */






14.5.3.6  SHOW CREATE TABLE Syntax










SHOW CREATE TABLE tbl_name





Shows a CREATE TABLE statement that will create the given table.
It was added in MySQL 3.23.20.





mysql> SHOW CREATE TABLE t\G
*************************** 1. row ***************************
       Table: t
Create Table: CREATE TABLE t (
  id INT(11) default NULL auto_increment,
  s char(60) default NULL,
  PRIMARY KEY (id)
) TYPE=MyISAM






SHOW CREATE TABLE quotes table and column names according to
the value of the SQL_QUOTE_SHOW_CREATE option.
section 14.5.3.1  SET Syntax.







14.5.3.7  SHOW CREATE VIEW Syntax










SHOW CREATE VIEW view_name





This statement shows a CREATE VIEW statement that will create
the given view.





mysql> SHOW CREATE VIEW v;
+-------+----------------------------------------------------+
| Table | Create Table                                       |
+-------+----------------------------------------------------+
| v     | CREATE VIEW `test`.`v` AS select 1 AS `a`,2 AS `b` |
+-------+----------------------------------------------------+





This statement was added in MySQL 5.0.1.







14.5.3.8  SHOW DATABASES Syntax











SHOW {DATABASES | SCHEMAS} [LIKE 'pattern']





SHOW DATABASES lists the databases on the MySQL server host.
You can also get this list using the @command{mysqlshow} command.
As of MySQL 4.0.2, you will see only those databases for which you have some
kind of privilege, if you don't have the global SHOW DATABASES
privilege.





If the server was started with the --skip-show-database option, you
cannot use this statement at all unless you have the SHOW DATABASES
privilege.





SHOW SCHEMAS can be used as of MySQL 5.0.2







14.5.3.9  SHOW ENGINES Syntax












SHOW [STORAGE] ENGINES





SHOW ENGINES shows you status information about the storage engines.
This is particularly useful for checking whether a storage engine is
supported, or to see what the default engine is.
This statement is implemented in MySQL 4.1.2.
SHOW TABLE TYPES is a deprecated synonym.





mysql> SHOW ENGINES\G
*************************** 1. row ***************************
 Engine: MyISAM
Support: DEFAULT
Comment: Default engine as of MySQL 3.23 with great performance
*************************** 2. row ***************************
 Engine: HEAP
Support: YES
Comment: Alias for MEMORY
*************************** 3. row ***************************
 Engine: MEMORY
Support: YES
Comment: Hash based, stored in memory, useful for temporary tables
*************************** 4. row ***************************
 Engine: MERGE
Support: YES
Comment: Collection of identical MyISAM tables
*************************** 5. row ***************************
 Engine: MRG_MYISAM
Support: YES
Comment: Alias for MERGE
*************************** 6. row ***************************
 Engine: ISAM
Support: NO
Comment: Obsolete storage engine, now replaced by MyISAM
*************************** 7. row ***************************
 Engine: MRG_ISAM
Support: NO
Comment: Obsolete storage engine, now replaced by MERGE
*************************** 8. row ***************************
 Engine: InnoDB
Support: YES
Comment: Supports transactions, row-level locking, and foreign keys
*************************** 9. row ***************************
 Engine: INNOBASE
Support: YES
Comment: Alias for INNODB
*************************** 10. row ***************************
 Engine: BDB
Support: YES
Comment: Supports transactions and page-level locking
*************************** 11. row ***************************
 Engine: BERKELEYDB
Support: YES
Comment: Alias for BDB
*************************** 12. row ***************************
 Engine: NDBCLUSTER
Support: YES
Comment: Clustered, fault-tolerant, memory-based tables
*************************** 13. row ***************************
 Engine: NDB
Support: YES
Comment: Alias for NDBCLUSTER
*************************** 14. row ***************************
 Engine: EXAMPLE
Support: YES
Comment: Example storage engine
*************************** 15. row ***************************
 Engine: ARCHIVE
Support: YES
Comment: Archive storage engine





A Support value indicates whether the particular storage engine is
supported, and which is the default engine. For example, if the server
is started with the --default-table-type=InnoDB option, then
the Support value for the InnoDB row will have the value
DEFAULT.







14.5.3.10  SHOW ERRORS Syntax










SHOW ERRORS [LIMIT [offset,] row_count]
SHOW COUNT(*) ERRORS





This statement is similar to SHOW WARNINGS, except that instead
of displaying errors, warnings, and notes, it displays only errors.
SHOW ERRORS is available as of MySQL 4.1.0.





The LIMIT clause has the same syntax as for the SELECT
statement.
See section 14.1.7  SELECT Syntax.





The SHOW COUNT(*) ERRORS statement displays the number of errors.
You can also retrieve this number from the error_count variable:





SHOW COUNT(*) ERRORS;
SELECT @@error_count;





For more information, see section 14.5.3.21  SHOW WARNINGS Syntax.







14.5.3.11  SHOW GRANTS Syntax












SHOW GRANTS FOR user





This statement lists the GRANT statements that must be issued to
duplicate the privileges for a MySQL user account.





mysql> SHOW GRANTS FOR 'root'@'localhost';
+---------------------------------------------------------------------+
| Grants for root@localhost                                           |
+---------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION |
+---------------------------------------------------------------------+





As of MySQL 4.1.2, to list privileges for the current session, you can use
any of the following statements:





SHOW GRANTS;
SHOW GRANTS FOR CURRENT_USER;
SHOW GRANTS FOR CURRENT_USER();





Before MySQL 4.1.2,  you can find out what user the
session was authenticated as by selecting the value
of the CURRENT_USER() function (new in MySQL 4.0.6).
Then use that value in the SHOW GRANTS statement.
See section 13.8.3  Information Functions.





SHOW GRANTS is available as of MySQL 3.23.4.







14.5.3.12  SHOW INDEX Syntax












SHOW INDEX FROM tbl_name [FROM db_name]





SHOW INDEX returns table index information in a format that
resembles the SQLStatistics call in ODBC.





SHOW INDEX returns the following fields:







Table


The name of the table.


Non_unique


0 if the index can't contain duplicates, 1 if it can.


Key_name


The name of the index.


Seq_in_index


The column sequence number in the index, starting with 1.


Column_name


The column name.




Collation


How the column is sorted in the index. In MySQL, this can have values
`A' (Ascending) or NULL (Not sorted).




Cardinality


The number of unique values in the index.  This is updated by running
ANALYZE TABLE or @command{myisamchk -a}.  Cardinality is counted
based on statistics stored as integers, so it's not necessarily accurate for
small tables. The higher the cardinality, the greater the chance that MySQL
will use the index when doing joins.


Sub_part


The number of indexed characters if the column is only partly indexed.
NULL if the entire column is indexed.


Packed


Indicates how the key is packed.  NULL if it is not.


Null


Contains YES if the column may contain NULL, ''
if not.


Index_type


The index method used (BTREE, FULLTEXT, HASH,
RTREE).


Comment


Various remarks.  Before MySQL 4.0.2 when the Index_type column was
added, Comment indicates whether an index is FULLTEXT.






The Packed and Comment columns were added in MySQL 3.23.0.
The Null and Index_type columns were added in MySQL 4.0.2.





You can use db_name.tbl_name as an alternative to the tbl_name
FROM db_name syntax.  These two statements are equivalent:





mysql> SHOW INDEX FROM mytable FROM mydb;
mysql> SHOW INDEX FROM mydb.mytable;





SHOW KEYS is a synonym for SHOW INDEX.  You can also list
a table's indexes with the @command{mysqlshow -k db_name tbl_name} command.







14.5.3.13  SHOW INNODB STATUS Syntax




SHOW INNODB STATUS





This statement shows extensive information about the state of the
InnoDB storage engine.







14.5.3.14  SHOW LOGS Syntax




SHOW [BDB] LOGS





SHOW LOGS displays status information about existing log files.
It was implemented in MySQL 3.23.29. Currently, it displays only information
about Berkeley DB log files, so an alias for it (available as of MySQL
4.1.1) is SHOW BDB LOGS.





SHOW LOGS returns the following fields:







File


The full path to the log file.


Type


The log file type (BDB for Berkeley DB log files).


Status


The status of the log file (FREE if the file can be removed, or
IN USE if the file is needed by the transaction subsystem)







14.5.3.15  SHOW PRIVILEGES Syntax










SHOW PRIVILEGES





SHOW PRIVILEGES shows the list of system privileges that the underlying 
MySQL server supports.
This statement is implemented as of MySQL 4.1.0.





mysql> SHOW PRIVILEGES\G
*************************** 1. row ***************************
Privilege: Select
  Context: Tables
  Comment: To retrieve rows from table
*************************** 2. row ***************************
Privilege: Insert
  Context: Tables
  Comment: To insert data into tables
*************************** 3. row ***************************
Privilege: Update
  Context: Tables
  Comment: To update existing rows 
*************************** 4. row ***************************
Privilege: Delete
  Context: Tables
  Comment: To delete existing rows
*************************** 5. row ***************************
Privilege: Index
  Context: Tables
  Comment: To create or drop indexes
*************************** 6. row ***************************
Privilege: Alter
  Context: Tables
  Comment: To alter the table
*************************** 7. row ***************************
Privilege: Create
  Context: Databases,Tables,Indexes
  Comment: To create new databases and tables
*************************** 8. row ***************************
Privilege: Drop
  Context: Databases,Tables
  Comment: To drop databases and tables
*************************** 9. row ***************************
Privilege: Grant
  Context: Databases,Tables
  Comment: To give to other users those privileges you possess
*************************** 10. row ***************************
Privilege: References
  Context: Databases,Tables
  Comment: To have references on tables
*************************** 11. row ***************************
Privilege: Reload
  Context: Server Admin
  Comment: To reload or refresh tables, logs and privileges
*************************** 12. row ***************************
Privilege: Shutdown
  Context: Server Admin
  Comment: To shutdown the server
*************************** 13. row ***************************
Privilege: Process
  Context: Server Admin
  Comment: To view the plain text of currently executing queries
*************************** 14. row ***************************
Privilege: File
  Context: File access on server
  Comment: To read and write files on the server






14.5.3.16  SHOW PROCESSLIST Syntax



















SHOW [FULL] PROCESSLIST





SHOW PROCESSLIST shows you which threads are running.
You can also get this information using the @command{mysqladmin processlist}
statement.  If you have the SUPER privilege, you can see all
threads.  Otherwise, you can see only your own threads (that is, threads
associated with the MySQL account that you are using).
See section 14.5.4.3  KILL Syntax.
If you don't use the FULL keyword, only the first 100
characters of each query are shown.





Starting from MySQL 4.0.12, the statement reports the hostname for TCP/IP
connections in host_name:client_port format to make it easier to
determine which client is doing what.





This statement is very useful if you get the "too many connections" error
message and want to find out what is going on. MySQL reserves one extra
connection to be used by accounts that have the SUPER privilege, to
ensure that administrators should always be able to connect and check the
system (assuming that you are not giving this privilege to all your users).





Some states commonly seen in the output from SHOW PROCESSLIST:







Checking table


The thread is performing (automatic) checking of the table.


Closing tables


Means that the thread is flushing the changed table data to disk and
closing the used tables. This should be a fast operation.  If not, then
you should verify that you don't have a full disk and that the disk is not
in very heavy use.


Connect Out


Slave connecting to master.


Copying to tmp table on disk


The temporary result set was larger than tmp_table_size and the
thread is now changing the temporary table from in-memory to disk-based
format to save memory.


Creating tmp table


The thread is creating a temporary table to hold a part of the result for
the query.


deleting from main table


The server is executing the first part of a multiple-table delete and
deleting only from the first table.


deleting from reference tables


The server is executing the second part of a multiple-table delete and deleting
the matched rows from the other tables.


Flushing tables


The thread is executing FLUSH TABLES and is waiting for all
threads to close their tables.


Killed


Someone has sent a kill to the thread and it should abort next time it
checks the kill flag.  The flag is checked in each major loop in MySQL,
but in some cases it might still take a short time for the thread to die.
If the thread is locked by some other thread, the kill takes effect
as soon as the other thread releases its lock.


Locked


The query is locked by another query.


Sending data


The thread is processing rows for a SELECT statement and
also is sending data to the client.


Sorting for group


The thread is doing a sort to satisfy a GROUP BY.


Sorting for order


The thread is doing a sort to satisfy a ORDER BY.


Opening tables


The thread is trying to open a table. This is should be very fast procedure,
unless something prevents opening. For example, an ALTER TABLE or a
LOCK TABLE statement can prevent opening a table until the statement
is finished.


Removing duplicates


The query was using SELECT DISTINCT in such a way that MySQL
couldn't optimize away the distinct operation at an early stage. Because of
this, MySQL requires an extra stage to remove all duplicated rows before
sending the result to the client.


Reopen table


The thread got a lock for the table, but noticed after getting the lock
that the underlying table structure changed. It has freed the lock,
closed the table, and is now trying to reopen it.


Repair by sorting


The repair code is using sorting to create indexes.


Repair with keycache


The repair code is using creating keys one by one through the key cache.
This is much slower than Repair by sorting.


Searching rows for update


The thread is doing a first phase to find all matching rows before
updating them.  This has to be done if the UPDATE is changing
the index that is used to find the involved rows.


Sleeping


The thread is waiting for the client to send a new statement to it.


System lock


The thread is waiting to get an external system lock for the
table. If you are not using multiple @command{mysqld} servers that are accessing
the same tables, you can disable system locks with the
--skip-external-locking option.


Upgrading lock


The INSERT DELAYED handler is trying to get a lock for the table
to insert rows.


Updating


The thread is searching for rows to update and updating them.


User Lock


The thread is waiting on a GET_LOCK().


Waiting for tables


The thread got a notification that the underlying structure for a table
has changed and it needs to reopen the table to get the new structure.
However, to be able to reopen the table, it must wait until all other
threads have closed the table in question.

This notification happens if another thread has used FLUSH TABLES
or one of the following statements on the table in question: FLUSH
TABLES tbl_name, ALTER TABLE, RENAME TABLE,
REPAIR TABLE, ANALYZE TABLE, or OPTIMIZE TABLE.


waiting for handler insert


The INSERT DELAYED handler has processed all pending inserts and is
waiting for new ones.






Most states correspond to very quick operations. If a thread stays in any
of these states for many seconds, there might be a problem that
needs to be investigated.





There are some other states that are not mentioned in the preceding list,
but many of them are useful only for finding bugs in the server.







14.5.3.17  SHOW STATUS Syntax











SHOW STATUS [LIKE 'pattern']






SHOW STATUS provides server status information.  This information
also can be obtained using the @command{mysqladmin extended-status} command.





Partial output is shown here. The list of variables and their values may be
different for your server. The meaning of each variable is given in
See section 5.2.4  Server Status Variables.





mysql> SHOW STATUS;
+--------------------------+------------+
| Variable_name            | Value      |
+--------------------------+------------+
| Aborted_clients          | 0          |
| Aborted_connects         | 0          |
| Bytes_received           | 155372598  |
| Bytes_sent               | 1176560426 |
| Connections              | 30023      |
| Created_tmp_disk_tables  | 0          |
| Created_tmp_tables       | 8340       |
| Created_tmp_files        | 60         |
...
| Open_tables              | 1          |
| Open_files               | 2          |
| Open_streams             | 0          |
| Opened_tables            | 44600      |
| Questions                | 2026873    |
...
| Table_locks_immediate    | 1920382    |
| Table_locks_waited       | 0          |
| Threads_cached           | 0          |
| Threads_created          | 30022      |
| Threads_connected        | 1          |
| Threads_running          | 1          |
| Uptime                   | 80380      |
+--------------------------+------------+





With a LIKE clause, the statement displays only those variables that
match the pattern:





mysql> SHOW STATUS LIKE 'Key%';
+--------------------+----------+
| Variable_name      | Value    |
+--------------------+----------+
| Key_blocks_used    | 14955    |
| Key_read_requests  | 96854827 |
| Key_reads          | 162040   |
| Key_write_requests | 7589728  |
| Key_writes         | 3813196  |
+--------------------+----------+






14.5.3.18  SHOW TABLE STATUS Syntax












SHOW TABLE STATUS [FROM db_name] [LIKE 'pattern']





SHOW TABLE STATUS (new in MySQL 3.23) works likes SHOW
TABLE, but provides a lot of information about each table. You can
also get this list using the @command{mysqlshow --status db_name} command.





SHOW TABLE STATUS returns the following fields:







Name


The name of the table.


Engine


The storage engine for the table.
Before MySQL 4.1.2, this value is labeled as Type.
See section 15  MySQL Storage Engines and Table Types.


Version


The version number of the table's
`.frm' file.


Row_format


The row storage format (Fixed, Dynamic, or Compressed).


Rows


The number of rows.
Some storage engines, such as MyISAM and ISAM, store the exact
count.
For other storage engines, such as InnoDB, this value is an
approximation.


Avg_row_length


The average row length.


Data_length


The length of the data file.


Max_data_length


The maximum length of the data file.
For fixed-row formats, this is the maximum number of rows in the table.
For dynamic-row formats, this is the total number of data bytes that
can be stored in the table, given the data pointer size used.


Index_length


The length of the index file.


Data_free


The number of allocated but unused bytes.


Auto_increment


The next AUTO_INCREMENT value.


Create_time


When the table was created.


Update_time


When the data file was last updated.


Check_time


When the table was last checked.


Collation


The table's character set and collation. (New in 4.1.1)


Checksum


The live checksum value (if any). (New in 4.1.1)


Create_options


Extra options used with CREATE TABLE.


Comment


The comment used when creating the table (or some information why MySQL
couldn't access the table information).






In the table comment, InnoDB tables will report the free space of
the tablespace to which the table belongs.  For a table located in the
shared tablespace, this is the free space of the shared tablespace. If
you are using multiple tablespaces and the table has its own tablespace,
the freespace is for just that table.





For MEMORY (HEAP) tables, the Data_length,
Max_data_length, and Index_length values approximate the
actual amount of allocated memory. The allocation algorithm reserves memory
in large amounts to reduce the number of allocation operations.







14.5.3.19  SHOW TABLES Syntax











SHOW [FULL|OPEN] TABLES [FROM db_name] [LIKE 'pattern']





SHOW TABLES lists the non-TEMPORARY tables in a given
database.  You can also get this list using the @command{mysqlshow db_name}
command.





Before MySQL 5.0.1, the output from SHOW TABLES contains a single
column of table names.  Beginning with MySQL 5.0.1, also lists the views in
the database. As of MySQL 5.0.2, the FULL modifier is supported such
that SHOW FULL TABLES displays a second output column.  Values in the
second column are BASE TABLE for a table and VIEW for a view.





Note: If you have no privileges for a table, the table
will not show up in the output from SHOW TABLES or @command{mysqlshow
db_name}.






SHOW OPEN TABLES lists the tables that are currently open in
the table cache. See section 7.4.8  How MySQL Opens and Closes Tables.  The Comment field in the output
tells how many times the table is cached and in_use.
OPEN can be used from MySQL 3.23.33 on.







14.5.3.20  SHOW VARIABLES Syntax













SHOW [GLOBAL | SESSION] VARIABLES [LIKE 'pattern']






SHOW VARIABLES shows the values of some MySQL system variables.  This
information also can be obtained using the @command{mysqladmin variables}
command.





The GLOBAL and SESSION options are new in MySQL 4.0.3.
With GLOBAL, you will get the values that will be used for new
connections to MySQL. With SESSION, you will get the values that
are in effect for the current connection. If you use neither option, the
default SESSION. LOCAL is a synonym for SESSION.





If the default values are unsuitable, you can set most of these variables
using command-line options when @command{mysqld} starts or at runtime with
the SET statement.
See section 5.2.1  @command{mysqld} Command-Line Options and section 14.5.3.1  @code{SET} Syntax.





Partial output is shown here. The list of variables and their values may be
different for your server. The meaning of each variable is given in
See section 5.2.3  Server System Variables.
Information about tuning them is provided in section 7.5.2  Tuning Server Parameters.





mysql> SHOW VARIABLES;
+---------------------------------+------------------------------+
| Variable_name                   | Value                        |
+---------------------------------+------------------------------|
| back_log                        | 50                           |
| basedir                         | /usr/local/mysql             |
| bdb_cache_size                  | 8388572                      |
| bdb_log_buffer_size             | 32768                        |
| bdb_home                        | /usr/local/mysql             |
...
| max_connections                 | 100                          |
| max_connect_errors              | 10                           |
| max_delayed_threads             | 20                           |
| max_error_count                 | 64                           |
| max_heap_table_size             | 16777216                     |
| max_join_size                   | 4294967295                   |
| max_relay_log_size              | 0                            |
| max_sort_length                 | 1024                         |
...
| timezone                        | EEST                         |
| tmp_table_size                  | 33554432                     |
| tmpdir                          | /tmp/:/mnt/hd2/tmp/          |
| version                         | 4.0.4-beta                   |
| wait_timeout                    | 28800                        |
+---------------------------------+------------------------------+





With a LIKE clause, the statement displays only those variables that
match the pattern:





mysql> SHOW VARIABLES LIKE 'have%';
+--------------------+----------+
| Variable_name      | Value    |
+--------------------+----------+
| have_bdb           | YES      |
| have_innodb        | YES      |
| have_isam          | YES      |
| have_raid          | NO       |
| have_symlink       | DISABLED |
| have_openssl       | YES      |
| have_query_cache   | YES      |
+--------------------+----------+






14.5.3.21  SHOW WARNINGS Syntax










SHOW WARNINGS [LIMIT [offset,] row_count]
SHOW COUNT(*) WARNINGS





SHOW WARNINGS shows the error, warning, and note messages that
resulted from the last statement that generated messages, or nothing if the
last statement that used a table generated no messages.
This statement is implemented as of MySQL 4.1.0.
A related statement, SHOW ERRORS, shows only the errors.
See section 14.5.3.10  SHOW ERRORS Syntax.





The list of messages is reset for each new statement that uses a table.





The SHOW COUNT(*) WARNINGS statement displays the total number of
errors, warnings, and notes. You can also retrieve this number from the
warning_count variable:





SHOW COUNT(*) WARNINGS;
SELECT @@warning_count;





The value of warning_count might be greater than the number of
messages displayed by SHOW WARNINGS if the max_error_count
system variable is set low enough that not all messages are stored. An
example shown later in this section demonstrates how this can happen.





The LIMIT clause has the same syntax as for the SELECT
statement.
See section 14.1.7  SELECT Syntax.





The MySQL server sends back the total number of errors, warnings, and
notes resulting from the last statement.
If you are using the C API, this value can be obtained by calling
mysql_warning_count().
See section 21.2.3.240  mysql_warning_count().





Note that the framework for warnings was added in MySQL 4.1.0, at which
point many statements did not generate warnings.  In 4.1.1, the situation is
much improved, with warnings generated for statements such as LOAD
DATA INFILE and DML statements such as INSERT, UPDATE,
CREATE TABLE, and ALTER TABLE.





The following DROP TABLE statement results in a note:





mysql> DROP TABLE IF EXISTS no_such_table;
mysql> SHOW WARNINGS;
+-------+------+-------------------------------+
| Level | Code | Message                       |
+-------+------+-------------------------------+
| Note  | 1051 | Unknown table 'no_such_table' |
+-------+------+-------------------------------+





Here is a simple example that shows a syntax warning for CREATE TABLE
and conversion warnings for INSERT:





mysql> CREATE TABLE t1 (a TINYINT NOT NULL, b CHAR(4)) TYPE=MyISAM;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Warning
   Code: 1287
Message: 'TYPE=storage_engine' is deprecated, use
         'ENGINE=storage_engine' instead
1 row in set (0.00 sec)

mysql> INSERT INTO t1 VALUES(10,'mysql'),(NULL,'test'),
    -> (300,'open source');
Query OK, 3 rows affected, 4 warnings (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 4

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Warning
   Code: 1265
Message: Data truncated for column 'b' at row 1
*************************** 2. row ***************************
  Level: Warning
   Code: 1263
Message: Data truncated, NULL supplied to NOT NULL column 'a' at row 2
*************************** 3. row ***************************
  Level: Warning
   Code: 1264
Message: Data truncated, out of range for column 'a' at row 3
*************************** 4. row ***************************
  Level: Warning
   Code: 1265
Message: Data truncated for column 'b' at row 3
4 rows in set (0.00 sec)





The maximum number of error, warning, and note messages to store is controlled
by the max_error_count system variable.  By default, its value is 64.
To change the number of messages you want stored, change the value of
max_error_count.
In the following example, the ALTER TABLE statement produces three
warning messages, but only one is stored because max_error_count
has been set to 1:





mysql> SHOW VARIABLES LIKE 'max_error_count';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| max_error_count | 64    |
+-----------------+-------+
1 row in set (0.00 sec)

mysql> SET max_error_count=1;
Query OK, 0 rows affected (0.00 sec)

mysql> ALTER TABLE t1 MODIFY b CHAR;
Query OK, 3 rows affected, 3 warnings (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 3

mysql> SELECT @@warning_count;
+-----------------+
| @@warning_count |
+-----------------+
|               3 |
+-----------------+
1 row in set (0.01 sec)

mysql> SHOW WARNINGS;
+---------+------+----------------------------------------+
| Level   | Code | Message                                |
+---------+------+----------------------------------------+
| Warning | 1263 | Data truncated for column 'b' at row 1 |
+---------+------+----------------------------------------+
1 row in set (0.00 sec)





To disable warnings, set max_error_count to 0. In this case,
warning_count still indicates how many warnings have occurred, but
none of the messages are stored.







14.5.4  Other Administrative Statements





14.5.4.1  CACHE INDEX Syntax

















CACHE INDEX
  tbl_index_list [, tbl_index_list] ...
  IN key_cache_name

tbl_index_list:
  tbl_name [[INDEX|KEY] (index_name[, index_name] ...)]





The CACHE INDEX statement assigns table indexes to a specific key
cache. It is used only for MyISAM tables.





The following statement assigns indexes from the tables t1,
t2, and t3 to the key cache named hot_cache:





mysql> CACHE INDEX t1, t2, t3 IN hot_cache;
+---------+--------------------+----------+----------+
| Table   | Op                 | Msg_type | Msg_text |
+---------+--------------------+----------+----------+
| test.t1 | assign_to_keycache | status   | OK       |
| test.t2 | assign_to_keycache | status   | OK       |
| test.t3 | assign_to_keycache | status   | OK       |
+---------+--------------------+----------+----------+





The syntax of CACHE INDEX allows you to specify that only particular
indexes from a table should be assigned to the cache.  However, the current
implementation assigns all the table's indexes to the cache, so there is no
reason to specify anything other than the table name.





The key cache referred to in a CACHE INDEX statement can be created
by setting its size with a parameter setting statement or in the server
parameter settings. For example:





mysql> SET GLOBAL keycache1.key_buffer_size=128*1024;





Key cache parameters can be accessed as members of a structured system
variable.
See section 10.4.1  Structured System Variables.





A key cache must exist before you can assign indexes to it:





mysql> CACHE INDEX t1 in non_existent_cache;
ERROR 1283 (HY000): Unknown key cache 'non_existent_cache'





By default, table indexes are assigned to the main (default) key cache
created at the server startup. When a key cache is destroyed, all
indexes assigned to it become assigned to the default key cache again.





Index assignment affects the server globally: If one client
assigns an index to a given cache, this cache is used for all queries
involving the index, no matter what client issues the queries.





CACHE INDEX was added in MySQL 4.1.1.







14.5.4.2  FLUSH Syntax


















FLUSH [LOCAL | NO_WRITE_TO_BINLOG] flush_option [, flush_option] ...





You should use the FLUSH statement if you want to clear some of the
internal caches MySQL uses.  To execute FLUSH, you must have
the RELOAD privilege.





flush_option can be any of the following:







HOSTS


Empties the host cache tables.  You should flush the
host tables if some of your hosts change IP number or if you get the
error message Host ... is blocked.  When more than
max_connect_errors errors occur successively for a given host while
connecting to the MySQL server, MySQL assumes that
something is wrong and blocks the host from further connection requests.
Flushing the host tables allows the host to attempt to connect
again. See section A.2.5  Host 'host_name' is blocked. You can start @command{mysqld} with
--max_connect_errors=999999999 to avoid this error message.


DES_KEY_FILE


Reloads the DES keys from the file that was
specified with the --des-key-file option at server startup time.


LOGS


Closes and reopens all log files.
If you have specified an update log file or a binary log file without
an extension, the extension number of the log file will be incremented
by one relative to the previous file. If you have used an extension in
the file name, MySQL will close and reopen the update log or binary log file.
See section 5.9.3  The Update Log. On Unix, this is the same thing as sending a SIGHUP
signal to the @command{mysqld} server (except on some Mac OS X 10.3 versions
where mysqld will ignore SIGHUP and SIGQUIT).


PRIVILEGES


Reloads the privileges from the grant tables in
the mysql database.


QUERY CACHE


Defragment the query cache to better utilize its
memory.  This statement does not remove any queries from the cache, unlike
RESET QUERY CACHE.


STATUS


Resets most status variables to zero. This is something you should use only when debugging a query. See section 1.7.1.3  How to Report Bugs or Problems.


{TABLE | TABLES} [tbl_name [, tbl_name] ...]


When no tables are named, closes all open tables and forces all tables in use to be closed. This also flushes the query cache.  With one or more table names, flushes only the given tables.
FLUSH TABLES also removes all query results from the query cache, like
the RESET QUERY CACHE statement.


TABLES WITH READ LOCK


Closes all open tables and locks all
tables for all databases with a read lock until you execute UNLOCK
TABLES. This is very convenient way to get backups if you have a filesystem
such as Veritas that can take snapshots in time.


USER_RESOURCES


Resets all user resources to zero. This
enables clients that have reached their hourly connection, query, or update
limits to resume activity.
See section 14.5.1.2  GRANT and REVOKE Syntax.






Before MySQL 4.1.1, FLUSH statements are not written to the binary
log. As of MySQL 4.1.1, they are written to the binary log unless the
optional NO_WRITE_TO_BINLOG keyword (or its alias LOCAL) is
used.  Exceptions are that FLUSH LOGS, FLUSH MASTER,
FLUSH SLAVE, and FLUSH TABLES WITH READ LOCK are not logged in
any case because they would cause problems if replicated to a slave.





You can also access some of these statements with the @command{mysqladmin}
utility, using the flush-hosts, flush-logs, flush-privileges,
flush-status, or flush-tables commands.





Take also a look at the RESET statement used with replication.
See section 14.5.4.5  RESET Syntax.







14.5.4.3  KILL Syntax
















KILL [CONNECTION | QUERY] thread_id





Each connection to @command{mysqld} runs in a separate thread.  You can see
which threads are running with the SHOW PROCESSLIST statement and kill
a thread with the KILL thread_id statement.





As of MySQL 5.0.0, KILL allows the optional CONNECTION or
QUERY modifiers:








If you have the PROCESS privilege, you can see all threads.
If you have the SUPER privilege, you can kill all threads and
statements.  Otherwise, you can see and kill only your own threads and
statements.





You can also use the @command{mysqladmin processlist} and @command{mysqladmin kill}
commands to examine and kill threads.





Note: You currently cannot use KILL with the Embedded MySQL
Server library, because the embedded server merely runs inside the threads
of the host application, it does not create connection threads of its own.





When you do a KILL, a thread-specific kill flag is set for
the thread.
In most cases, it might take some time for the thread to die, because the kill
flag is checked only at specific intervals:









14.5.4.4  LOAD INDEX INTO CACHE Syntax




LOAD INDEX INTO CACHE
  tbl_index_list [, tbl_index_list] ...

tbl_index_list:
  tbl_name
    [[INDEX|KEY] (index_name[, index_name] ...)]
    [IGNORE LEAVES]





The LOAD INDEX INTO CACHE statement preloads a table index into the
key cache to which it has been assigned by an explicit CACHE INDEX
statement, or into the default key cache otherwise.  LOAD INDEX INTO
CACHE is used only for MyISAM tables.





The IGNORE LEAVES modifier causes only blocks for the non-leaf
nodes of the index to be preloaded.





The following statement preloads nodes (index blocks) of indexes of the
tables t1 and t2:





mysql> LOAD INDEX INTO CACHE t1, t2 IGNORE LEAVES;
+---------+--------------+----------+----------+
| Table   | Op           | Msg_type | Msg_text |
+---------+--------------+----------+----------+
| test.t1 | preload_keys | status   | OK       |
| test.t2 | preload_keys | status   | OK       |
+---------+--------------+----------+----------+





This statement preloads all index blocks from t1. It preloads only
blocks for the non-leaf nodes from t2.





The syntax of LOAD INDEX INTO CACHE allows you to specify that only
particular indexes from a table should be preloaded.  However, the current
implementation preloads all the table's indexes into the cache, so there is
no reason to specify anything other than the table name.





LOAD INDEX INTO CACHE was added in MySQL 4.1.1.







14.5.4.5  RESET Syntax




RESET reset_option [, reset_option] ...





The RESET statement is used to clear the state of various server
operations. It also acts as a stronger
version of the FLUSH statement.  See section 14.5.4.2  FLUSH Syntax.





To execute RESET, you must have the RELOAD privilege.





reset_option can be any of the following:







MASTER


Deletes all binary logs listed in the index file, resets the binary log
index file to be empty, and creates a new binary log file. Previously named
FLUSH MASTER.
See section 14.6.1  SQL Statements for Controlling Master Servers.


QUERY CACHE


Removes all query results from the query cache.


SLAVE


Makes the slave forget its replication position in the master
binary logs.  Previously named FLUSH SLAVE.
See section 14.6.2  SQL Statements for Controlling Slave Servers.







14.6  Replication Statements




This section describes replication-related SQL statements.
One group of statements is used for controlling master servers.
The other is used for controlling slave servers.







14.6.1  SQL Statements for Controlling Master Servers












Replication can be controlled through the SQL interface. This section
discusses statements for managing master replication servers.
section 14.6.2  SQL Statements for Controlling Slave Servers discusses statements for managing slave servers.







14.6.1.1  PURGE MASTER LOGS Syntax










PURGE {MASTER | BINARY} LOGS TO 'log_name'
PURGE {MASTER | BINARY} LOGS BEFORE 'date'





Deletes all the binary logs listed in the log
index that are strictly prior to the specified log or date.
The logs also are removed from the list recorded in the log index file,
so that the given log becomes the first. 





Example:





PURGE MASTER LOGS TO 'mysql-bin.010';
PURGE MASTER LOGS BEFORE '2003-04-02 22:46:26';





The BEFORE variant is available as of MySQL 4.1. Its date argument
can be in 'YYYY-MM-DD hh:mm:ss' format.
MASTER and BINARY are synonyms, but BINARY can be used
only as of MySQL 4.1.1.





If you have an active slave that currently is reading one of the logs you
are trying to delete, this statement does nothing and fails with an error.
However, if a slave is dormant and you happen to purge one of
the logs it wants to read, the slave will be unable to replicate once it
comes up.  The statement is safe to run while slaves are replicating. You
do not need to stop them.





To purge logs, follow this procedure:





    

  1. 

On each slave server, use SHOW SLAVE STATUS to check which log it is
reading.

  2. 

Obtain a listing of the logs on the master server with SHOW MASTER LOGS.

  3. 

Determine the earliest log among all the slaves. This is the target log.
If all the slaves are up to date, this will be the last log on the list.

  4. 

Make a backup of all the logs you are about to delete.  (The step is optional,
but a good idea.)

  5. 

Purge all logs up to but not including the target log.






14.6.1.2  RESET MASTER Syntax










RESET MASTER





Deletes all binary logs listed in the index file,
resets the binary log index file to be empty, and creates a new binary log
file.





This statement was named FLUSH MASTER before MySQL 3.23.26.







14.6.1.3  SET SQL_LOG_BIN Syntax










SET SQL_LOG_BIN = {0|1}





Disables or enables binary logging for the current connection 
(SQL_LOG_BIN is a session variable)
if the client connects using an account that has the SUPER privilege.
The statement is refused with an error if the client does not have that
privilege. (Before MySQL 4.1.2, the statement was simply ignored in that case.)







14.6.1.4  SHOW BINLOG EVENTS Syntax










SHOW BINLOG EVENTS
   [IN 'log_name'] [FROM pos] [LIMIT [offset,] row_count]





Shows the events in the binary log. 
If you do not specify 'log_name', the first binary log will be displayed.





The LIMIT clause has the same syntax as for the SELECT
statement.
See section 14.1.7  SELECT Syntax.





This statement is available as of MySQL 4.0.







14.6.1.5  SHOW MASTER LOGS Syntax











SHOW MASTER LOGS
SHOW BINARY LOGS





Lists the binary log files on the server. This statement is used as part of
the procedure described in section 14.6.1.1  PURGE MASTER LOGS Syntax for determining which logs
can be purged.





SHOW MASTER LOGS was added in MySQL 3.23.38.  As of MySQL 4.1.1,
you can also use SHOW BINARY LOGS, which is equivalent.







14.6.1.6  SHOW MASTER STATUS Syntax










SHOW MASTER STATUS





Provides status information on the binary log files of the master.







14.6.1.7  SHOW SLAVE HOSTS Syntax










SHOW SLAVE HOSTS





Displays a list of slaves currently registered with the master. 
Any slave not started with the --report-host=slave_name 
option will not be visible in that list.







14.6.2  SQL Statements for Controlling Slave Servers












Replication can be controlled through the SQL interface. This section
discusses statements for managing slave replication servers.
section 14.6.1  SQL Statements for Controlling Master Servers discusses statements for managing master servers.







14.6.2.1  CHANGE MASTER TO Syntax











CHANGE MASTER TO master_def [, master_def] ...

master_def:
      MASTER_HOST = 'host_name'
    | MASTER_USER = 'user_name'
    | MASTER_PASSWORD = 'password'
    | MASTER_PORT = port_num
    | MASTER_CONNECT_RETRY = count
    | MASTER_LOG_FILE = 'master_log_name'
    | MASTER_LOG_POS = master_log_pos
    | RELAY_LOG_FILE = 'relay_log_name'
    | RELAY_LOG_POS = relay_log_pos
    | MASTER_SSL = {0|1}
    | MASTER_SSL_CA = 'ca_file_name'
    | MASTER_SSL_CAPATH = 'ca_directory_name'
    | MASTER_SSL_CERT = 'cert_file_name'
    | MASTER_SSL_KEY = 'key_file_name'
    | MASTER_SSL_CIPHER = 'cipher_list'





Changes the parameters that the slave server uses for connecting to and
communicating with the master server.





MASTER_USER, MASTER_PASSWORD, MASTER_SSL,
MASTER_SSL_CA, MASTER_SSL_CAPATH, MASTER_SSL_CERT,
MASTER_SSL_KEY, and MASTER_SSL_CIPHER provide information for
the slave about how to connect to its master.





The relay log options (RELAY_LOG_FILE and RELAY_LOG_POS) are
available beginning with MySQL 4.0.





The SSL options
(MASTER_SSL,
MASTER_SSL_CA,
MASTER_SSL_CAPATH,
MASTER_SSL_CERT,
MASTER_SSL_KEY,
and
MASTER_SSL_CIPHER)
are available beginning with MySQL 4.1.1.
You can change these options even on slaves that are compiled without SSL
support. They are saved to the `master.info' file, but is ignored
until you use a server that has SSL support enabled.





If you don't specify a given parameter, it keeps its old
value, except as indicated in the following discussion. For example, if the password to connect to your MySQL master has
changed, you just need to issue these statements
to tell the slave about the new password:





mysql> STOP SLAVE; -- if replication was running
mysql> CHANGE MASTER TO MASTER_PASSWORD='new3cret';
mysql> START SLAVE; -- if you want to restart replication





There is no need to specify the parameters that do
not change (host, port, user, and so forth).





MASTER_HOST and MASTER_PORT are the hostname (or IP address) of
the master host and its TCP/IP port. Note that if MASTER_HOST is
equal to localhost, then, like in other parts of MySQL, the port
may be ignored (if Unix socket files can be used, for example).





If you specify MASTER_HOST or MASTER_PORT,
the slave assumes that the master server is different than
before (even if you specify a host or port value that is
the same as the current value.) In this case, the old values for the master
binary log name and position are considered no longer applicable, so if you
do not specify MASTER_LOG_FILE and MASTER_LOG_POS in the
statement, MASTER_LOG_FILE='' and MASTER_LOG_POS=4 are
silently appended to it. 





MASTER_LOG_FILE and MASTER_LOG_POS are the coordinates
at which the slave I/O thread should begin reading from the master the
next time the thread starts.
If you specify either of them, you can't specify RELAY_LOG_FILE or
RELAY_LOG_POS.
If neither of MASTER_LOG_FILE or MASTER_LOG_POS are
specified, the slave uses the last coordinates of the slave SQL thread
before CHANGE MASTER was issued. This ensures that
replication has no discontinuity, even if the slave SQL thread was late
compared to the slave I/O thread, when you just want to change, say, the
password to use. This safe behavior was introduced starting from MySQL
4.0.17 and 4.1.1. (Before these versions, the coordinates used were 
the last coordinates of the slave I/O thread before CHANGE MASTER
was issued. This caused the SQL thread to possibly lose some events
from the master, thus breaking replication.)





CHANGE MASTER deletes all relay log files and starts
a new one, unless you specify RELAY_LOG_FILE or
RELAY_LOG_POS. In that case, relay logs are kept;
as of MySQL 4.1.1 the relay_log_purge global variable
is set silently to 0.





CHANGE MASTER TO updates the contents of the `master.info' and
`relay-log.info' files.





CHANGE MASTER is useful for setting up a slave when you have
the snapshot of the master and have recorded the log and the offset
corresponding to it.  After loading the snapshot into the slave, you
can run CHANGE MASTER TO MASTER_LOG_FILE='log_name_on_master',
MASTER_LOG_POS=log_offset_on_master on the slave.





Examples:





mysql> CHANGE MASTER TO
    ->     MASTER_HOST='master2.mycompany.com',
    ->     MASTER_USER='replication',
    ->     MASTER_PASSWORD='bigs3cret',
    ->     MASTER_PORT=3306,
    ->     MASTER_LOG_FILE='master2-bin.001',
    ->     MASTER_LOG_POS=4,
    ->     MASTER_CONNECT_RETRY=10;

mysql> CHANGE MASTER TO
    ->     RELAY_LOG_FILE='slave-relay-bin.006',
    ->     RELAY_LOG_POS=4025;





The first example changes the master and master's binary log
coordinates. This is used when you want to set up the slave to replicate
the master.





The second example shows an operation that is less frequently used. It
is done when the slave has relay logs that you want it to execute again
for some reason.  To do this, the master need not be reachable. You just
have to use CHANGE MASTER TO and start the SQL thread (START
SLAVE SQL_THREAD).





You can even use the second operation in a non-replication setup with a
standalone, non-slave server, to recover after a crash.  Suppose that your
server has crashed and you have restored a backup.  You want to replay
the server's own binary logs (not relay logs, but regular binary logs),
supposedly named `myhost-bin.*'. First, make a backup copy of these
binary logs in some safe place, in case you don't exactly follow the
procedure below and accidentally have the server purge the binary logs.
If using MySQL 4.1.1 or newer, use SET GLOBAL relay_log_purge=0 for
additional safety.  Then start the server without the --log-bin
option. Before MySQL 4.0.19, start it with a new (different from before) server
id; in newer versions there is no need, just use the
--replicate-same-server-id option. Start it with
--relay-log=myhost-bin (to make the server believe that these
regular binary logs are relay logs) and with --skip-slave-start. After
the server starts, issue these statements:





mysql> CHANGE MASTER TO
    ->     RELAY_LOG_FILE='myhost-bin.153',
    ->     RELAY_LOG_POS=410,
    ->     MASTER_HOST='some_dummy_string';
mysql> START SLAVE SQL_THREAD;





The server will read and execute its own binary logs, thus achieving
crash recovery.  Once the recovery is finished, run STOP SLAVE,
shut down the server, delete `master.info' and `relay-log.info',
and restart the server with its original options.





For the moment, specifying MASTER_HOST (even with a dummy value) is
required to make the server think it is a slave. In the future, we plan to add
options to get rid of these small constraints.







14.6.2.2  LOAD DATA FROM MASTER Syntax










LOAD DATA FROM MASTER





Takes a snapshot of the master and copies it to the slave.  It updates the
values of MASTER_LOG_FILE and MASTER_LOG_POS so that the slave
will start replicating from the correct position. Any table and database
exclusion rules specified with the --replicate-*-do-* and
--replicate-*-ignore-* options are honored.
--replicate-rewrite-db is not taken into account (because one user
could, with this option, set up a non-unique mapping such as
--replicate-rewrite-db=db1->db3 and
--replicate-rewrite-db=db2->db3, which would confuse the slave when
it loads the master's tables).





Use of this statement is subject to the following conditions:








In the future, it is planned to make this statement work with
InnoDB tables and to remove the need for a global read lock by using
non-blocking online backup.





If you are loading big tables, you might have to increase the values
of net_read_timeout and net_write_timeout
on both your master and slave servers.
See section 5.2.3  Server System Variables.





Note that LOAD DATA FROM MASTER does not copy any
tables from the mysql database.  This makes it easy to have
different users and privileges on the master and the slave.





The LOAD DATA FROM MASTER statement
requires the replication account that is used to connect to the master
to have the RELOAD and SUPER privileges on the master and the
SELECT privilege for all master tables you want to load. All
master tables for which the user does not have the SELECT privilege are
ignored by LOAD DATA FROM MASTER. This is because the
master will hide them from the user: LOAD DATA FROM MASTER calls
SHOW DATABASES to know the master databases to load, but
SHOW DATABASES returns only databases for which the user has
some privilege.
See section 14.5.3.8  SHOW DATABASES Syntax.
On the slave's side, the user that issues LOAD DATA FROM MASTER should
have grants to drop and create the databases and tables that are copied.







14.6.2.3  LOAD TABLE tbl_name FROM MASTER Syntax










LOAD TABLE tbl_name FROM MASTER





Transfers a copy of the table from master to the slave. This statement is
implemented mainly for debugging of LOAD DATA FROM MASTER.
It requires that the account used for connecting to the master server has the
RELOAD and SUPER privileges on the master and the
SELECT privilege on the master table to load.
On the slave side, the user that issues LOAD TABLE FROM MASTER should
have privileges to drop and create the table.





The conditions for LOAD DATA FROM MASTER apply here, too. For
example, LOAD TABLE FROM MASTER works only for MyISAM tables.
The timeout notes for LOAD DATA FROM MASTER apply as well.







14.6.2.4  MASTER_POS_WAIT() Syntax










SELECT MASTER_POS_WAIT('master_log_file', master_log_pos)





This is a function, not a statement. It is used to ensure that the slave
has read and executed events up to a given position in the master's binary log.
See section 13.8.4  Miscellaneous Functions for a full description.







14.6.2.5  RESET SLAVE Syntax










RESET SLAVE





Makes the slave forget its replication position in the master's binary logs.
This statement is meant to be used for a clean start: It deletes the
`master.info' and `relay-log.info' files, all the relay logs,
and starts a new relay log.





Note: All relay logs are deleted, even if they have not been
totally executed by the slave SQL thread.  (This is a condition likely to
exist on a replication slave if you have issued a STOP SLAVE
statement or if the slave is highly loaded.)





Connection information stored in the `master.info' file is immediately
reset using any values specified in the corresponding startup options.
This information includes values such as master host, master port, master
user, and master password.  If the slave SQL thread was in the middle of
replicating temporary tables when it was stopped, and RESET SLAVE
is issued, these replicated temporary tables are deleted on the slave.





This statement was named FLUSH SLAVE before MySQL 3.23.26.







14.6.2.6  SET GLOBAL SQL_SLAVE_SKIP_COUNTER Syntax










SET GLOBAL SQL_SLAVE_SKIP_COUNTER = n





Skip the next n events from the master. This is
useful for recovering from replication stops caused by a statement.





This statement is valid only when the slave thread is not running.
Otherwise, it produces an error.





Before MySQL 4.0, omit the GLOBAL keyword from the statement.







14.6.2.7  SHOW SLAVE STATUS Syntax










SHOW SLAVE STATUS





Provides status information on
essential parameters of the slave threads. If you issue this statement using
the
@command{mysql} client, you can use a \G statement terminator rather than
semicolon to get a more readable vertical layout:





mysql> SHOW SLAVE STATUS\G
*************************** 1. row ***************************
       Slave_IO_State: Waiting for master to send event
          Master_Host: localhost
          Master_User: root
          Master_Port: 3306
        Connect_Retry: 3
      Master_Log_File: gbichot-bin.005
  Read_Master_Log_Pos: 79
       Relay_Log_File: gbichot-relay-bin.005
        Relay_Log_Pos: 548
Relay_Master_Log_File: gbichot-bin.005
     Slave_IO_Running: Yes
    Slave_SQL_Running: Yes
      Replicate_Do_DB:
  Replicate_Ignore_DB:
           Last_Errno: 0
           Last_Error:
         Skip_Counter: 0
  Exec_Master_Log_Pos: 79
      Relay_Log_Space: 552
      Until_Condition: None
       Until_Log_File:
        Until_Log_Pos: 0
   Master_SSL_Allowed: No
   Master_SSL_CA_File:
   Master_SSL_CA_Path:
      Master_SSL_Cert:
    Master_SSL_Cipher:
       Master_SSL_Key:
Seconds_Behind_Master: 8





Depending on your version of MySQL, you may not see all the fields just
shown. In particular, several fields are present only as of MySQL 4.1.1.





SHOW SLAVE STATUS returns the following fields:







Slave_IO_State


A copy of the State field of
the output of SHOW PROCESSLIST for the slave I/O thread. This
tells you if the thread is trying to connect to the master, waiting
for events from the master, reconnecting to the master, and so on. Possible
states are listed in section 6.3  Replication Implementation Details. Looking at
this field is necessary because, for example, the thread can be running
but unsuccessfully trying to connect to the master; only this field
will make you aware of the connection problem.
The state of the SQL thread is not copied because it is simpler.
If it is running, there is no
problem; if it is not, you will find the error in the
Last_Error field (described below). 

This field is present beginning with MySQL 4.1.1.


Master_Host


The current master host.


Master_User


The current user used to connect to the master.


Master_Port


The current master port.


Connect_Retry


The current value of the --master-connect-retry option.


Master_Log_File


The name of the master binary log file from which the I/O thread is
currently reading.


Read_Master_Log_Pos


The position up to which the I/O thread has read in the current master binary log.


Relay_Log_File


The name of the relay log file from which the SQL thread is currently
reading and executing.


Relay_Log_Pos


The position up to which the SQL thread has read and executed in the current relay log.


Relay_Master_Log_File


The name of the master binary log file that contains the
last event executed by the SQL thread.


Slave_IO_Running


Whether or not the I/O thread is started.


Slave_SQL_Running


Whether or not the SQL thread is started.


Replicate_Do_DB, Replicate_Ignore_DB


The lists of databases that were specified with the
--replicate-do-db and --replicate-ignore-db options, if any.

These fields are present beginning with MySQL 4.1.1.


Replicate_Do_Table, Replicate_Ignore_Table, Replicate_Wild_Do_Table, Replicate_Wild_Ignore_Table


The lists of tables that were specified with the
--replicate-do-table,
--replicate-ignore-table,
--replicate-wild-do-table,
and
--replicate-wild-ignore_table
options, if any.

These fields are present beginning with MySQL 4.1.1.


Last_Errno, Last_Error


The error number and error message returned by the most recently executed query.
An error number of 0 and message of the empty string mean ``no error.''
If the Last_Error value is not empty, it will also appear as a
message in the slave's error log.

For example:



Last_Errno: 1051
Last_Error: error 'Unknown table 'z'' on query 'drop table z'



The message indicates that
the table z existed on the master and was dropped there, but it
did not exist on the slave, so DROP TABLE failed on the slave.
(This might occur, for example, if you forget to copy the table to the
slave when setting up replication.)


Skip_Counter


The last used value for SQL_SLAVE_SKIP_COUNTER.


Exec_Master_Log_Pos


The position of the last event executed by the SQL thread
from the master's binary log (Relay_Master_Log_File).
(Relay_Master_Log_File, Exec_Master_Log_Pos) in the
master's binary log corresponds to
(Relay_Log_File, Relay_Log_Pos)
in the relay log.


Relay_Log_Space


The total combined size of all existing relay logs.


Until_Condition, Until_Log_File, Until_Log_Pos


The values specified in the UNTIL clause of the START SLAVE
statement.

Until_Condition has these values:



    

  • 

None if no UNTIL clause was specified

  • 

Master if the slave is reading until a given position in the master's
binary logs

  • 

Relay if the slave is reading until a given position in its relay logs



Until_Log_File and Until_Log_Pos indicate the log filename and
position values that define the point at which the SQL thread will stop
executing.

These fields are present beginning with MySQL 4.1.1.


Master_SSL_Allowed, Master_SSL_CA_File, Master_SSL_CA_Path, Master_SSL_Cert, Master_SSL_Cipher, Master_SSL_Key


These fields show the
the SSL parameters used by the slave to connect to the master, if any.

Master_SSL_Allowed has these values:



    

  • 

Yes if an SSL connection to the master is allowed

  • 

No if an SSL connection to the master is not allowed

  • 

Ignored if an SSL connection is allowed but the slave server does not
have SSL support enabled



The values of the other SSL-related fields correspond to the values of the
--master-ca,
--master-capath,
--master-cert,
--master-cipher,
and
--master-key
options.

These fields are present beginning with MySQL 4.1.1.


Seconds_Behind_Master


The number of seconds that have elapsed since the timestamp of the last
master's event executed by the slave SQL thread. This will be NULL
when no event has been executed yet, or after CHANGE MASTER and
RESET SLAVE. This field can be used to know how ``late'' your slave
is. It will work even though your master and slave don't have identical
clocks.

This field is present beginning with MySQL 4.1.1.







14.6.2.8  START SLAVE Syntax










START SLAVE [thread_type [, thread_type] ... ]
START SLAVE [SQL_THREAD] UNTIL
    MASTER_LOG_FILE = 'log_name', MASTER_LOG_POS = log_pos
START SLAVE [SQL_THREAD] UNTIL
    RELAY_LOG_FILE = 'log_name', RELAY_LOG_POS = log_pos

thread_type: IO_THREAD | SQL_THREAD





START SLAVE with no options starts both of the slave threads.
The I/O thread reads queries from the master server and stores them in the
relay log.  The SQL thread reads the relay log and executes the
queries.
START SLAVE requires the SUPER privilege.





If START SLAVE succeeds in starting the slave threads, it
returns without any error. However, even in that case, it might be that the slave
threads start and then later stop (for example, because they don't manage to
connect to the master or read its binary logs, or some other
problem). START SLAVE will not warn you about this. You must
check your slave's error log for error messages generated by
the slave threads, or check that they are running fine with SHOW
SLAVE STATUS.






As of MySQL 4.0.2, you can add IO_THREAD and SQL_THREAD
options to the statement to name which of the threads to start.





As of MySQL 4.1.1, an UNTIL clause may be added to specify that
the slave should start and run until the SQL thread reaches a given point in
the master binary logs or in the slave relay logs. When the SQL thread reaches
that point, it stops.  If the SQL_THREAD option is specified in the
statement, it starts only the SQL thread.  Otherwise, it starts both slave
threads.  If the SQL thread is already running, the UNTIL clause is
ignored and a warning is issued.





With an UNTIL clause, you must specify both a log filename and
position. Do not mix master and relay log options.





Any UNTIL condition is reset by a subsequent STOP SLAVE
statement, a START SLAVE statement that includes no UNTIL
clause, or a server restart.





The UNTIL clause can be useful for debugging replication, or to
cause replication to proceed until just before the point where you want
to avoid having the slave replicate a statement. For example, if an unwise
DROP TABLE statement was executed on the master, you can use
UNTIL to tell the slave to execute up to that point but no farther.
To find what the event is, use @command{mysqlbinlog} with the master logs or
slave relay logs, or by using a SHOW BINLOG EVENTS statement.





If you are using UNTIL to have the slave process replicated queries in
sections, it is recommended that you start the slave with the
--skip-slave-start option to prevent the SQL thread from running
when the slave server starts.  It is probably best to use this option in an
option file rather than on the command line, so that an unexpected server
restart does not cause it to be forgotten.





The SHOW SLAVE STATUS statement includes output fields that display
the current values of the UNTIL condition.





This statement is called SLAVE START before MySQL 4.0.5.
For the moment, SLAVE START is still accepted for backward
compatibility, but is deprecated.







14.6.2.9  STOP SLAVE Syntax










STOP SLAVE [thread_type [, thread_type] ... ]

thread_type: IO_THREAD | SQL_THREAD





Stops the slave threads.
STOP SLAVE requires the SUPER privilege.





Like START SLAVE, as of MySQL 4.0.2, this statement
may be used with the IO_THREAD and SQL_THREAD options to name
the thread or threads to stop.






This statement is called SLAVE STOP before MySQL 4.0.5.
For the moment, SLAVE STOP is still accepted for backward
compatibility, but is deprecated.







14.7  SQL Syntax for Prepared Statements












Support for server-side prepared statements was added in MySQL 4.1.
This support takes advantage of the efficient client/server binary protocol,
provided that you use an appropriate client programming interface. Candidate
interfaces include the MySQL C API client library (for C programs) or
MySQL Connector/J (for Java programs).  For example, the C API provides
a set of function calls that make up its prepared statement API.
See section 21.2.4  C API Prepared Statements.
Other language interfaces can provide support for prepared statements that
use the binary protocol by linking in the C client library.  (The mysqli
extension in PHP 5.0 does this, for example.)






Beginning with MySQL 4.1.3, an alternative interface to prepared statements
is available:  SQL syntax for prepared statements.  This interface is not as
efficient as using the binary protocol through a prepared statement API, but
requires no programming because it is available directly at the SQL level:








SQL syntax for prepared statements is intended to be used for situations
such as these:








SQL syntax for prepared statements is based on three SQL statements:





PREPARE stmt_name FROM preparable_stmt;

EXECUTE stmt_name [USING @var_name [, @var_name] ...];

DEALLOCATE PREPARE stmt_name;





The PREPARE statement prepares a statement and assigns it
a name, stmt_name, by which to refer to the statement later.
preparable_stmt is either a string literal or a user variable that
contains the text of the statement.  The text must represent a single SQL
statement, not multiple statements.  Within the statement, `?'
characters can be used as parameter markers to indicate where data values
are to be bound to the query later when you execute it. The `?'
characters should not be enclosed within quotes, even if you intend to bind
them to string values.





If a prepared statement already exists with the same name, it is deallocated
implicitly before the new statement is prepared. This means that if the new
statement contains an error and cannot be prepared, an error is returned
and no statement with the given name will exist.





The scope of a prepared statement is the client session within which it is
created. Other clients cannot see it.





After preparing a statement, you execute it with an EXECUTE
statement that refers to the prepared statement name. If the prepared
statement contains any parameter markers, you must supply a USING
clause that lists user variables containing the values to be bound to
the parameters.
Parameter values can be supplied only by user variables, and the USING
clause must name exactly as many variables as the number of parameter markers
in the statement.





You can execute a given prepared statement multiple times, passing it
different variables or setting the variables to different values before
each execution.





To deallocate a prepared statement, use the DEALLOCATE PREPARE
statement.  Attempting to execute a prepared statement after deallocating
it results in an error.





If you terminate a client session without deallocating a previously prepared
statement, the server deallocates it automatically.





The following examples show two equivalent ways of preparing a statement that
computes the hypotenuse of a triangle given the lengths of the two sides.





The first example shows how to create a prepared statment by using a string
literal to supply the text of the statement:





mysql> PREPARE stmt1 FROM 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
mysql> SET @a = 3;
mysql> SET @b = 4;
mysql> EXECUTE stmt1 USING @a, @b;
+------------+
| hypotenuse |
+------------+
|          5 |
+------------+
mysql> DEALLOCATE PREPARE stmt1;





The second example is similar, but supplies the text of the statement
with a user variable:





mysql> SET @s = 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
mysql> PREPARE stmt2 FROM @s;
mysql> SET @a = 6;
mysql> SET @b = 8;
mysql> EXECUTE stmt2 USING @a, @b;
+------------+
| hypotenuse |
+------------+
|         10 |
+------------+
mysql> DEALLOCATE PREPARE stmt2;





SQL syntax for prepared statements cannot be used in nested fashion.
That is, a statement passed to PREPARE cannot itself be a
PREPARE, EXECUTE, or DEALLOCATE PREPARE statement.





Also, SQL syntax for prepared statements is distinct from using
prepared statement API calls.  For example, you cannot use the
mysql_stmt_prepare() C API function to prepare a PREPARE,
EXECUTE, or DEALLOCATE PREPARE statement.







15  MySQL Storage Engines and Table Types






























MySQL supports several storage engines that act as handlers for different
table types. MySQL storage engines include both those that handle
transaction-safe tables and those that handle non-transaction-safe tables:








This chapter describes each of the MySQL storage engines except for
InnoDB, which is covered in section 16  The InnoDB Storage Engine and NDB Cluster which is covered in section 17  MySQL Cluster.





When you create a new table, you can tell MySQL what type of table to create
by adding an ENGINE or TYPE table option to the CREATE
TABLE statement:





CREATE TABLE t (i INT) ENGINE = INNODB;
CREATE TABLE t (i INT) TYPE = MEMORY;





ENGINE is the preferred term, but cannot be used before MySQL 4.0.18.
TYPE is available beginning with MySQL 3.23.0, the first
version of MySQL for which multiple storage engines were available.





If you omit the ENGINE or TYPE option,
the default table type is usually MyISAM.
This can be changed by setting the table_type system variable.





To convert a table from one type to another, use an ALTER TABLE
statement that indicates the new type:





ALTER TABLE t ENGINE = MYISAM;
ALTER TABLE t TYPE = BDB;





See section 14.2.6  CREATE TABLE Syntax and
section 14.2.2  ALTER TABLE Syntax.






If you try to use a storage engine that is not compiled in or that is
compiled in but deactivated, MySQL instead creates a table of type
MyISAM.  This behavior is convenient when you want to copy tables
between MySQL servers that support different storage engines. (For example, in
a replication setup, perhaps your
master server supports transactional storage engines for increased safety,
but the slave servers use only non-transactional storage engines for greater
speed.)





This automatic automatic substitution of the MyISAM table type when an
unavailable type is specified can be confusing for new MySQL
users.  In MySQL 4.1 and up, a warning is generated when a table type is
automatically changed.





MySQL always creates an `.frm' file to hold the table and column
definitions. The table's index and data may be stored in one or more other
files, depending on the table type.
The server creates the `.frm' file above the storage engine level.
Individual storage engines create any additional files required for the tables
that they manage.





A database may contain tables of different types.





Transaction-safe tables (TSTs) have several advantages over
non-transaction-safe tables (NTSTs):








Note that to use the InnoDB storage engine in MySQL 3.23, you
must configure at least the innodb_data_file_path startup option.
In 4.0 and up, InnoDB uses default configuration values if you specify
none.
See section 16.4  InnoDB Configuration.





Non-transaction-safe tables have several advantages of their own, all
of which occur because there is no transaction overhead:








You can combine transaction-safe and non-transaction-safe tables in the same statements to get the best
of both worlds. However, within a transaction with autocommit disabled,
changes to non-transaction-safe tables still are committed immediately and
cannot be rolled back.







15.1  The MyISAM Storage Engine












MyISAM is the default storage engine as of MySQL 3.23. It is
based on the ISAM code but has many useful extensions.





Each MyISAM table is stored on disk in three files.  The files have
names that begin with the table name and have an extension to indicate the
file type.  An `.frm' file stores the table definition.  The data file
has an `.MYD' (MYData) extension.  The index file has an `.MYI'
(MYIndex) extension,





To specify explicitly that you want a MyISAM table, indicate that with
an ENGINE or TYPE table option:





CREATE TABLE t (i INT) ENGINE = MYISAM;
CREATE TABLE t (i INT) TYPE = MYISAM;





Normally, the ENGINE or TYPE option is unnecessary;
MyISAM is the default storage engine unless the default has been
changed.





You can check or repair MyISAM tables with the @command{myisamchk}
utility. See section 5.7.2.7  Using @command{myisamchk} for Crash Recovery. You can compress MyISAM tables with
@command{myisampack} to take up much less space.
@xref{myisampack, , @command{myisampack}}.





The following characteristics of the MyISAM storage engine are
improvements over the older ISAM engine:








MyISAM also supports the following features, which MySQL
will be able to use in the near future:









15.1.1  MyISAM Startup Options




The following options to @command{mysqld} can be used to change the behavior of
MyISAM tables:







--myisam-recover=mode


Set the mode for automatic recovery of crashed MyISAM tables.


--delay-key-write=ALL


Don't flush key buffers between writes for any MyISAM table.

Note: If you do this, you should not use MyISAM tables from
another program (such as from another MySQL server or with @command{myisamchk}) when
the table is in use. Doing so will lead to index corruption.

Using --external-locking will not help for tables that use
--delay-key-write.






See section 5.2.1  @command{mysqld} Command-Line Options.





The following system variables affect the behavior of
MyISAM tables:







bulk_insert_buffer_size


The size of the tree cache used in bulk insert
optimization. Note: This is a limit per thread!


myisam_max_extra_sort_file_size


Used to help MySQL to decide when to use the slow but safe key cache index
creation method. Note: This parameter is given in megabytes
before MySQL 4.0.3, and in bytes as of 4.0.3.


myisam_max_sort_file_size


Don't use the fast sort index method to create an index if the temporary
file would become larger than this.  Note: This parameter is
given in megabytes before MySQL 4.0.3, and in bytes as of 4.0.3.


myisam_sort_buffer_size


Set the size of the buffer used when recovering tables.






See section 5.2.3  Server System Variables.





Automatic recovery is activated if you start @command{mysqld} with the
--myisam-recover option. In this case, when the server opens a
MyISAM table, it checks whether the table
is marked as crashed or whether the open count variable for the
table is not 0 and you are running the server with
--skip-external-locking.  If either of these conditions is true, the
following happens:








If the recovery wouldn't be able to recover all rows from a previous
completed statement and you didn't specify FORCE in the value of the
--myisam-recover option, automatic repair aborts with an error
message in the error log:





Error: Couldn't repair table: test.g00pages





If you specify FORCE, a warning like this is written instead:





Warning: Found 344 of 354 rows when repairing ./test/g00pages





Note that if the automatic recovery value includes BACKUP, the
recovery process creates files with names of the form
`tbl_name-datetime.BAK'.  You should have a @command{cron} script that
automatically moves these files from the database directories to backup
media.







15.1.2  Space Needed for Keys










MyISAM tables use B-tree indexes. You can roughly calculate
the size for the index file as (key_length+4)/0.67, summed over
all keys.  This is for the worst case when all keys are inserted in
sorted order and the table doesn't have any compressed keys.





String indexes are space compressed. If the first index part is a
string, it will also be prefix compressed.  Space compression makes the
index file smaller than the worst-case figure if the string column has a lot
of trailing space or is a VARCHAR column that is not always used
to the full length. Prefix compression is used on keys that start
with a string.  Prefix compression helps if there are many strings
with an identical prefix.





In MyISAM tables, you can also prefix compress numbers by specifying
PACK_KEYS=1 when you create the table.  This helps when you have
many integer keys that have an identical prefix when the numbers are stored
high-byte first.







15.1.3  MyISAM Table Storage Formats




MyISAM supports three different storage formats. Two of them (fixed
and dynamic format) are chosen automatically depending on the type of
columns you are using. The third, compressed format, can be created only
with the @command{myisampack} utility.





When you CREATE or ALTER a table that has no BLOB
or TEXT columns, you can force the table format to FIXED or
DYNAMIC with the ROW_FORMAT table option. This causes
CHAR and VARCHAR columns to become CHAR for
FIXED format or VARCHAR for DYNAMIC format.





In the future, you will be able to compress or decompress tables by specifying
ROW_FORMAT={COMPRESSED | DEFAULT} to ALTER TABLE.
See section 14.2.6  CREATE TABLE Syntax.







15.1.3.1  Static (Fixed-Length) Table Characteristics




Static format is the default for MyISAM tables.  It is used when the
table contains no variable-length columns (VARCHAR, BLOB, or
TEXT). Each row is stored using a fixed number of bytes.





Of the three MyISAM storage formats, static format is the simplest
and most secure (least subject to corruption). It is also the fastest of the
on-disk formats. The speed comes from the easy way that rows in the data file
can be found on disk: When looking up a row based on a row number in the
index, multiply the row number by the row length.  Also, when scanning a
table, it is very easy to read a constant number of records with each disk
read operation.





The security is evidenced if your computer crashes while the MySQL server is
writing to a fixed-format MyISAM file. In this case, @command{myisamchk}
can easily determine where each row starts and ends, so it can usually
reclaim all records except the partially written one. Note that MyISAM
table
indexes can always be reconstructed based on the data rows.





General characteristics of static format tables:









15.1.3.2  Dynamic Table Characteristics











Dynamic storage format is used if a MyISAM table contains any
variable-length columns (VARCHAR, BLOB, or TEXT), or if
the table was created with the ROW_FORMAT=DYNAMIC option.





This format is a little more complex because each row has a header that
indicates
how long it is. One record can also end up at more than one location when it
is made longer as a result of an update.






You can use OPTIMIZE TABLE or @command{myisamchk} to defragment a
table. If you have fixed-length columns that you access or change frequently
in a table that also contains some variable-length columns, it might be
a good idea to move the variable-length columns to other tables just to
avoid fragmentation.





General characteristics of dynamic-format tables:









15.1.3.3  Compressed Table Characteristics













Compressed storage format is a read-only format that is generated with the
@command{myisampack} tool.





All MySQL distributions as of version 3.23.19 include @command{myisampack} by
default. (This version is when MySQL was placed under the GPL.) For earlier
versions, @command{myisampack} was included only with licenses or support
agreements, but the server still can read tables that were compressed
with @command{myisampack}.  Compressed tables can be uncompressed with
@command{myisamchk}.  (For the ISAM storage engine, compressed tables
can be created with @command{pack_isam} and uncompressed with @command{isamchk}.)





Compressed tables have the following characteristics:









15.1.4  MyISAM Table Problems




The file format that MySQL uses to store data has been extensively
tested, but there are always circumstances that may cause database tables
to become corrupted.







15.1.4.1  Corrupted MyISAM Tables




Even though the MyISAM table format is very reliable (all changes to
a table made by an SQL statement are written before the statement returns),
you can still get corrupted tables if some of the following things happen:








Typical symptoms for a corrupt table are:








You can check whether a MyISAM table is okay with the CHECK
TABLE statement.  You can repair a corrupted MyISAM table with
REPAIR TABLE.  When @command{mysqld} is not running, you can also
check or repair a table with the @command{myisamchk} command.
See section 14.5.2.3  CHECK TABLE Syntax,
section 14.5.2.6  REPAIR TABLE Syntax, and section 5.7.2.1  @command{myisamchk} Invocation Syntax.





If your tables become corrupted frequently, you should try to determine why
this is happening.  The most important thing to know is whether the table
became corrupted as a result of a server crash. You can verify this easily
by looking for a recent restarted mysqld message in the error log.
If there is such a message, it is likely that that table corruption is a
result of the server dying.  Otherwise, corruption may have occurred during
normal operation, which is a bug. You should try to create a reproducible
test case that demonstrates the problem.
See section A.4.2  What to Do If MySQL Keeps Crashing and section D.1.6  Making a Test Case If You Experience Table Corruption.







15.1.4.2  Problems from Tables Not Being Closed Properly




Each MyISAM index (`.MYI') file has a counter in the header
that can be used to check whether a table has been closed properly.
If you get the following warning from CHECK TABLE or @command{myisamchk},
it means that this counter has gone out of sync:





clients are using or haven't closed the table properly





This warning doesn't necessarily mean that the table is corrupted, but
you should at least check the table to verify that it's okay.





The counter works as follows:








In other words, the counter can go out of sync only under these conditions:









15.2  The MERGE Storage Engine





















The MERGE storage engine was introduced in MySQL 3.23.25. It
is also known as the MRG_MyISAM engine. The code is now reasonably
stable.





A MERGE table is a collection of identical MyISAM tables that
can be used as one.  ``Identical'' means that all tables have
identical column and index information.  You can't merge tables in which the
columns are listed in a different order, don't have exactly the same columns, or
have the indexes in different order.  However, any or all of the tables can be
compressed with @command{myisampack}.
@xref{myisampack, , @command{myisampack}}.
Differences in table options such as AVG_ROW_LENGTH, MAX_ROWS,
or PACK_KEYS do not matter.






When you create a MERGE table, MySQL creates two files on disk.
The files have names that begin with the table name and have an extension
to indicate the file type.  An `.frm' file stores the table definition,
and an `.MRG' file contains the names of the tables that should be
used as one.  (Originally, all used tables had to be in the same database
as the MERGE table itself. This restriction has been lifted as of
MySQL 4.1.1.)





You can use SELECT, DELETE, UPDATE, and (as of MySQL
4.0) INSERT on the collection of tables.  For the moment, you must
have SELECT, UPDATE, and DELETE privileges on the
tables that you map to a MERGE table.





If you DROP the MERGE table, you are dropping only the
MERGE specification. The underlying tables are not affected.





When you create a MERGE table, you must specify a
UNION=(list-of-tables) clause that indicates which tables you want to
use as one. You can optionally specify an INSERT_METHOD option if you
want inserts for the MERGE table to happen in the first or last table
of the UNION list. If you don't specify any INSERT_METHOD
option or specify it with a value of NO, attempts to insert records
into the MERGE table result in an error.





The following example shows how to create a MERGE table:





mysql> CREATE TABLE t1 (
    ->    a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    ->    message CHAR(20));
mysql> CREATE TABLE t2 (
    ->    a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    ->    message CHAR(20));
mysql> INSERT INTO t1 (message) VALUES ('Testing'),('table'),('t1');
mysql> INSERT INTO t2 (message) VALUES ('Testing'),('table'),('t2');
mysql> CREATE TABLE total (
    ->    a INT NOT NULL AUTO_INCREMENT,
    ->    message CHAR(20), INDEX(a))
    ->    TYPE=MERGE UNION=(t1,t2) INSERT_METHOD=LAST;





Note that the a column is indexed in the MERGE table, but is
not declared as a PRIMARY KEY as it is in the underlying
MyISAM tables. This is necessary because a MERGE table cannot
enforce uniqueness over the set of underlying tables.





After creating the MERGE table, you can do things like this:





mysql> SELECT * FROM total;
+---+---------+
| a | message |
+---+---------+
| 1 | Testing |
| 2 | table   |
| 3 | t1      |
| 1 | Testing |
| 2 | table   |
| 3 | t2      |
+---+---------+





Note that you can also manipulate the `.MRG' file directly from
outside of the MySQL server:





shell> cd /mysql-data-directory/current-database
shell> ls -1 t1 t2 > total.MRG
shell> mysqladmin flush-tables





To remap a MERGE table to a different collection of MyISAM
tables, you can do one of the following:








MERGE tables can help you solve the following problems:








The disadvantages of MERGE tables are:









15.2.1  MERGE Table Problems




The following are the known problems with MERGE tables:









15.3  The MEMORY (HEAP) Storage Engine















The MEMORY storage engine creates tables with contents that are stored
in memory. Before MySQL 4.1, MEMORY tables are called HEAP
tables. As of 4.1, HEAP is a synonym for MEMORY, and
MEMORY is the preferred term.





Each MEMORY table is associated with one disk file.  The filename
begins with the table name and has an extension of `.frm' to indicate
that it stores the table definition.





To specify explicitly that you want a MEMORY table, indicate that with
an ENGINE or TYPE table option:





CREATE TABLE t (i INT) ENGINE = MEMORY;
CREATE TABLE t (i INT) TYPE = HEAP;





MEMORY tables are stored in memory and use hash indexes.  This makes
them very fast, and very useful for creating temporary tables!  However,
when the server shuts down, all data stored in MEMORY tables is lost.
The tables continue to exist because their definitions are stored in the
`.frm' files on disk, but their contents will be empty when the server
restarts.





Here is an example that shows how you might create, use, and remove a
MEMORY table:





mysql> CREATE TABLE test TYPE=MEMORY
    ->     SELECT ip,SUM(downloads) AS down
    ->     FROM log_table GROUP BY ip;
mysql> SELECT COUNT(ip),AVG(down) FROM test;
mysql> DROP TABLE test;





MEMORY tables have the following characteristics:









15.4  The BDB (BerkeleyDB) Storage Engine
















Sleepycat Software has provided MySQL with the Berkeley DB transactional
storage engine. This storage engine typically is called BDB for short.
Support for the BDB storage engine is included in MySQL source
distributions starting from version 3.23.34a and is activated in MySQL-Max
binary distributions.





BDB tables may have a greater chance of surviving crashes and are also
capable of COMMIT and ROLLBACK operations on transactions.
The MySQL source distribution comes with a BDB distribution that has a
couple of small patches to make it work more smoothly with MySQL.
You can't use a non-patched BDB version with MySQL.





We at MySQL AB are working in close cooperation with Sleepycat to keep the
quality of the MySQL/BDB interface high.  (Even though Berkeley DB is in
itself very tested and reliable, the MySQL interface is still considered
gamma quality.  We are improving and optimizing it.)





When it comes to support for any problems involving BDB tables, we
are committed to helping our users locate the problem and create a
reproducible test case.  Any such test case will be forwarded to Sleepycat,
which in turn will help us find and fix the problem.  As this is a two-stage
operation, any problems with BDB tables may take a little longer for
us to fix than for other storage engines.  However, we anticipate no
significant difficulties with this procedure because the Berkeley DB code
itself is used in many applications other than MySQL.
See section 1.4.1  Support Offered by MySQL AB.





For general information about Berkeley DB, please visit the Sleepycat Web site,
http://www.sleepycat.com/.







15.4.1  Operating Systems Supported by BDB




Currently, we know that the BDB storage engine works with the following
operating systems:








BDB does not work with the following operating systems:








Note: The preceding lists are not complete. We will update them as we
receive more information.





If you build MySQL from source with support for BDB tables, but the
following error occurs when you start @command{mysqld}, it means BDB
is not supported for your architecture:





bdb: architecture lacks fast mutexes: applications cannot be threaded
Can't init databases





In this case, you must rebuild MySQL without BDB table support or
start the server with the --skip-bdb option.







15.4.2  Installing BDB




If you have downloaded a binary version of MySQL that includes support for
Berkeley DB, simply follow the usual binary distribution installation
instructions. (MySQL-Max distributions include BDB support.)





If you build MySQL from source, you can enable BDB support by running
@command{configure} with the --with-berkeley-db option in addition
to any other options that you normally use.  Download a distribution for
MySQL 3.23.34 or newer, change location into its top-level directory,
and run this command:





shell> ./configure --with-berkeley-db [other-options]





For more information, see
section 2.2.5  Installing MySQL on Other Unix-Like Systems,
@ref{mysqld-max, , @command{mysqld-max}}, and
See section 2.3  MySQL Installation Using a Source Distribution.







15.4.3  BDB Startup Options




The following options to @command{mysqld} can be used to change the behavior of
the BDB storage engine:







--bdb-home=path


The base directory for BDB tables.  This should be the same directory
you
use for --datadir.


--bdb-lock-detect=method


The BDB lock detection method. The option value should be 
DEFAULT, OLDEST, RANDOM, or YOUNGEST.


--bdb-logdir=path


The BDB log file directory.


--bdb-no-recover


Don't start Berkeley DB in recover mode.


--bdb-no-sync


Don't synchronously flush the BDB logs.


--bdb-shared-data


Start Berkeley DB in multi-process mode.  (Don't use DB_PRIVATE when
initializing Berkeley DB.)


--bdb-tmpdir=path


The BDB temporary file directory.


--skip-bdb


Disable the BDB storage engine.






See section 5.2.1  @command{mysqld} Command-Line Options.





The following system variable affects the behavior of
BDB tables:







bdb_max_lock


The maximum number of locks you can have active on a BDB table.






See section 5.2.3  Server System Variables.





If you use the --skip-bdb option, MySQL will not initialize the Berkeley DB
library and this will save a lot of memory.  However, if you use this
option, you cannot use BDB tables.  If you try to create a BDB
table, MySQL will create a MyISAM table instead.





Normally, you should start @command{mysqld} without the --bdb-no-recover
option if you intend to use BDB tables.  However, this may give you
problems when you try to start @command{mysqld} if the BDB log files are
corrupted.
See section 2.4.2.3  Starting and Troubleshooting the MySQL Server.





With the bdb_max_lock variable, you can specify the maximum number of
locks that can be active on a BDB table.  The default is
10,000. You should increase this if errors such as the following occur
when you perform long transactions or when @command{mysqld} has to examine
many rows to execute a query:





bdb: Lock table is out of available locks
Got error 12 from ...





You may also want to change the binlog_cache_size and
max_binlog_cache_size variables if you are using large
multiple-statement transactions.
See section 5.9.4  The Binary Log.







15.4.4  Characteristics of BDB Tables




Each BDB table is stored on disk in two files.  The files have
names that begin with the table name and have an extension to indicate the
file type.  An `.frm' file stores the table definition, and a `.db'
file contains the table data and indexes.





To specify explicitly that you want a BDB table, indicate that with
an ENGINE or TYPE table option:





CREATE TABLE t (i INT) ENGINE = BDB;
CREATE TABLE t (i INT) TYPE = BDB;





BerkeleyDB is a synonym for BDB in the ENGINE or
TYPE option.





The BDB storage engine provides transactional tables. The way you use
these tables depends on the autocommit mode:








See section 14.4.1  START TRANSACTION, COMMIT, and ROLLBACK Syntax.





The BDB storage engine has the following characteristics:









15.4.5  Things We Need to Fix for BDB








15.4.6  Restrictions on BDB Tables




The following list indicates restrictions that you must observe when using
BDB tables:









15.4.7  Errors That May Occur When Using BDB Tables








15.5  The ISAM Storage Engine












The original storage engine in MySQL was the ISAM engine. It was the
only storage engine available until MySQL 3.23, when the improved
MyISAM engine was introduced as the default. ISAM now is
deprecated.  As of MySQL 4.1, it's included in the source but not enabled in
binary distributions.  It will disappear in MySQL 5.0.
Embedded MySQL server versions do not support ISAM tables by default.





Due to the deprecated status of ISAM, and because MyISAM is
an improvement over ISAM, you are advised to convert any remaining
ISAM tables to MySAM as soon as possible.  To convert an
ISAM table to a MyISAM table, use an ALTER TABLE
statement:





mysql> ALTER TABLE tbl_name TYPE = MYISAM;





For more information about MyISAM, see
section 15.1  The MyISAM Storage Engine.





Each ISAM table is stored on disk in three files.  The files have
names that begin with the table name and have an extension to indicate the
file type.  An `.frm' file stores the table definition.  The data file
has an `.ISD' extension.  The index file has an `.ISM'
extension.





ISAM uses B-tree indexes.





You can check or repair ISAM tables with the @command{isamchk} utility.
See section 5.7.2.7  Using @command{myisamchk} for Crash Recovery.





ISAM has the following properties:








Many of the properties of MyISAM tables are also true for ISAM
tables. However, there are also many differences. The following list
describes some of the ways that ISAM is distinct from MyISAM:









16  The InnoDB Storage Engine




NOTE: CRITICAL BUG in 4.1.2 if you specify innodb_file_per_table in
`my.cnf' on Unix. In crash recovery InnoDB will skip the crash recovery
for all `.ibd' files and those tables become CORRUPT! The symptom is
a message
Unable to lock ...ibd with lock 1, error: 9: fcntl: Bad file descriptor
in the `.err' log in crash recovery.
















16.1  InnoDB Overview













InnoDB provides MySQL with a transaction-safe (ACID compliant)
storage engine with commit, rollback, and crash recovery capabilities.
InnoDB does locking on the row level and also provides an Oracle-style
consistent
non-locking read in SELECT statements. These features increase
multi-user concurrency and performance. There is no need for
lock escalation in InnoDB
because row-level locks in InnoDB fit in very little space.
InnoDB also supports FOREIGN KEY constraints.
In SQL queries you can freely mix InnoDB type tables with other
table types of MySQL, even within the same query.





InnoDB has been designed for maximum performance when processing
large data volumes. Its CPU efficiency is probably not
matched by any other disk-based relational database engine.





Fully integrated with MySQL Server, the InnoDB storage engine maintains
its own buffer pool for caching data and indexes in main memory.
InnoDB stores its tables and indexes in a tablespace, which
may consist of several files (or raw disk partitions).
This is different from, for example,
MyISAM tables where each table is stored using separate files.
InnoDB tables can be of any size even on operating
systems where file size is limited to 2GB.





InnoDB is included in binary distributions by default as of MySQL 4.0.
For information about InnoDB support in MySQL 3.23, see
section 16.3  InnoDB in MySQL 3.23.





InnoDB is used in production at numerous
large database sites requiring high performance.
The famous Internet news site Slashdot.org runs on InnoDB.
Mytrix, Inc. stores over 1TB of data in InnoDB,
and another site handles an average
load of 800 inserts/updates per second in InnoDB.





InnoDB is published under the same GNU GPL License Version 2
(of June 1991) as MySQL. If you distribute MySQL/InnoDB, and your application
does not satisfy the provisions of the GPL license, you must purchase a
commercial
MySQL Pro license from https://order.mysql.com/?sub=pg&pg_no=1.







16.2  InnoDB Contact Information




Contact information for Innobase Oy, producer of the InnoDB engine:





Web site: http://www.innodb.com/
Email: [email protected]
Phone: +358-9-6969 3250 (office)
       +358-40-5617367 (mobile)

Innobase Oy Inc.
World Trade Center Helsinki
Aleksanterinkatu 17
P.O.Box 800
00101 Helsinki
Finland






16.3  InnoDB in MySQL 3.23




Beginning with MySQL 4.0, InnoDB is enabled by default, so
the following information applies only to MySQL 3.23.





InnoDB tables are included in the MySQL source distribution
starting from 3.23.34a and are activated in the MySQL-Max
binaries of the 3.23 series.
For Windows, the MySQL-Max binaries are included in the
standard distribution.





If you have downloaded a binary version of MySQL that includes support for
InnoDB, simply follow the instructions of the MySQL manual for
installing a binary version of MySQL. If you already have MySQL 3.23
installed, the simplest way to install MySQL-Max is to replace the
executable @command{mysqld} server with the corresponding executable from the
MySQL-Max distribution.  MySQL and MySQL-Max differ only in the server
executable.
See section 2.2.5  Installing MySQL on Other Unix-Like Systems and
@ref{mysqld-max, , @command{mysqld-max}}.





To compile the MySQL source code with InnoDB support,
download MySQL 3.23.34a or newer from
http://www.mysql.com/
and configure MySQL with the
--with-innodb option. 
See section 2.3  MySQL Installation Using a Source Distribution.





To use InnoDB tables with MySQL 3.23, you must specify configuration
parameters in the [mysqld] section of the `my.cnf' option file.
On Windows, you can use `my.ini' instead. If you do not configure
InnoDB in the option file, InnoDB will not start. (From MySQL
4.0 on, InnoDB uses default parameters if you do not specify any.
However, to get best performance, it is still recommended that
you use
parameters appropriate for your system, as discussed in
section 16.4  InnoDB Configuration.)





In MySQL 3.23, you must specify at the minimum an innodb_data_file_path
value to configure the InnoDB data files. For example, to configure
InnoDB to use a single 500MB data file, place the
following setting in the [mysqld] section of your option file:





[mysqld]
innodb_data_file_path=ibdata1:500M





InnoDB will create the `ibdata1' file in the MySQL data directory
by default. To specify the location explicitly, specify an
innodb_data_home_dir setting.
See section 16.4  InnoDB Configuration.







16.4  InnoDB Configuration




To enable InnoDB tables in MySQL 3.23, see
section 16.3  InnoDB in MySQL 3.23.





From MySQL 4.0 on, the InnoDB storage engine is enabled
by default. If you don't want to use InnoDB tables, you can
add the skip-innodb option to your MySQL option file.





Two important disk-based resources managed by the InnoDB storage
engine are its tablespace data files and its log files.





If you specify no InnoDB configuration options, MySQL 4.0 and
above creates an auto-extending 10MB data file named `ibdata1' and
two 5MB log files named `ib_logfile0' and `ib_logfile1' in the MySQL data directory.
(In MySQL 4.0.0 and 4.0.1, the data file is 64MB and not auto-extending.)
In MySQL 3.23, InnoDB will not start if you provide no configuration
options.





Note: To get good performance, you should explicitly provide
InnoDB parameters as discussed in the following examples. Naturally,
you should edit the settings to suit your hardware and requirements.





To set up the InnoDB tablespace files, use the
innodb_data_file_path option in the [mysqld] section of the
`my.cnf' option file.  On Windows, you can use `my.ini' instead.
The value of innodb_data_file_path should be a list of one or more
data file specifications.  If you name more than one data file, separate them
by semicolon (`;') characters:





innodb_data_file_path=datafile_spec1[;datafile_spec2]...





For example, a setting that explicitly creates a tablespace having the
same characteristics as the MySQL 4.0 default is as follows:





[mysqld]
innodb_data_file_path=ibdata1:10M:autoextend





This setting configures a single 10MB data file named `ibdata1' that is
auto-extending.  No location for the file is given, so the default is the
MySQL data directory.





Sizes are specified using M or G suffix letters to indicate
units of MB or GB.





A tablespace containing a fixed-size 50MB data file named `ibdata1' and
a 50MB auto-extending file named ibdata2 in the data directory can be
configured like this:





[mysqld]
innodb_data_file_path=ibdata1:50M;ibdata2:50M:autoextend





The full syntax for a data file specification includes the filename, its size,
and several optional attributes:





file_name:file_size[:autoextend[:max:max_file_size]]





The autoextend attribute and those following can be used only for the
last data file in the innodb_data_file_path line.  autoextend
is available starting from MySQL 3.23.50 and 4.0.2.





If you specify the autoextend option for the last data file,
InnoDB extends the data file if it runs out of free space in the
tablespace. The increment is 8MB at a time.





If the disk becomes full, you might want to add another data file on
another disk. Instructions for reconfiguring an existing tablespace are
given in section 16.8  Adding and Removing InnoDB Data and Log Files.





InnoDB is not aware of the maximum file size, so
be cautious on filesystems where the maximum file size is 2GB.
To specify a maximum size for an auto-extending data file, use the max
attribute. The following configuration allows `ibdata1' to grow up to a
limit of 500MB:





[mysqld]
innodb_data_file_path=ibdata1:10M:autoextend:max:500M





InnoDB creates tablespace files in the MySQL data directory by default.
To specify a location explicitly, use the innodb_data_home_dir option.
For example, to use two files named `ibdata1' and `ibdata2' but
create them in the `/ibdata' directory, configure InnoDB like this:





[mysqld]
innodb_data_home_dir = /ibdata
innodb_data_file_path=ibdata1:50M;ibdata2:50M:autoextend





Note: InnoDB does not create directories, so make sure that
the `/ibdata' directory exists before you start the server.  This is
also true of any log file directories that you configure.  Use the Unix or
DOS mkdir command to create any necessary directories.





InnoDB forms the directory path for each data file by textually
concatenating the value of innodb_data_home_dir to the data file name,
adding a slash or backslash between if needed.  If the
innodb_data_home_dir option is not mentioned in `my.cnf' at all,
the default value is the ``dot'' directory `./', which means the MySQL
data directory.





If you specify innodb_data_home_dir as an empty string, you can
specify absolute paths for the data files listed in the
innodb_data_file_path value. The following example is equivalent to
the preceding one:





[mysqld]
innodb_data_home_dir =
innodb_data_file_path=/ibdata/ibdata1:50M;/ibdata/ibdata2:50M:autoextend





A simple `my.cnf' example.
Suppose that you have a computer
with 128MB RAM and one hard disk. The following example shows
possible configuration parameters in `my.cnf' or
`my.ini' for InnoDB. The example assumes the use of
MySQL-Max 3.23.50 or later or MySQL 4.0.2 or later because it makes use of the
autoextend attribute.





This example suits most users, both on Unix and Windows,
who do not want to distribute InnoDB data files and
log files on several disks. It creates an
auto-extending data file `ibdata1' and two InnoDB log files
`ib_logfile0' and `ib_logfile1' in the
MySQL data directory.  Also, the small archived InnoDB log file
`ib_arch_log_0000000000' that InnoDB creates automatically ends
up in the data directory.





[mysqld]
# You can write your other MySQL server options here
# ...
# Data files must be able to hold your data and indexes.
# Make sure that you have enough free disk space.
innodb_data_file_path = ibdata1:10M:autoextend
#
# Set buffer pool size to 50-80% of your computer's memory
set-variable = innodb_buffer_pool_size=70M
set-variable = innodb_additional_mem_pool_size=10M
#
# Set the log file size to about 25% of the buffer pool size
set-variable = innodb_log_file_size=20M
set-variable = innodb_log_buffer_size=8M
#
innodb_flush_log_at_trx_commit=1





Make sure that the MySQL server has the proper access rights to create files
in the data directory. More generally, the server must have access rights in
any directory where it needs to create data files or log files.





Note that data files must be less than 2GB in some filesystems.
The combined size of the log files must be less than 4GB. The combined
size of data files must be at least 10MB.





When you create an InnoDB tablespace for the first time, it is
best that you start the MySQL server from the command prompt.
InnoDB will then print the information about the database
creation to the screen, so you can see what is happening.
For example, on Windows, if @command{mysqld-max} is located in
`C:\mysql\bin', you can start it like this:





C:\> C:\mysql\bin\mysqld-max --console





If you do not send server output to the screen, check the server's error log
to see what InnoDB prints during the startup process.





See section 16.6  Creating the InnoDB Tablespace for an example of what the
information displayed by InnoDB should look like.





Where to specify options on Windows?
The rules for option files on Windows are as follows:








Where to specify options on Unix?
On Unix, @command{mysqld} reads options from the following files, if they exist,
in the following order: 









DATADIR represents the MySQL data directory that was
specified as a @command{configure} option when @command{mysqld}
was compiled
(typically `/usr/local/mysql/data' for a binary installation or `/usr/local/var' for a source installation).





If you want to make sure that @command{mysqld} reads options only from a
specific file, you can use the --defaults-option as the first option
on the command line when starting the server:





mysqld --defaults-file=your_path_to_my_cnf





An advanced `my.cnf' example.
Suppose that you have a Linux computer
with 2GB RAM and three 60GB hard disks
(at directory paths `/', `/dr2' and
`/dr3'). The following example shows possible
configuration parameters in `my.cnf' for InnoDB. 





[mysqld]
# You can write your other MySQL server options here
# ...
innodb_data_home_dir =
#
# Data files must be able to hold your data and indexes
innodb_data_file_path = /ibdata/ibdata1:2000M;/dr2/ibdata/ibdata2:2000M:autoextend
#
# Set buffer pool size to 50-80% of your computer's memory,
# but make sure on Linux x86 total memory usage is < 2GB
set-variable = innodb_buffer_pool_size=1G
set-variable = innodb_additional_mem_pool_size=20M
innodb_log_group_home_dir = /dr3/iblogs
#
# innodb_log_arch_dir must be the same as innodb_log_group_home_dir
# (starting from 4.0.6, you can omit it)
innodb_log_arch_dir = /dr3/iblogs
set-variable = innodb_log_files_in_group=2
#
# Set the log file size to about 25% of the buffer pool size
set-variable = innodb_log_file_size=250M
set-variable = innodb_log_buffer_size=8M
#
innodb_flush_log_at_trx_commit=1
set-variable = innodb_lock_wait_timeout=50
#
# Uncomment the next lines if you want to use them
#innodb_flush_method=fdatasync
#set-variable = innodb_thread_concurrency=5





Note that the example places the two data files on different disks.
InnoDB will fill the tablespace beginning with the first data file.
In some cases, it will
improve the performance of the database if all data is not placed
on the same physical disk. Putting log files on a different disk from
data is very often beneficial for performance.
You can also use raw disk partitions (raw devices)
as InnoDB data files, which may speed up I/O. See section 16.15.2  Using Raw Devices for the Tablespace.





Warning: On GNU/Linux x86, you must be careful not to set memory
usage too high.  glibc will allow the process heap to grow over
thread stacks, which will crash your server. It is a risk if the value of
the following expression is close to or exceeds 2GB:





innodb_buffer_pool_size
+ key_buffer_size
+ max_connections*(sort_buffer_size+read_buffer_size+binlog_cache_size)
+ max_connections*2MB





Each thread will use a stack (often 2MB, but only 256KB in MySQL AB binaries)
and in the worst case also uses sort_buffer_size + read_buffer_size
additional memory.





Starting from MySQL 4.1, you can use up to 64GB of physical memory in 32-bit
Windows.  See the description for innodb_buffer_pool_awe_mem_mb in
section 16.5  InnoDB Startup Options.





How to tune other @command{mysqld server parameters?}
The following values are typical and suit most users:





[mysqld]
skip-external-locking
set-variable = max_connections=200
set-variable = read_buffer_size=1M
set-variable = sort_buffer_size=1M
#
# Set key_buffer to 5 - 50% of your RAM depending on how much
# you use MyISAM tables, but keep key_buffer_size + InnoDB
# buffer pool size < 80% of your RAM
set-variable = key_buffer_size=...






16.5  InnoDB Startup Options




This section describes the InnoDB-related server options.  In MySQL
4.0 and up, all of them can be specified in --opt_name=value form on
the command line or in option files.  Before MySQL 4.0, numeric options
should be specified using --set-variable=opt_name=value or -O
opt_name=value syntax.








innodb_additional_mem_pool_size


The size of a memory pool InnoDB uses to store data dictionary
information and other internal data structures.  The more tables you have in
your application, the more memory you will need to allocate here. If
InnoDB runs out of memory in this pool, it will start to allocate
memory from the operating system, and write warning messages to the MySQL
error log. The default value is 1MB.


innodb_autoextend_increment


The increment size (in megabytes) for extending the size of an autoextending
tablespace when it becomes full.  The default value is 8.
This option is available starting from MySQL 4.1.5.


innodb_buffer_pool_awe_mem_mb


The size of the buffer pool (in MB), if it is placed in the AWE memory of
32-bit Windows. Available from MySQL 4.1.0 and relevant only in 32-bit Windows. If
your 32-bit Windows operating system supports more than 4GB memory, so-called ``Address
Windowing Extensions,'' you can allocate the InnoDB buffer pool into the
AWE physical memory using this parameter. The maximum possible value for this
is 64000. If this parameter is specified, innodb_buffer_pool_size
is the window in the 32-bit address space of @command{mysqld} where InnoDB
maps that AWE memory. A good value for innodb_buffer_pool_size
is 500MB.


innodb_buffer_pool_size


The size of the memory buffer InnoDB uses to cache data and indexes
of its tables.  The larger you set this value, the less disk I/O is needed
to access data in tables. On a dedicated database server, you may set this
to up to 80% of the machine physical memory size. However, do not set it
too large because competition for the physical memory might cause paging
in the operating system.


innodb_data_file_path


The paths to individual data files and their sizes. The full directory path
to each data file is acquired by concatenating innodb_data_home_dir
to each path specified here. The file sizes are specified in megabytes
or gigabytes (1024MB) by appending M or G to the size
value.  The sum of the sizes of the files must be at least 10MB. On some
operating systems, files must be less than 2GB. If you do not specify
innodb_data_file_path, the default behavior starting from 4.0 is to
create a single 10MB auto-extending data file named `ibdata1'. Starting
from 3.23.44, you can set the file size bigger than 4GB on those operating
systems that support big files. You can also use raw disk partitions as
data files. See section 16.15.2  Using Raw Devices for the Tablespace.


innodb_data_home_dir


The common part of the directory path for all InnoDB data files. If
you do not set this value, the default is the MySQL data directory. You can
specify this also as an empty string, in which case you can use absolute
file paths in innodb_data_file_path.


innodb_fast_shutdown


By default, InnoDB does a full purge and an insert buffer merge
before a shutdown. These operations can take minutes, or even hours in extreme
cases.  If you set this parameter to 1, InnoDB
skips these operations at shutdown. This option is available starting from
MySQL 3.23.44 and 4.0.1. Its default value is 1 starting from 3.23.50.


innodb_file_io_threads


The number of file I/O threads in InnoDB. Normally this should be
left at the default value of 4, but disk I/O on Windows may benefit from
a larger number. On Unix, increasing the number has no effect; InnoDB
always uses the default value.
This option is available as of MySQL 3.23.37.


innodb_file_per_table


NOTE: CRITICAL BUG in 4.1.2 if you specify innodb_file_per_table in
`my.cnf' on Unix. In crash recovery InnoDB will skip the crash recovery
for all `.ibd' files and those tables become CORRUPT! The symptom is
a message
Unable to lock ...ibd with lock 1, error: 9: fcntl: Bad file descriptor
in the `.err' log in crash recovery.
This option causes InnoDB to create each new table
using its own `.ibd' file for storing data and indexes, rather than in
the shared tablespace. See section 16.7.6  Using Per-Table Tablespaces.
This option is available as of MySQL 4.1.1.


innodb_locks_unsafe_for_binlog


Normally InnoDB uses an algorithm called ``next-key locking.'' 
InnoDB does the row-level locking in such a way that when it 
searches or scans an index of a table, it sets shared or exclusive 
locks on the index records it encounters. Thus the row-level locks 
are actually index record locks. The locks InnoDB sets on index 
records also affect the ``gap'' before that index record. 
If a user has a shared or exclusive lock on record R in an index, 
another user cannot insert a new index record immediately before 
R in the index order. This option causes InnoDB not to 
use next-key locking in searches or index scans. Next-key locking is 
still used to ensure foreign key constraints and duplicate key checking.
Note that using this option may cause phantom problems: Suppose that you 
want to read and lock all children from the child table with an 
identifier value larger than 100, with the intent of updating 
some column in the selected rows later:



SELECT * FROM child WHERE id > 100 FOR UPDATE;



Suppose that there is an index on the id column. The query will scan 
that index starting from the first record where id is bigger than 100. 
Now, if the locks set on the index records do not lock out inserts 
made in the gaps, a new row will meanwhile be inserted to the table. 
If you now execute the same SELECT within the same transaction, 
you will see a new row in the result set returned by the query. 
This option is available as of MySQL 4.1.4. 


innodb_flush_log_at_trx_commit


Normally you set this to 1, meaning that at a transaction commit, the log
is flushed to disk, and the modifications made by the transaction become
permanent and survive a database crash. If you are willing to compromise
this safety, and you are running small transactions, you may set this to
0 or 2 to reduce disk I/O to the logs. A value of 0 means that the log is
only written to the log file and the log file flushed to disk approximately
once per second. A value of 2 means the log is written to the log file at
each commit, but the log file is only flushed to disk approximately once
per second.  The default value is 1 (prior to MySQL 4.0.13, the default is 0).


innodb_flush_method


This option is relevant only on Unix systems. If set to fdatasync,
InnoDB uses fsync() to flush both the data and log
files. If set to O_DSYNC, InnoDB uses O_SYNC
to open and flush the log files, but uses fsync() to flush the
data files. If O_DIRECT is specified (available on some GNU/Linux
versions  starting from MySQL 4.0.14), InnoDB uses O_DIRECT
to open the data files, and uses fsync() to flush both the data
and log files. Note that InnoDB does not use fdatasync or
O_DSYNC by default because there have been problems with them on
many Unix flavors.  This option is available as of MySQL 3.23.40.


innodb_force_recovery


Warning: This option should be defined only in an emergency situation when
you want to dump your tables from a corrupt database! Possible values are
from 1 to 6. The meanings of these values are described in section 16.9.1  Forcing Recovery. As a safety measure, InnoDB prevents a user from modifying
data when this option is greater than 0. This option is available starting
from MySQL 3.23.44.


innodb_lock_wait_timeout


The timeout in seconds an InnoDB transaction may wait for a lock
before being rolled back.  InnoDB automatically detects transaction
deadlocks in its own lock table and rolls back the transaction. Beginning
with MySQL 4.0.20 and 4.1.2, InnoDB notices locks set using the LOCK
TABLES statement. Before that, if you
use the LOCK TABLES statement, or other transaction-safe storage
engines than InnoDB in the same transaction, a deadlock may arise
that InnoDB cannot notice. In cases like this, the timeout is useful
to resolve the situation. The default is 50 seconds.


innodb_log_arch_dir


The directory where fully written log files would be archived if we used log
archiving.  The value of this parameter should currently be set the same as
innodb_log_group_home_dir. Starting from MySQL 4.0.6, you may omit
this option.


innodb_log_archive


This value should currently be set to 0.  Because recovery from a backup
is done by MySQL using its own log files, there is currently no need to
archive InnoDB log files. The default for this option is 0.


innodb_log_buffer_size


The size of the buffer that InnoDB uses to write to the log files
on disk.  Sensible values range from 1MB to 8MB.  The default is 1MB. A large
log buffer allows large transactions to run without a need to write the log
to disk before the transactions commit. Thus, if you have big transactions,
making the log buffer larger will save disk I/O.


innodb_log_file_size


The size of each log file in a log group. The combined size of
log files must be less than 4GB on 32-bit computers. The default is 5MB.
Sensible values range from 1MB to 1/N-th of the size of the buffer pool,
below, where N is the number of log files in the group. The larger
the value, the less checkpoint flush activity is needed in the buffer
pool, saving disk I/O. But larger log files also mean that recovery will
be slower in case of a crash.


innodb_log_files_in_group


The number of log files in the log group. InnoDB writes to the
files in a circular fashion. The default is 2 (recommended).


innodb_log_group_home_dir


The directory path to the InnoDB log files.  It must have the same
value as innodb_log_arch_dir.  If you do not specify any InnoDB
log parameters, the default is to create two 5MB files names `ib_logfile0'
and `ib_logfile1' in the MySQL data directory.


innodb_max_dirty_pages_pct


This is an integer in the range from 0 to 100. The default is 90. The main
thread in InnoDB tries to flush pages from the buffer pool so that
at most this many percent of pages may not yet flushed been flushed at
any particular time. Available starting from 4.0.13 and 4.1.1. If you have the
SUPER privilege, this percentage can be changed while the server is
running:



SET GLOBAL innodb_max_dirty_pages_pct = value;




innodb_mirrored_log_groups


The number of identical copies of log groups we keep for the
database. Currently this should be set to 1.


innodb_open_files


This option is relevant only if you use multiple tablespaces in
InnoDB. It specifies the maximum number of `.ibd' files that
InnoDB can keep open at one time. The minimum value is 10. The
default is 300.  This option is available as of MySQL 4.1.1.

The file descriptors used for `.ibd' files are for InnoDB only.
They are independent of those specified by the --open-files-limit
server option, and do not affect the operation of the table cache.


innodb_thread_concurrency


InnoDB tries to keep the number of operating system threads
concurrently inside InnoDB less than or equal to the limit given
by this parameter. The default value is 8. If you have low performance
and SHOW INNODB STATUS reveals many threads waiting for semaphores,
you may have thread thrashing and should try setting this parameter lower
or higher.  If you have a computer with many processors and disks, you
can try setting the value higher to better utilize the resources of you
computer. A recommended value is the sum of the number of processors and
disks your system has.  A value of 500 or greater disables the concurrency
checking.  This option is available starting from MySQL 3.23.44 and 4.0.1.


innodb_status_file


This option causes InnoDB to create a file
`<datadir>/innodb_status.<pid>' for periodical
SHOW INNODB STATUS output.  This option is available as of MySQL 4.0.21.







16.6  Creating the InnoDB Tablespace




Suppose that you have installed MySQL and have edited your option file so
that it contains the necessary InnoDB configuration parameters.
Before starting MySQL, you should verify that the directories you have
specified for InnoDB data files and log files exist and that the
MySQL server has access rights to those directories. InnoDB cannot
create directories, only files. Check also that you have enough disk space
for the data and log files.





It is best to run the MySQL server @command{mysqld} from the command prompt
when you create an InnoDB database, not from the @command{mysqld_safe}
wrapper or as a Windows service. When you run from a command prompt you
see what @command{mysqld} prints and what is happening. On Unix, just invoke
@command{mysqld}. On Windows, use the --console option.





When you start the MySQL server after initially configuring InnoDB in
your option file, InnoDB creates your
data files and log files. InnoDB will print something like the following:





InnoDB: The first specified datafile /home/heikki/data/ibdata1
did not exist:
InnoDB: a new database to be created!
InnoDB: Setting file /home/heikki/data/ibdata1 size to 134217728
InnoDB: Database physically writes the file full: wait...
InnoDB: datafile /home/heikki/data/ibdata2 did not exist:
new to be created
InnoDB: Setting file /home/heikki/data/ibdata2 size to 262144000
InnoDB: Database physically writes the file full: wait...
InnoDB: Log file /home/heikki/data/logs/ib_logfile0 did not exist:
new to be created
InnoDB: Setting log file /home/heikki/data/logs/ib_logfile0 size
to 5242880
InnoDB: Log file /home/heikki/data/logs/ib_logfile1 did not exist:
new to be created
InnoDB: Setting log file /home/heikki/data/logs/ib_logfile1 size
to 5242880
InnoDB: Doublewrite buffer not found: creating new
InnoDB: Doublewrite buffer created
InnoDB: Creating foreign key constraint system tables
InnoDB: Foreign key constraint system tables created
InnoDB: Started
mysqld: ready for connections





A new InnoDB database has now been created. You can connect to the MySQL
server with the usual MySQL client programs like @command{mysql}.
When you shut down the MySQL server with @command{mysqladmin shutdown},
the output will be like the following:





010321 18:33:34  mysqld: Normal shutdown
010321 18:33:34  mysqld: Shutdown Complete
InnoDB: Starting shutdown...
InnoDB: Shutdown completed





You can now look at the data file and log directories and you
will see the files created. The log directory will also contain
a small file named `ib_arch_log_0000000000'. That file
resulted from the database creation, after which InnoDB switched off
log archiving.
When MySQL is started again, the data files and log files will already have
been created, so the output will be much briefer:





InnoDB: Started
mysqld: ready for connections






16.6.1  Dealing with InnoDB Initialization Problems




If InnoDB prints an operating system error in a file operation,
usually the problem is one of the following:








If something goes wrong when InnoDB attempts to initialize
its tablespace or its log files, you should delete all files created
by InnoDB. This means all data files, all log files, and the small
archived log file. In case you already created some InnoDB tables,
delete the corresponding `.frm' files for these tables (and any
`.ibd' files if you are using multiple tablespaces) from the
MySQL database directories as well. Then you can try the InnoDB
database creation again.  It is best to start the MySQL server from a
command prompt so that you see what is happening.







16.7  Creating InnoDB Tables




Suppose that you have started the MySQL client with the command mysql test.
To create an InnoDB table, you must specify and ENGINE = InnoDB
or TYPE = InnoDB option in the table creation SQL statement:





CREATE TABLE customers (a INT, b CHAR (20), INDEX (a)) ENGINE=InnoDB;
CREATE TABLE customers (a INT, b CHAR (20), INDEX (a)) TYPE=InnoDB;





The SQL statement creates a table and an index on column a in the
InnoDB tablespace that consists of the data files you specified in
`my.cnf'. In addition, MySQL creates a file `customers.frm'
in the `test' directory under the MySQL database directory.
Internally, InnoDB adds to its own data dictionary an entry for
table 'test/customers'. This means you can create a table of the same
name customers in some other database, and the table names will
not collide inside InnoDB.





You can query the amount of free space in the InnoDB tablespace
by issuing a SHOW TABLE STATUS statement for any InnoDB table.
The amount of free
space in the tablespace appears in the Comment section in the
output of SHOW TABLE STATUS. An example:





SHOW TABLE STATUS FROM test LIKE 'customers'





Note that the statistics SHOW gives about InnoDB tables
are only approximate. They are used in SQL optimization.  Table and index
reserved sizes in bytes are accurate, though.







16.7.1  How to Use Transactions in InnoDB with Different APIs




By default, each client that connects to the MySQL server begins with autocommit
mode enabled, which automatically commits every SQL statement you run.
To use multiple-statement transactions, you can switch autocommit off
with the SQL statement SET AUTOCOMMIT = 0 and use COMMIT
and ROLLBACK to commit or roll back your transaction.
If you want to leave autocommit on, you can enclose your transactions
between START TRANSACTION and COMMIT or ROLLBACK.
Before MySQL 4.0.11, you have to use the keyword BEGIN
instead of START TRANSACTION. The following example shows two
transactions. The first is committed and the second is rolled back.





shell> mysql test
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 5 to server version: 3.23.50-log
Type 'help;' or '\h' for help. Type '\c' to clear the buffer.
mysql> CREATE TABLE CUSTOMER (A INT, B CHAR (20), INDEX (A))
    -> TYPE=InnoDB;
Query OK, 0 rows affected (0.00 sec)
mysql> BEGIN;
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO CUSTOMER VALUES (10, 'Heikki');
Query OK, 1 row affected (0.00 sec)
mysql> COMMIT;
Query OK, 0 rows affected (0.00 sec)
mysql> SET AUTOCOMMIT=0;
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO CUSTOMER VALUES (15, 'John');
Query OK, 1 row affected (0.00 sec)
mysql> ROLLBACK;
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT * FROM CUSTOMER;
+------+--------+
| A    | B      |
+------+--------+
|   10 | Heikki |
+------+--------+
1 row in set (0.00 sec)
mysql>





In APIs like PHP, Perl DBI/DBD, JDBC, ODBC, or the standard C call
interface of MySQL, you can send transaction control statements such as
COMMIT to the MySQL server as strings just like any other
SQL statements such as SELECT or INSERT.
Some APIs also offer separate special transaction commit and rollback functions
or methods.







16.7.2  Converting MyISAM Tables to InnoDB




Important: You should not convert MySQL system tables in the mysql
database (such as user or host) to the InnoDB type. The
system tables must always be of the MyISAM type.





If you want all your (non-system) tables to be created as InnoDB
tables, you can, starting from the MySQL 3.23.43, add the line
default-table-type=innodb to the [mysqld] section of
your `my.cnf' or `my.ini' file.





InnoDB does not have a special optimization for separate index
creation the way the MyISAM storage engine does.  Therefore, it does
not pay to export and import the table and create indexes afterward.
The fastest way to alter a table to InnoDB is to do the inserts
directly to an InnoDB table. That is, use ALTER TABLE
... TYPE=INNODB, or create an empty InnoDB table with identical
definitions and insert the rows with INSERT INTO ... SELECT *
FROM ....





If you have UNIQUE constraints on secondary keys, starting from
MySQL 3.23.52, you can speed up a table import by turning off the uniqueness
checks temporarily during the import session: SET UNIQUE_CHECKS=0;
For big tables, this saves a lot of disk I/O because InnoDB can then
use its insert buffer to write secondary index records in a batch.





To get better control over the insertion process, it might be good to insert
big tables in pieces:





INSERT INTO newtable SELECT * FROM oldtable
   WHERE yourkey > something AND yourkey <= somethingelse;





After all records have been inserted, you can rename the tables.





During the conversion of big tables, you should increase the size of the
InnoDB buffer pool to reduce disk I/O. Do not use more than 80% of the
physical memory, though. You can also increase the sizes of the InnoDB
log files and the log files.





Make sure that you do not fill up the tablespace: InnoDB tables require
a lot more disk space than MyISAM tables. If an ALTER TABLE
runs out of space, it will start a rollback, and that can take hours if
it is disk-bound.  For inserts, InnoDB uses the insert buffer to
merge secondary index records to indexes in batches. That saves a lot of
disk I/O. In rollback, no such mechanism is used, and the rollback can take
30 times longer than the insertion.





In the case of a runaway rollback, if you do not have valuable data in your
database, it may be advisable to kill the database process rather than wait
for millions of disk I/O operations to complete.
For the complete procedure, see section 16.9.1  Forcing Recovery.







16.7.3  How an AUTO_INCREMENT Column Works in InnoDB




If you specify an AUTO_INCREMENT column for a table, the InnoDB
table handle in the data dictionary will contain a special counter called
the auto-increment counter that is used in assigning new values for the
column. The auto-increment counter is stored only in main memory, not
on disk.





InnoDB uses the following algorithm to initialize the auto-increment
counter for a table T that contains an AUTO_INCREMENT column
named ai_col: After a server startup, when a user first does an insert
to a table T, InnoDB executes the equivalent of this statement:





SELECT MAX(ai_col) FROM T FOR UPDATE;





The value retrieved by the statement is incremented by one and assigned to
the column and the auto-increment counter of the table. If the table is
empty, the value 1 is assigned. If the auto-increment counter is not
initialized and the user invokes a SHOW TABLE STATUS statement that
displays output for the table T, the counter is initialized (but not
incremented) and stored for use by later inserts.
Note that in this initialization
we do a normal exclusive-locking read on the table and the lock lasts to the end
of the transaction.





InnoDB follows the same procedure for initializing the auto-increment
counter for a freshly created table.





Note that if the user specifies NULL or 0 for the
AUTO_INCREMENT column in an INSERT, InnoDB treats the
row as if the value had not been specified and generates a new value for it.





After the auto-increment counter has been initialized, if a user inserts
a row that explicitly specifies the column value, and the value is
bigger than the current counter value, the counter is set to the specified
column value. If the user does not explicitly specify a value, InnoDB
increments the counter by one and assigns the new value to the column.





When accessing the auto-increment counter, InnoDB uses a special table
level AUTO-INC lock that it keeps to the end of the current SQL
statement, not to the end of the transaction.
The special lock release strategy was introduced to improve concurrency
for inserts into a table containing an AUTO_INCREMENT column. Two
transactions cannot have the AUTO-INC lock on the same table
simultaneously.





Note that you may see gaps in the sequence of values assigned to the
AUTO_INCREMENT column if you roll back transactions that have gotten
numbers from the counter.





The behavior of the auto-increment mechanism is not defined if a user
assigns a negative value to the column or if the value becomes bigger
than the maximum integer that can be stored in the specified integer type.







16.7.4  FOREIGN KEY Constraints




Starting from MySQL 3.23.44, InnoDB features foreign key constraints.





The syntax of a foreign key constraint definition in InnoDB looks like
this:





[CONSTRAINT symbol] FOREIGN KEY [id] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name, ...)
    [ON DELETE {RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT}]
    [ON UPDATE {RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT}]





Both tables must be InnoDB type.  In the referencing table, there must
be an index where the foreign key columns are listed as the first
columns in the same order. In the referenced table, there must be an index
where the referenced columns are listed as the first columns in
the same order. Index prefixes on foreign key columns are not supported.





InnoDB needs indexes on foreign keys and referenced keys so that
foreign key checks can be fast and not require a table scan.  Starting with
MySQL 4.1.2, these indexes are created automatically.  In older versions,
the indexes must be created explicitly or the creation of foreign key
constraints will fail.





Corresponding columns in the foreign key and the referenced key must
have similar internal data types inside InnoDB so that they can be
compared without a type conversion.
The size and the signedness of integer types has to be the same.
The length of string types need not be the same.
If you specify a SET NULL action, make sure that you
have not declared the columns in the child table as
NOT NULL.





If MySQL reports an error number 1005 from a CREATE TABLE statement,
and the error message string refers to errno 150, this means that the table
creation failed because a foreign key constraint was not correctly formed.
Similarly, if an ALTER TABLE fails and it refers to errno 150,
that means a foreign key definition would be incorrectly formed for the
altered table.  Starting from MySQL 4.0.13, you can use SHOW INNODB
STATUS to display a detailed explanation of the latest InnoDB
foreign key error in the server.





Starting from MySQL 3.23.50, InnoDB does not check foreign key
constraints on those foreign key or referenced key values
that contain a NULL column.





A deviation from SQL standards: If in the parent table
there are several rows that have the same referenced key value,
then InnoDB acts in foreign key checks as if the other parent
rows with the same key value do not exist. For example,
if you have defined a RESTRICT type constraint, and there
is a child row with several parent rows, InnoDB does not allow
the deletion of any of those parent rows. 





Starting from MySQL 3.23.50, you can also associate the
ON DELETE CASCADE or ON DELETE SET NULL clause with
the foreign key constraint. Corresponding ON UPDATE options
are available starting from 4.0.8. If ON DELETE CASCADE is
specified, and a row in the parent table is deleted, InnoDB
automatically deletes also all those rows in the child table
whose foreign key values are equal to the referenced key value in
the parent row. If ON DELETE SET NULL is specified, the
child rows are automatically updated so that the columns in the
foreign key are set to the SQL NULL value.
SET DEFAULT is parsed but ignored.





InnoDB performs cascading operations through a depth-first algorithm,
based on records in the indexes corresponding to the foreign key
constraints.





A deviation from SQL standards: If
ON UPDATE CASCADE or ON UPDATE SET NULL recurses to
update the same table it has already updated during the cascade,
it acts like RESTRICT. This means that you cannot use
self-referential ON UPDATE CASCADE or
ON UPDATE SET NULL operations.
This is to prevent infinite loops resulting from cascaded updates.
A self-referential ON DELETE SET NULL, on the other hand,
is possible from 4.0.13.
A self-referential ON DELETE CASCADE has been possible since ON
DELETE was implemented.  Since 4.0.21, cascading operations may not be nested
more than 15 levels.





A deviation from SQL standards:
Like MySQL in general, in an SQL statement that inserts, deletes, or
updates many rows, InnoDB checks UNIQUE and FOREIGN KEY
constraints row-by-row. According to the SQL standard, the default behavior
should be that constraints are only checked after the WHOLE SQL statement
has been processed.





A simple example that relates parent and child tables through a
single-column foreign key:





CREATE TABLE parent(id INT NOT NULL,
                    PRIMARY KEY (id)
) TYPE=INNODB;
CREATE TABLE child(id INT, parent_id INT,
                   INDEX par_ind (parent_id),
                   FOREIGN KEY (parent_id) REFERENCES parent(id)
                     ON DELETE CASCADE
) TYPE=INNODB;





A more complex example in which a product_order table has foreign keys
for two other tables. One foreign key references a two-column index in the
product table. The other references a single-column index in the
customer table:





CREATE TABLE product (category INT NOT NULL, id INT NOT NULL,
                      price DECIMAL,
                      PRIMARY KEY(category, id)) TYPE=INNODB;
CREATE TABLE customer (id INT NOT NULL,
                      PRIMARY KEY (id)) TYPE=INNODB;
CREATE TABLE product_order (no INT NOT NULL AUTO_INCREMENT,
                      product_category INT NOT NULL,
                      product_id INT NOT NULL,
                      customer_id INT NOT NULL,
                      PRIMARY KEY(no),
                      INDEX (product_category, product_id),
                      FOREIGN KEY (product_category, product_id)
                        REFERENCES product(category, id)
                        ON UPDATE CASCADE ON DELETE RESTRICT,
                      INDEX (customer_id),
                      FOREIGN KEY (customer_id)
                        REFERENCES customer(id)) TYPE=INNODB;





Starting from MySQL 3.23.50, InnoDB allows you to add a new
foreign key constraint to a table by using ALTER TABLE:





ALTER TABLE yourtablename
    ADD [CONSTRAINT symbol] FOREIGN KEY [id] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name, ...)
    [ON DELETE {RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT}]
    [ON UPDATE {RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT}]





Remember to create the required indexes first.
You can also add a self-referential foreign key constraint to a
table using ALTER TABLE.








Starting from MySQL 4.0.13, InnoDB supports the use of ALTER
TABLE to drop foreign keys:





ALTER TABLE yourtablename DROP FOREIGN KEY fk_symbol;





If the FOREIGN KEY clause included a CONSTRAINT name when you
created the foreign key, you can refer to that name to drop the foreign key.
(A constraint name can be given as of MySQL 4.0.18.)  Otherwise, the
fk_symbol value is internally generated by InnoDB when the
foreign key is created. To find out the symbol when you want to drop a foreign
key, use the SHOW CREATE TABLE statement.
An example:





mysql> SHOW CREATE TABLE ibtest11c\G
*************************** 1. row ***************************
       Table: ibtest11c
Create Table: CREATE TABLE `ibtest11c` (
  `A` int(11) NOT NULL auto_increment,
  `D` int(11) NOT NULL default '0',
  `B` varchar(200) NOT NULL default '',
  `C` varchar(175) default NULL,
  PRIMARY KEY  (`A`,`D`,`B`),
  KEY `B` (`B`,`C`),
  KEY `C` (`C`),
  CONSTRAINT `0_38775` FOREIGN KEY (`A`, `D`)
REFERENCES `ibtest11a` (`A`, `D`)
ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `0_38776` FOREIGN KEY (`B`, `C`)
REFERENCES `ibtest11a` (`B`, `C`)
ON DELETE CASCADE ON UPDATE CASCADE
) TYPE=InnoDB CHARSET=latin1
1 row in set (0.01 sec)

mysql> ALTER TABLE ibtest11c DROP FOREIGN KEY 0_38775;





Starting from MySQL 3.23.50, the InnoDB parser allows you to use
backticks around table and column names in a
FOREIGN KEY ... REFERENCES ... clause.
Starting from MySQL 4.0.5, the InnoDB parser also takes into account
the lower_case_table_names system variable setting.





Before MySQL 3.23.50, ALTER TABLE or CREATE INDEX
should not be used in connection with tables that have foreign
key constraints or that are referenced in foreign key constraints:
Any ALTER TABLE removes all foreign key
constraints defined for the table. You should not use
ALTER TABLE with the referenced table, either. Instead,
use DROP TABLE and CREATE TABLE to modify the
schema. When MySQL does an ALTER TABLE it may internally
use RENAME TABLE, and that will confuse the
foreign key constraints that refer to the table.
In MySQL, a CREATE INDEX statement is processed as an
ALTER TABLE, so the same considerations apply.





Starting from MySQL 3.23.50, InnoDB returns the foreign key
definitions of a table as part of the output of the SHOW CREATE TABLE
statement:





SHOW CREATE TABLE tbl_name;





From this version, @command{mysqldump} also produces correct definitions
of tables to the dump file, and does not forget about the foreign keys.





You can display the foreign key constraints for a table like this:


SHOW TABLE STATUS FROM db_name LIKE 'tbl_name'





The foreign key constraints are listed in the Comment column of
the output.





When performing foreign key checks, InnoDB sets shared row
level locks on child or parent records it has to look at.
InnoDB checks foreign key constraints immediately; the check
is not deferred to transaction commit.





To make it easier to reload dump files for tables that have foreign key
relationships, @command{mysqldump} automatically includes a statement in the
dump output to set FOREIGN_KEY_CHECKS to 0 as of MySQL 4.1.1. This
avoids problems with tables having to be reloaded in a particular order
when the dump is reloaded.  For earlier versions, you can disable the
variable manually within @command{mysql} when loading the dump file like this:





mysql> SET FOREIGN_KEY_CHECKS = 0;
mysql> SOURCE dump_file_name
mysql> SET FOREIGN_KEY_CHECKS = 1;





This allows you to import the tables in any order if the dump file contains
tables that are not correctly ordered for foreign keys. It also speeds
up the import operation.  FOREIGN_KEY_CHECKS is available starting
from MySQL 3.23.52 and 4.0.3.





Setting FOREIGN_KEY_CHECKS to 0 can also be useful for ignoring
foreign key constraints during LOAD DATA operations.





InnoDB allows you to drop any table, even though that
would break the foreign key constraints that reference
the table. When you drop a table, the constraints that
were defined in its create statement are also dropped.





If you re-create a table that was dropped, it must have
a definition that conforms to the foreign key constraints
referencing it. It must have the right column names and types,
and it must have indexes on the referenced keys, as stated earlier.
If these are not satisfied, MySQL returns error number 1005
and refers to errno 150 in the error message string.







16.7.5  InnoDB and MySQL Replication




MySQL replication works for InnoDB tables as it does
for MyISAM tables. It is also possible to use replication in
a way where the table type on the slave is not the same as the original
table type on the master.  For example, you can replicate modifications to
an InnoDB table on the master to a MyISAM table on the slave.





To set up a new slave for a master, you have to make a copy of the
InnoDB tablespace and the log files, as well as the `.frm' files
of the InnoDB tables, and move the copies to the slave.
For the proper procedure to do this, see section 16.10  Moving an InnoDB Database to Another Machine.





If you can shut down the master or an existing slave, you can take a
cold backup of the InnoDB tablespace and log files and use that to
set up a slave.
To make a new slave without taking down any server you can also use
the non-free (commercial) InnoDB Hot Backup tool.





There are minor limitations in InnoDB replication:








Most of these limitations can be eliminated by using more recent server
versions for which the limitations do not apply.





Transactions that fail on the master do not affect replication at all.
MySQL replication is based on the binary log where MySQL writes SQL
statements that modify data. A slave reads the binary log of the master and
executes the same SQL statements. However, statements that occur within a
transaction are not written to the binary log until the transaction commits,
at which point all statements in the transaction are written at once.
If a statement fails, for example, because of a foreign key violation,
or if a transaction is rolled back, no SQL statements are written to the
binary log, and the transaction is not executed on the slave at all.







16.7.6  Using Per-Table Tablespaces




NOTE: CRITICAL BUG in 4.1.2 if you specify innodb_file_per_table in
`my.cnf' on Unix. In crash recovery InnoDB will skip the crash recovery
for all `.ibd' files and those tables become CORRUPT! The symptom is
a message
Unable to lock ...ibd with lock 1, error: 9: fcntl: Bad file descriptor
in the `.err' log in crash recovery.





Starting from MySQL 4.1.1, you can store each InnoDB table and its
indexes into its own file. This feature is called ``multiple tablespaces''
because in effect each table has its own tablespace.





Important note: If you upgrade to MySQL 4.1.1 or higher, it is
difficult to downgrade back to 4.0 or 4.1.0! That is because, for earlier
versions, InnoDB is not aware of multiple tablespaces.





If you need to downgrade to 4.0, you have to take table dumps and
re-create the whole InnoDB tablespace. If you have not created new
InnoDB tables under MySQL 4.1.1 or later, and need to downgrade
quickly, you can also do a direct downgrade to the MySQL
4.0.18 or later in the 4.0 series.
Before doing the direct downgrade to 4.0.x, you have to end all client
connections to the @command{mysqld} server that is to be downgraded, and let it
run the purge and insert buffer merge operations to completion, so that
SHOW INNODB STATUS shows the main thread in the state
waiting for server activity. Then you can shut down
@command{mysqld} and start 4.0.18 or later in the 4.0 series.
A direct downgrade is not recommended, however, because it has not
been extensively tested.





You can enable multiple tablespaces by adding a line to the
[mysqld] section of `my.cnf':





[mysqld]
innodb_file_per_table





After restarting the server, InnoDB will store each newly created
table into its own file `tbl_name.ibd' in the database directory
where the table belongs.  This is similar to what the MyISAM
storage engine does, but MyISAM divides the table into a data
file `tbl_name.MYD' and the index file `tbl_name.MYI'.
For InnoDB, the data and the indexes are stored together in the
`.ibd' file.  The `tbl_name.frm' file is still created as usual.





If you remove the innodb_file_per_table line from `my.cnf' and
restart the server, InnoDB creates tables inside the shared tablespace
files again.





innodb_file_per_table affects only table creation. If you start the
server with this option, new tables are created using `.ibd' files, but you can still access tables that exist in the shared
tablespace. If you remove the option, new tables are created in the shared
tablespace, but you can still access any tables that
were created using multiple tablespaces.





InnoDB always needs the shared tablespace. The `.ibd'
files are not sufficient for InnoDB to operate. The shared tablespace
consists of the familiar `ibdata' files where InnoDB puts its
internal data dictionary and undo logs.





You cannot freely move `.ibd' files around between database
directories the way you can with
MyISAM table files. This is because the table definition is stored
in the InnoDB shared tablespace, and also because InnoDB
must preserve the consistency of transaction IDs and log sequence numbers.





Within a given MySQL installation, you can move an `.ibd' file and the
associated table from one database to another with the familiar RENAME
TABLE statement:





RENAME TABLE old_db_name.tbl_name TO new_db_name.tbl_name;







If you have a ``clean'' backup of an `.ibd' file,
you can restore it to the MySQL installation from which it originated as
follows:





    

  1. 

Issue this ALTER TABLE statement:



    ALTER TABLE tbl_name DISCARD TABLESPACE;

    

Caution: This deletes the current `.ibd' file.


  2. 

Put the backup `.ibd' file back in the proper database directory.


  3. 

Issue this ALTER TABLE statement:



    ALTER TABLE tbl_name IMPORT TABLESPACE;

    






In this context, a ``clean'' `.ibd' file backup means:








You can make such a clean backup `.ibd' file with the following method:





    

  1. 

Stop all activity from the @command{mysqld} server and commit all transactions.

  2. 

Wait until SHOW INNODB STATUS shows that there are no active
transactions in the database, and the main thread status of InnoDB is
Waiting for server activity. Then you can make a copy of the
`.ibd' file. 





Another method for making a clean copy of an `.ibd' file is to
use the commercial InnoDB Hot Backup tool:





    

  1. 

Use InnoDB Hot Backup to back up the InnoDB installation.

  2. 

Start a second @command{mysqld} server on the backup and let it clean up
the `.ibd' files in the backup. 





It is in the TODO to also allow moving clean `.ibd' files to
another MySQL installation. This requires resetting of
transaction IDs and log sequence numbers in the `.ibd' file.







16.8  Adding and Removing InnoDB Data and Log Files




This section describes what you can do when your InnoDB tablespace runs
out of room or when you want to change the size of the log files.





From MySQL 3.23.50 and 4.0.2, the easiest way to increase the size of the
InnoDB tablespace is to configure it from the beginning to be
auto-extending.  Specify the autoextend attribute for the last
data file in the tablespace definition. Then InnoDB will increase the
size of that file automatically in 8MB increments when it runs out of space.
Starting with MySQL 4.1.5, the increment size can be
configured with the option innodb_autoextend_increment, in megabytes.
The default value is 8.





Alternatively, you can increase the size of your tablespace by adding another
data file. To do this, you have to shut down the MySQL server,
edit the `my.cnf' file to add a new data file to the end of
innodb_data_file_path, and start the server again.





If your last data file already was defined with the keyword autoextend,
the procedure to edit `my.cnf' must take into account the size to
which the last data file has grown. You have to
look at the size of the data file, round the size downward to the
closest multiple of 1024 * 1024 bytes (= 1MB), and specify the rounded
size explicitly in innodb_data_file_path. Then you can add
another data file.  Remember that only the last data file in the
innodb_data_file_path can be specified as auto-extending.





As an example, assume that the tablespace has just one auto-extending data file
`ibdata1':





innodb_data_home_dir =
innodb_data_file_path = /ibdata/ibdata1:10M:autoextend





Suppose that that this data file, over time, has grown to 988MB. Below is the
configuration line after adding another auto-extending data file.





innodb_data_home_dir =
innodb_data_file_path = /ibdata/ibdata1:988M;/disk2/ibdata2:50M:autoextend





When you add a new file to the tablespace, make sure that it does not
exist. InnoDB will create and initialize it when you restart
the server.





Currently, you cannot remove a data file from the tablespace. To decrease the
size of your tablespace, use this procedure:





    

  1. 

Use @command{mysqldump} to dump all your InnoDB tables.

  2. 

Stop the server.

  3. 

Remove all the existing tablespace files.

  4. 

Configure a new tablespace.

  5. 

Restart the server.

  6. 

Import the dump files.





If you want to change the number or the size of your InnoDB log
files, you have to stop the MySQL server and make sure that it shuts down
without errors.  Then copy the old log files into a safe place just in case
something went wrong in the shutdown and you will need them to recover the
tablespace. Delete the old log files from the log file directory, edit
`my.cnf' to change the log file configuration, and start the MySQL
server again. @command{mysqld} will see that the no log files exist at startup
and tell you that it is creating new ones.







16.9  Backing Up and Recovering an InnoDB Database




The key to safe database management is taking regular backups.





InnoDB Hot Backup is an online backup tool you can use to backup
your InnoDB database while it is running. InnoDB Hot Backup does not
require you to shut down your database and it does not set any locks or
disturb your normal database processing. InnoDB Hot Backup
is a non-free (commercial) additional tool whose annual license fee is
390 euros per computer where the MySQL server is run.
See the InnoDB Hot Backup home page
for detailed information and screenshots.





If you are able to shut down your MySQL server, you can make
a ``binary'' backup that consists of all files used by InnoDB to manage
its tables. Use the following procedure:





    

  1. 

Shut down your MySQL server and make sure that it shuts down without errors.

  2. 

Copy all your data files into a safe place.

  3. 

Copy all your InnoDB log files to a safe place.

  4. 

Copy your `my.cnf' configuration file or files to a safe place.

  5. 

Copy all the `.frm' files for your InnoDB tables to a safe place.





Replication works with InnoDB type tables, so you can use MySQL
replication capabilities to keep a copy of your database at database sites
requiring high availability.





In addition to taking binary backups as just described,
you should also regularly take dumps of your tables with
@command{mysqldump}. The reason for this is that a binary file
might be corrupted without you noticing it. Dumped tables are stored
into text files that are human-readable, so spotting table corruption
becomes easier. Also, since the format is simpler, the chance for
serious data corruption is smaller.
@command{mysqldump} also has a --single-transaction option that
you can use to take a consistent snapshot without locking out other
clients.





To be able to recover your InnoDB database to the present from the
binary backup described above, you have to run your MySQL server
with binary logging turned on. Then you can apply the binary log to the
backup database to achieve point-in-time recovery:





mysqlbinlog yourhostname-bin.123 | mysql





To recover from a crash of your MySQL server process, the only thing
you have to do is to restart it. InnoDB will automatically check the
logs and perform a roll-forward of the database to the present.
InnoDB will automatically roll back uncommitted transactions that were
present at the time of the crash. During recovery, @command{mysqld} will
display output something like this:





InnoDB: Database was not shut down normally.
InnoDB: Starting recovery from log files...
InnoDB: Starting log scan based on checkpoint at
InnoDB: log sequence number 0 13674004
InnoDB: Doing recovery: scanned up to log sequence number 0 13739520
InnoDB: Doing recovery: scanned up to log sequence number 0 13805056
InnoDB: Doing recovery: scanned up to log sequence number 0 13870592
InnoDB: Doing recovery: scanned up to log sequence number 0 13936128
...
InnoDB: Doing recovery: scanned up to log sequence number 0 20555264
InnoDB: Doing recovery: scanned up to log sequence number 0 20620800
InnoDB: Doing recovery: scanned up to log sequence number 0 20664692
InnoDB: 1 uncommitted transaction(s) which must be rolled back
InnoDB: Starting rollback of uncommitted transactions
InnoDB: Rolling back trx no 16745
InnoDB: Rolling back of trx no 16745 completed
InnoDB: Rollback of uncommitted transactions completed
InnoDB: Starting an apply batch of log records to the database...
InnoDB: Apply batch completed
InnoDB: Started
mysqld: ready for connections





If your database gets corrupted or your disk fails, you have
to do the recovery from a backup. In the case of corruption, you should
first find a backup that is not corrupted. After restoring the base
backup, do the recovery from the binary log files.





In some cases of database corruption it is enough just to dump, drop,
and re-create one or a few corrupt tables. You can use the
CHECK TABLE SQL statement to check whether a table is corrupt, although
CHECK TABLE naturally cannot detect every possible kind of corruption.
You can use innodb_tablespace_monitor to check the integrity of
the file space management inside the tablespace files.





In some cases, apparent database page corruption is actually due to the
operating system corrupting its own file cache, and the data
on disk may be okay. It is best first to try restarting your computer.
It may eliminate errors that appeared to be database page corruption.







16.9.1  Forcing Recovery




If there is database page corruption, you may want to dump
your tables from the database with SELECT INTO OUTFILE,
and usually most of the data
is intact and correct. But the corruption may cause
SELECT * FROM tbl_name or InnoDB
background operations to crash or assert, or even the InnoDB
roll-forward recovery to crash. Starting from MySQL
3.23.44, there is an InnoDB variable that you can use to
force the InnoDB storage engine to start up, and you can also
prevent background operations from running, so that
you will be able to dump your tables. For example, you can add the following
line to the [mysqld] section of your option file before restarting the
server:





[mysqld]
innodb_force_recovery = 4





Before MySQL 4.0, use this syntax instead:





[mysqld]
set-variable = innodb_force_recovery=4





The allowable non-zero values for innodb_force_recovery follow.
A larger number includes all precautions of lower numbers. If you are
able to dump your tables with an option value of at most 4, then you are
relatively safe that only some data on corrupt individual pages
is lost. A value of 6 is more dramatic, because database pages are left
in an obsolete state, which in turn may introduce more corruption
into B-trees and other database structures.









The database must not otherwise be used with any of these options enabled!  As a safety
measure, InnoDB prevents users from doing INSERT,
UPDATE, or DELETE when innodb_force_recovery is set to
a value greater than 0.





Starting from MySQL 3.23.53 and 4.0.4, you are allowed to DROP or
CREATE a table even if forced recovery is used. If you know that a
certain table is causing a crash in rollback, you can drop it.  You can use
this also to stop a runaway rollback caused by a failing mass import or
ALTER TABLE.  You can kill the @command{mysqld} process and set
innodb_force_recovery to 3 to bring your database up without
the rollback.  Then DROP the table that is causing the runaway
rollback.







16.9.2  Checkpoints




InnoDB implements a checkpoint mechanism called a ``fuzzy
checkpoint.'' InnoDB will flush modified database pages from the buffer
pool in small batches. There is no need to flush the buffer pool
in one single batch, which would in practice stop processing
of user SQL statements for a while.





In crash recovery, InnoDB looks for a checkpoint label written
to the log files. It knows that all modifications to the database
before the label are already present in the disk image of the database.
Then InnoDB scans the log files forward from the place of the
checkpoint,
applying the logged modifications to the database.





InnoDB writes to the log files in a circular fashion.
All committed modifications that make the database pages in the buffer
pool different from the images on disk must be available in the log files
in case InnoDB has to do a recovery. This means that when InnoDB starts
to reuse a log file in the circular fashion, it has to make sure that the
database page images on disk already contain the modifications
logged in the log file InnoDB is going to reuse. In other words,
InnoDB
has to make a checkpoint and often this involves flushing of
modified database pages to disk.





The preceding description explains why making your log files very big may save
disk I/O in checkpointing. It can make sense to set
the total size of the log files as big as the buffer pool or even bigger.
The drawback of big log files is that crash recovery can take longer
because there will be more logged information to apply to the database.







16.10  Moving an InnoDB Database to Another Machine




On Windows, InnoDB internally always stores database and
table names in lowercase. To move databases in a binary format 
from Unix to Windows or from Windows to Unix, you should have all table
and database names in lowercase. A convenient way to accomplish this
on Unix is to add the following line to the [mysqld] section of
your `my.cnf' before you start creating your databases and tables:





[mysqld]
set-variable = lower_case_table_names=1





On Windows, lower_case_table_names is set to 1 by default.





Like MyISAM data files, InnoDB data and log files are
binary-compatible on all platforms if the floating-point number format on
the machines is the same.  You can move an InnoDB database simply by
copying all the relevant files, which were listed in section 16.9  Backing Up and Recovering an InnoDB Database.  If
the floating-point formats on the machines are different but you have not
used FLOAT or DOUBLE data types in your tables, then the
procedure is the same: Just copy the relevant files. If the formats are
different and your tables contain floating-point data, you have to use
@command{mysqldump} to dump your tables on one machine and then import the dump
files on the other machine.





A performance tip is to switch off autocommit mode when you import
data into your database, assuming that your tablespace has enough space for
the big rollback segment the big import transaction will generate.
Do the commit only after importing a whole table or a segment of
a table.







16.11  InnoDB Transaction Model and Locking




In the InnoDB transaction model, the goal has been to combine the
best properties of a multi-versioning database with traditional two-phase
locking.  InnoDB does locking on the row level and runs queries as
non-locking consistent reads by default, in the style of Oracle.  The lock
table in InnoDB is stored so space-efficiently that lock escalation
is not needed: Typically several users are allowed to lock every row in the
database, or any random subset of the rows, without InnoDB running
out of memory.







16.11.1  InnoDB and AUTOCOMMIT




In InnoDB, all user activity occurs inside a transaction. If the
autocommit mode is enabled, each SQL statement forms a
single transaction on its own. MySQL always starts a new connection
with autocommit enabled.





If the autocommit mode is switched off with SET AUTOCOMMIT = 0,
then we can consider that a user always has a transaction open. An
SQL COMMIT or ROLLBACK statement ends the
current transaction and a new one starts. Both statements will release
all InnoDB locks that were set during the current transaction.
A COMMIT means that the changes made in the current transaction
are made permanent and become visible to other users.
A ROLLBACK statement, on the other hand, cancels all
modifications made by the current transaction.





If the connection has autocommit enabled, the user can still
perform a multiple-statement transaction by starting it with an
explicit START TRANSACTION or BEGIN statement and ending it with
COMMIT or ROLLBACK.







16.11.2  InnoDB and TRANSACTION ISOLATION LEVEL




In terms of the SQL:1992 transaction isolation levels,
the InnoDB default is REPEATABLE READ.
Starting from MySQL 4.0.5, InnoDB offers all four different
transaction isolation levels described by the SQL standard.
You can set the default isolation level for all connections by using the
--transaction-isolation option on the command line or in option files.
For example, you can set the option
in the [mysqld] section of `my.cnf' like this:





[mysqld]
transaction-isolation = {READ-UNCOMMITTED | READ-COMMITTED
                         | REPEATABLE-READ | SERIALIZABLE}





A user can change the isolation level of a single session or
all new incoming connections with the SET TRANSACTION
statement. Its syntax is as follows:





SET [SESSION | GLOBAL] TRANSACTION ISOLATION LEVEL
                       {READ UNCOMMITTED | READ COMMITTED
                        | REPEATABLE READ | SERIALIZABLE}





Note that there are hyphens in the level names for the
--transaction-isolation option, but not for the SET
TRANSACTION statement.





The default behavior is to set the isolation level for the next (not
started) transaction. If you use the GLOBAL keyword, the statement
sets the default transaction level globally for all new connections
created from that point on (but not existing connections).
You need the SUPER privilege to do this.  Using the SESSION
keyword sets the default transaction level for all future transactions
performed on the current connection.





Any client is free to change the session isolation level (even in the
middle of a transaction), or the isolation level for the next
transaction.





Before MySQL 3.23.50, SET TRANSACTION had no effect
on InnoDB tables. Before 4.0.5, only REPEATABLE READ
and SERIALIZABLE were available.





You can query the global and session transaction isolation levels with these
statements:





SELECT @@global.tx_isolation;
SELECT @@tx_isolation;





In row-level locking, InnoDB uses so-called ``next-key locking.''
That means that besides index records, InnoDB can also lock
the ``gap'' before an index record to block insertions by other users
immediately before the index record. A next-key lock refers to
a lock that locks an index record and the gap before it.
A gap lock refers to a lock that only locks a gap before some
index record.





A detailed description of each isolation level in InnoDB:










16.11.3  Consistent Non-Locking Read




A consistent read means that InnoDB uses its multi-versioning to
present to a query a snapshot of the database at a point in time.
The query will see the changes made by exactly those transactions
that committed before that point of time, and no changes made by
later or uncommitted transactions. The exception to this rule
is that the query will see the changes made by the transaction
itself that issues the query. 





If you are running with the default REPEATABLE READ isolation level,
then all consistent reads within the same transaction read the snapshot
established by the first such read in that transaction. You can get a
fresher snapshot for your queries by committing the current transaction
and after that issuing new queries. 





Consistent read is the default mode in which InnoDB processes
SELECT statements in READ COMMITTED and
REPEATABLE READ isolation levels. A consistent read
does not set any locks on the tables it accesses, and
therefore other users are free to modify those tables at
the same time a consistent read is being performed on the table. 







16.11.4  Locking Reads SELECT ... FOR UPDATE and SELECT ... LOCK IN SHARE MODE




In some circumstances, a consistent read is not convenient.
For example, you might want to add a new row into your table child,
and make sure that the child already has a parent in table
parent.  The following example shows how to implement referential
integrity in your application code.





Suppose that you use a consistent read to read the table parent
and indeed see the parent of the child in the table. Can you now safely
add the child row to table child? No, because it may
happen that meanwhile some other user deletes the parent row
from the table parent, without you being aware of it.





The solution is to perform the SELECT in a locking
mode using LOCK IN SHARE MODE:
  


SELECT * FROM parent WHERE NAME = 'Jones' LOCK IN SHARE MODE;





Performing a read in share mode means that we read the latest
available data, and set a shared mode lock on the rows we read.
A shared mode lock prevents others from updating or deleting
the row we have read.  Also,
if the latest data belongs to a yet uncommitted transaction of another
client connection, we will wait until that transaction commits.
After we see that the preceding query returns
the parent 'Jones', we can safely add the child record
to the child table and commit our transaction.





Let us look at another example: We have an integer counter field in
a table child_codes that we use to assign
a unique identifier to each child added to table child.
Obviously, using a consistent read or a shared mode read
to read the present value of the counter is not a good idea, since
two users of the database may then see the same value for the
counter, and a duplicate-key error will occur if two users attempt to add
children with the same identifier to the table.





Here, LOCK IN SHARE MODE is not a good solution because if two
users read the counter at the same time, at least one of them
will end up in deadlock when attempting to update the counter.





In this case, there are two good ways to implement the reading and
incrementing of the counter:
(1) update the counter first by incrementing it by 1 and only after
that read it, or
(2) read the counter first with a lock mode FOR UPDATE, and
increment after that. The latter approach can be implemented as follows:





SELECT counter_field FROM child_codes FOR UPDATE;
UPDATE child_codes SET counter_field = counter_field + 1;





A SELECT ... FOR UPDATE reads the latest
available data, setting exclusive locks on each row it reads.
Thus it sets the same locks a searched SQL UPDATE would set
on the rows.





Please note that the above is merely an example of how SELECT ... FOR
UPDATE works. In MySQL, the specific task of generating a unique identifier
actually can be accomplished using only a single access to the table:





UPDATE child_codes SET counter_field = LAST_INSERT_ID(counter_field + 1);
SELECT LAST_INSERT_ID();





The SELECT statement merely retrieves the identifier information
(specific to the current connection). It does not access any table.







16.11.5  Next-Key Locking: Avoiding the Phantom Problem




In row-level locking, InnoDB uses an algorithm called ``next-key locking.''
InnoDB does the row-level locking in such a way that when it searches
or scans an index of a table, it sets shared or exclusive locks
on the index records it encounters. Thus the row-level locks are
actually index record locks.





The locks InnoDB sets on index records also affect the ``gap'' before
that index record. If a user has a shared or exclusive lock on record
R in an index, another user cannot insert a new index record
immediately before R in the index order.  This locking of gaps is
done to prevent the so-called ``phantom problem.'' Suppose that you want to read and
lock all children from the child table with an identifier value
larger than 100, with the intent of updating some column in the selected
rows later:





SELECT * FROM child WHERE id > 100 FOR UPDATE;





Suppose that there is an index on the id column. The query will scan that
index starting from the first record where id is bigger than 100.
Now, if the locks set on the index records would not lock out inserts made
in the gaps, a new row might meanwhile be inserted to the table. If you now
execute the same SELECT within the same transaction, you would see a
new row in the result set returned by the query.  This is contrary the
isolation principle of transactions: A transaction should be able to run so
that the data it has read does not change during the transaction. If we
regard a set of rows as a data item, the new ``phantom'' child would violate
this isolation principle.





When InnoDB scans an index, it can also lock the gap
after the last record in the index. Just that happens in the previous
example: The locks set by InnoDB prevent any insert to
the table where id would be bigger than 100.





You can use next-key locking to implement a uniqueness
check in your application: If you read your data in share mode
and do not see a duplicate for a row you are going to insert,
then you can safely insert your row and know that the next-key
lock set on the successor of your row during the read will prevent
anyone meanwhile inserting a duplicate for your row. Thus the next-key
locking allows you to ``lock'' the non-existence of something in your
table.







16.11.6  An Example of How the Consistent Read Works in InnoDB




Suppose that you are running in the default REPEATABLE READ isolation level.
When you issue a consistent read, that is, an ordinary SELECT
statement, InnoDB will give your transaction a timepoint according
to which your query sees the database. If another transaction deletes
a row and commits after your timepoint was assigned, you will
not see the row as having been deleted. Inserts and updates are treated
similarly.





You can advance your timepoint by committing your transaction
and then doing another SELECT.





This is called ``multi-versioned concurrency control.''





               User A                 User B

           SET AUTOCOMMIT=0;      SET AUTOCOMMIT=0;
time
|          SELECT * FROM t;
|          empty set
|                                 INSERT INTO t VALUES (1, 2);
|
v          SELECT * FROM t;
           empty set
                                  COMMIT;

           SELECT * FROM t;
           empty set

           COMMIT;

           SELECT * FROM t;
           ---------------------
           |    1    |    2    |
           ---------------------
           1 row in set





In this example,
user A sees the row inserted by B only when B has committed the
insert and A has committed as well, so that the timepoint
is advanced past the commit of B.





If you want to see the ``freshest'' state of the database, you should
use either the READ COMMITTED isolation level or a locking read:





SELECT * FROM t LOCK IN SHARE MODE;






16.11.7  Locks Set by Different SQL Statements in InnoDB




A locking read, an UPDATE, or a DELETE
generally set record locks on every index record that is scanned in
the processing of the SQL query. It does not matter if there are
WHERE conditions in the query that would exclude the row
from the result set of the query. InnoDB does not remember the exact
WHERE condition, but only knows which index ranges were
scanned. The record locks are normally next-key locks that also block
inserts to the ``gap'' immediately before the record.





If the locks to be set are exclusive,
then InnoDB always retrieves also the clustered index record and sets
a lock on it.





If you do not have indexes suitable for your query and
MySQL has to scan the whole table to process the query,
every row of the table will become locked, which in turn blocks all
inserts by other users to the table. It is important to create good indexes
so that your queries do not unnecessarily need to scan many rows.









16.11.8  When Does MySQL Implicitly Commit or Roll Back a Transaction?




MySQL begins each client connection with autocommit mode enabled by default.
When autocommit is enabled, MySQL does a commit after each SQL statement if
that statement did not return an error.





If you have the autocommit mode off and close a connection
without calling an explicit commit of your transaction,
then MySQL will roll back your transaction.





If a SQL statement returns an error, the commit/rollback
behavior depends on the error.
See section 16.16  Error Handling.





The following SQL statements (and any synonyms for them) cause an implicit
commit of the current transaction in MySQL:









16.11.9  Deadlock Detection and Rollback




InnoDB automatically detects a deadlock of transactions and rolls back a
transaction or transactions to prevent the deadlock. Starting from
MySQL 4.0.5, InnoDB tries to pick small transactions to roll
back. The size of a transaction is determined by the number of rows
it has inserted, updated, or deleted. Prior to 4.0.5, InnoDB
always rolled back the transaction whose lock request was the last
one to build a deadlock, that is, a cycle in the ``waits-for'' graph
of transactions. 





Beginning with MySQL 4.0.20 and 4.1.2, InnoDB is aware of
table locks if innodb_table_locks=1, and the MySQL
layer above InnoDB knows about row-level locks.  Before that,
InnoDB cannot detect deadlocks where a table lock set by a MySQL
LOCK TABLES statement is involved, or if a lock set
by another storage engine than InnoDB is involved. You have to resolve
these situations by setting the value of the  innodb_lock_wait_timeout
system variable.





When InnoDB performs a complete rollback of a transaction, all the
locks of the transaction are released. However, if just a single SQL
statement is rolled back as a result of an error, some of the locks
set by the SQL statement may be preserved. This is because InnoDB
stores row locks in a format such it cannot know afterward which lock was
set by which SQL statement.







16.11.10  How to Cope with Deadlocks




Deadlocks are a classic problem in transactional databases, but they
are not dangerous unless they are so frequent that you cannot run
certain transactions at all. Normally, you must write your
applications so that they are always prepared to re-issue a
transaction if it gets rolled back because of a deadlock. 





InnoDB uses automatic row-level locking. You can get deadlocks
even in the case of transactions that just insert or delete a
single row. That is because these operations are not really ``atomic'';
they automatically set locks on the (possibly several) index
records of the row inserted or deleted.





You can cope with deadlocks and reduce the likelihood of their occurrence
with the following techniques: 









16.12  InnoDB Performance Tuning Tips








16.12.1  SHOW INNODB STATUS and the InnoDB Monitors




Starting from MySQL 3.23.42, InnoDB includes InnoDB Monitors
that print information about the InnoDB internal state.  Starting from
MySQL 3.23.52 and 4.0.3, you can use the SQL statement SHOW INNODB
STATUS to fetch the output of the standard InnoDB Monitor to your
SQL client.  The information is useful in performance tuning. If you are
using the @command{mysql} interactive SQL client, the output is more readable
if you replace the usual semicolon statement terminator by \G:





mysql> SHOW INNODB STATUS\G





Another way to use InnoDB Monitors is to let them continuously write
data to the standard output of the server @command{mysqld}.  In this case,
no output is sent to clients.  When switched on, InnoDB Monitors
print data about every 15 seconds. Server output usually is directed
to the `.err' log in the MySQL data directory.  This data is useful in
performance tuning.  On Windows, you must start the server from a command
prompt in a console window with the --console option if you want
to direct the output to the window rather than to the error log.





Monitor output includes information of the following types:








To cause the standard InnoDB Monitor to write to the standard
output of @command{mysqld}, use the following SQL statement:





CREATE TABLE innodb_monitor(a INT) TYPE=InnoDB;





The monitor can be stopped by issuing the following statement:





DROP TABLE innodb_monitor;





The CREATE TABLE syntax is just a way to pass a command
to the InnoDB engine through the MySQL SQL parser: The only things
that matter are the table name innodb_monitor and that it be an
InnoDB table. The structure of the
table is not relevant at all for the InnoDB Monitor. If you shut down
the server when the monitor is running, and you want to start
the monitor again, you have to drop the
table before you can issue a new CREATE TABLE
statement to start the monitor.
This syntax may change in a future release.





In a similar way, you can start innodb_lock_monitor, which is
otherwise the same as innodb_monitor but also prints a
lot of lock information. A separate innodb_tablespace_monitor
prints a list of created file segments existing in the tablespace
and also validates the tablespace allocation data structures.
Starting from 3.23.44, there is innodb_table_monitor with which
you can print the contents of the InnoDB internal data dictionary.





A sample of InnoDB Monitor output:





mysql> SHOW INNODB STATUS\G
*************************** 1. row ***************************
Status:
=====================================
030709 13:00:59 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 18 seconds
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 413452, signal count 378357
--Thread 32782 has waited at btr0sea.c line 1477 for 0.00 seconds the semaphore:
X-lock on RW-latch at 41a28668 created in file btr0sea.c line 135
a writer (thread id 32782) has reserved it in mode wait exclusive
number of readers 1, waiters flag 1
Last time read locked in file btr0sea.c line 731
Last time write locked in file btr0sea.c line 1347
Mutex spin waits 0, rounds 0, OS waits 0
RW-shared spins 108462, OS waits 37964; RW-excl spins 681824, OS waits 375485
------------------------
LATEST FOREIGN KEY ERROR
------------------------
030709 13:00:59 Transaction:
TRANSACTION 0 290328284, ACTIVE 0 sec, process no 3195, OS thread id 34831 inser
ting
15 lock struct(s), heap size 2496, undo log entries 9
MySQL thread id 25, query id 4668733 localhost heikki update
insert into ibtest11a (D, B, C) values (5, 'khDk' ,'khDk')
Foreign key constraint fails for table test/ibtest11a:
,
  CONSTRAINT `0_219242` FOREIGN KEY (`A`, `D`) REFERENCES `ibtest11b` (`A`, `D`)
 ON DELETE CASCADE ON UPDATE CASCADE
Trying to add in child table, in index PRIMARY tuple:
 0: len 4; hex 80000101; asc ....;; 1: len 4; hex 80000005; asc ....;; 2: len 4;
 hex 6b68446b; asc khDk;; 3: len 6; hex 0000114e0edc; asc ...N..;; 4: len 7; hex
 00000000c3e0a7; asc .......;; 5: len 4; hex 6b68446b; asc khDk;;
But in parent table test/ibtest11b, in index PRIMARY,
the closest match we can find is record:
RECORD: info bits 0 0: len 4; hex 8000015b; asc ...[;; 1: len 4; hex 80000005; a
sc ....;; 2: len 3; hex 6b6864; asc khd;; 3: len 6; hex 0000111ef3eb; asc ......
;; 4: len 7; hex 800001001e0084; asc .......;; 5: len 3; hex 6b6864; asc khd;;
------------------------
LATEST DETECTED DEADLOCK
------------------------
030709 12:59:58
*** (1) TRANSACTION:
TRANSACTION 0 290252780, ACTIVE 1 sec, process no 3185, OS thread id 30733 inser
ting
LOCK WAIT 3 lock struct(s), heap size 320, undo log entries 146
MySQL thread id 21, query id 4553379 localhost heikki update
INSERT INTO alex1 VALUES(86, 86, 794,'aA35818','bb','c79166','d4766t','e187358f'
,'g84586','h794',date_format('2001-04-03 12:54:22','%Y-%m-%d %H:%i'),7
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 48310 n bits 568 table test/alex1 index symbole
trx id 0 290252780 lock mode S waiting
Record lock, heap no 324 RECORD: info bits 0 0: len 7; hex 61613335383138; asc a
a35818;; 1:
*** (2) TRANSACTION:
TRANSACTION 0 290251546, ACTIVE 2 sec, process no 3190, OS thread id 32782 inser
ting
130 lock struct(s), heap size 11584, undo log entries 437
MySQL thread id 23, query id 4554396 localhost heikki update
REPLACE INTO alex1 VALUES(NULL, 32, NULL,'aa3572','','c3572','d6012t','', NULL,'
h396', NULL, NULL, 7.31,7.31,7.31,200)
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 0 page no 48310 n bits 568 table test/alex1 index symbole
trx id 0 290251546 lock_mode X locks rec but not gap
Record lock, heap no 324 RECORD: info bits 0 0: len 7; hex 61613335383138; asc a
a35818;; 1:
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 48310 n bits 568 table test/alex1 index symbole
trx id 0 290251546 lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 82 RECORD: info bits 0 0: len 7; hex 61613335373230; asc aa
35720;; 1:
*** WE ROLL BACK TRANSACTION (1)
------------
TRANSACTIONS
------------
Trx id counter 0 290328385
Purge done for trx's n:o < 0 290315608 undo n:o < 0 17
Total number of lock structs in row lock hash table 70
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0 0, not started, process no 3491, OS thread id 42002
MySQL thread id 32, query id 4668737 localhost heikki
show innodb status
---TRANSACTION 0 290328384, ACTIVE 0 sec, process no 3205, OS thread id 38929 in
serting
1 lock struct(s), heap size 320
MySQL thread id 29, query id 4668736 localhost heikki update
insert into speedc values (1519229,1, 'hgjhjgghggjgjgjgjgjggjgjgjgjgjgggjgjgjlhh
gghggggghhjhghgggggghjhghghghghghhhhghghghjhhjghjghjkghjghjghjghjfhjfh
---TRANSACTION 0 290328383, ACTIVE 0 sec, process no 3180, OS thread id 28684 co
mmitting
1 lock struct(s), heap size 320, undo log entries 1
MySQL thread id 19, query id 4668734 localhost heikki update
insert into speedcm values (1603393,1, 'hgjhjgghggjgjgjgjgjggjgjgjgjgjgggjgjgjlh
hgghggggghhjhghgggggghjhghghghghghhhhghghghjhhjghjghjkghjghjghjghjfhjf
---TRANSACTION 0 290328327, ACTIVE 0 sec, process no 3200, OS thread id 36880 st
arting index read
LOCK WAIT 2 lock struct(s), heap size 320
MySQL thread id 27, query id 4668644 localhost heikki Searching rows for update
update ibtest11a set B = 'kHdkkkk' where A = 89572
------- TRX HAS BEEN WAITING 0 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 65556 n bits 232 table test/ibtest11a index PRIM
ARY trx id 0 290328327 lock_mode X waiting
Record lock, heap no 1 RECORD: info bits 0 0: len 9; hex 73757072656d756d00; asc
 supremum.;;
------------------
---TRANSACTION 0 290328284, ACTIVE 0 sec, process no 3195, OS thread id 34831 ro
llback of SQL statement
ROLLING BACK 14 lock struct(s), heap size 2496, undo log entries 9
MySQL thread id 25, query id 4668733 localhost heikki update
insert into ibtest11a (D, B, C) values (5, 'khDk' ,'khDk')
---TRANSACTION 0 290327208, ACTIVE 1 sec, process no 3190, OS thread id 32782
58 lock struct(s), heap size 5504, undo log entries 159
MySQL thread id 23, query id 4668732 localhost heikki update
REPLACE INTO alex1 VALUES(86, 46, 538,'aa95666','bb','c95666','d9486t','e200498f
','g86814','h538',date_format('2001-04-03 12:54:22','%Y-%m-%d %H:%i'),
---TRANSACTION 0 290323325, ACTIVE 3 sec, process no 3185, OS thread id 30733 in
serting
4 lock struct(s), heap size 1024, undo log entries 165
MySQL thread id 21, query id 4668735 localhost heikki update
INSERT INTO alex1 VALUES(NULL, 49, NULL,'aa42837','','c56319','d1719t','', NULL,
'h321', NULL, NULL, 7.31,7.31,7.31,200)
--------
FILE I/O
--------
I/O thread 0 state: waiting for i/o request (insert buffer thread)
I/O thread 1 state: waiting for i/o request (log thread)
I/O thread 2 state: waiting for i/o request (read thread)
I/O thread 3 state: waiting for i/o request (write thread)
Pending normal aio reads: 0, aio writes: 0,
 ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0
Pending flushes (fsync) log: 0; buffer pool: 0
151671 OS file reads, 94747 OS file writes, 8750 OS fsyncs
25.44 reads/s, 18494 avg bytes/read, 17.55 writes/s, 2.33 fsyncs/s
-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf for space 0: size 1, free list len 19, seg size 21,
85004 inserts, 85004 merged recs, 26669 merges
Hash table size 207619, used cells 14461, node heap has 16 buffer(s)
1877.67 hash searches/s, 5121.10 non-hash searches/s
---
LOG
---
Log sequence number 18 1212842764
Log flushed up to   18 1212665295
Last checkpoint at  18 1135877290
0 pending log writes, 0 pending chkp writes
4341 log i/o's done, 1.22 log i/o's/second
----------------------
BUFFER POOL AND MEMORY
----------------------
Total memory allocated 84966343; in additional pool allocated 1402624
Buffer pool size   3200
Free buffers       110
Database pages     3074
Modified db pages  2674
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages read 171380, created 51968, written 194688
28.72 reads/s, 20.72 creates/s, 47.55 writes/s
Buffer pool hit rate 999 / 1000
--------------
ROW OPERATIONS
--------------
0 queries inside InnoDB, 0 queries in queue
Main thread process no. 3004, id 7176, state: purging
Number of rows inserted 3738558, updated 127415, deleted 33707, read 755779
1586.13 inserts/s, 50.89 updates/s, 28.44 deletes/s, 107.88 reads/s
----------------------------
END OF INNODB MONITOR OUTPUT
============================
1 row in set (0.05 sec)





Some notes on the output:








Beginning with MySQL 4.0.19, InnoDB sends diagnostic output to stderr
or files instead of stdout or fixed-size memory buffers, to avoid potential
buffer overflow errors.  As a side effect, the output of SHOW INNODB
STATUS is written to a status file every fifteen seconds.  The name of the
file is `innodb_status.pid', where pid is the server
process ID. This file is created in the MySQL data directory.  InnoDB
removes the file for a normal shutdown.  If abnormal shutdowns have occurred,
instances of these status files may be present and must be removed manually.
Before removing them, you might want to examine them to see if they contain
useful information to the cause of abnormal shutdowns.  Beginning with
MySQL 4.0.21, the `innodb_status.pid' file will only be created
if the configuration option innodb_status_file=1 is set.







16.13  Implementation of Multi-Versioning




Because InnoDB is a multi-versioned database, it must keep information
about old versions of rows in the tablespace. This information is stored
in a data structure called a rollback segment after an analogous
data structure in Oracle.





Internally, InnoDB adds two fields to each row stored in the database.
A 6-byte field indicates the transaction identifier for the last
transaction that inserted or updated the row. Also, a deletion
is treated internally as an update where a special bit in the row
is set to mark it as deleted. Each row also contains a 7-byte
field called the roll pointer. The roll pointer points to an
undo log record written to the rollback segment. If the row was
updated, the undo log record contains the information necessary
to rebuild the content of the row before it was updated.





InnoDB uses the information in the rollback segment to perform the
undo operations needed in a transaction rollback. It also uses the
information to build earlier versions of a row for a consistent
read.





Undo logs in the rollback segment are divided into insert and update
undo logs. Insert undo logs are needed only in transaction rollback
and can be discarded as soon as the transaction commits. Update undo logs
are used also in consistent reads, and they can be discarded only after
there is no transaction present for which InnoDB has assigned
a snapshot that in a consistent read could need the information
in the update undo log to build an earlier version of a database
row.





You must remember to commit your transactions regularly, including those
transactions that only issue consistent reads.  Otherwise, InnoDB
cannot discard data from the update undo logs, and the rollback segment
may grow too big, filling up your tablespace.





The physical size of an undo log record in the rollback segment
is typically smaller than the corresponding inserted or updated
row. You can use this information to calculate the space need
for your rollback segment.





In the InnoDB multi-versioning scheme, a row is not physically removed from
the database immediately when you delete it with an SQL statement.
Only when InnoDB can discard the update undo log record written for
the deletion can it also physically remove the corresponding row and
its index records from the database. This removal operation is
called a purge, and it is quite fast, usually taking the same order of
time as the SQL statement that did the deletion.





In a scenario where the user inserts and deletes rows in smallish
batches at about the same rate in the table, it is possible that the
purge thread will start to lag behind, and the table grows bigger and
bigger, making everything disk-bound and very slow.  Even if the table
would carry just 10 MB of useful data, it may grow to occupy
10 GB with all the dead rows.  In such a case, it would be good
to throttle new row operations, and allocate more resources for the
purge thread.





The InnoDB transaction system maintains a list of transactions that
have delete-marked index records by UPDATE or DELETE
operations.  Let the length of this list be purge_lag.





Starting with MySQL/InnoDB-4.1.6, there is a startup option and
settable global variable innodb_max_purge_lag, which is zero by
default.  When this parameter is nonzero, InnoDB may delay new row
operations.  When the purge_lag exceeds
innodb_max_purge_lag, each INSERT, UPDATE and
DELETE operation will be delayed by
purge_lag/innodb_max_purge_lag*10-5 milliseconds.  The
delay is computed in the beginning of a purge batch, every ten
seconds.  The operations will not be delayed if purge cannot run
because of an old consistent read view that could see the rows to be
purged.  A typical setting for a problematic workload might be 1
million, assuming that our transactions are small, only 100 bytes in
size, and we can allow 100 MB of unpurged rows in our tables.






16.14  Table and Index Structures




MySQL stores its data dictionary information for tables in `.frm'
files in database directories. This is true for all MySQL storage engines.
But every InnoDB table
also has its own entry in InnoDB internal data dictionaries
inside the tablespace. When MySQL drops a table or a database,
it has to delete both an `.frm' file or files, and
the corresponding entries inside the InnoDB data dictionary.
This is the reason why you cannot move InnoDB tables between
databases simply by moving the `.frm' files. It is also the reason why
DROP DATABASE did not work for InnoDB type tables
before MySQL 3.23.44.





Every InnoDB table has a special index called the clustered index
where the data of the rows is stored. If you define a
PRIMARY KEY on your table, the index of the primary key
will be the clustered index.





If you do not define a PRIMARY KEY for
your table, MySQL picks the first UNIQUE index that has only
NOT NULL columns as the primary key and
InnoDB uses it as the clustered index.
If there is no such index in the table,
InnoDB internally generates a clustered index
where the rows are ordered by the row ID that InnoDB assigns
to the rows in such a table. The row ID is a 6-byte field that
increases monotonically as new rows are inserted. Thus the rows
ordered by the row ID will be physically in the insertion order.





Accessing a row through the clustered index is fast because the row data
will be on the same page where the index search leads.  If a table is large,
the clustered index architecture often saves a disk I/O when compared to
the traditional solution.  (In many databases, the data is traditionally
stored on a different page from the index record.)





In InnoDB, the records in non-clustered indexes (also called secondary
indexes) contain the primary key value for the row. InnoDB uses
this primary key value to search for the row from the clustered index. Note
that if the primary key is long, the secondary indexes use more space.





InnoDB compares CHAR and VARCHAR strings
of different lengths such that the remaining length in the shorter
string is treated as if padded with spaces.







16.14.1  Physical Structure of an Index




All indexes in InnoDB are B-trees where the index records are
stored in the leaf pages of the tree. The default size of an index
page is 16KB. When new records are inserted, InnoDB tries to
leave 1/16 of the page free for future insertions and updates
of the index records.





If index records are inserted in a sequential order (ascending or descending),
the resulting index pages will be about 15/16 full.
If records are inserted in a random order, the pages will be from
1/2 to 15/16 full. If the fillfactor of an index page drops below 1/2,
InnoDB tries to contract the index tree to free the page.







16.14.2  Insert Buffering




It is a common situation in a database application that the
primary key is a unique identifier and new rows are inserted in the
ascending order of the primary key. Thus the insertions to the
clustered index do not require random reads from a disk.





On the other hand, secondary indexes are usually non-unique, and insertions
into secondary indexes happen in a relatively random order.  This would
cause a lot of random disk I/O operations without a special mechanism used
in InnoDB.





If an index record should be inserted to a non-unique secondary index,
InnoDB checks whether the secondary index page is already in the buffer
pool. If that is the case, InnoDB does the insertion directly to
the index page. If the index page is not found in the buffer
pool, InnoDB inserts the record to a special insert buffer structure.
The insert buffer is kept so small that it fits entirely in the buffer
pool, and insertions can be done very fast.





Periodically, the insert buffer is merged into the secondary index trees in
the database. Often it is possible to merge several insertions to the same
page of the index tree, saving disk I/O operations.  It has been measured
that the insert buffer can speed up insertions into a table up to 15 times.







16.14.3  Adaptive Hash Indexes




If a table fits almost entirely in main memory, the fastest way
to perform queries on it is to use hash indexes. InnoDB has an
automatic mechanism that monitors index searches made to the indexes
defined for a table. If InnoDB notices that queries could
benefit from building a hash index, it does so automatically.





Note that the hash index is always built based on an existing
B-tree index on the table. InnoDB can build a hash index on a prefix
of any length of the key defined for the B-tree, depending on the
pattern of searches that InnoDB observes for the B-tree index.
A hash index can be partial: It is not required that the whole
B-tree index is cached in the buffer pool. InnoDB will build
hash indexes on demand for those pages of the index that are
often accessed.





In a sense, InnoDB tailors itself through the adaptive hash index
mechanism to ample main memory, coming closer to the architecture of main
memory databases.







16.14.4  Physical Record Structure




Records in InnoDB tables have the following characteristics:









16.15  File Space Management and Disk I/O





16.15.1  Disk I/O




InnoDB uses simulated asynchronous disk I/O: InnoDB creates a number of
threads to take care of I/O operations, such as read-ahead.





There are two read-ahead heuristics in InnoDB:








Starting from MySQL 3.23.40b, InnoDB uses a novel file flush technique
called doublewrite.
It adds safety to crash recovery after an operating system crash
or a power outage, and improves performance on most Unix flavors
by reducing the need for fsync() operations.





Doublewrite means that before writing pages to a data file, InnoDB
first writes them to a contiguous tablespace area called the
doublewrite buffer. Only after the write and the flush to the doublewrite
buffer has completed does InnoDB write the pages to their proper
positions in the data file. If the operating system crashes in the
middle of a page write, InnoDB later will find a good
copy of the page from the doublewrite buffer during recovery.







16.15.2  Using Raw Devices for the Tablespace




Starting from MySQL 3.23.41, you can use raw disk partitions as tablespace
data files.
By using a raw disk, you can perform non-buffered I/O on Windows and on
some Unix systems without filesystem overhead, which might improve performance.





When you create a new data file, you must put the keyword newraw
immediately after the data file size in innodb_data_file_path.
The partition must be at least as large as the size that you specify.
Note that 1MB in InnoDB is 1024 * 1024 bytes, whereas 1MB usually
means 1,000,000 bytes in disk specifications.





[mysqld]
innodb_data_home_dir=
innodb_data_file_path=/dev/hdd1:3Gnewraw;/dev/hdd2:2Gnewraw





The next time you start the server, InnoDB notices the newraw
keyword and initializes the new partition. However, do not create or change
any InnoDB tables yet. Otherwise, when you next restart the server,
InnoDB will reinitialize the partition and your changes will be lost.
(Starting from 3.23.44, as a safety measure InnoDB prevents users from
modifying data when any partition with newraw is specified.)





After InnoDB has initialized the new partition, stop the server, change
newraw in the data file specification to raw:





[mysqld]
innodb_data_home_dir=
innodb_data_file_path=/dev/hdd1:5Graw;/dev/hdd2:2Graw





Then restart the server and InnoDB will allow changes to be made.





On Windows, starting from 4.1.1, you can allocate a disk partition
as a data file like this:





[mysqld]
innodb_data_home_dir=
innodb_data_file_path=//./D::10Gnewraw





The
`//./'
corresponds to the Windows syntax of
`\\.\'
for accessing physical drives.





When you use raw disk partitions, be sure that they have permissions that
allow read and write access by the account used for running the MySQL
server.







16.15.3  File Space Management




The data files you define in the configuration file form the tablespace of
InnoDB. The files are simply concatenated to form the tablespace.
There is no striping in use.  Currently you cannot define where in the
tablespace your tables will be allocated. However, in a newly created
tablespace, InnoDB allocates space starting from the first
data file.





The tablespace consists of database pages with a default size of 16KB.
The pages are grouped into extents of 64 consecutive pages. The ``files''
inside a tablespace are called segments in InnoDB.
The name of the ``rollback segment'' is somewhat confusing because it
actually contains many segments in the tablespace.





Two segments are allocated for each index in InnoDB.  One is for
non-leaf nodes of the B-tree, the other is for the leaf nodes. The idea
here is to achieve better sequentiality for the leaf nodes, which contain
the data.





When a segment grows inside the tablespace, InnoDB allocates the
first 32 pages to it individually. After that InnoDB starts
to allocate whole extents to the segment.
InnoDB can add to a large segment up to 4 extents at a time to ensure
good sequentiality of data.





Some pages in the tablespace contain bitmaps of other pages, and
therefore a few extents in an InnoDB tablespace cannot be
allocated to segments as a whole, but only as individual pages.





When you ask for available free space in the tablespace by issuing
a SHOW TABLE STATUS, InnoDB
reports the extents that are definitely free in the tablespace.
InnoDB always reserves some extents for clean-up and other internal
purposes; these reserved extents are not included in the free space.





When you delete data from a table, InnoDB will contract the corresponding
B-tree indexes. It depends on the pattern of deletes whether that frees
individual pages or extents to the tablespace, so that the freed
space becomes available for other users. Dropping a table or deleting
all rows from it is guaranteed to release the space to other users,
but remember that deleted rows will be physically removed only in an
(automatic) purge operation after they are no longer needed in
transaction rollback or consistent read.







16.15.4  Defragmenting a Table




If there are random insertions into or deletions from
the indexes of a table, the indexes
may become fragmented. Fragmentation means that the physical ordering
of the index pages on the disk is not close to the index ordering
of the records on the pages, or that there are many unused pages in the
64-page blocks that were allocated to the index.





A symptom of fragmentation is that a table takes more space than it 'should
take'. How much exactly is that, is difficult to determine. All InnoDB data
and indexes are stored in B-trees, and their fillfactor may vary
50 % - 100 %. Another symptom of fragmentation is that a table scan:


SELECT COUNT(*) FROM t WHERE a_non_indexed_column <> 12345;





takes more time than 'it should take'. (Above we are fooling the SQL
optimizer to
scan the clustered index, not a secondary index.) Most disks can read 10
- 50 MB/s. That can be used to estimate how fast a table scan should run.





It can speed up index scans if you periodically perform a ``null''
ALTER TABLE operation:





ALTER TABLE tbl_name TYPE=InnoDB





That causes MySQL to rebuild the table. Another way to perform a
defragmention operation is to use @command{mysqldump} to dump the table to a text
file, drop the table, and reload it from the dump file.





If the insertions to an index are always ascending and records are deleted
only from the end, the InnoDB file space management algorithm
guarantees that fragmentation in the index will not occur.






16.16  Error Handling




Error handling in InnoDB is not always the same as specified in the
SQL standard. According to the standard, any error during an SQL statement
should cause the rollback of that statement. InnoDB sometimes rolls
back only part of the statement, or the whole transaction.  The following
items describe how InnoDB performs error handling:









16.16.1  InnoDB Error Codes




The following is a non-exhaustive list of common
InnoDB-specific errors that you may encounter, with information
about why they occur and how to resolve the problem.







1005 (ER_CANT_CREATE_TABLE)


Cannot create table. If the error message string refers to
errno 150, table creation failed because a foreign key
constraint was not correctly formed.


1016 (ER_CANT_OPEN_FILE)


Cannot find the InnoDB table from the InnoDB data files though
the `.frm' file for the table exists.
See section 16.18.1  Troubleshooting InnoDB Data Dictionary Operations.


1114 (ER_RECORD_FILE_FULL)


InnoDB has run out of free space in the tablespace.
You should reconfigure the tablespace to add a new data file.


1205 (ER_LOCK_WAIT_TIMEOUT)


Lock wait timeout expired. Transaction was rolled back.


1213 (ER_LOCK_DEADLOCK)


Transaction deadlock. You should rerun the transaction.


1216 (ER_NO_REFERENCED_ROW)


You are trying to add a row but there is no parent row, and a
foreign key constraint fails. You should add the parent row first.


1217 (ER_ROW_IS_REFERENCED)


You are trying to delete a parent row that has children, and a
foreign key constraint fails. You should delete the children first.






16.16.2  Operating System Error Codes




To print the meaning of an operating system error number,
use the @command{perror} program that comes with the MySQL distribution.





The following table provides a list of some common Linux system error codes.
For a more complete list, see
Linux source code.







1 (EPERM)


Operation not permitted

2 (ENOENT)


No such file or directory

3 (ESRCH)


No such process

4 (EINTR)


Interrupted system call

5 (EIO)


I/O error

6 (ENXIO)


No such device or address

7 (E2BIG)


Arg list too long

8 (ENOEXEC)


Exec format error

9 (EBADF)


Bad file number

10 (ECHILD)


No child processes

11 (EAGAIN)


Try again

12 (ENOMEM)


Out of memory

13 (EACCES)


Permission denied

14 (EFAULT)


Bad address

15 (ENOTBLK)


Block device required

16 (EBUSY)


Device or resource busy

17 (EEXIST)


File exists

18 (EXDEV)


Cross-device link

19 (ENODEV)


No such device

20 (ENOTDIR)


Not a directory

21 (EISDIR)


Is a directory

22 (EINVAL)


Invalid argument

23 (ENFILE)


File table overflow

24 (EMFILE)


Too many open files

25 (ENOTTY)


Inappropriate ioctl for device

26 (ETXTBSY)


Text file busy

27 (EFBIG)


File too large

28 (ENOSPC)


No space left on device

29 (ESPIPE)


Illegal seek

30 (EROFS)


Read-only file system

31 (EMLINK)


Too many links





The following table provides a list of some common Windows system error codes.
For a complete list see the
Microsoft website.







1 (ERROR_INVALID_FUNCTION)


Incorrect function.

2 (ERROR_FILE_NOT_FOUND)


The system cannot find the file specified.

3 (ERROR_PATH_NOT_FOUND)


The system cannot find the path specified.

4 (ERROR_TOO_MANY_OPEN_FILES)


The system cannot open the file.

5 (ERROR_ACCESS_DENIED)


Access is denied.

6 (ERROR_INVALID_HANDLE)


The handle is invalid.

7 (ERROR_ARENA_TRASHED)


The storage control blocks were destroyed.

8 (ERROR_NOT_ENOUGH_MEMORY)


Not enough storage is available to process this command.

9 (ERROR_INVALID_BLOCK)


The storage control block address is invalid.

10 (ERROR_BAD_ENVIRONMENT)


The environment is incorrect.

11 (ERROR_BAD_FORMAT)


An attempt was made to load a program with an incorrect format.

12 (ERROR_INVALID_ACCESS)


The access code is invalid.

13 (ERROR_INVALID_DATA)


The data is invalid.

14 (ERROR_OUTOFMEMORY)


Not enough storage is available to complete this operation.

15 (ERROR_INVALID_DRIVE)


The system cannot find the drive specified.

16 (ERROR_CURRENT_DIRECTORY)


The directory cannot be removed.

17 (ERROR_NOT_SAME_DEVICE)


The system cannot move the file to a different disk drive.

18 (ERROR_NO_MORE_FILES)


There are no more files.

19 (ERROR_WRITE_PROTECT)


The media is write protected.

20 (ERROR_BAD_UNIT)


The system cannot find the device specified.

21 (ERROR_NOT_READY)


The device is not ready.

22 (ERROR_BAD_COMMAND)


The device does not recognize the command.

23 (ERROR_CRC)


Data error (cyclic redundancy check).

24 (ERROR_BAD_LENGTH)


The program issued a command but the command length is incorrect.

25 (ERROR_SEEK)


The drive cannot locate a specific area or track on the disk.

26 (ERROR_NOT_DOS_DISK)


The specified disk or diskette cannot be accessed.

27 (ERROR_SECTOR_NOT_FOUND)


The drive cannot find the sector requested.

28 (ERROR_OUT_OF_PAPER)


The printer is out of paper.

29 (ERROR_WRITE_FAULT)


The system cannot write to the specified device.

30 (ERROR_READ_FAULT)


The system cannot read from the specified device.

31 (ERROR_GEN_FAILURE)


A device attached to the system is not functioning.

32 (ERROR_SHARING_VIOLATION)


The process cannot access the file because it is being used by another process.

33 (ERROR_LOCK_VIOLATION)


The process cannot access the file because another process has locked a portion of the file.

34 (ERROR_WRONG_DISK)


The wrong diskette is in the drive. Insert %2 (Volume Serial Number: %3) into drive %1.

36 (ERROR_SHARING_BUFFER_EXCEEDED)


Too many files opened for sharing.

38 (ERROR_HANDLE_EOF)


Reached the end of the file.

39 (ERROR_HANDLE_DISK_FULL)


The disk is full.

112 (ERROR_DISK_FULL)


The disk is full.

123 (ERROR_INVALID_NAME)


The filename, directory name, or volume label syntax is incorrect.

1450 (ERROR_NO_SYSTEM_RESOURCES)


Insufficient system resources exist to complete the requested service.






16.17  Restrictions on InnoDB Tables








16.18  InnoDB Troubleshooting








16.18.1  Troubleshooting InnoDB Data Dictionary Operations




A specific issue with tables is that the MySQL server keeps data
dictionary information in `.frm' files it stores in the database
directories, while InnoDB also stores the information into its own
data dictionary inside the tablespace files.
If you move `.frm' files around, or use DROP DATABASE in
MySQL versions before 3.23.44, or the server crashes in the middle of a
data dictionary operation, the `.frm' files may end up
out of sync with the InnoDB internal data dictionary.





A symptom of an out-of-sync data dictionary is that a CREATE TABLE
statement fails. If this occurs, you should look in the server's error
log. If the log says that the table already exists inside the InnoDB
internal data dictionary, you have an orphaned table inside the InnoDB
tablespace files that has no corresponding `.frm' file. The error message
looks like this:





InnoDB: Error: table test/parent already exists in InnoDB internal
InnoDB: data dictionary. Have you deleted the .frm file
InnoDB: and not used DROP TABLE? Have you used DROP DATABASE
InnoDB: for InnoDB tables in MySQL version <= 3.23.43?
InnoDB: See the Restrictions section of the InnoDB manual.
InnoDB: You can drop the orphaned table inside InnoDB by
InnoDB: creating an InnoDB table with the same name in another
InnoDB: database and moving the .frm file to the current database.
InnoDB: Then MySQL thinks the table exists, and DROP TABLE will
InnoDB: succeed.





You can drop the orphaned table by following the instructions given in
the error message.





Another symptom of an out-of-sync data dictionary is that MySQL prints
an error that it cannot open an `.InnoDB' file:





ERROR 1016: Can't open file: 'child2.InnoDB'. (errno: 1)





In the error log you will find a message like this:





InnoDB: Cannot find table test/child2 from the internal data dictionary
InnoDB: of InnoDB though the .frm file for the table exists. Maybe you
InnoDB: have deleted and recreated InnoDB data files but have forgotten
InnoDB: to delete the corresponding .frm files of InnoDB tables?





This means that there is an orphaned `.frm' file without a
corresponding table inside InnoDB. You can drop the orphaned
`.frm' file by deleting it manually.





If MySQL crashes in the middle of an ALTER TABLE operation, you
may end up with an orphaned temporary table inside the InnoDB
tablespace. With innodb_table_monitor you see a table whose
name is `#sql-...'. Starting from MySQL 4.0.6, you can perform
SQL statements also on tables whose name contains the character `#'
if you enclose the name in backticks.
Thus, you can drop such an orphaned table like any other orphaned
table with the method described above. Note that to copy or rename a file
in the Unix shell, you need to put the file name in double quotes if the
file name contains `#'.





Older MySQL versions did not allow accessing any
table with a name containing `#'. The solution in older MySQL
versions is to use a special InnoDB mechanism available starting from
MySQL 3.23.48. When you have an orphaned table `#sql-id' inside
the tablespace, you can cause InnoDB to rename it to
`rsql-id_recover_innodb_tmp_table' with the following statement:





CREATE TABLE `rsql-id_recover_innodb_tmp_table`(...) TYPE=InnoDB;






17  MySQL Cluster




MySQL Cluster uses the new NDB Cluster storage engine to enable running
several MySQL servers in a cluster.
The NDB Cluster storage engine is available in the BitKeeper from MySQL
release 4.1.2, and in binary releases from MySQL-Max 4.1.3.





Currently, supported operating systems are Linux, Mac OS X, and Solaris.
We are working to make NDB Cluster run on all of
the operating systems that MySQL itself also runs on, including Windows.





This chapter represents work in progress.
Other documents describing MySQL Cluster can be found at
http://www.mysql.com/cluster/ and
http://dev.mysql.com/doc/#cluster.





You may also wish to subscribe to the MySQL Cluster mailing list.
See http://lists.mysql.com/.







17.1  MySQL Cluster Overview




MySQL Cluster is a new technology to enable clustering of in-memory
databases in a shared-nothing system. The shared-nothing architecture allows
the system to work with very inexpensive hardware, without any specific
requirement on hardware or software.  It also does not have any single
point of failure because each component has its own memory and disk.





MySQL Cluster is an integration of the standard MySQL server with an
in-memory clustered storage engine, called NDB. In our
documentation, the term NDB refers to the storage engine specific
part of the setup, whereas MySQL Cluster refers to the combination
of MySQL and the new storage engine.





A MySQL Cluster consists of computers with a set of processes executing
several MySQL servers, storage nodes for NDB Cluster, management servers
and possibly also specialized data access programs. All these programs work
together to form MySQL Cluster. When data is stored in the NDB Cluster storage
engine, the tables are stored in the storage nodes for NDB Cluster.
Those tables are directly accessible also from all other MySQL servers in
the cluster. Thus, if one application updates the salary of an employee, all
other MySQL servers that query this data can see it immediately.





The data stored in the storage nodes for MySQL Cluster can be mirrored
and can handle failures of storage nodes with no impact other than
that a number of transactions are aborted due to loosing the transaction
state. This causes no problems since transactional applications
should be written to handle transaction failure.





By bringing MySQL Cluster to the open source world, MySQL makes clustered
data management with high availability, high performance, and scalability
available to all who need it.







17.2  Basic MySQL Cluster Concepts




NDB is an in-memory storage engine offering high-availability
and data-persistence features.





The NDB storage engine can be configured with a range of fail-over and
load-balancing options, but it is easiest to start with the storage
engine at the cluster level. The NDB storage engine of MySQL Cluster
contains a complete set of data, dependent only on other data within
the cluster itself. 





We will now describe how to set up
a MySQL Cluster consisting of an NDB storage engine and some
MySQL servers.





The cluster part of MySQL Cluster is currently configured independently
from the MySQL servers. In a MySQL Cluster, each part of the cluster
is considered to be a node.





Note: A node is in many contexts a computer, but for
MySQL Cluster it is a process.  There can be any number of nodes on
a single computer.





Each node has a type, and there can be multiple nodes in the MySQL
Cluster of each type. In a minimal MySQL Cluster configuration, there
will be at least three nodes:








We refer to these cluster processes as nodes in the cluster. Setting up the
configuration of the cluster involves configuring each individual node in
the cluster and setting up each individual communication link between the
nodes in the cluster. MySQL Cluster currently is designed with the
intention that storage nodes are homogenous in terms of processor power,
memory space, and communication bandwidth. Also, to enable one point of
configuration, the entire cluster configuration is located in
one configuration file.





The management server manages the cluster configuration file and the
cluster log.  All nodes in the cluster contact the management server to
retrieve their part of the configuration, so they need a way to determine
where the management server resides.  When interesting events occur in
the storage nodes, they transfer the information of these events to the
management server, which then writes the information to the cluster log.





In addition, there are any number of clients to the cluster. These
are of two types.









17.3  MySQL Cluster Configuration




A MySQL server that is part of MySQL Cluster differs in only one aspect
from what we are used to. It has an additional storage engine
(NDB or NDBCLUSTER).





Except for this, the MySQL server is not much different than what we are
used to from previous MySQL releases, except any other new 4.1 features,
of course. By default, the server is configured with the NDB storage
engine disabled (so as not to allocate resources needlessly). To enable
NDB, you need to modify `my.cnf'.





Moreover, since the MySQL server is a part of the cluster, it needs to know
how to access a MGM node to get the cluster configuration.  There is a
default behavior to look for the MGM node on localhost.  However if you need
to specify its location elsewhere, this is also
done in `my.cnf' or on the MySQL server command line. Before the storage
engine NDB may be used, a MGM node and the DB nodes must be up and 
running. One MGM node is sufficient to start.







17.3.1  Building from Source Code




NDB Cluster is available in binary distributions from MySQL-Max 4.1.3
on.





If you choose to build from a source tarball or the MySQL 4.1 BitKeeper tree,
make sure you use the --with-ndbcluster option when running
@command{configure}.
You could also simply use the BUILD/compile-pentium-max build script.
This script also includes OpenSSL, so you either have to get OpenSSL
of modify the build script to exclude it.





Apart from these things, you can just follow the standard instructions to
build your own binaries, run the tests and perform the installation procedure.
See section 2.3.3  Installing from the Development Source Tree.







17.3.2  Installing the Software




It is easiest if you have all the MGM and DB nodes up and running first,
and this will probably be the most time-consuming part of the configuration
(because we will assume that you are already familiar
with MySQL to a certain extent).
As for the MySQL configuration and the `my.cnf' file, this is very
straightforward, and this section only covers the differences from
configuring MySQL without clustering.







17.3.3  Quick Test Setup of MySQL Cluster




This section describes how to quickly configure and start the simplest
possible MySQL Cluster setup. This will make you familiar with the basic
concepts. Then, read the other sections and design your desired setup.





One directory must be created. The example uses `/var/lib/mysql-cluster'.
Execute the following command as root:





shell> mkdir /var/lib/mysql-cluster





In this directory, create a file `config.ini' with the
following contents. Substitute HostName and DataDir values that
are appropriate for your system.





# file "config.ini" - showing minimal setup consisting of 1 DB node,
# 1 management server, and 3 MySQL servers.
# The empty default sections are not needed, and are shown only for clarity.
# Storage nodes are required to provide a host name but MySQL Servers
# are not. Thus the configuration can be dynamic as to setting up the
# MySQL Servers.
# If you don't know the host name of your machine, use localhost.
# The DataDir parameter also has a default value, but it is recommended to
# set it explicitly.
# NDBD, MYSQLD, and NDB_MGMD are aliases for DB, API, and MGM respectively
#
[NDBD DEFAULT]
NoOfReplicas= 1

[MYSQLD DEFAULT]
[NDB_MGMD DEFAULT]
[TCP DEFAULT]

[NDB_MGMD]
HostName= myhost.example.com 

[NDBD]
HostName= myhost.example.com 
DataDir= /var/lib/mysql-cluster

[MYSQLD]
[MYSQLD]
[MYSQLD]





You can now start the management server. Do this as follows:





shell> cd /var/lib/mysql-cluster
shell> ndb_mgmd





Then start a single DB node in this simple setup by running the @command{ndbd}
program. If this is the very first time you are starting @command{ndbd}, use
the --initial option:





shell> ndbd --initial





For subsequent @command{ndbd} starts, do not use that option:





shell> ndbd





By default, @command{ndbd} will look for the management server at
localhost at port 2200.





Please note that if you installed from the binary tarball, you will need
to explicitly specify the path of the @command{ndb_mgmd} and @command{ndbd}
servers. They would be in the `/usr/local/mysql/bin' directory.





Finally, go to the MySQL data directory (this might be a location such as
`/var/lib/mysql' or
`/usr/local/mysql/data'). Ensure that the `my.cnf' file contains
the option necessary to enable the NDB Cluster storage engine:





[mysqld]
ndbcluster





You can now start the MySQL server as usual:





shell> mysqld_safe --user=mysql &





Wait a moment to make sure the MySQL server is running properly.
If you see a notice ``mysql ended'', check the server's
`.err' file to find out what went wrong.





If all went well so far, you now can start using the cluster:





shell> mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 1 to server version: 4.1.7-gamma

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> SHOW STORAGE ENGINES;
+------------+---------+------------------------------------------------------------+
| Engine     | Support | Comment                                                    |
+------------+---------+------------------------------------------------------------+
...
| NDBCLUSTER | DEFAULT | Clustered, fault-tolerant, memory-based tables             |
| NDB        | YES     | Alias for NDBCLUSTER                                       |
...

mysql> USE test;
Database changed

mysql> CREATE TABLE ctest (i INT) ENGINE=NDBCLUSTER;
Query OK, 0 rows affected (0.09 sec)

mysql> show create table ctest \G
*************************** 1. row ***************************
       Table: ctest
Create Table: CREATE TABLE `ctest` (
  `i` int(11) default NULL
) ENGINE=ndbcluster DEFAULT CHARSET=latin1
1 row in set (0.00 sec)





If you like to check that your nodes was set-up properly you can then
start the management client.





shell> ndb_mgm





Try using the SHOW command to get an overview of the cluster.





NDB> show
Cluster Configuration
---------------------
[ndbd(NDB)]     1 node(s)
id=2    @127.0.0.1  (Version: 3.5.3, Nodegroup: 0, Master)

[ndb_mgmd(MGM)] 1 node(s)
id=1    @127.0.0.1  (Version: 3.5.3)

[mysqld(API)]   3 node(s)
id=3    @127.0.0.1  (Version: 3.5.3)
id=4 (not connected, accepting connect from any host)
id=5 (not connected, accepting connect from any host)





Clustering is yours. Enjoy.






17.3.4  Configuration File




The configuration of MySQL Cluster is contained in a configuration file
read by the management server and distributed to all processes being
part part of the cluster. This file contains a description of all involved nodes
in the cluster, configuration parameters for the storage nodes and
configuration parameters for all connections between the nodes in the
cluster.







17.3.4.1  An Example Configuration in a MySQL Cluster




Currently the configuration file is in INI format, and is named
`config.ini' by default.  It is placed in the start directory of
@command{ndb_mgmd} (the management server).





Preset defaults are available for most parameters, and defaults can be
specified in the `config.ini' file itself. To create a default value
section, add the word DEFAULT to the section name. For example, DB
nodes are coconfigured using [DB] sections. If all DB nodes use the
same data memory size, and this size is not the same as the preset default
size, create a [DB DEFAULT] section that contains a DataMemory
line to specify the default data memory size for all DB nodes.





The INI format consists of sections preceded by section headings (surrounded
by square brackets), followed by settings and values. One deviation from the
standard format being that the setting name and value can be separated by a
colon (`:') in addition to the equal sign (`='), and another is
that the sections aren't unique. Instead, unique entries (such as two nodes
of the same type) are identified by a unique ID.





A minimalistic configuration file needs to define the computers involved
in the cluster and which nodes are involved in the cluster and what
computers these nodes are placed on.





An example of a still fairly minimalistic configuration file for a
cluster with one management server, two storage nodes and two MySQL
servers is shown below:





# file "config.ini" - 2 DB nodes and 2 mysqld
# This file is placed in the start directory of ndb_mgmd,
# the management server.
# The first MySQL Server can be started from any host and the second
# can only be started at the host mysqld_5.mysql.com
# NDBD, MYSQLD, and NDB_MGMD are aliases for DB, API, and MGM respectively
#
[NDBD DEFAULT]
NoOfReplicas= 2
DataDir= /var/lib/mysql-cluster

[NDB_MGMD]
Hostname= ndb_mgmd.mysql.com
DataDir= /var/lib/mysql-cluster

[NDBD]
HostName= ndbd_2.mysql.com

[NDBD]
HostName= ndbd_3.mysql.com

[MYSQLD]
[MYSQLD]
HostName= mysqld_5.mysql.com





There are six different sections in the config file. [COMPUTER]
defines the computers in the cluster. [DB|NDBD] defines the storage nodes
in the cluster.  [API|MYSQLD] defines the MySQL server nodes in the cluster.
[MGM|NDB_MGMD] defines the management server node in the cluster. [TCP]
defines TCP/IP connections between nodes in the cluster, TCP/IP is the
default connection mechanism between two nodes. [SHM] defines
shared-memory connections between nodes.  This is only available in binaries
that have been built with the --with-ndb-shm option.





For each section one can define DEFAULTs. Parameters are case insensitive
as of MySQL 4.1.5.






17.3.4.2  Defining the Computers in a MySQL Cluster




The [COMPUTER] section has no real significance other than serving as
a way to avoid the need of defining host names for each node in the system.
All parameters mentioned here are mandatory.







[COMPUTER]Id


This is an internal identity in the configuration file. Later on in the
file one refers to this computer by the ID. It is an integer.


[COMPUTER]HostName


This is the host name of the computer. It is also possible to use an
IP address rather than the host name.






17.3.4.3  Defining the Management Server in a MySQL Cluster




The [MGM] section (with its alias [NDB_MGMD]) is used to
configure the behavior of the management server in various aspects. The
mandatory parameter is either ExecuteOnComputer or HostName.
All other parameters can be left out and will in that case receive the
default value.







[MGM]Id


This identity is the node ID used as the address of the node in all
cluster internal messages. This is an integer between 1 and 63. Each
node in the cluster has a unique identity.


[MGM]ExecuteOnComputer


This is referring to one of the computers defined in the computer section.


[MGM]PortNumber


This is the port number which the management server will listen on for
configuration requests and management commands.


[MGM]LogDestination


This parameter specifies where to send the cluster log. There are three
possible places to send it to and they can be sent to all three in parallel
if desired. These three are CONSOLE, SYSLOG, and FILE.
One assigns all of these three in one string with each part separated by a
';'.

CONSOLE means putting it to stdout, no more parameters are needed.



CONSOLE



SYSLOG means sending it to a syslog facility. It is necessary to
specify the facility for this parameter.  The poossible facilities are
auth, authpriv, cron, daemon, ftp,
kern, lpr, mail, news, syslog,
user, uucp, local0, local1, local2,
local3, local4, local5, local6, local7.
Note that every facility is not necessarily supported by every operating
system.



SYSLOG:facility=syslog



FILE means sending the cluster log to a regular file on the machine. It
is necessary to specify the name of this file, the maximum size of the
file until a new file is opened and the old is renamed with filename
extended by `.x' where x is the next number not used yet on the
file. It is also necessary to specify maximum number of rolled files.



FILE:filename=cluster.log,maxsize=1000000,maxfiles=6



Multiple log destinations can be specified as in the following example.



CONSOLE;SYSLOG:facility=local0;FILE:filename=/var/log/mgmd



The default value of this parameter is FILE:filename=cluster.log,maxsize=1000000,maxfiles=6.


[MGM]ArbitrationRank


This parameter is used to define which nodes can act as an arbitrator.
MGM nodes and API nodes can be arbitrators. 0 means it isn't used as
arbitrator, 1 high priority and 2 low priority. A normal configuration
uses the management server as arbitrator setting the ArbitrationRank
to 1 (which is the default) and setting all APIs to 0 (not default
in MySQL 4.1.3).


[MGM]ArbitrationDelay


If setting this to anything else than 0 it means that the management
server will delay responses to the arbitration requests. Default is no
delay and this should not be necessary to change.


[MGM]DataDir


This sets the directory where output files from the management server will
be placed. These files include cluster log files, file for printouts from
the process and pid file for daemon.






17.3.4.4  Defining the Storage Nodes in a MySQL Cluster




The [DB] section (with its alias [NDBD]) is used to configure
the behavior of the storage nodes.  There are many parameters specified that
controls the buffer sizes, pool sizes, timeout parameters and so forth. The
only mandatory parameter is either ExecuteOnComputer or
HostName and the parameter NoOfReplicas which need to be
defined in the [DB DEFAULT] section. Most parameters should be set in
the [DB DEFAULT] section. Only parameters explicitly stated as
possible to have local values are allowed to be changed in the [DB]
section. HostName, Id and ExecuteOnComputer needs to be
defined in the local [DB] section.





The Id value, or the node identity of the storage node can now be
allocated at start of the node. It is still possible to assign a node ID in
the config file.





For each parameter it is possible to use k, M, or G as suffixes to indicate
units of 1024, (1024*1024), or (1024* 1024*1024). For example, 100k means
102400.  Parameters and values are currently case sensitive.







[DB]Id


This identity is the node ID used as the address of the node in all
cluster internal messages. This is an integer between 1 and 63. Each
node in the cluster has a unique identity.


[DB]ExecuteOnComputer


This is referring to one of the computers defined in the computer section.


[DB]HostName


This parameter is similar to specifying a computer to execute on. It defines
the host name of the computer the storage node is to reside on. Either this
parameter or the ExecuteOnComputer is mandatory.


[DB]ServerPort


Each node in the cluster will use one port as the port other nodes use to
connect the transporters to each other. This port is used also for non-TCP
transporters in the connection setup phase. The default port will be
calculated to ensure that no nodes on the same computer receive the same
port number. The minimum port number will be 2202.


[DB]NoOfReplicas


This parameter can be set only in the [DB DEFAULT] section because it is a
global parameter. It defines the number of replicas for each table
stored in the cluster. This parameter also specifies the size of node
groups. A node group is a set of nodes that all store the same information.

Node groups are formed implicitly. The first node group is formed by the
storage nodes with the lowest node identities. And the next by the next
lowest node identities. As an example presume we have 4 storage nodes and
NoOfReplicas is set to 2. The four storage nodes have node IDs 2, 3,
4 and 5. Then the first node group will be formed by node 2 and node 3. The
second node group will be formed by node 4 and node 5. It is important to
configure the cluster in such a manner such that nodes in the same node
groups are not placed on the same computer. This would cause a single HW
failure to cause a cluster crash.

If no node identities are provided then the order of the storage nodes will
be the determining factor for the node group. The actual node group assigned
will be printed by the SHOW command in the management client.

There is no default value and the maximum number is 4.


[DB]DataDir


This parameter specifies the directory where trace files, log files, pid files
and error logs are placed.


[DB]FileSystemPath


This parameter specifies the directory where all files created for metadata,
REDO logs, UNDO logs and data files are placed. The default value is to use
the same directory as the DataDir. The directory must be created before
starting the @command{ndbd} process.

If you use the recommended directory hierarchy, you will use a directory
`/var/lib/mysql-cluster'. Under this directory a directory `ndb_2_fs'
will be created (if node ID was 2) which will be the file system for that
node.


[DB]BackupDataDir


It is possible also to specify the directory where backups will be placed.
By default, the directory FileSystemPath/`BACKUP' will be choosen.





DataMemory and IndexMemory are the parameters that specify
the size of memory segments used to store the actual records and their
indexes. It is important to understand how DataMemory and
IndexMemory are used to understand how to set these parameters.
For most uses, they need to be updated to reflect the usage of the
cluster.







[DB]DataMemory


This parameter is one of the most important parameters because it defines the
space available to store the actual records in the database. The entire
DataMemory will be allocated in memory so it is important that the
machine contains enough memory to handle the DataMemory size.

The DataMemory is used to store two things. It stores the actual
records. Each record is currently of fixed size. So VARCHAR columns are
stored as fixed size columns. There is an overhead on each record of 16
bytes normally. Additionally each record is stored in a 32KB page with
128 byte page overhead.  There will also be a small amount of waste for
each page because records are only stored in one page.  The maximum record
size for the columns currently is 8052 bytes.

The DataMemory is also used to store ordered indexes. Ordered
indexes uses about 10 bytes per record. Each record in the table is always
represented in the ordered index.

The DataMemory consists of 32KB pages. These pages are allocated
to partitions of the tables. Each table is normally partitioned with the
same number of partitions as there are storage nodes in the cluster.  Thus
for each node there are the same number of partitions (=fragments) as the
NoOfReplicas is set to.  Once a page has been allocated to a
partition it is currently not possible to bring it back to the pool of
free pages. The method to restore pages to the pool is by deleting the
table. Performing a node recovery also will compress the partition because
all records are inserted into an empty partition from another live node.

Another important aspect is that the DataMemory also contains UNDO
information for records. For each update of a record a copy record is
allocated in the DataMemory. Also each copy record will also have an
instance in the ordered indexes of the table. Unique hash indexes are
updated only when the unique index columns are updated and in that case a new
entry in the index table is inserted and at commit the old entry is
deleted. Thus it is necessary also to allocate memory to be able to handle
the largest transactions which are performed in the cluster.

Performing large transactions has no advantage in MySQL Cluster other than
the consistency of using transactions which is the whole idea of
transactions. It is not faster and consumes large amounts of memory.

The default DataMemory size is 80MB. The minimum size is 1MB.
There is no maximum size, but in reality the
maximum size has to be adapted so that the process doesn't start swapping
when using the maximum size of the memory.


[DB]IndexMemory


The IndexMemory is the parameter that controls the amount of storage
used for hash indexes in MySQL Cluster. Hash indexes are always used for
primary key indexes, unique indexes, and unique constraints.  Actually
when defining a primary key and a unique index there will be two indexes
created in MySQL Cluster.  One index is a hash index which is used for all
tuple accesses and also for lock handling. It is also used to ensure unique
constraints.

The size of the hash index is 25 bytes plus the size of the primary key.
For primary keys larger than 32 bytes another 8 bytes is added for some
internal references.

Thus for a table defined as



CREATE TABLE example
(
    a INT NOT NULL,
    b INT NOT NULL,
    c INT NOT NULL,
    PRIMARY KEY(a),
    UNIQUE(b)
) ENGINE=NDBCLUSTER;



We will have 12 bytes overhead (having no nullable columns saves 4 bytes of
overhead) plus 12 bytes of data per record. In addition we will have two
ordered indexes on a and b consuming about 10 bytes each per record. We
will also have a primary key hash index in the base table with roughly 29
bytes per record. The unique constraint is implemented by a separate table
with b as primary key and a as a column. This table will consume another 29
bytes of index memory per record in the table and also 12 bytes of overhead
plus 8 bytes of data in the record part.

Thus for one million records, we will need 58MB of index memory to handle
the hash indexes for the primary key and the unique constraint. For the
DataMemory part we will need 64MB of memory to handle the
records of the base table and the unique index table plus the two ordered
index tables.

The conclusion is that hash indexes takes up a fair amount of memory space
but in return they provide very fast access to the data. They are also used
in MySQL Cluster to handle uniqueness constraints.

Currently the only partitioning algorithm is hashing and the ordered
indexes are local to each node and can thus not be used to handle
uniqueness constraints in the general case.

An important point for both IndexMemory and DataMemory is
that the total database size is the the sum of all DataMemory and
IndexMemory in each node group. Each node group is used to store
replicated information, so if there are four nodes with 2 replicas there
will be two node groups and thus the total DataMemory available is
2*DataMemory in each of the nodes.

Another important point is about changes of DataMemory and
IndexMemory.  First of all, it is highly recommended to have the same
amount of DataMemory and IndexMemory in all nodes. Since data
is distributed evenly over all nodes in the cluster the size available is
no better than the smallest sized node in the cluster times the number of
node groups.

DataMemory and IndexMemory can be changed, but it is dangerous to
decrease them because that can easily lead to a node that will not be able to
restart or even a cluster not being able to restart since there is not
enough memory space for the tables needed to restore into the starting
node. Increasing them should be quite okay, but it is recommended that such
upgrades are performed in the same manner as a software upgrade where first
the configuration file is updated, then the management server is restarted
and then one storage node at a time is restarted by command.

More IndexMemory is not used due to updates but inserts are inserted
immediately and deletes are not deleted until the transaction is committed.

The default IndexMemory size is 18MB. The minimum size is 1MB.





The next three parameters are important because they affect the number of
parallel transactions and the sizes of transactions that can be handled by
the system. MaxNoOfConcurrentTransactions sets the number of
parallel transactions possible in a node and
MaxNoOfConcurrentOperations sets the number of records that can be
in update phase or locked simultaneously.





Both of these parameters and particularly
MaxNoOfConcurrentOperations are likely targets for users setting
specific values and not using the default value. The default value is set
for systems using small transactions and to ensure not using too much
memory in the default case.







[DB]MaxNoOfConcurrentTransactions


For each active transaction in the cluster there needs to be also a
transaction record in one of the nodes in the cluster. The role of
transaction coordination is spread among the nodes and thus the total
number of transactions records in the cluster is the amount in one times
the number of nodes in the cluster.

Actually transaction records are allocated to MySQL servers, normally there
is at least one transaction record allocated in the cluster per connection
that uses or have used a table in the cluster. Thus one should ensure that
there is more transaction records in the cluster than there are concurrent
connections to all MySQL servers in the cluster.

This parameter has to be the same in all nodes in the cluster.

Changing this parameter is never safe and can cause a cluster crash. When a
node crashes one of the node (actually the oldest surviving node) will
build up the transaction state of all transactions ongoing in the crashed
node at the time of the crash. It is thus important that this node has as
many transaction records as the failed node.

The default value for this parameter is 4096.


[DB]MaxNoOfConcurrentOperations


This parameter is likely to be subject for change by users. Users
performing only short, small transactions don't need to set this parameter very
high. Applications desiring to be able to perform rather large transactions
involving many records need to set this parameter higher.

For each transaction that updates data in the cluster it is required to
have operation records. There are operation records both in the transaction
coordinator and in the nodes where the actual updates are performed.

The operation records contain state information needed to be able to find
UNDO records for rollback, lock queues, and much other state information.

To dimension the cluster to handle transactions where one million records are
updated simultaneously one should set this parameter to one million divided
by the number of nodes. Thus for a cluster with four storage nodes one
should set this parameter to 250000.

Also read queries which set locks use up operation records. Some extra
space is allocated in the local nodes to cater for cases where the
distribution is not perfect over the nodes.

When queries translate into using the unique hash index there will actually
be two operation records used per record in the transaction. The first one
represents the read in the index table and the second handles the operation
on the base table.

The default value for this parameter is 32768.

This parameter actually handles two parts that can be configured separately.
The first part specifies how many operation records are to be placed in the
transaction coordinator part. The second part specifies how many operation
records that are to be used in the local database part.

If a very big transaction is performed on a 8-node cluster then this will
need as many operation records in the transaction coordinator as there are
reads, updates, deletes involved in the transaction. The transaction will
however spread the operation records of the actual reads, updates, and
inserts over all eight nodes. Thus if it is necessary to configure the
system for one very big transaction then it is a good idea to configure
those separately. MaxNoOfConcurrentOperations will always be used
to calculate the number of operation records in the transaction coordinator
part of the node.

It is also important to have an idea of the memory requirements for those
operation records. In MySQL 4.1.5, operation records consume
about 1KB per record. This figure will shrink in future 5.x versions.


[DB]MaxNoOfLocalOperations


By default this parameter is calculated as 1.1 * 
MaxNoOfConcurrentOperations
which fits systems with many simultaneous, not very large transactions. If
the configuration needs to handle one very large transaction at a time and
there are many nodes then it is a good idea to configure this separately.





The next set of parameters are used for temporary storage in the midst of
executing a part of a query in the cluster. All of these records will have
been released when the query part is completed and is waiting for the
commit or rollback.





Most of the defaults for these parameters will be okay for most users. Some
high-end users might want to increase those to enable more parallelism in
the system and some low-end users might want to decrease them to save
memory.







[DB]MaxNoOfConcurrentIndexOperations


For queries using a unique hash index another set of operation records are
temporarily used in the execution phase of the query. This parameter sets
the size of this pool. Thus this record is only allocated while executing a
part of a query, as soon as this part has been executed the record is
released. The state needed to handle aborts and commits is handled by the
normal operation records where the pool size is set by the parameter
MaxNoOfConcurrentOperations.

The default value of this parameter is 8192. Only in rare cases of
extremely high parallelism using unique hash indexes should this parameter
be necessary to increase. To decrease could be performed for memory savings
if the DBA is certain that such high parallelism is not occurring in the
  cluster.


[DB]MaxNoOfFiredTriggers


The default value of MaxNoOfFiredTriggers is 4000. Normally this
value should be sufficient for most systems. In some cases it could be
decreased if the DBA feels certain the parallelism in the cluster is not so
high.

This record is used when an operation is performed that affects a unique
hash index. Updating a column that is part of a unique hash index or
inserting/deleting a record in a table with unique hash indexes will fire
an insert or delete in the index table. This record is used to represent
this index table operation while its waiting for the original operation
that fired it to complete.  Thus it is short lived but can still need a
fair amount of records in its pool for temporary situations with many
parallel write operations on a base table containing a set of unique hash
indexes.


[DB]TransactionBufferMemory


This parameter is also used for keeping fired operations to update index
tables. This part keeps the key and column information for the fired
operations. It should be very rare that this parameter needs to be updated.

Also normal read and write operations use a similar buffer. This buffer is
even more short term in its usage so this is a compile time parameter set to
4000*128 bytes (500KB). The parameter is ZATTRBUF_FILESIZE in
Dbtc.hpp. A similar buffer for key info exists which contains 4000*16
bytes, 62.5KB of buffer space. The parameter in this case is
ZDATABUF_FILESIZE in Dbtc.hpp. Dbtc is the module for
handling the transaction coordination.

Similar parameters exist in the Dblqh module taking care of the reads
and updates where the data is located. In `Dblqh.hpp' with
ZATTRINBUF_FILESIZE set to 10000*128 bytes (1250KB) and
ZDATABUF_FILE_SIZE, set to 10000*16 bytes (roughly 156KB) of buffer
space. No known instances of that any of those compile time limits haven't
been big enough has been reported so far or discovered by any of our
extensive test suites.

The default size of the TransactionBufferMemory is 1MB.


[DB]MaxNoOfConcurrentScans


This parameter is used to control the amount of parallel scans that can be
performed in the cluster.  Each transaction coordinator can handle the
amount of parallel scans defined by this parameter. Each scan query is
performed by scanning all partitions in parallel. Each partition scan will
use a scan record in the node where the partition is located. The number of
those records is the size of this parameter times the number of nodes so
that the cluster should be able to sustain maximum number of scans in
parallel from all nodes in the cluster.

Scans are performed in two cases. The first case is when no hash or ordered
indexes exists to handle the query. In this case the query is executed by
performing a full table scan. The second case is when there is no hash
index to support the query but there is an ordered index. Using the ordered
index means executing a parallel range scan. Since the order is only kept
on the local partitions it is necessary to perform the index scan on all
partitions.

The default value of MaxNoOfConcurrentScans is 256. The maximum value
is 500.

This parameter will always specify the number of scans possible in the
transaction coordinator. If the number of local scan records is not provided
it is calculated as the product of MaxNoOfConcurrentScans and the number
of storage nodes in the system.


[DB]MaxNoOfLocalScans


Possible to specify the number of local scan records if many scans are not
fully parallelized.


[DB]BatchSizePerLocalScan


This parameter is used to calculate the number of lock records which needs
to be there to handle many concurrent scan operations.

The default value is 64 and this value has a strong connection to the
ScanBatchSize defined in the API nodes.


[DB]LongMessageBuffer


This is an internal buffer used for message passing internally in the node
and for messages between nodes in the system. It is highly unlikely that
anybody would need to change this parameter but for configurability it is
now configurable. By default it is set to 1MB.







[DB]NoOfFragmentLogFiles


This is an important parameter that states the size of the REDO
log files in the node. REDO log files are organized in a ring such that it
is important that the tail and the head doesn't meet.  When the tail and
head have come to close the each other the node will start aborting all
updating transactions because there is no room for the log records.

REDO log records aren't removed until three local checkpoints have
completed since the log record was inserted. The speed of checkpoint is
controlled by a set of other parameters so these parameters are all glued
together.

The default parameter value is 8, which means 8 sets of 4 16MB files. Thus in
total 512MB. Thus the unit is 64MB of REDO log space. In high
update scenarios this parameter needs to be set very high.  Test cases
where it has been necessary to set it to over 300 have been performed.

If the checkpointing is slow and there are so many writes to the database
that the log files are full and the log tail cannot be cut for recovery
reasons then all updating transactions will be aborted with internal error
code 410 which will be translated to Out of log file space temporarily.
This condition will prevail until a checkpoint has completed and the log tail
can be moved forward.


[DB]MaxNoOfSavedMessages


This parameter sets the maximum number of trace files that will be kept
before overwriting old trace files. Trace files are generated when the node
crashes for some reason.

The default is 25 trace files.





The next set of parameters defines the pool sizes for metadata objects. It
is necessary to define the maximum number of attributes, tables, indexes,
and trigger objects used by indexes, events and replication between
clusters.







[DB]MaxNoOfAttributes


This parameter defines the number of attributes that can be defined in the
cluster.

The default value of this parameter is 1000. The minimum value is 32 and there is
no maximum. Each attribute consumes around 200 bytes of storage in each node
because metadata is fully replicated in the servers.


[DB]MaxNoOfTables


A table object is allocated for each table, for each unique hash index, and
for each ordered index.  This parameter sets the maximum number of table
objects in the cluster.

For each attribute that has a BLOB data type an extra table is used to
store most of the BLOB data. These tables also must be taken into account
when defining the number of tables.

The default value of this parameter is 128. The minimum is 8 and there is no
maximum. There are internal limitations though that limits the maximum number
of tables to 1600. Each table object consumes around 20KB in each node.


[DB]MaxNoOfOrderedIndexes


For each ordered index in the cluster, objects are allocated to describe what
it is indexing and its storage parts. By default each index defined will have
an ordered index also defined. Unique indexes and primary key indexes have
both an ordered index and a hash index.

The default value of this parameter is 128. Each object consumes around 10KB
of data per node.


[DB]MaxNoOfUniqueHashIndexes


For each unique index (not for primary keys) a special table is allocated that
maps the unique key to the primary key of the indexed table. By default there
will be an ordered index also defined for each unique index. To avoid this,
you must use the USING HASH option in the unique index definition.

The default value is 64. Each index will consume around 15KB per node.


[DB]MaxNoOfTriggers


For each unique hash index an internal update, insert and delete trigger is
allocated. Thus three triggers per unique hash index. Ordered indexes use
only one trigger object. Backups also use three trigger objects for each
normal table in the cluster. When replication between clusters is supported
it will also use internal triggers.

This parameter sets the maximum number of trigger objects in the cluster.

The default value of this parameter is 768.


[DB]MaxNoOfIndexes


This parameter was deprecated in MySQL 4.1.5. Now you should use
MaxNoOfOrderedIndexes and MaxNoOfUniqueHashIndexes instead.

This parameter is only used by unique hash indexes. There needs to be one
record in this pool for each unique hash index defined in the cluster.

The default value of this parameter is 128.





There is a set of boolean parameters affecting the behavior of storage
nodes. Boolean parameters can be specified to true by setting it to Y or 1
and to false by setting it to N or 0.







[DB]LockPagesInMainMemory


For a number of operating systems such as Solaris and Linux it is possible
to lock a process into memory and avoid all swapping problems. This is an
important feature to provide real-time characteristics of the cluster.

The default is that this feature is not enabled.


[DB]StopOnError


This parameter states whether the process is to exit on error condition or
whether it is perform an automatic restart.

The default is that this feature is enabled.


[DB]Diskless


In the internal interfaces it is possible to set tables as diskless tables
meaning that the tables are not checkpointed to disk and no logging occur.
They only exist in main memory. The tables will still exist after a crash
but not the records in the table.

This feature makes the entire cluster Diskless, in this case even
the tables doesn't exist anymore after a crash. Enabling this feature can
be done by either setting it to Y or 1.

When this feature is enabled, backups will be performed but will not be
stored because there is no "disk". In future releases it is likely to make
the backup diskless a separate configurable parameter.

The default is that this feature is not enabled.


[DB]RestartOnErrorInsert


This feature is only accessible when building the debug version where it is
possible to insert errors in the execution of various code parts to test
failure cases.

The default is that this feature is not enabled.





There are quite a few parameters specifying timeouts and time intervals
between various actions in the storage nodes. Most of the timeouts are
specified in milliseconds with a few exceptions which will be mentioned
below.







[DB]TimeBetweenWatchDogCheck


To ensure that the main thread doesn't get stuck in an eternal loop
somewhere there is a watch dog thread which checks the main thread. This
parameter states the number of milliseconds between each check. After three
checks and still being in the same state the process is stopped by the
watch dog thread.

This parameter can easily be changed and can be different in the nodes
although there seems to be little reason for such a difference.

The default timeout is 4000 milliseconds (4 seconds).


[DB]StartPartialTimeout


This parameter specifies the time that the cluster will wait for all
storage nodes to come up before the algorithm to start the cluster is
invocated. This time out is used to avoid starting only a partial cluster
if possible.

The default value is 30000 milliseconds (30 seconds). 0 means eternal time
out. Thus only start if all nodes are available.


[DB]StartPartitionedTimeout


If the cluster is ready start after waiting StartPartialTimeout but
is still in a possibly partitioned state one waits until also this timeout
has passed.

The default timeout is 60000 milliseconds (60 seconds).


[DB]StartFailureTimeout


If the start is not completed within the time specified by this parameter
the node start will fail. Setting this parameter to 0 means no time out is
applied on the time to start the cluster.

The default value is 60000 milliseconds (60 seconds). For storage nodes
containing large data sets this parameter needs to be increased because it
could very well take 10-15 minutes to perform a node restart of a storage
node with a few gigabytes of data.


[DB]HeartbeatIntervalDbDb


One of the main methods of discovering failed nodes is by heartbeats. This
parameter states how often heartbeat signals are sent and how often to
expect to receive them. After missing three heartbeat intervals in a row,
the node is declared dead. Thus the maximum time of discovering a failure
through the heartbeat mechanism is four times the heartbeat interval.

The default heartbeat interval is 1500 milliseconds (1.5 seconds). This
parameter must not be changed drastically. If one node uses 5000
milliseconds and the node watching it uses 1000 milliseconds then obviously
the node will be declared dead very quickly. So this parameter can be
changed in small steps but not in large steps.


[DB]HeartbeatIntervalDbApi


In a similar manner each storage node sends heartbeats to each of the
connected MySQL servers to ensure that they behave properly. If a MySQL
server doesn't send a heartbeat in time (same algorithm as for storage node
with three heartbeats missed causing failure) it is declared down and all
ongoing transactions will be completed and all resources will be released
and the MySQL server cannot reconnect until the completion of all
activities started by the previous MySQL instance has been completed.

The default interval is 1500 milliseconds. This interval can be different
in the storage node because each storage node independently of all other
storage nodes watches the MySQL servers connected to it.


[DB]TimeBetweenLocalCheckpoints


This parameter is an exception in that it doesn't state any time to wait
before starting a new local checkpoint. This parameter is used to ensure
that in a cluster where not so many updates are taking place that we don't
perform local checkpoints. In most clusters with high update rates it is
likely that a new local checkpoint is started immediately after the
previous was completed. 

The size of all write operations executed since the start of the previous
local checkpoints is added.  This parameter is specified as the logarithm
of the number of words. So the default value 20 means 4MB of write
operations, 21 would mean 8MB and so forth up until the maximum value
31 which means 8GB of write operations.

All the write operations in the cluster are added together.  Setting it to
6 or lower means that local checkpoints will execute continuosly without
any wait between them independent of the workload in the cluster.


[DB]TimeBetweenGlobalCheckpoints


When a transaction is committed it is committed in main memory in all nodes
where mirrors of the data existed. The log records of the transaction are
not forced to disk as part of the commit however. The reasoning here is
that having the transaction safely committed in at least two independent
computers should be meeting standards of durability.

At the same time it is also important to ensure that even the worst of
cases when the cluster completely crashes is handled properly. To ensure
this all transactions in a certain interval is put into a global
checkpoint. A global checkpoint is very similar to a grouped commit of
transactions. An entire group of transactions is sent to disk. Thus as part
of the commit the transaction was put into a global checkpoint group. Later
this groups log records are forced to disk and then the entire group of
transaction is safely committed also on all computers disk storage as well.

This parameter states the interval between global checkpoints. The default
time is 2000 milliseconds.


[DB]TimeBetweenInactiveTransactionAbortCheck


Time-out handling is performed by checking each timer on each transaction
every period of time in accordance with this parameter. Thus if this
parameter is set to 1000 milliseconds, then every transaction will be
checked for timeout once every second.

The default for this parameter is 1000 milliseconds (1 second). 


[DB]TransactionInactiveTimeout


If the transaction is currently not performing any queries but is waiting
for further user input, this parameter states the maximum time that the
  user can wait before the transaction is aborted.

The default for this parameter is no timeout. For a real-time database
that needs to control that no transaction keeps locks for a too long time
this parameter should be set to a much smaller value. The unit is
milliseconds.


[DB]TransactionDeadlockDetectionTimeout


When a transactioon is involved in executing a query it waits for other
nodes. If the other nodes doesn't respond it could depend on three things.
First, the node could be dead, second the operation could have entered a
lock queue and finally the node requested to perform the action could be
heavily overloaded.  This timeout parameter states how long the
transaction coordinator will wait until it aborts the transaction when
waiting for query execution of another node.

Thus this parameter is important both for node failure handling and for
deadlock detection. Setting it too high would cause a non-desirable
behavior at deadlocks and node failures.

The default time out is 1200 milliseconds (1.2 seconds).


[DB]NoOfDiskPagesToDiskAfterRestartTUP


When executing a local checkpoint the algorithm sends all data pages to
disk during the local checkpoint.  Simply sending them there as quickly as
possible will cause unnecessary load on both processors, networks, and
disks. Thus to control the write speed this parameter specifies how many
pages per 100 milliseconds is to be written. A page is here defined as 8KB.
The unit this parameter is specified in is thus 80KB per
second. So setting it to 20 means writing 1.6MB of data pages to disk
per second during a local checkpoint. Also writing of UNDO log records for
data pages is part of this sum. Writing of index pages (see IndexMemory to
understand what index pages are used for) and their UNDO log records is
handled by the parameter NoOfDiskPagesToDiskAfterRestartACC. This
parameter handles the limitation of writes from the DataMemory.

So this parameter specifies how quickly local checkpoints will be executed.
This parameter is important in connection with NoOfFragmentLogFiles,
DataMemory, IndexMemory.

The default value is 40 (3.2MB of data pages per second). 


[DB]NoOfDiskPagesToDiskAfterRestartACC


This parameter has the same unit as
NoOfDiskPagesToDiskAfterRestartTUP but limits the speed of writing
index pages from IndexMemory.

The default value of this parameter is 20 (1.6MB per second).


[DB]NoOfDiskPagesToDiskDuringRestartTUP


This parameter specifies the same things as
NoOfDiskPagesToDiskAfterRestartTUP and
NoOfDiskPagesToDiskAfterRestartACC, only it does it for local
checkpoints executed in the node as part of a local checkpoint when the
node is restarting. As part of all node restarts a local checkpoint is
always performed. Since during a node restart it is possible to use a
higher speed of writing to disk because fewer activities are performed in the
node due to the restart phase.

This parameter handles the DataMemory part.

The default value is 40 (3.2MB per second).


[DB]NoOfDiskPagesToDiskDuringRestartACC


During Restart for IndexMemory part of local checkpoint.

The default value is 20 (1.6MB per second).


[DB]ArbitrationTimeout


This parameter specifies the time that the storage node will wait for a
response from the arbitrator when sending an arbitration message in the
case of a split network.

The default value is 1000 milliseconds (1 second).





A number of new configuration parameters were introduced in MySQL 4.1.5.
These correspond to values that
previously were compile time parameters. The main reason for this is to
enable the advanced user to have more control of the size of the process
and adjust various buffer sizes according to his needs.





All of these buffers are used as front-ends to the file system when
writing log records of various kinds to disk. If the node runs with
Diskless then these parameters can most definitely be set to their
minimum values because all disk writes are faked as okay by the file system
abstraction layer in the NDB storage engine.







[DB]UndoIndexBuffer


This buffer is used during local checkpoints. The NDB storage engine
uses a recovery scheme based on a consistent checkpoint together with an
operational REDO log. In order to produce a consistent checkpoint without
blocking the entire system for writes, UNDO logging is done while performing
the local checkpoint. The UNDO logging is only activated on one fragment of
one table at a time. This optimization is possible because tables are
entirely stored in main memory.

This buffer is used for the updates on the primary key hash index. Inserts
and deletes rearrange the hash index and the NDB storage engine writes
UNDO log records that map all physical changes to an index page such that
they can be undone at a system restart. It also logs all active insert
operations at the start of a local checkpoint for the fragment.

Reads and updates only set lock bits and update a header in the hash index
entry. These changes are handled by the page write algorithm to ensure that
these operations need no UNDO logging.

This buffer is 2MB by default. The minimum value is 1MB. For most
applications this is good enough. Applications doing extremely heavy inserts
and deletes together with large transactions using large primary keys
might need to extend this buffer.

If this buffer is too small, the NDB storage engine issues the internal
error code 677 which will be translated into "Index UNDO buffers overloaded".


[DB]UndoDataBuffer


This buffer has exactly the same role as the UndoIndexBuffer but is
used for the data part. This buffer is used during local checkpoint of a
fragment and inserts, deletes, and updates use the buffer.

Since these UNDO log entries tend to be bigger and more things are logged,
the buffer is also bigger by default. It is set to 16MB by default.
For some applications this might be too conservative and they might want
to decrease this size, the minimum size is 1MB. It should be rare that
applications need to increase this buffer size. If there is a need for this
it is a good idea to check if the disks can actually handle the load that
the update activity in the database causes. If they cannot then no size of
this buffer will be big enough.

If this buffer is too small and gets congested, the NDB storage engine 
issues the internal error code 891 which will be translated to
"Data UNDO buffers overloaded".


[DB]RedoBuffer


All update activities also need to be logged. This enables a replay of these
updates at system restart. The recovery algorithm uses a consistent
checkpoint produced by a "fuzzy" checkpoint of the data together with UNDO
logging of the pages. Then it applies the REDO log to play back all changes
up until the time that will be restored in the system restart.

This buffer is 8MB by default. The minimum value is 1MB.

If this buffer is too small, the NDB storage engine issues the internal
error code 1221 which will be translated into "REDO log buffers overloaded".





For cluster management, it is important to be able to control the
amount of log messages sent to stdout for various event types. The possible
events will be listed in this manual soon.  There are 16 levels possible
from level 0 to level 15. Setting event reporting to level 15 means
receiving all event reports of that category and setting it to 0 means
getting no event reports in that category.





The reason why most defaults are set to 0 and thus not causing any output
to stdout is that the same message is sent to the cluster log in the
management server. Only the startup message is by default generated to
stdout.





A similar set of levels can be set in management client to define what
levels to record in the cluster log.







[DB]LogLevelStartup


Events generated during startup of the process.

The default level is 1.


[DB]LogLevelShutdown


Events generated as part of graceful shutdown of a node.

The default level is 0.


[DB]LogLevelStatistic


Statistical events such as how many primary key reads, updates, inserts and
many other statistical information of buffer usage, and so forth.

The default level is 0.


[DB]LogLevelCheckpoint


Events generated by local and global checkpoints.

The default level is 0.


[DB]LogLevelNodeRestart


Events generated during node restart.

The default level is 0.


[DB]LogLevelConnection


Events generated by connections between nodes in the cluster.

The default level is 0.


[DB]LogLevelError


Events generated by errors and warnings in the cluster. These are errors
not causing a node failure but still considered worth reporting.

The default level is 0.


[DB]LogLevelInfo


Events generated for information about state of cluster and so forth.

The default level is 0.





There is a set of parameters defining memory buffers that are set aside for
online backup execution.







[DB]BackupDataBufferSize


When executing a backup there are two buffers used for sending data to the
disk. This buffer is used to fill in data recorded by scanning the tables
in the node. When filling this to a certain level the pages are sent to
disk. This level is specified by the BackupWriteSize parameter.
When sending data to the disk, the backup can continue filling this buffer
until it runs out of buffer space. When running out of buffer space, it will
simply stop the scan and wait until some disk writes return and thus free
up memory buffers to use for further scanning.

The default value is 2MB.


[DB]BackupLogBufferSize


This parameter has a similar role but instead used for writing a log of all
writes to the tables during execution of the backup. The same principles
apply for writing those pages as for BackupDataBufferSize except
that when this part runs out of buffer space, it causes the backup to
fail due to lack of backup buffers. Thus the size of this buffer must be
big enough to handle the load caused by write activities during the backup
execution.

The default parameter should be big enough. Actually it is more likely that
a backup failure is caused by a disk not able to write as quickly as it
should. If the disk subsystem is not dimensioned for the write load caused
by the applications this will create a cluster which will have great
difficulties to perform the desired actions.

It is important to dimension the nodes in such a manner that the processors
becomes the bottleneck rather than the disks or the network connections.

The default value is 2MB.


[DB]BackupMemory


This parameter is simply the sum of the two previous, the
BackupDataBufferSize and BackupLogBufferSize.

The default value is 4MB.


[DB]BackupWriteSize


This parameter specifies the size of the write messages to disk for the log
and data buffer used for backups.

The default value is 32KB.






17.3.4.5  Defining the MySQL Servers in a MySQL Cluster




The [API] section (with its alias [MYSQLD]) defines the
behavior of the MySQL server. No parameter is mandatory. If no computer or
host name is provided, then any host can use this API node.







[API]Id


This identity is the node ID used as the address of the node in all cluster
internal messages. This is an integer between 1 and 63. Each node in the
cluster must have a unique identity.


[API]ExecuteOnComputer


This is referring to one of the computers defined in the computer section.


[API]ArbitrationRank


This parameter is used to define which nodes can act as an arbitrator. MGM
nodes and API nodes can be arbitrators. 0 means it isn't used as
arbitrator, 1 high priority and 2 low priority. A normal configuration uses
the management server as arbitrator setting the ArbitrationRank to 1 (which
is the default) and setting all APIs to 0 (not default in MySQL 4.1.3).


[API]ArbitrationDelay


If setting this to anything else than 0 it means that the management server
will delay responses to the arbitration requests. Default is no delay and
this should not be necessary to change.

[API]BatchByteSize


For queries that get translated into full table scans or range scans on
indexes, it is important for best performance to fetch records in properly
sized batches. It is possible to set the proper size both in terms of
number of records and in terms of bytes. Real batch size will be limited
by both parameters.

Performance of queries can vary more than 40% due to how this parameter is
set. In future releases, the MySQL Server will make educated guesses on
what to set these parameters to, based on the query type.

This parameter is measured in bytes and is by default equal to 32KB.

[API]BatchSize


This parameter is measured in number of records and is by default set to
64. The maximum size is 992.

[API]MaxScanBatchSize


The batch size is the size of each batch sent from each storage node. Most
scans are performed in parallel so to protect the MySQL Server from getting
too much data from many nodes in parallel, this parameter sets a limit to
the total batch size over all nodes.

The default value of this parameter is set to 256KB. Its maximum size
is 16MB.






17.3.4.6  Defining TCP/IP Connections in a MySQL Cluster




TCP/IP is the default transport mechanism for establishing connections in
MySQL Cluster. It
is actually not necessary to define any connection because there will be a
one connection setup between each of the storage nodes, between each
storage node, and all MySQL server nodes and between each storage node and
the management server.





It is only necessary to define a connection if it is necessary to change
the default values of the connection.  In that case it is necessary to
define at least NodeId1, NodeId2 and the parameters to
change.





It is also possible to change the default values by setting the parameters
in the [TCP DEFAULT] section.







[TCP]NodeId1



[TCP]NodeId2


To identify a connection between two nodes it is necessary to provide the
node identity for both of them in NodeId1 and NodeId2.


[TCP]SendBufferMemory


TCP transporters use a buffer all messages before performing the send call
to the operating system. When this buffer reaches 64KB it sends the
buffer, the buffer is also sent when a round of messages have been
executed. To handle temporary overload situations it is also possible to
define a bigger send buffer.  The default size of the send buffer is 256KB.


[TCP]SendSignalId


To be able to retrace a distributed message diagram it is necessary to
identify each message with an identity. By setting this parameter to Y
these message identities are also transported over the network.  This
feature is not enabled by default.


[TCP]Checksum


This parameter is also a Y/N parameter which is not enabled by default.
When enabled all messages are checksummed before put into the send buffer.
This feature enables control that messages are not corrupted while waiting
in the send buffer. It is also a double check that the transport mechanism
haven't corrupted the data.


[TCP]PortNumber


This is the port number to use for listening to connections from other
nodes. This port number should be specified in the [TCP DEFAULT] section
normally.

This parameter should no longer be used. Use the parameter ServerPort on
storage nodes instead.


[TCP]ReceiveBufferMemory


This parameter specifies the size of the buffer used when receiving data
from the TCP/IP socket. There is seldom any need to change this parameter
from its default value of 64KB. One possible reason could be to save memory.






17.3.4.7  Defining Shared-Memory Connections in a MySQL Cluster




Shared memory segments are currently supported only for special builds of
MySQL Cluster using the @command{configure} parameter --with-ndb-shm. Its
implementation will most likely change. When defining shared memory as the
connection method it is necessary to define at least NodeId1,
NodeId2 and ShmKey. All other parameters have default values
that will work out fine in most cases.







[SHM]NodeId1



[SHM]NodeId2


To identify a connection between two nodes it is necessary to provide the
node identity for both of them in NodeId1 and NodeId2.


[SHM]ShmKey


When setting up shared memory segments an identifier is used to uniquely
identify the shared memory segment to use for the communication. This is an
integer which does not have a default value.


[SHM]ShmSize


Each connection has a shared memory segment where messages between the
nodes are put by the sender and read by the reader. This segment has a size
defined by this parameter. Default value is 1MB.


[SHM]SendSignalId


To be able to retrace a distributed message diagram it is necessary to
identify each message with an identity. By setting this parameter to Y
these message identities are also transported over the network.  This
feature is not enabled by default.


[SHM]Checksum


This parameter is also a Y/N parameter which is not enabled by default.
When enabled all messages are checksummed before put into the send buffer.
This feature enables control that messages are not corrupted while waiting
in the send buffer. It is also a double check that the transport mechanism
haven't corrupted the data.






17.3.4.8  Defining SCI Transporter Connections in a MySQL Cluster




SCI Transporters as connection between nodes in the MySQL Cluster is only
supported for special builds of MySQL Cluster using the @command{configure}
parameter --with-ndb-sci=/your/path/to/SCI. The path should point
to a directory that contains at least a lib and a include directory where
SISCI libraries and header files are provided.





It is strongly recommended to only use SCI Transporters for communication
between ndbd processes. Also using SCI Transporters will mean that the
ndbd process will never sleep so use SCI Transporters only for machines
with at least 2 CPU's which are dedicated for use by ndbd process(es). There
should be at least 1 CPU per ndbd process in this case and in addition at 
least one more is needed to also handle OS activities.







[SCI]NodeId1



[SCI]NodeId2


To identify a connection between two nodes it is necessary to provide the
node identity for both of them in NodeId1 and NodeId2.


[SCI]Host1SciId0


This identifies the SCI node id on the first node identified by NodeId1.


[SCI]Host1SciId1


It is possible to set-up SCI Transporters for fail-over between two SCI
cards which then should use separate networks between the nodes. This
identifies the node id and the second SCI card to be used on the first
node.


[SCI]Host2SciId0


This identifies the SCI node id on the second node identified by NodeId2.


[SCI]Host2SciId1


It is possible to set-up SCI Transporters for fail-over between two SCI
cards which then should use separate networks between the nodes. This
identifies the node id and the second SCI card to be used on the second
node.


[SCI]SharedBufferSize


Each SCI transporter has a shared memory segment between the two nodes. With
this segment set to the default 1 MB most applications should be ok. Smaller
sizes such as 256 kB has problems when performing many parallel inserts.
If the buffer is too small it can cause crashes of the ndbd process.


[SCI]SendLimit


A small buffer in front of the SCI media buffers up messages before sending
them over the SCI network. By default this is set to 8kB. Most benchmark
measurements shows that tops are reached at 64 kB but 16kB reaches within a
few percent of the performance and for all MySQL Cluster benchmarks it was
no measurable difference in increasing it beyond 8kB.


[SCI]SendSignalId


To be able to retrace a distributed message diagram it is necessary to
identify each message with an identity. By setting this parameter to Y
these message identities are also transported over the network.  This
feature is not enabled by default.


[SCI]Checksum


This parameter is also a Y/N parameter which is not enabled by default.
When enabled all messages are checksummed before put into the send buffer.
This feature enables control that messages are not corrupted while waiting
in the send buffer. It is also a double check that the transport mechanism
haven't corrupted the data.






17.4  Process Management in MySQL Cluster




There are four processes that are important to know about when using MySQL
Cluster. We will cover how to work with those processes, which options to
use when starting and so forth.







17.4.1  MySQL Server Process Usage for MySQL Cluster




@command{mysqld} is the traditional MySQL server process. To be used with
MySQL Cluster it needs to be built with support for the NDB Cluster storage
engine. If the @command{mysqld} binary has been built in such a manner, the
NDB Cluster storage engine is still disabled by default.





To enable the NDB Cluster storage engine there are two ways. Either use
--ndbcluster as a startup option when starting @command{mysqld} or
insert a line with ndbcluster in the [mysqld] section of your
`my.cnf' file.
An easy way to verify that your server runs with support for the
NDB Cluster storage engine is to issue the command
SHOW ENGINES from a @command{mysql} client.
You should see YES for the row listing NDBCLUSTER.
If you see NO, you are not running a @command{mysqld} that is compiled
with NDB Cluster support enabled. If you see DISABLED, then you
need to enable it in the my.cnf configuration file.





The MySQL server needs to know how to get the configuration of the cluster.
To access this configuration, it needs to know three things:








The node ID can be skipped from MySQL 4.1.5 on, because a node ID can be
dynamically allocated.





There are currently three possible ways to provide this information to the
@command{mysqld} process. The recommended option is to set the connect
string as a mysqld parameter ndb-connectstring either when
starting mysqld or in my.cnf.
You may alternatively include this information in a
file called `Ndb.cfg'. This file should reside in the data directory
of the MySQL Server.  As another option you set an environment variable
called NDB_CONNECTSTRING. The string is the same in all cases,
"[nodeid=<id>;][host=]<host>:<port>". If no information is supplied
then it will default to "host=localhost:2200".





shell> mysqld --ndb-connectstring=ndb_mgmd.mysql.com:2200





ndb_mgmd.mysql.com is the host where the management server resides,
and it is listening to port 2200.





With this setup the MySQL server will be a full citizen of MySQL Cluster
and will be fully aware of all storage nodes in the cluster and their
status. It will setup connection to all storage nodes and will be
able to use any storage node as a transaction coordinator and to
access their data for reading and updating.







17.4.2  @command{ndbd}, the Storage Engine Node Process




@command{ndbd} is the process that is used to handle all the data in the
tables using the NDB Cluster storage engine. This is the process that
contains all the logic of distributed transaction handling, node recovery,
checkpointing to disk, online backup, and lots of other functionality.





In a cluster there is a set of @command{ndbd} processes cooperating in
handling the data. These processes can execute on the same computer or on
different computers, in a completely configurable manner.





Before MySQL 4.1.5, @command{ndbd} process should start from a separate
directory. The reason for this was that @command{ndbd} generates a set of log
files in its starting directory.





In MySQL 4.1.5, this was changed such that the files are placed in the
directory specified by DataDir in the configuration file. Thus
@command{ndbd} can be started from anywhere.





These log files are (the 2 is the node ID):








It is recommended to not use a directory mounted through NFS because in some
environments that can cause problems with the lock on the pid-file remaining
even after the process has stopped.





Also when starting the @command{ndbd} process it may be necessary to specify
the hostname of the management server and the port it is listening to,
optionally one may specify node ID the process is to use.
Again there are three ways of specifying this
information.  Either in a string in the file `Ndb.cfg', this file
should be stored in the starting directory of the @command{ndbd} process. The
second option is to set the environment variable NDB_CONNECTSTRING
before starting the process. The third option is to use a command line parameter
as in the example below. See previous section for connect string format.





shell> ndbd --connect-string="nodeid=2;host=ndb_mgmd.mysql.com:2200"





When @command{ndbd} starts it will actually start two processes. The starting
process is called the "angel" and its only job is to discover when the
execution process has completed, and then restart the @command{ndbd} process if
configured to do so.  Thus if one attempts to kill @command{ndbd} through the
@command{kill} command in Unix, it is necessary to kill both processes.  A
more proper way to handle the stopping of @command{ndbd} processes is to use
the management client and stop the process from there.





The execution process uses one thread for all activities in reading,
writing, and scanning data and all other activities. This thread is designed
with asynchronous programming so it can easily handle thousands of
concurrent activites. In addition there is a watch-dog thread supervising
the execution thread to ensure it doesn't stop in an eternal loop or other
problem. There is a pool of threads handling file I/O. Each thread can
handle one open file. In addition threads can be used for connection
activities of the transporters in the @command{ndbd} process. Thus in a system
that performs a large number of activities including update activities the
@command{ndbd} process will consume up to about 2 CPUs if allowed to. Thus in
a large SMP box with many CPUs it is recommended to use several
@command{ndbd} processes which are configured to be part of different node
groups.






17.4.3  @command{ndb_mgmd}, the Management Server Process




The management server is the process that reads the configuration file of
the cluster and distributes this information to all nodes in the cluster
requesting it. It also maintains the log of cluster activities.
Management clients can connect to the management server and use commands to
check status of the cluster in various aspects.





As of MySQL 4.1.5, it is no longer to specify a connect string when starting
the management server.  If however you are using several management servers
a connect string should be provided and all nodes in the cluster should specify
their nodeid's explicitly.





The following files are created or used by @command{ndb_mgmd} in its starting
directory of @command{ndb_mgmd}. From MySQL 4.1.5, the log and PID files will
be placed in the DataDir specified in the configuration file:









17.4.4  @command{ndb_mgm}, the Management Client Process




The final important process to know about is the management client. This
process is not needed to run the cluster. Its value lies in its ability to
check status of the cluster, start backups, and perform other management
activities.  It does so by providing access to a set of commands.





Actually the management client is using a C API that is used to access the
management server so for advanced users it is also possible to program
dedicated management processes which can do similar things as the
management client can do.





When starting the management client, it is necessary to state the hostname
and port of the management server as in the example below. The default is
localhost as host and port number 2200.





shell> ndb_mgm localhost 2200






17.4.5  Command Options for MySQL Cluster Processes





17.4.5.1  MySQL Cluster-Related Command Options for @command{mysqld}






--ndbcluster


If the binary includes support for the NDB Cluster storage engine the
default disabling of support for the NDB storage engine can be
overruled by using this option. Using the NDB Cluster storage engine
is necessary for using MySQL Cluster.


--skip-ndbcluster


Disable the NDB Cluster storage engine. This is disabled by default
for binaries where it is included. So this option only applies if the
server was configured to use the NDB Cluster storage engine.


--ndb-connectstring=connect_string


When using the NDB storage engine, it is possible to point out the
management server that distributes the cluster configuration by setting the
connect string option.






17.4.5.2  Command Options for @command{ndbd}






-?, --usage


These options only starts the program to print its command options.


-c connect_string, --connect-string connect_string


For @command{ndbd} it is also possible to set the connect string to the
management server as a command option.



shell> ndbd --connect-string="nodeid=2;host=ndb_mgmd.mysql.com:2200"




-d, --daemon


Instructs @command{ndbd} to execute as a daemon process. From MySQL 4.1.5 on, this
is the default behavior.


--nodaemon


Instructs @command{ndbd} not to start as a daemon process. Useful when
@command{ndbd} is debugged and one wants printouts on the screen.


--initial


Instructs @command{ndbd} to perform an initial start. An initial start will
erase any files created by earlier @command{ndbd} instances for recovery. It
will also recreate recovery log files which on some operating systems can
take a substantial amount of time.

An initial start is only to be used at the very first start of the
@command{ndbd} process. It removes all files from the file system and
creates all REDO log files. When performing a software upgrade which has
changed the file contents on any files it is also necessary to use this
option when restarting the node with a new software version of
@command{ndbd}. Finally it could also be used as a final resort if for some
reason the node restart or system restart doesn't work. In this case be
aware that destroying the contents of the file system means that this node
can no longer be used to restore data.

This option does not affect any backup files created.

The previous possibility to use -i for this option was removed to
ensure that this option is not used by mistake.


--nostart


Instructs @command{ndbd} not to automatically start. @command{ndbd} will connect
to the management server and get the configuration and initialise
communication objects. It will not start the execution engine until
requested to do so by the management server. The management server can
request by command issued by the management client.


-v, --version


Prints the version number of the @command{ndbd} process. The version number is
the MySQL Cluster version number. It is important because at
startup the MySQL Cluster processes verifies that the versions of the
processes in the cluster can co-exist in the cluster. It is also important
for online software upgrade of MySQL Cluster (see section
Software Upgrade of MySQL Cluster).


--debug=options


This can be used only for versions compiled with debug information. It is
used to enable printouts from debug calls in the same manner as for the
@command{mysqld} process.


-? --usage


Prints a short description of the available command options.






17.4.5.3  Command Options for @command{ndb_mgmd}






-?, --usage


These options only start the program to print its command options.


-c filename --config-file=filename


Instructs the management server which file to use as configuration file.
This option must be specified. The file name defaults to config.ini.


-d --daemon


Instructs @command{ndb_mgmd} to start as a daemon process. This is the
default behavior.


-nodaemon


Instructs the management server not to start as a daemon process.


-v --version


Prints the version number of the management server. The version number is
the MySQL Cluster version number. The management server can check that only
versions capable of working with its versions are accepted and provided
with the configuration information.


--debug=options


This can only be used for versions compiled with debug information. It is
used to enable printouts from debug calls in the same manner as for the
@command{mysqld} process.






17.4.5.4  Command Options for @command{ndb_mgm}






-?, --usage


These options only starts the program to print its command options.


[host_name [port_num]]


To start the management client it is necessary to specify where the
management server resides. This means specifying the hostname and the port.
The default hostname is localhost and the default port is 2200.


--try-reconnect=number


If the connection to the management server is broken it is possible to
perform only a specified amount of retries before reporting a fault code to
the user. The default is that it keeps retrying every 5 seconds until it
succeeds.






17.5  Management of MySQL Cluster




Managing a MySQL Cluster involves a number of activities.
The first activity is to configure and startup MySQL Cluster.
This is covered by the sections section 17.3  MySQL Cluster Configuration
and section 17.4  Process Management in MySQL Cluster.
This section covers how to manage a running MySQL Cluster.





There are essentially two ways of actively managing a running MySQL Cluster.
The first is by commands entered into the management client where status of
cluster can be checked, log levels changed, backups started and stopped and
nodes can be stopped and started. The second method is to study the output
in the cluster log. The cluster log is directed towards the
`ndb_2_cluster.log' in the DataDir directory of the management
server.  The cluster log contains event reports generated from the
@command{ndbd} processes in the cluster. It is also possible to send the
cluster log entries to a Unix system log.







17.5.1  Commands in the Management Client




In addition to the central configuration file, the cluster
may also be controlled through a command line interface. The command
line interface is available through a separate management client process.
This is the main administrative interface to a running cluster.





The management client has the following basic commands.
Below, <id> denotes either a database node id (e.g. 21) or the
keyword ALL that indicates that the command should be applied
to all database nodes in the cluster.







HELP


Prints information on all available commands.

SHOW


Prints information on the status of the cluster.

<id> START


Start a database node identified with <id> or all database nodes.

<id> STOP


Stop a database node identified with <id> or all database nodes.

<id> RESTART [-N] [-I]


Restart a database node identified with <id> or all database nodes.

<id> STATUS


Displays status information for database node identified with <id>
(or ALL database nodes).

ENTER SINGLE USER MODE <id>


Enters single user mode where only the API with node <id> is
allowed to access the database system.

EXIT SINGLE USER MODE


Exists single user mode allowing all APIs to access the database system.

QUIT


Terminates the management client.

SHUTDOWN


Shuts down all cluster nodes (except mysql servers) and exits.





Commands for the event logs are given in the next section and commands
for backup and restore are given in a separate section on these topics.







17.5.2  Event Reports Generated in MySQL Cluster




MySQL Cluster has two event logs, the cluster log and the node log.








Note: The cluster log is the recommended log.  The node log is only
intended to be used during application development or for debugging
application code.





Each reportable event has the following properties:






The two logs (the cluster log and the node log) can be filtered on
these properties.







17.5.2.1  Logging Management Commands




The following management commands are related to the cluster log:







CLUSTERLOG ON


Turn cluster log on.

CLUSTERLOG OFF


Turn cluster log off.

CLUSTERLOG INFO


Information about cluster log settings.

<id> CLUSTERLOG <category>=<threshold>


Log category events with priority less than or equal to threshold in
the cluster log.

CLUSTERLOG FILTER <severity>


Toggles cluster logging of the specified severity type on/off.





The following table describes the default setting (for all database
nodes) of the cluster log category threshold.  If an event has a
priority with a value lower than or equal to the priority threshold,
then it is reported in the cluster log.





Note that the events are reported per database node and that the
thresholds can be set differently on different nodes.






Category  Default threshold (All database nodes)



STARTUP  7



SHUTDOWN  7



STATISTICS  7



CHECKPOINT  7



NODERESTART  7



CONNECTION  7



ERROR  15



INFO  7







The threshold is used to filter events within each category.
For example: a STARTUP event with a priority of 3 is never sent
unless the threshold for STARTUP is changed to 3 or lower.
Only events with priority 3 or lower are sent if the threshold is 3.
The event severities are (corresponds to UNIX syslog levels):






1  ALERT  A condition that should be corrected immediately, such as a corrupted system database



2  CRITICAL  Critical conditions, such as device errors or out of resources



3  ERROR  Conditions that should be corrected, such as configuration errors



4  WARNING  Conditions that are not error conditions but might require handling



5  INFO  Informational messages



6  DEBUG  Messages used during development of NDB Cluster







Syslog's LOG_EMERG and LOG_NOTICE are not used/mapped.





The event severities can be turned on or off.  If the severity is on
then all events with priority less than or equal to the category
thresholds are logged. If the severity is off then no events belonging
to the severity are logged.






The following commands are related to the node log:









17.5.2.2  Log Events




All reportable events are listed below.






Event  Category  Priority  Severity  Description




DB nodes connected  CONNECTION  8  INFO 



DB nodes disconnected  CONNECTION  8  INFO 



Communication closed  CONNECTION  8  INFO  API & DB nodes connection closed



Communication opened  CONNECTION  8  INFO  API & DB nodes connection opened




Global checkpoint started  CHECKPOINT  9  INFO  Start of a GCP, i.e., REDO log is written to disk



Global checkpoint completed  CHECKPOINT  10  INFO  GCP finished



Local checkpoint started  CHECKPOINT  7  INFO  Start of local check pointing, i.e., data is written to disk. LCP Id and GCI Ids (keep and oldest restorable)



Local checkpoint completed  CHECKPOINT  8  INFO  LCP finished



LCP stopped in calc keep GCI  CHECKPOINT  0  ALERT  LCP stopped!



Local checkpoint fragment completed  CHECKPOINT  11  INFO  A LCP on a fragment has been completed



Report undo log blocked  CHECKPOINT  7  INFO  Reports undo logging blocked due buffer near to overflow




DB node start phases initiated  STARTUP  1  INFO  NDB Cluster starting



DB node all start phases completed  STARTUP  1  INFO  NDB Cluster started



Internal start signal received STTORRY  STARTUP  15  INFO  Internal start signal to blocks received after restart finished



DB node start phase X completed  STARTUP  4  INFO  A start phase has completed



Node has been successfully included into the cluster  STARTUP  3  INFO  President node, own node and dynamic id is shown



Node has been refused to be included into the cluster  STARTUP  8  INFO 



DB node neighbours  STARTUP  8  INFO  Show left and right DB nodes neighbours



DB node shutdown initiated  STARTUP  1  INFO 



DB node shutdown aborted  STARTUP  1  INFO 



New REDO log started  STARTUP  10  INFO  GCI keep X, newest restorable GCI Y



New log started  STARTUP  10  INFO  Log part X, start MB Y, stop MB Z



Undo records executed  STARTUP  15  INFO 




Completed copying of dictionary information  NODERESTART  8  INFO 



Completed copying distribution information  NODERESTART  8  INFO 



Starting to copy fragments  NODERESTART  8  INFO 



Completed copying a fragment  NODERESTART  10  INFO 



Completed copying all fragments  NODERESTART  8  INFO 



Node failure phase completed  NODERESTART  8  ALERT  Reports node failure phases



Node has failed,  node state was X  NODERESTART  8  ALERT  Reports that a node has failed




Report whether an arbitrator is found or not  NODERESTART  6  INFO  7 different cases



    - President restarts arbitration thread [state=X]



    - Prepare arbitrator node X [ticket=Y]



    - Receive arbitrator node X [ticket=Y]



    - Started arbitrator node X [ticket=Y]



    - Lost arbitrator node X - process failure [state=Y]



    - Lost arbitrator node X - process exit [state=Y]



    - Lost arbitrator node X <error msg>[state=Y]




Report arbitrator results  NODERESTART  2  ALERT  8 different results



    - Arbitration check lost - less than 1/2 nodes left



    - Arbitration check won - node group majority



    - Arbitration check lost - missing node group



    - Network partitioning - arbitration required



    - Arbitration won - positive reply from node X



    - Arbitration lost - negative reply from node X



    - Network partitioning - no arbitrator available



    - Network partitioning - no arbitrator configured




GCP take over started  NODERESTART  7  INFO 



GCP take over completed  NODERESTART  7  INFO 



LCP take over started  NODERESTART  7  INFO 



LCP take completed (state = X)  NODERESTART  7  INFO 




Report transaction statistics  STATISTICS  8  INFO  # of: transactions, commits, reads, simple reads, writes, concurrent operations, attribute info, aborts



Report operations  STATISTICS  8  INFO  # of operations



Report table create  STATISTICS  7  INFO 



Report job scheduling statistics  STATISTICS  9  INFO  Mean Internal job scheduling statistics



Sent # of bytes  STATISTICS  9  INFO  Mean # of bytes sent to node X



Received # of bytes  STATISTICS  9  INFO  Mean # of bytes received from node X



Memory usage  STATISTICS  5  INFO  Data and Index memory usage (80%, 90% and 100%)




Transporter errors  ERROR  2  ERROR 



Transporter warnings  ERROR  8  WARNING 



Missed heartbeats  ERROR  8  WARNING  Node X missed heartbeat # Y



Dead due to missed heartbeat  ERROR  8  ALERT  Node X declared dead due to missed heartbeat



General warning events  ERROR  2  WARNING 




Sent heartbeat  INFO  12  INFO  Heartbeat sent to node X



Create log bytes  INFO  11  INFO  Log part, log file, MB



General info events  INFO  2  INFO 







An event report has the following format in the logs:


<date & time in GMT> [<any string>] <event severity> -- <log message>

09:19:30 2003-04-24 [NDB] INFO -- Node 4 Start phase 4 completed






17.5.3  Single User Mode




Single user mode allows the database administrator to restrict access
to the database system to only one application (API node).  When
entering single user mode all connections to all APIs will be
gracefully closed and no transactions are allowed to be started.
All running transactions are aborted.





When the cluster has entered single user mode (use the all status
command to see when the state has entered the single user mode), only
the allowed API node is granted access to the database.





Example:


ENTER SINGLE USER MODE 5





After executing this command and after cluster has entered the single
user mode, the API node with node id 5 becomes the single user of the
cluster.





The node specified in the command above must be a MySQL Server node.
Any attempt to specify any other type of node will be rejected.





Note: if the node with id 5 is running when executing
ENTER SINGLE USER MODE 5, all transactions running on node 5
will be aborted, the connection is closed, and the server must be
restarted.





The command EXIT SINGLE USER MODE alters the state of the
cluster DB nodes from ``single user mode'' to ``started''.
MySQL Servers waiting for a connection, i.e. for the cluster to become
ready, are now allowed to connect.  The MySQL Server denoted as the
single user continues to run, if it is connected, during and after the
state transition. 





Example:


EXIT SINGLE USER MODE





Best practice in case of node failures when running in single user
mode is to:





    

  1. Finish all single user mode transactions


  2. Issue the command exit single user mode


  3. Restart database nodes






Or restart database nodes prior to entering single user mode.







17.5.4  On-line Backup of MySQL Cluster




This section describes how to create a backup and later restore the
backup to a database.







17.5.4.1  Cluster Backup Concepts




A backup is a snapshot of the database at a given time.
The backup contains three main parts:





    

  1. Meta data (what tables exists etc)


  2. Table records (data in tables)


  3. A log of committed transactions






Each of these parts is saved on all nodes participating in a backup. 





During backup each node saves these three parts to disk into three files:







`BACKUP-<BackupId>.<NodeId>.ctl'


The control file which contain control information and meta data.

`BACKUP-<BackupId>-0.<NodeId>.data'


The data file that contain the table records.

`BACKUP-<BackupId>.<NodeId>.log'


The log file that contain the committed transactions.





Above <BackupId> is an identifier for the backup and <NodeId> is the
node id of the node creating the file.







Meta data


The meta data consists of table definitions. All nodes have the exact
same table definitions saved on disk.

Table records


The table records are saved per fragment. Each fragment contains a
header that describes which table the records belong to.  After the
list of records there is a footer that contains a checksum for the
records. Different nodes save different fragments during the backup.

Committed log


The committed log contains committed transaction made during the
backup.  Only transactions on tables stored in the backup are stored
in the log.  The different nodes in the backup saves different log
records as they host different database fragments.






17.5.4.2  Using The Management Server to Create a Backup




Before starting make sure that the cluster is properly configured for
backups.





    

  1. 

Start management server.

  2. 

Execute the command START BACKUP.

  3. 

The management server will reply with a message
``Start of backup ordered''.
This means that the management server has submitted the request to the
cluster, but has not yet received any response.

  4. 

The management server will reply ``Backup <BackupId> started'' where
<BackupId> is the unique identifier for this particular backup.
This will also be saved in the cluster log (if not configured otherwise).
This means that the cluster has received and processed the backup
request. It does not mean that the backup has completed.

  5. 

The management server will when the backup is finished reply
``Backup <BackupId> completed''.





Using the management server to abort a backup:





    

  1. 

Start management server.

  2. 

Execute the command ABORT BACKUP <BACKUPID>.
The number <BackupId> is the identifier of the backup that is included
in the response of the management server when the backup is started,
i.e. in the message ``Backup <BackupId> started''.
The identifier is also saved in the cluster log (cluster.log).

  3. 

The management server will reply ``Abort of backup <BackupId> ordered''
This means that it has submitted the request to the cluster, but has
not received any response.

  4. 

The management server will reply ``Backup <BackupId> has been aborted
reason XYZ''. This means that the cluster has aborted the backup and
removed everything belonging to it, including the files in the file
system.





Note that if there is not any backup with id <BackupId> running when
it is aborted, the management server will not reply anything.  However
there will be a line in the cluster.log mentioning that an ``invalid''
abort command has been filed.







17.5.4.3  How to Restore a Cluster Backup




The restore program is implemented as seperate command line utility.
It reads the files created from the backup and inserts the stored
information into the database. The restore program has to be executed
once for each set of backup files, i.e. as many times as there were
database nodes running when the backup we created.





The first time you run the restore program you also need to restore
the meta data, i.e. create tables. The restore program acts as an
API to the cluster and therefore needs a free connection to connect
to. This can be verified with the @command{ndb_mgm} command SHOW. In the 
same way as with other API nodes, e.g. the mysqld, the `Ndb.cfg', 
the NDB_CONNECTSTRING or the switch -c <connectstring> may 
be used to locate the MGM node.  The backup files must be present in 
the directory given as an argument to the restore program.  The backup 
can be restored to a database with a different configuration than it was
created from.  For example, consider if a backup (with id 12) created
in a cluster with two database nodes (with node id 2 and node id 3)
that should be restored to a cluster with four nodes.  The restore
program then has to be executed two times (one for each database node
in the cluster where the backup was taken) as described in the box
below.





Note: for rapid restore, the data may be restored in parallell
(provided that there are enough free API connections available). Note
however that the data files must always be applied before the logs.





Note: the cluster should have an empty database when starting to
restore a backup. 







17.5.4.4  Configuration for Cluster Backup




There are four configuration parameters for backup:







BackupDataBufferSize


Amount of memory (out of the total memory) used to buffer data before
it is written to disk.

BackupLogBufferSize


Amount of memory (out of the total memory) used to buffer log records
before these are written to disk.

BackupMemory


Total memory allocated in a database node for backups.  This should be
the sum of the memory allocated for the two buffers.

BackupWriteSize


Size of blocks written to disk.  This applies for both the data buffer
and the log buffer.






17.5.4.5  Backup Troubleshooting




If an error code is returned when issuing a backup request, then check
that there is enough memory allocated for the backup (i.e. the
configuration parameters).  Also check that there is enough space on
the hard drive partition of the backup target.







17.6  Using High-Speed Interconnects with MySQL Cluster




Already before design of NDB Cluster started in 1996 it was evident that one
of the major problems of building parallel databases is the communication
between the nodes in the network. Thus from the very beginning NDB Cluster
was designed with a transporter concept to allow for different transporters.





At the moment the code base includes 4 different transporters where 3 of them
are currently working. Most users today uses TCP/IP over Ethernet since this
exists in all machines. This is also by far the most well-tested transporter
in MySQL Cluster.





Within MySQL we are working hard to ensure that communication with the ndbd
process is made in as large chunks as possible since this will benefit all
communication medias since all means of transportation benefits from sending
large messages over small messages.





For users which desire top performance it is however also possible to use
cluster interconnects to increase performance even further. There are two
ways to achieve this, either a transporter can be designed to handle this
case, or one can use socket implementations that bypass the TCP/IP stack to
a small or large extent.





We have made some experiments with both those variants using SCI technology
developed by Dolphin (www.dolphinics.no).






17.6.1  Configuring MySQL Cluster to use SCI Sockets




In this section we will show how one can use a cluster configured for normal
TCP/IP communication to instead use SCI Sockets. Prerequisites for doing this
is that the machines to communicate needs to be equipped with SCI cards. This
documentation is based on the SCI Socket version 2.3.0 as of 1 october 2004.





To use SCI Sockets one can use any version of MySQL Cluster. The tests were
performed on an early 4.1.6 version. No special builds are needed since
it uses normal socket calls which is the normal configuration set-up for
MySQL Cluster. SCI Sockets are only supported on Linux 2.4 and 2.6 kernels at the
moment. SCI Transporters works on more OS's although only Linux 2.4 have been
verified.





There are essentially four things needed to enable SCI Sockets. First it is
necessary to build the SCI Socket libraries. Second the SCI Socket kernel
libraries need to be installed. Third one or two configuration files needs to
be installed.
At last the the SCI Socket kernel library needs to be enabled either for the
entire machine or for the shell where the MySQL Cluster processes are started
from. This process needs to be repeated for each machine in cluster which will
use SCI Sockets to communicate.





Two packages need to be retrieved to get SCI Sockets working. The first package
builds the libraries which SCI Sockets are built upon and the second is the
actual SCI Socket libraries. Currently the distribution is only in source
code format.





The latest versions of these packages is currently found at. Check


http://www.dolphinics.no/support/downloads.html





for latest versions.





http://www.dolphinics.no/ftp/source/DIS_GPL_2_5_0_SEP_10_2004.tar.gz
http://www.dolphinics.no/ftp/source/SCI_SOCKET_2_3_0_OKT_01_2004.tar.gz





The next step is to unpack those directories, SCI Sockets is unpacked below
the DIS code. Then the code base is compiled. The example below shows the
commands used in Linux/x86 to perform this.





shell> tar xzf DIS_GPL_2_5_0_SEP_10_2004.tar.gz
shell> cd DIS_GPL_2_5_0_SEP_10_2004/src/
shell> tar xzf ../../SCI_SOCKET_2_3_0_OKT_01_2004.tar.gz
shell> cd ../adm/bin/Linux_pkgs
shell> ./make_PSB_66_release





If the build is made on an Opteron box and is to use the 64 bit extensions
then use make_PSB_66_X86_64_release instead, if the build is made on an
Itanium box then use make_PSB_66_IA64_release instead. The X86-64 variant
should work for Intel EM64T architectures but no known tests of this exists
yet.





After building the code base it has been put into a zipped tar filed DIS
and OS and date. It is now time to install the package in the proper place.
In this example we will place the installation in /opt/DIS. These actions
will most likely require you to log in as root-user.





shell> cp DIS_Linux_2.4.20-8_181004.tar.gz /opt/
shell> cd /opt
shell> tar xzf DIS_Linux_2.4.20-8_181004.tar.gz
shell> mv DIS_Linux_2.4.20-8_181004 DIS





Now that all the libraries and binaries are in their proper place we need to
ensure that SCI cards gets proper node identities within the SCI address space.
Since SCI is a networking gear it is necessary to decide on the network structure
at first.





There are three types of network structures, the first is a simple
one-dimensional ring, the second uses SCI switch(es) with one ring per switch
port and finally there are 2D/3D torus. Each has its standard of providing
node ids.





A simple ring uses simply node ids displaced by 4.


4, 8, 12, ....





The next possibility uses switch(es). The SCI switch has 8 ports. On each port
it is possible to place a ring. It is here necessary to ensure that the
rings on the switch uses different node id spaces. So the first port uses
node ids below 64 and the next 64 node ids are allocated for the next port and
so forth.


4,8, 12, ... , 60  Ring on first port
68, 72, .... , 124 Ring on second port
132, 136, ..., 188 Ring on third port
..
452, 456, ..., 508 Ring on the eight port





2D/3D torus network structures takes into account where each node is in each
dimension, increment by 4 for each node in the first dimension, by 64 in the
second dimension and by 1024 in the third dimension. Please look in the Dolphin
for more thorough documentation on this.





In our testing we have used switches. Most of the really big cluster
installations uses 2D/3D torus. The extra feature which switches provide
is that with dual SCI cards and dual switches we can easily build a
redundant network where failover times on the SCI network is around 100
microseconds. This feature is supported by the SCI transporter and is
currently also developed for the SCI Socket implementation.





Failover for 2D/3D torus is also possible but requires sending out new
routing indexes to all nodes. Even this will complete in around 100
milliseconds and should be ok for most high-availability cases.





By placing the NDB nodes in proper places in the switched architecture it is
possible to use 2 switches to build a structure where 16 computers can be
interconnected and no single failure can hamper more than one computer.
With 32 computers and 2 switches it is possible to configure the cluster in
such a manner that no single failure can hamper more than two nodes and
in this case it is also known which pair will be hit. Thus by placing those
two in separate NDB node groups it is possible to build a safe MySQL Cluster
installation. We won't go into details in how this is done, since it is
likely to be only of interest for users wanting to go real deep into this.





To set the node id of an SCI card use the following command still being
in the /opt/DIS/sbin directory. -c 1 refers to the number of the SCI
card, where 1 is this number if only 1 card is in the machine. In this case
use adapter 0 always (set by -a 0). 68 is the node id set in this example.


shell> ./sciconfig -c 1 -a 0 -n 68





In case you have several SCI cards in your machine the only safe to discover
which card has which slot is by issuing the following command


shell> ./sciconfig -c 1 -gsn





This will give the serial number which can be found at the back of the SCI
card and on the card itself. Do this then for -c 2 and onwards as many cards
there are in the machine. This will identify which cards uses which id. Then
set node ids for all cards.





Now we have installed the necessary libraries and binaries. We have also set the
SCI node ids. The next step is to set the mapping from hostnames (or IP addresses)
to SCI node ids.





The configuration file for SCI Sockets is to be placed in the file
/etc/sci/scisock.conf. This file contains a mapping from hostnames
(or IP addresses) to SCI node ids. The SCI node id will map the hostname
to communicate through the proper SCI card. Below is a very simple such
configuration file.


#host           #nodeId 
alpha           8
beta            12
192.168.10.20   16





It is also possible to limit this configuration to only apply for a subset of
the ports of these hostnames. To do this another configuration is used which
is placed in /etc/sci/scisock_opt.conf.





#-key                        -type        -values
EnablePortsByDefault		          yes
EnablePort                  tcp           2200
DisablePort                 tcp           2201
EnablePortRange             tcp           2202 2219 
DisablePortRange            tcp           2220 2231 





Now we ready to install the drivers. We need to first install the low-level drivers
and then the SCI Socket driver.





shell> cd DIS/sbin/
shell> ./drv-install add PSB66
shell> ./scisocket-install add





If desirable one can now check the installation by invoking a script which checks
that all nodes in the SCI Socket config files are accessible.





shell> cd /opt/DIS/sbin/
shell> ./status.sh





If you discover an error and need to change the SCI Socket config files then it is
necessary to use a program ksocketconfig to change the configuration.





shell> cd /opt/DIS/util
shell> ./ksocketconfig -f





To check that SCI Sockets are actually used you can use a test program
latency_bench which needs to have a server component and clients can
connect to the server to test the latency, whether SCI is enabled is very clear
from the latency you get. Before you use those programs you also need to set
the LD_PRELOAD variable in the same manner as shown below.





To set up a server use the command


shell> cd /opt/DIS/bin/socket
shell> ./latency_bench -server





To run a client use the following command


shell> cd /opt/DIS/bin/socket
shell> ./latency_bench -client hostname_of_server





Now the SCI Socket configuration is completed. MySQL Cluster is now
ready to use both SCI Sockets and the SCI transporter documented in
section 17.3.4.8  Defining SCI Transporter Connections in a MySQL Cluster.





The next step is to start-up MySQL Cluster. To enable usage of SCI Sockets
it is necessary to set the environment variable LD_PRELOAD before starting
the ndbd, mysqld and ndb_mgmd processes to use SCI Sockets. The LD_PRELOAD
variable should point to the kernel library for SCI Sockets.





So as an example to start up ndbd in a bash-shell use the following
commands.





bash-shell> export LD_PRELOAD=/opt/DIS/lib/libkscisock.so
bash-shell> ndbd





From a tcsh environment the same thing would be accomplished with
the following commands.





tcsh-shell> setenv LD_PRELOAD=/opt/DIS/lib/libkscisock.so
tcsh-shell> ndbd





Noteworthy here is that MySQL Cluster can only use the kernel variant of
SCI Sockets.






17.6.2  Low-level benchmarks to understand impact of cluster interconnects




The ndbd process has a number of simple constructs which are used to access
the data in MySQL Cluster. We made a very simple benchmark to check the
performance of each such statement and the effect various interconnects have
on their performance.





There are four access methods:




Primary key access


This is a simple access of one record through its primary key. In the simplest
case only one record is accessed at a time. This means that the full cost of
setting up a number of TCP/IP message and a number of costs for context
switching is taken by this single request. In a batched case where e.g. 32
primary key accesses are sent in one batch then those 32 accesses will share
the set-up cost of TCP/IP messages and context switches (if the TCP/IP are
for different destinations then naturally a number of TCP/IP messages needs
to be set up.


Unique key access


Unique key accesses are very similar to primary key accesses except that they
are executed as a read of an index table followed by a primary key access on
the table. However only one request is sent from the MySQL Server, the read
of the index table is handled by the ndbd process. Thus again these requests
benefit from being accessed in batches.


Full table scan


When no indexes exist for the lookup on a table, then a full scan of a table
is performed. This is one request to the ndbd process which divides the
table scan into a set of parallel scans on all ndbd processes in the cluster.
In future versions the MySQL server will be able to push down some filtering
in those scans.
When no indexes exist for the lookup on a table, then a full scan of a table
is performed. This is one request to the ndbd process which divides the
table scan into a set of parallel scans on all ndbd processes in the cluster.
In future versions the MySQL server will be able to push down some filtering
in those scans.


Range scan using ordered index


When an ordered index is used it will perform a scan in the same manner as the
full table scan except that it will only scan those records which are in the
range used by the query set-up by the MySQL server. In future versions a
special optimisation will ensure that when all index attributes that are
bound includes all attributes in the partitioning key then only one partition
will be scanned instead of all in parallel.





To check the base performance of these access methods we developed a set of
benchmarks. One such benchmark, testReadPerf issues, simple primary and unique
key access, batched primary and unique key accesses. The benchmark also
measures the set-up cost of range scans by issuing scans returning a single
record and finally there is a variant which uses a range scan to fetch a
batch of records.





In this manner we can test the cost of issuing single key access and single
record scan accesses and measure the impact of the communication media
implementation of these base access methods.





We executed those base benchmark both using a normal transporter using TCP/IP
sockets and a similar set-up using SCI sockets. The figures reported below
is for small accesses of 20 records per of data per access. The difference
between serial and batched goes down by a factor of 3-4 when using 2 kB
records instead. SCI Sockets were not tested with 2 kB record2 kB records. Tests
were performed on a 2-node cluster with 2 dual CPU machines equipped with
AMD MP1900+ processors.





Access type:         TCP/IP sockets           SCI Socket
Serial pk access:    400 microseconds         160 microseconds
Batched pk access:    28 microseconds          22 microseconds
Serial uk access:    500 microseconds         250 microseconds
Batched uk access:    70 microseconds          36 microseconds
Indexed eq-bound:   1250 microseconds         750 microseconds
Index range:          24 microseconds          12 microseconds





We did also another set of tests to check the performance of SCI Sockets
compared to using the SCI transporter and all compared to the TCP/IP
transporter. All these tests used primary key accesses either serially,
multi-threaded and multi-threaded and batched simultaneously.





More or less all of these tests showed that SCI sockets were about
100% faster compared to TCP/IP. The SCI transporter was faster in most cases
compared to SCI sockets. One notable case however with many threads in the
test program showed that the SCI transporter behaved really bad if used in the
mysqld process.





Thus our conclusion overall is that for most benchmarks SCI sockets improves
performance with around 100% compared to TCP/IP except in rare cases when
communication performance is not an issue such as when scan filters make up
most of processing time or when very large batches of primary key accesses are
achieved. In that case the CPU processing in the ndbd processes becomes a fairly
large part of the cost.





Using the SCI transporter instead of SCI Sockets is only of interest in
communicating between ndbd processes. Using the SCI transporter is also only
of interest if a CPU can be dedicated for the ndbd process since the SCI
transporter ensures that the ndbd will never go to sleep. It is also
important to ensure that the ndbd process priority is set in such a way that
the process doesn't lose in priority due to running for a long time (as
can be done by locking processes to CPU's in Linux 2.6). If this is a
possible configuration then ndbd process will benefit by 10-70% compared to
using SCI sockets (the larger figures when performing updates and probably
also on parallel scan activities).





There are several other implementations of optimised socket variants for
clusters reported in various papers. These include optimised socket
variants for Myrinet, Gigabit Ethernet, Infiniband and the VIA interface.
We have only tested MySQL Cluster so far with SCI sockets and we also
include documentation above on how to set-up SCI sockets using ordinary
TCP/IP configuration for MySQL Cluster.







18  Introduction to MaxDB




MaxDB is an enterprise-level database. MaxDB is the new name of a database management system formerly called SAP DB.







18.1  History of MaxDB




The  history of SAP DB goes back to the early 1980s when it was developed as
a commercial product (Adabas). The database has changed names several times
since then. When SAP AG, a company based in Walldorf, Germany, took over the
development of that database system, it was called SAP DB.





SAP developed that database system to serve as a storage system
for all heavy-duty SAP applications, namely R/3. SAP DB was meant
to provide an alternative to third-party database systems such as
Oracle, Microsoft SQL Server, and DB2 by IBM. In October 2000, SAP
AG released SAP DB under the GNU GPL license (see section G  GNU General Public License), 
thus making it open source software. In October 2003, more than 2,000 
customers of SAP AG were using SAP DB as their main database system, 
and more than another 2,000 customers were using it as a separate 
database system besides their main database, as part of the 
APO/LiveCache solution.





In May 2003, a technology partnership was formed between MySQL AB
and SAP AG. That partnership entitles MySQL AB to further develop SAP
DB, rename it, and sell commercial licenses of the renamed SAP DB
to customers who do not want to be bound to the restrictions imposed
on them when using that database system under the GNU GPL (see section G  GNU General Public License). In August 2003, SAP DB was renamed MaxDB by MySQL AB.






18.2  Licensing and Support




MaxDB can be used under the same licenses available for the other
products distributed by MySQL AB (section 1.4.3  MySQL Licenses). Thus,
MaxDB will be available under the GNU General Public License
(section G  GNU General Public License), and a commercial license (section 1.4  MySQL Support and Licensing).





MySQL will offer MaxDB support to non-SAP customers.





The first rebranded version was MaxDB 7.5.00, which was
released in November 2003.







18.3  MaxDB-Related Links




The main page for information about MaxDB is
http://www.mysql.com/products/maxdb. Information formerly
available at http://www.sapdb.org has been moved there.






18.4  Basic Concepts of MaxDB




MaxDB operates as a client/server product. It was developed to meet
the demands of installations processing a high volume of online
transactions. Both online backup and expansion of the database are
supported. Microsoft Clustered Server is supported directly for
multiple server implementations; other failover solutions must be
scripted manually. Database management tools are provided in both
Windows and browser-based implementations.






18.5  Feature Differences Between MaxDB and MySQL




The following list provides a short summary of the main differences
between MaxDB and MySQL; it is not complete.









18.6  Interoperability Features Between MaxDB and MySQL




The following features will be included in MaxDB versions to be
released shortly after the first 7.5.00 version. These features
will allow interoperation between MaxDB and MySQL.









18.7  Reserved Words in MaxDB




Like MySQL, MaxDB has a number of reserved words that have special meanings.
Normally, they cannot be used
as names of identifiers, such as database or table names. The following
table lists reserved words in MaxDB, indicates the context in which
those words are used, and indicates whether or not they have
counterparts in MySQL. If such a counterpart exists, the meaning
in MySQL might be identical or differing in some aspects. The main
purpose is to list in which respects MaxDB differs from MySQL;
therefore, this list is not complete.





For the list of reserved words in MySQL, see section 10.6  Treatment of Reserved Words in MySQL.






Reserved in MaxDB
  Context of usage in MaxDB
  MySQL counterpart



@
  Can prefix identifier, like ``@table''
  Not allowed



ADDDATE()
  SQL function
  ADDDATE(); new in MySQL 4.1.1



ADDTIME()
  SQL function
  ADDTIME(); new in MySQL 4.1.1



ALPHA
  SQL function
  Nothing comparable



ARRAY
  Data type
  Not implemented



ASCII()
  SQL function
  ASCII(), but implemented with a different meaning



AUTOCOMMIT
  Transactions; ON by default
  Transactions; OFF by default



BOOLEAN
  Column types; BOOLEAN accepts as values only TRUE, FALSE, and NULL
  BOOLEAN was added in MySQL 4.1.0; it is a synonym for BOOL which is mapped to TINYINT(1). It accepts integer values in the same range as TINYINT as well as NULL. TRUE and FALSE can be used as aliases for 1 and 0.



CHECK
  CHECK TABLE
  CHECK TABLE; similar, but not identical usage



COLUMN
  Column types
  COLUMN; noise word



CHAR()
  SQL function
  CHAR(); identical syntax; similar, not identical usage



COMMIT
  Implicit commits of transactions happen when data definition statements are issued
  Implicit commits of transactions happen when data definition statements are issued, and also with a number of other statements



COSH()
  SQL function
  Nothing comparable



COT()
  SQL function
  COT(); identical syntax and implementation



CREATE
  SQL, data definition language
  CREATE



DATABASE
  SQL function
  DATABASE(); DATABASE is used in a different context; for example, CREATE DATABASE



DATE()
  SQL function
  CURRENT_DATE



DATEDIFF()
  SQL function
  DATEDIFF(); new in MySQL 4.1.1



DAY()
  SQL function
  Nothing comparable



DAYOFWEEK()
  SQL function
  DAYOFWEEK(); by default, 1 represents Monday in MaxDB and Sunday in MySQL



DISTINCT
  SQL functions AVG, MAX, MIN, SUM
  DISTINCT; but used in a different context: SELECT DISTINCT



DROP
  DROP INDEX, for example
  DROP INDEX; similar, but not identical usage



EBCDIC()
  SQL function
  Nothing comparable



EXPAND()
  SQL function
  Nothing comparable



EXPLAIN
  Optimization
  EXPLAIN; similar, but not identical usage



FIXED()
  SQL function
  Nothing comparable



FLOAT()
  SQL function
  Nothing comparable



HEX()
  SQL function
  HEX(); similar, but not identical usage



INDEX()
  SQL function
  INSTR() or LOCATE(); similar, but not identical syntaxes and meanings



INDEX
  USE INDEX, IGNORE INDEX and similar hints are used right after SELECT; for example, SELECT ... USE INDEX
  USE INDEX, IGNORE INDEX and similar hints are used in the FROM clause of a SELECT query; for example, in SELECT ... FROM ... USE INDEX



INITCAP()
  SQL function
  Nothing comparable



LENGTH()
  SQL function
  LENGTH(); identical syntax, but slightly different implementation



LFILL()
  SQL function
  Nothing comparable



LIKE
  Comparisons
  LIKE; but the extended LIKE MaxDB provides rather resembles the MySQL REGEX



LIKE wildcards
  MaxDB supports ``%'', ``_'', ``Control-underline'', ``Control-up arrow'', ``*'', and ``?'' as wildcards in LIKE comparisons
  MySQL supports ``%'', and ``_'' as wildcards in LIKE comparisons



LPAD()
  SQL function
  LPAD(); slightly different implementation



LTRIM()
  SQL function
  LTRIM(); slightly different implementation



MAKEDATE()
  SQL function
  MAKEDATE(); new in MySQL 4.1.1



MAKETIME()
  SQL function
  MAKETIME(); new in MySQL 4.1.1



MAPCHAR()
  SQL function
  Nothing comparable



MICROSECOND()
  SQL function
  MICROSECOND(); new in MySQL 4.1.1



NOROUND()
  SQL function
  Nothing comparable



NULL
  Column types; comparisons
  NULL; MaxDB supports special NULL values that are returned by arithmetic operations that lead to an overflow or a division by zero; MySQL does not support such special values



PI
  SQL function
  PI(); identical syntax and implementation, but parentheses are mandatory in MySQL



REF
  Data type
  Nothing comparable



RFILL()
  SQL function
  Nothing comparable



ROWNO
  Predicate in WHERE clause
  Similar to LIMIT clause



RPAD()
  SQL function
  RPAD(); slightly different implementation



RTRIM()
  SQL function
  RTRIM(); slightly different implementation



SEQUENCE
  CREATE SEQUENCE, DROP SEQUENCE
  AUTO_INCREMENT; similar concept, but different implementation



SINH()
  SQL function
  Nothing comparable



SOUNDS()
  SQL function
  SOUNDEX(); slightly different syntax



STATISTICS
  UPDATE STATISTICS
  ANALYZE TABLE; similar concept, but different implementation



SUBSTR()
  SQL function
  SUBSTRING(); slightly different implementation



SUBTIME()
  SQL function
  SUBTIME(); new in MySQL 4.1.1



SYNONYM
  Data definition language: CREATE [PUBLIC] SYNONYM, RENAME SYNONYM, DROP SYNONYM
  Nothing comparable



TANH()
  SQL function
  Nothing comparable



TIME()
  SQL function
  CURRENT_TIME



TIMEDIFF()
  SQL function
  TIMEDIFF(); new in MySQL 4.1.1



TIMESTAMP()
  SQL function
  TIMESTAMP(); new in MySQL 4.1.1



TIMESTAMP() as argument to DAYOFMONTH() and DAYOFYEAR()
  SQL function
  Nothing comparable



TIMEZONE()
  SQL function
  Nothing comparable



TRANSACTION()
  Returns the ID of the current transaction
  Nothing comparable



TRANSLATE()
  SQL function
  REPLACE(); identical syntax and implementation



TRIM()
  SQL function
  TRIM(); slightly different implementation



TRUNC()
  SQL function
  TRUNCATE(); slightly different syntax and implementation



USE
  Switches to a new database instance; terminates the connection to the 
	current database instance; all subsequent commands are referred to this 
	database instance
  USE; identical syntax, but does not terminate the connection to
	the current database



USER
  SQL function
  USER(); identical syntax, but slightly different implementation, and parentheses are mandatory in MySQL



UTC_DIFF()
  SQL function
  UTC_DATE(); provides a means to calculate the same result as UTC_DIFF()



VALUE()
  SQL function, alias for COALESCE()
  COALESCE(); identical syntax and implementation



VARIANCE()
  SQL function
  VARIANCE(); new in MySQL 4.1.0



WEEKOFYEAR()
  SQL function
  WEEKOFYEAR(); new in MySQL 4.1.1








19  Spatial Extensions in MySQL










MySQL 4.1 introduces spatial extensions to allow the
generation, storage, and analysis of geographic features.
Currently, these features are available for MyISAM tables only.





This chapter covers the following topics:









19.1  Introduction

















MySQL implements spatial extensions following the specification of
the Open GIS Consortium (OGC). This is an international consortium
of more than 250 companies, agencies, and universities participating
in the development of publicly available conceptual solutions that can be
useful with all kinds of applications that manage spatial data.
The OGC maintains a Web site at http://www.opengis.org/.





In 1997, the Open GIS Consortium published the
OpenGIS (R) Simple Features Specifications For SQL, a document that
proposes several conceptual ways for extending an SQL RDBMS to support spatial
data. This specification is available from the Open GIS Web site at
http://www.opengis.org/docs/99-049.pdf.
It contains additional information relevant to this chapter.





MySQL implements a subset of the SQL with Geometry Types
environment proposed by OGC.
This term refers to an SQL environment that has been extended with a
set of geometry types. A geometry-valued SQL column is implemented as
a column that has a geometry type. The specifications describe a set of SQL
geometry types, as well as functions on those types to create and
analyze geometry values.











A geographic feature is anything in the world that has a location.
A feature can be:














You can also find documents that use the term geospatial feature to
refer to geographic features.











Geometry is another word that denotes a geographic feature.
Originally the word geometry meant measurement of the earth.
Another meaning comes from cartography, referring to the geometric
features that cartographers use to map the world.





This chapter uses all of these terms synonymously:
geographic feature, geospatial feature,
feature, or geometry.
The term most commonly used here is geometry.





Let's define a geometry as a point or an aggregate of
points representing anything in the world that has a location.







19.2  The OpenGIS Geometry Model




The set of geometry types proposed by OGC's SQL with Geometry Types
environment is based on the OpenGIS Geometry Model. In this model,
each geometric object has the following general properties:









19.2.1  The Geometry Class Hierarchy




The geometry classes define a hierarchy as follows:








It is not possible to create objects in non-instantiable classes.
It is possible to create objects in instantiable classes.
All classes have properties, and instantiable classes may also
have assertions (rules that define valid class instances).





Geometry is the base class. It's an abstract class.
The instantiable subclasses of Geometry are restricted to zero-, one-,
and two-dimensional geometric objects that exist in
two-dimensional coordinate space. All instantiable geometry classes are
defined so that valid instances of a geometry class are topologically closed
(that is, all defined geometries include their boundary).





The base Geometry class has subclasses for Point,
Curve, Surface, and GeometryCollection:








Geometry, Curve, Surface, MultiCurve,
and MultiSurface are defined as non-instantiable classes.
They define a common set of methods for their subclasses and
are included for extensibility.





Point, LineString, Polygon, GeometryCollection,
MultiPoint, MultiLineString, and
MultiPolygon are instantiable classes.






19.2.2  Class Geometry




Geometry is the root class of the hierarchy.  It is a
non-instantiable class but has a number of properties that are common to
all geometry values created from any of the Geometry subclasses.
These properties are described in the following list.  (Particular
subclasses have their own specific properties, described later.)






19.2.2.1  Geometry Properties




A geometry value has the following properties:









19.2.3  Class Point




A Point is a geometry that represents a single
location in coordinate space.





19.2.3.1  Point Examples







19.2.3.2  Point Properties








19.2.4  Class Curve




A Curve is a one-dimensional geometry, usually represented by a sequence
of points. Particular subclasses of Curve define the type of
interpolation between points. Curve is a non-instantiable class.





19.2.4.1  Curve Properties








19.2.5  Class LineString




A LineString is a Curve with linear interpolation between points.





19.2.5.1  LineString Examples







19.2.5.2  LineString Properties








19.2.6  Class Surface




A Surface is a two-dimensional geometry. It is a non-instantiable
class. Its only instantiable subclass is Polygon.






19.2.6.1  Surface Properties








19.2.7  Class Polygon




A Polygon is a planar Surface representing a multisided
geometry. It is defined by a single exterior boundary and zero or more
interior boundaries, where
each interior boundary defines a hole in the Polygon.





19.2.7.1  Polygon Examples







19.2.7.2  Polygon Assertions







The preceding assertions make a Polygon a simple geometry.







19.2.8  Class GeometryCollection




A GeometryCollection is a geometry that is a collection of one or more
geometries of any class.





All the elements in a GeometryCollection must be in
the same Spatial Reference System (that is, in the same coordinate system).
There are no other constraints on the elements of a GeometryCollection,
although the
subclasses of GeometryCollection described in the following sections
may restrict membership. Restrictions may be based on:









19.2.9  Class MultiPoint




A MultiPoint is a geometry collection composed of
Point elements. The points are not connected or ordered
in any way.





19.2.9.1  MultiPoint Examples







19.2.9.2  MultiPoint Properties








19.2.10  Class MultiCurve




A MultiCurve is a geometry collection composed of
Curve elements. MultiCurve is a non-instantiable class.





19.2.10.1  MultiCurve Properties








19.2.11  Class MultiLineString




A MultiLineString is a MultiCurve geometry collection composed
of LineString elements.





19.2.11.1  MultiLineString Examples








19.2.12  Class MultiSurface




A MultiSurface is a geometry collection composed of surface elements.
MultiSurface is a non-instantiable class. Its only instantiable
subclass is MultiPolygon.





19.2.12.1  MultiSurface Assertions








19.2.13  Class MultiPolygon




A MultiPolygon is a MultiSurface object composed of
Polygon elements.





19.2.13.1  MultiPolygon Examples







19.2.13.2  MultiPolygon Assertions







19.2.13.3  MultiPolygon Properties








19.3  Supported Spatial Data Formats




This section describes the standard spatial data formats that are used
to represent geometry objects in queries.
They are:








Internally, MySQL stores geometry values in a format that is not identical
to either WKT or WKB format.







19.3.1  Well-Known Text (WKT) Format











The Well-Known Text (WKT) representation of Geometry is designed to
exchange geometry data in ASCII form.






Examples of WKT representations of geometry objects are:








A Backus-Naur grammar that specifies the formal production rules for writing
WKT values can be found in the OGC specification document referenced near the
beginning of this chapter.






19.3.2  Well-Known Binary (WKB) Format











The Well-Known Binary (WKB) representation for geometric values is defined by
the OpenGIS specifications.
It is also defined in the ISO ``SQL/MM Part 3: Spatial'' standard.





WKB is used to exchange geometry data as binary streams represented by
BLOB values containing geometric WKB information.





WKB uses one-byte unsigned integers, four-byte unsigned integers, and eight-byte
double-precision numbers (IEEE 754 format). A byte is eight bits.





For example, a WKB value that corresponds to POINT(1 1) consists of
this sequence of 21 bytes (each represented here by two hex digits):





0101000000000000000000F03F000000000000F03F





The sequence may be broken down into these components:





Byte order : 01
WKB type   : 01000000
X          : 000000000000F03F
Y          : 000000000000F03F





Component representation is as follows:








WKB values for more complex geometry values are represented by more complex
data structures, as detailed in the OpenGIS specification.






19.4  Creating a Spatially Enabled MySQL Database




This section describes the data types you can use for representing
spatial data in MySQL, and the functions available for creating and retrieving
spatial values.







19.4.1  MySQL Spatial Data Types

















MySQL has data types that correspond to OpenGIS classes.
Some of these types hold single geometry values:








GEOMETRY can store geometry values of any type.
The other single-value types,
POINT and LINESTRING and POLYGON,
restrict their values to a particular geometry type.





The other data types hold collections of values:








GEOMETRYCOLLECTION can store a collection of objects
of any type. The other collection types,
MULTIPOINT and MULTILINESTRING and MULTIPOLYGON and GEOMETRYCOLLECTION,
restrict collection members to those having a particular geometry type.







19.4.2  Creating Spatial Values




This section describes how to create spatial values using Well-Known Text
and Well-Known Binary functions that are defined in the OpenGIS standard,
and using MySQL-specific functions.







19.4.2.1  Creating Geometry Values Using WKT Functions




MySQL provides a number of functions that take as input parameters a
Well-Known Text representation and, optionally, a spatial reference
system identifier (SRID). They return the corresponding geometry.





GeomFromText() accepts a WKT of any geometry type as its first
argument.  An implementation also provides type-specific construction
functions for construction of geometry values of each geometry type.








GeomCollFromText(wkt[,srid])



GeometryCollectionFromText(wkt[,srid])




Constructs a GEOMETRYCOLLECTION value using its WKT representation and SRID.


GeomFromText(wkt[,srid])



GeometryFromText(wkt[,srid])




Constructs a geometry value of any type using its WKT representation and SRID.


LineFromText(wkt[,srid])



LineStringFromText(wkt[,srid])




Constructs a LINESTRING value using its WKT representation and SRID.


MLineFromText(wkt[,srid])



MultiLineStringFromText(wkt[,srid])




Constructs a MULTILINESTRING value using its WKT representation and SRID.


MPointFromText(wkt[,srid])



MultiPointFromText(wkt[,srid])




Constructs a MULTIPOINT value using its WKT representation and SRID.


MPolyFromText(wkt[,srid])



MultiPolygonFromText(wkt[,srid])




Constructs a MULTIPOLYGON value using its WKT representation and SRID.


PointFromText(wkt[,srid])



Constructs a POINT value using its WKT representation and SRID.


PolyFromText(wkt[,srid])



PolygonFromText(wkt[,srid])




Constructs a POLYGON value using its WKT representation and SRID.






The OpenGIS specification also describes optional functions for constructing
Polygon or MultiPolygon values based on the WKT representation
of a collection of rings or closed LineString values. These values
may intersect.  MySQL does not implement these functions:







BdMPolyFromText(wkt,srid)



Constructs a MultiPolygon value from a
MultiLineString value in WKT format containing
an arbitrary collection of closed LineString values.


BdPolyFromText(wkt,srid)



Constructs a Polygon value from a
MultiLineString value in WKT format containing
an arbitrary collection of closed LineString values.







19.4.2.2  Creating Geometry Values Using WKB Functions




MySQL provides a number of functions that take as input parameters a
BLOB containing a Well-Known Binary representation
and, optionally, a spatial reference
system identifier (SRID). They return the corresponding geometry.





GeomFromWKT() accepts a WKB of any geometry type as its first
argument.  An implementation also provides type-specific construction
functions for construction of geometry values of each geometry type.








GeomCollFromWKB(wkb[,srid])



GeometryCollectionFromWKB(wkt[,srid])




Constructs a GEOMETRYCOLLECTION value using its WKB representation and SRID.


GeomFromWKB(wkb[,srid])



GeometryFromWKB(wkt[,srid])




Constructs a geometry value of any type using its WKB representation and SRID.


LineFromWKB(wkb[,srid])



LineStringFromWKB(wkb[,srid])




Constructs a LINESTRING value using its WKB representation and SRID.


MLineFromWKB(wkb[,srid])



MultiLineStringFromWKB(wkb[,srid])




Constructs a MULTILINESTRING value using its WKB representation and SRID.


MPointFromWKB(wkb[,srid])



MultiPointFromWKB(wkb[,srid])




Constructs a MULTIPOINT value using its WKB representation and SRID.


MPolyFromWKB(wkb[,srid])



MultiPolygonFromWKB(wkb[,srid])




Constructs a MULTIPOLYGON value using its WKB representation and SRID.


PointFromWKB(wkb[,srid])



Constructs a POINT value using its WKB representation and SRID.


PolyFromWKB(wkb[,srid])



PolygonFromWKB(wkb[,srid])




Constructs a POLYGON value using its WKB representation and SRID.






The OpenGIS specification also describes optional functions for constructing
Polygon or MultiPolygon values based on the WKB representation
of a collection of rings or closed LineString values. These values
may intersect.  MySQL does not implement these functions:







BdMPolyFromWKB(wkb,srid)



Constructs a MultiPolygon value from a
MultiLineString value in WKB format containing
an arbitrary collection of closed LineString values.


BdPolyFromWKB(wkb,srid)



Constructs a Polygon value from a
MultiLineString value in WKB format containing
an arbitrary collection of closed LineString values.







19.4.2.3  Creating Geometry Values Using MySQL-Specific Functions




Note: MySQL does not implement the functions listed in this
section.





MySQL provides a set of useful functions for creating geometry WKB
representations. The functions described in this section are MySQL
extensions to the OpenGIS specifications. The results of these
functions are BLOB values containing WKB representations of geometry
values with no SRID.
The results of these functions can be substituted as the first argument
for any function in the GeomFromWKB() function family.







GeometryCollection(g1,g2,...)



Constructs a WKB GeometryCollection. If any argument is not a
well-formed WKB representation of a geometry, the return value is
NULL.


LineString(pt1,pt2,...)



Constructs a WKB LineString value from a number of WKB Point
arguments.  If any argument is not a WKB Point, the return value
is NULL.  If the number of Point arguments is less than two,
the return value is NULL.


MultiLineString(ls1,ls2,...)



Constructs a WKB MultiLineString value using using WKB LineString
arguments.  If any argument is not a WKB LineString, the return
value is NULL.


MultiPoint(pt1,pt2,...)



Constructs a WKB MultiPoint value using WKB Point arguments.
If any argument is not a WKB Point, the return value is NULL.


MultiPolygon(poly1,poly2,...)



Constructs a WKB MultiPolygon value from a set of WKB Polygon
arguments.
If any argument is not a WKB Polygon, the rerurn value is NULL.


Point(x,y)



Constructs a WKB Point using its coordinates.


Polygon(ls1,ls2,...)



Constructs a WKB Polygon value from a number of WKB LineString
arguments. If any argument does not represent the WKB of a LinearRing
(that is, not a closed and simple LineString) the return value
is NULL.







19.4.3  Creating Spatial Columns




MySQL provides a standard way of creating spatial columns for
geometry types, for example, with CREATE TABLE or ALTER TABLE.
Currently, spatial columns are supported only for MyISAM tables.









19.4.4  Populating Spatial Columns




After you have created spatial columns, you can populate them
with spatial data.





Values should be stored in internal geometry format, but you can convert them
to that format from either Well-Known Text (WKT) or Well-Known Binary (WKB)
format. The following examples demonstrate how to insert geometry values into
a table by converting WKT values into internal geometry format.





You can perform the conversion directly in the INSERT statement:





INSERT INTO geom VALUES (GeomFromText('POINT(1 1)'));

SET @g = 'POINT(1 1)';
INSERT INTO geom VALUES (GeomFromText(@g));





Or you can perform the conversion prior to the INSERT:





SET @g = GeomFromText('POINT(1 1)');
INSERT INTO geom VALUES (@g);





The following examples insert more complex geometries into the table:





SET @g = 'LINESTRING(0 0,1 1,2 2)';
INSERT INTO geom VALUES (GeomFromText(@g));

SET @g = 'POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7, 5 5))';
INSERT INTO geom VALUES (GeomFromText(@g));

SET @g =
'GEOMETRYCOLLECTION(POINT(1 1),LINESTRING(0 0,1 1,2 2,3 3,4 4))';
INSERT INTO geom VALUES (GeomFromText(@g));





The preceding examples all use GeomFromText() to create geometry
values.  You can also use type-specific functions:





SET @g = 'POINT(1 1)';
INSERT INTO geom VALUES (PointFromText(@g));

SET @g = 'LINESTRING(0 0,1 1,2 2)';
INSERT INTO geom VALUES (LineStringFromText(@g));

SET @g = 'POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7, 5 5))';
INSERT INTO geom VALUES (PolygonFromText(@g));

SET @g =
'GEOMETRYCOLLECTION(POINT(1 1),LINESTRING(0 0,1 1,2 2,3 3,4 4))';
INSERT INTO geom VALUES (GeomCollFromText(@g));





Note that if a client application program wants to use WKB representations
of geometry values, it is responsible for sending correctly formed WKB in
queries to the server. However, there are several ways of satisfying this
requirement. For example:









19.4.5  Fetching Spatial Data




Geometry values stored in a table can be fetched in internal
format. You can also convert them into WKT or WKB format.







19.4.5.1  Fetching Spatial Data in Internal Format




Fetching geometry values using internal format can be useful in
table-to-table transfers:





CREATE TABLE geom2 (g GEOMETRY) SELECT g FROM geom;






19.4.5.2  Fetching Spatial Data in WKT Format




The AsText() function converts a geometry from internal format into a WKT string.






mysql> SELECT AsText(g) FROM geom;
+-------------------------+
| AsText(p1)              |
+-------------------------+
| POINT(1 1)              |
| LINESTRING(0 0,1 1,2 2) |
+-------------------------+






19.4.5.3  Fetching Spatial Data in WKB Format




The AsBinary() function converts a geometry from internal format into a BLOB containing
the WKB value.





SELECT AsBinary(g) FROM geom;






19.5  Analyzing Spatial Information




After populating spatial columns with values, you are ready to
query and analyze them.  MySQL provides a set of functions to
perform various operations on spatial data. These functions can be
grouped into four major categories according to the type of operation
they perform:








Spatial analysis functions can be used in many contexts, such as:









19.5.1  Geometry Format Conversion Functions




MySQL supports the following functions for converting geometry values between
internal format and either WKT or WKB format:







AsBinary(g)



Converts a value in internal geometry format to its WKB representation
and returns the binary result.


AsText(g)



Converts a value in internal geometry format to its WKT representation
and returns the string result.



mysql> SET @g = 'LineString(1 1,2 2,3 3)';
mysql> SELECT AsText(GeomFromText(@g));
+--------------------------+
| AsText(GeomFromText(@G)) |
+--------------------------+
| LINESTRING(1 1,2 2,3 3)  |
+--------------------------+




GeomFromText(wkt[,srid])



Converts a string value from its WKT representation into internal geometry
format and returns the result.
A number of type-specific functions are also supported, such as
PointFromText() and LineFromText(); see
section 19.4.2.1  Creating Geometry Values Using WKT Functions.


GeomFromWKB(wkb[,srid])



Converts a binary value from its WKB representation into internal geometry
format and returns the result.
A number of type-specific functions are also supported, such as
PointFromWKB() and LineFromWKB(); see
section 19.4.2.2  Creating Geometry Values Using WKB Functions.







19.5.2  Geometry Functions




Each function that belongs to this group takes a geometry value as its
argument and returns some quantitative or qualitative property of the
geometry. Some functions restrict their argument type. Such functions
return NULL if the argument is of an incorrect geometry
type.  For example, Area() returns NULL if the object
type is neither Polygon nor MultiPolygon.







19.5.2.1  General Geometry Functions




The functions listed in this section do not restrict their argument and
accept a geometry value of any type.







Dimension(g)



Returns the inherent dimension of the geometry value g. The result
can be -1, 0, 1, or 2. (The meaning of these values is given in
section 19.2.2  Class Geometry.)



mysql> SELECT Dimension(GeomFromText('LineString(1 1,2 2)'));
+------------------------------------------------+
| Dimension(GeomFromText('LineString(1 1,2 2)')) |
+------------------------------------------------+
|                                              1 |
+------------------------------------------------+




Envelope(g)



Returns the Minimum Bounding Rectangle (MBR) for the geometry value g.
The result is returned as a Polygon value.



mysql> SELECT AsText(Envelope(GeomFromText('LineString(1 1,2 2)')));
+-------------------------------------------------------+
| AsText(Envelope(GeomFromText('LineString(1 1,2 2)'))) |
+-------------------------------------------------------+
| POLYGON((1 1,2 1,2 2,1 2,1 1))                        |
+-------------------------------------------------------+



The polygon is defined by the corner points of the bounding box:



POLYGON((MINX MINY, MAXX MINY, MAXX MAXY, MINX MAXY, MINX MINY))




GeometryType(g)



Returns as a string the name of the geometry type of which
the geometry instance g is a member.
The name will correspond to one of the instantiable Geometry subclasses.



mysql> SELECT GeometryType(GeomFromText('POINT(1 1)'));
+------------------------------------------+
| GeometryType(GeomFromText('POINT(1 1)')) |
+------------------------------------------+
| POINT                                    |
+------------------------------------------+




SRID(g)



Returns an integer indicating the Spatial Reference System ID for the geometry
value g.



mysql> SELECT SRID(GeomFromText('LineString(1 1,2 2)',101));
+-----------------------------------------------+
| SRID(GeomFromText('LineString(1 1,2 2)',101)) |
+-----------------------------------------------+
|                                           101 |
+-----------------------------------------------+








The OpenGIS specification also defines the following functions, which MySQL
does not implement:







Boundary(g)



Returns a geometry that is the closure of the combinatorial boundary of the
geometry value g.


IsEmpty(g)



Returns 1 if the geometry value g is the empty geometry, 0 if it is not
empty, and -1 if the argument is NULL.
If the geometry is empty, it represents the empty point set.


IsSimple(g)




Currently, this function is a placeholder and should not be used.
If implemented, its behavior will be as described in the next paragraph.

Returns 1 if the geometry value g has no anomalous geometric points,
such as self-intersection or self-tangency. IsSimple() returns 0 if the
argument is not simple, and -1 if it is NULL.

The description of each instantiable geometric class given earlier in
the chapter includes the specific conditions that cause an instance of
that class to be classified as not simple.







19.5.2.2  Point Functions




A Point consists of X and Y coordinates, which may be obtained
using the following functions:







X(p)



Returns the X-coordinate value for the point p as a double-precision
number.


mysql> SELECT X(GeomFromText('Point(56.7 53.34)'));
+--------------------------------------+
| X(GeomFromText('Point(56.7 53.34)')) |
+--------------------------------------+
|                                 56.7 |
+--------------------------------------+




Y(p)



Returns the Y-coordinate value for the point p as a double-precision
number.


mysql> SELECT Y(GeomFromText('Point(56.7 53.34)'));
+--------------------------------------+
| Y(GeomFromText('Point(56.7 53.34)')) |
+--------------------------------------+
|                                53.34 |
+--------------------------------------+









19.5.2.3  LineString Functions




A LineString consists of Point values. You can extract
particular points of a LineString, count the number of points that it
contains, or obtain its length.








EndPoint(ls)



Returns the Point that is the end point of the LineString value
ls.



mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
mysql> SELECT AsText(EndPoint(GeomFromText(@ls)));
+-------------------------------------+
| AsText(EndPoint(GeomFromText(@ls))) |
+-------------------------------------+
| POINT(3 3)                          |
+-------------------------------------+




GLength(ls)



Returns as a double-precision number the length of the LineString
value ls in its associated spatial reference.



mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
mysql> SELECT GLength(GeomFromText(@ls));
+----------------------------+
| GLength(GeomFromText(@ls)) |
+----------------------------+
|            2.8284271247462 |
+----------------------------+




IsClosed(ls)



Returns 1 if the LineString value ls is closed
(that is, its StartPoint() and EndPoint() values are the same).
Returns 0 if ls is not closed, and -1 if it is NULL.



mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
mysql> SELECT IsClosed(GeomFromText(@ls));
+-----------------------------+
| IsClosed(GeomFromText(@ls)) |
+-----------------------------+
|                           0 |
+-----------------------------+




NumPoints(ls)



Returns the number of points in the LineString value ls.



mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
mysql> SELECT NumPoints(GeomFromText(@ls));
+------------------------------+
| NumPoints(GeomFromText(@ls)) |
+------------------------------+
|                            3 |
+------------------------------+




PointN(ls,n)



Returns the n-th point in the Linestring value ls.
Point numbers begin at 1.



mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
mysql> SELECT AsText(PointN(GeomFromText(@ls),2));
+-------------------------------------+
| AsText(PointN(GeomFromText(@ls),2)) |
+-------------------------------------+
| POINT(2 2)                          |
+-------------------------------------+




StartPoint(ls)



Returns the Point that is the start point of the LineString value
ls.



mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
mysql> SELECT AsText(StartPoint(GeomFromText(@ls)));
+---------------------------------------+
| AsText(StartPoint(GeomFromText(@ls))) |
+---------------------------------------+
| POINT(1 1)                            |
+---------------------------------------+








The OpenGIS specification also defines the following function, which MySQL
does not implement:







IsRing(ls)



Returns 1 if the LineString value ls is closed
(that is, its StartPoint() and EndPoint() values are the same)
and is simple (does not pass through the same point more than once).
Returns 0 if ls is not a ring, and -1 if it is NULL.







19.5.2.4  MultiLineString Functions






GLength(mls)



Returns as a double-precision number
the length of the MultiLineString value mls. The length of
mls is equal to the sum of the lengths of its elements.



mysql> SET @mls = 'MultiLineString((1 1,2 2,3 3),(4 4,5 5))';
mysql> SELECT GLength(GeomFromText(@mls));
+-----------------------------+
| GLength(GeomFromText(@mls)) |
+-----------------------------+
|             4.2426406871193 |
+-----------------------------+




IsClosed(mls)



Returns 1 if the MultiLineString value mls is closed
(that is, the StartPoint() and EndPoint() values are the same
for each LineString in mls).
Returns 0 if mls is not closed, and -1 if it is NULL.



mysql> SET @mls = 'MultiLineString((1 1,2 2,3 3),(4 4,5 5))';
mysql> SELECT IsClosed(GeomFromText(@mls));
+------------------------------+
| IsClosed(GeomFromText(@mls)) |
+------------------------------+
|                            0 |
+------------------------------+









19.5.2.5  Polygon Functions






Area(poly)



Returns as a double-precision number the area of the Polygon value
poly, as measured in its spatial reference system.



mysql> SET @poly = 'Polygon((0 0,0 3,3 0,0 0),(1 1,1 2,2 1,1 1))';
mysql> SELECT Area(GeomFromText(@poly));
+---------------------------+
| Area(GeomFromText(@poly)) |
+---------------------------+
|                         4 |
+---------------------------+




ExteriorRing(poly)



Returns the exterior ring of the Polygon value poly
as a LineString.


mysql> SET @poly =
    -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
mysql> SELECT AsText(ExteriorRing(GeomFromText(@poly)));
+-------------------------------------------+
| AsText(ExteriorRing(GeomFromText(@poly))) |
+-------------------------------------------+
| LINESTRING(0 0,0 3,3 3,3 0,0 0)           |
+-------------------------------------------+




InteriorRingN(poly,n)



Returns the n-th interior ring for the Polygon value
poly as a LineString.
Ring numbers begin at 1.


mysql> SET @poly =
    -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
mysql> SELECT AsText(InteriorRingN(GeomFromText(@poly),1));
+----------------------------------------------+
| AsText(InteriorRingN(GeomFromText(@poly),1)) |
+----------------------------------------------+
| LINESTRING(1 1,1 2,2 2,2 1,1 1)              |
+----------------------------------------------+




NumInteriorRings(poly)



Returns the number of interior rings in the Polygon value poly.


mysql> SET @poly =
    -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
mysql> SELECT NumInteriorRings(GeomFromText(@poly));
+---------------------------------------+
| NumInteriorRings(GeomFromText(@poly)) |
+---------------------------------------+
|                                     1 |
+---------------------------------------+









19.5.2.6  MultiPolygon Functions






Area(mpoly)




Returns as a double-precision number the area of the MultiPolygon
value mpoly, as measured in its spatial reference system.



mysql> SET @mpoly =
    -> 'MultiPolygon(((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1)))';
mysql> SELECT Area(GeomFromText(@mpoly));
+----------------------------+
| Area(GeomFromText(@mpoly)) |
+----------------------------+
|                          8 |
+----------------------------+








The OpenGIS specification also defines the following functions, which MySQL
does not implement:







Centroid(mpoly)



Returns the mathematical centroid for the MultiPolygon value
mpoly as a Point.  The result is not guaranteed to be on
the MultiPolygon.


PointOnSurface(mpoly)



Returns a Point value that is guaranteed to be on the
MultiPolygon value mpoly.







19.5.2.7  GeometryCollection Functions






GeometryN(gc,n)



Returns the n-th geometry in the GeometryCollection value
gc.  Geometry numbers begin at 1.



mysql> SET @gc = 'GeometryCollection(Point(1 1),LineString(2 2, 3 3))';
mysql> SELECT AsText(GeometryN(GeomFromText(@gc),1));
+----------------------------------------+
| AsText(GeometryN(GeomFromText(@gc),1)) |
+----------------------------------------+
| POINT(1 1)                             |
+----------------------------------------+




NumGeometries(gc)



Returns the number of geometries in the GeometryCollection value
gc.



mysql> SET @gc = 'GeometryCollection(Point(1 1),LineString(2 2, 3 3))';
mysql> SELECT NumGeometries(GeomFromText(@gc));
+----------------------------------+
| NumGeometries(GeomFromText(@gc)) |
+----------------------------------+
|                                2 |
+----------------------------------+









19.5.3  Functions That Create New Geometries from Existing Ones





19.5.3.1  Geometry Functions That Produce New Geometries




In the section section 19.5.2  Geometry Functions,
we've already discussed some functions that can construct new geometries
from the existing ones:









19.5.3.2  Spatial Operators




OpenGIS proposes a number of other functions that can produce
geometries. They are designed to implement spatial operators.





These functions are not implemented in MySQL.
They may appear in future releases.







Buffer(g,d)



Returns a geometry that represents all points whose distance from the geometry
value g is less than or equal to a distance of d.


ConvexHull(g)



Returns a geometry that represents the convex hull of the geometry value
g.


Difference(g1,g2)



Returns a geometry that represents the point set difference of the geometry
value g1 with g2.


Intersection(g1,g2)



Returns a geometry that represents the point set intersection of the geometry
values g1 with g2.


SymDifference(g1,g2)



Returns a geometry that represents the point set symmetric difference of the
geometry value g1 with g2.


Union(g1,g2)



Returns a geometry that represents the point set union of the geometry values
g1 and g2.







19.5.4  Functions for Testing Spatial Relations Between Geometric Objects




The functions described in these sections take two geometries as input
parameters and return a qualitative or quantitative relation between them.






19.5.5  Relations on Geometry Minimal Bounding Rectangles (MBRs)












MySQL provides some functions that can test relations
between minimal bounding rectangles of two geometries g1 and g2.
They include:







MBRContains(g1,g2)



Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangle of
g1 contains the Minimum Bounding Rectangle of g2.



mysql> SET @g1 = GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0))');
mysql> SET @g2 = GeomFromText('Point(1 1)');
mysql> SELECT MBRContains(@g1,@g2), MBRContains(@g2,@g1);
----------------------+----------------------+
| MBRContains(@g1,@g2) | MBRContains(@g2,@g1) |
+----------------------+----------------------+
|                    1 |                    0 |
+----------------------+----------------------+




MBRDisjoint(g1,g2)



Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangles of
the two geometries g1 and g2 are disjoint (do not intersect).


MBREqual(g1,g2)



Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangles of
the two geometries g1 and g2 are the same.


MBRIntersects(g1,g2)



Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangles of
the two geometries g1 and g2 intersect.


MBROverlaps(g1,g2)



Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangles of
the two geometries g1 and g2 overlap.


MBRTouches(g1,g2)



Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangles of
the two geometries g1 and g2 touch.


MBRWithin(g1,g2)



Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangle
of g1 is within the Minimum Bounding Rectangle of g2.



mysql> SET @g1 = GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0))');
mysql> SET @g2 = GeomFromText('Polygon((0 0,0 5,5 5,5 0,0 0))');
mysql> SELECT MBRWithin(@g1,@g2), MBRWithin(@g2,@g1);
+--------------------+--------------------+
| MBRWithin(@g1,@g2) | MBRWithin(@g2,@g1) |
+--------------------+--------------------+
|                  1 |                  0 |
+--------------------+--------------------+









19.5.6  Functions That Test Spatial Relationships Between Geometries




The OpenGIS specification defines the following functions. Currently,
MySQL does not implement them according to the specification. Those that
are implemented return the same result as the corresponding
MBR-based functions.  This includes functions in the following list
other than Distance() and Related().






These functions may be implemented in future releases with full
support for spatial analysis, not just MBR-based support.





The functions operate on two geometry values g1 and g2.







Contains(g1,g2)



Returns 1 or 0 to indicate whether or not g1 completely contains
g2.


Crosses(g1,g2)



Returns 1 if g1 spatially crosses g2.
Returns NULL if g1 is a Polygon or a MultiPolygon,
or if g2 is a Point or a MultiPoint.
Otherwise, returns 0.

The term spatially crosses denotes a spatial relation between two given
geometries that has the following properties:



    


  • 

The two geometries intersect


  • 

Their intersection results in a geometry that has
a dimension that is one less than the maximum dimension of the two given
geometries


  • 

Their intersection is not equal to either of the two given geometries





Disjoint(g1,g2)



Returns 1 or 0 to indicate whether or not g1 is spatially disjoint
from (does not intersect) g2.


Distance(g1,g2)



Returns as a double-precision number
the shortest distance between any two points in the two geometries.


Equals(g1,g2)



Returns 1 or 0 to indicate whether or not g1 is spatially equal to
g2.


Intersects(g1,g2)



Returns 1 or 0 to indicate whether or not g1 spatially intersects
g2.


Overlaps(g1,g2)



Returns 1 or 0 to indicate whether or not g1 spatially overlaps
g2.
The term spatially overlaps is used if two
geometries intersect and their intersection results in a geometry of the
same dimension but not equal to either of the given geometries.


Related(g1,g2,pattern_matrix)



Returns 1 or 0 to indicate whether or not the spatial relationship specified
by pattern_matrix exists between g1 and g2.
Returns -1 if the arguments are NULL.
The pattern matrix is a string. Its specification will be noted here if this
function is implemented.


Touches(g1,g2)



Returns 1 or 0 to indicate whether or not g1 spatially touches
g2. Two geometries spatially touch if the interiors of
the geometries do not intersect, but the boundary of one of the geometries
intersects either the boundary or the interior of the other.


Within(g1,g2)



Returns 1 or 0 to indicate whether or not g1 is spatially within
g2.







19.6  Optimizing Spatial Analysis




Search operations in non-spatial databases can be optimized
using indexes. This is true for spatial databases as well.
With the help of a great variety of multi-dimensional indexing methods that
have already been designed, it is possible to optimize
spatial searches. The most typical of these are:








MySQL uses R-Trees with quadratic splitting to index
spatial columns. A spatial index is built using the MBR of a geometry.
For most geometries, the MBR is a minimum rectangle that
surrounds the geometries. For a horizontal or a vertical
linestring, the MBR is a rectangle degenerated into the linestring.
For a point, the MBR is a rectangle degenerated into the point.







19.6.1  Creating Spatial Indexes




MySQL can create spatial indexes using syntax similar to that for creating
regular indexes, but extended with the SPATIAL keyword.
Spatial columns that are indexed currently must be declared NOT NULL.
The following examples demonstrate how to create spatial indexes.








To drop spatial indexes, use ALTER TABLE or DROP INDEX:








Example: Suppose that a table geom contains more than 32,000 geometries,
which are stored in the column g of type GEOMETRY.
The table also has an AUTO_INCREMENT column fid for storing
object ID values.





mysql> DESCRIBE geom;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| fid   | int(11)  |      | PRI | NULL    | auto_increment |
| g     | geometry |      |     |         |                |
+-------+----------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> SELECT COUNT(*) FROM geom;
+----------+
| count(*) |
+----------+
|    32376 |
+----------+
1 row in set (0.00 sec)





To add a spatial index on the column g, use this statement:


mysql> ALTER TABLE geom ADD SPATIAL INDEX(g);
Query OK, 32376 rows affected (4.05 sec)
Records: 32376  Duplicates: 0  Warnings: 0






19.6.2  Using a Spatial Index




The optimizer investigates whether available spatial indexes can
be involved in the search for queries that use a function such as
MBRContains() or MBRWithin() in the WHERE clause.
For example, let's say we want to find all objects that are in the
given rectangle:





mysql> SELECT fid,AsText(g) FROM geom WHERE
mysql> MBRContains(GeomFromText('Polygon((30000 15000,31000 15000,31000 16000,30000 16000,30000 15000))'),g);
+-----+-----------------------------------------------------------------------------+
| fid | AsText(g)                                                                   |
+-----+-----------------------------------------------------------------------------+
|  21 | LINESTRING(30350.4 15828.8,30350.6 15845,30333.8 15845,30333.8 15828.8)     |
|  22 | LINESTRING(30350.6 15871.4,30350.6 15887.8,30334 15887.8,30334 15871.4)     |
|  23 | LINESTRING(30350.6 15914.2,30350.6 15930.4,30334 15930.4,30334 15914.2)     |
|  24 | LINESTRING(30290.2 15823,30290.2 15839.4,30273.4 15839.4,30273.4 15823)     |
|  25 | LINESTRING(30291.4 15866.2,30291.6 15882.4,30274.8 15882.4,30274.8 15866.2) |
|  26 | LINESTRING(30291.6 15918.2,30291.6 15934.4,30275 15934.4,30275 15918.2)     |
| 249 | LINESTRING(30337.8 15938.6,30337.8 15946.8,30320.4 15946.8,30320.4 15938.4) |
|   1 | LINESTRING(30250.4 15129.2,30248.8 15138.4,30238.2 15136.4,30240 15127.2)   |
|   2 | LINESTRING(30220.2 15122.8,30217.2 15137.8,30207.6 15136,30210.4 15121)     |
|   3 | LINESTRING(30179 15114.4,30176.6 15129.4,30167 15128,30169 15113)           |
|   4 | LINESTRING(30155.2 15121.4,30140.4 15118.6,30142 15109,30157 15111.6)       |
|   5 | LINESTRING(30192.4 15085,30177.6 15082.2,30179.2 15072.4,30194.2 15075.2)   |
|   6 | LINESTRING(30244 15087,30229 15086.2,30229.4 15076.4,30244.6 15077)         |
|   7 | LINESTRING(30200.6 15059.4,30185.6 15058.6,30186 15048.8,30201.2 15049.4)   |
|  10 | LINESTRING(30179.6 15017.8,30181 15002.8,30190.8 15003.6,30189.6 15019)     |
|  11 | LINESTRING(30154.2 15000.4,30168.6 15004.8,30166 15014.2,30151.2 15009.8)   |
|  13 | LINESTRING(30105 15065.8,30108.4 15050.8,30118 15053,30114.6 15067.8)       |
| 154 | LINESTRING(30276.2 15143.8,30261.4 15141,30263 15131.4,30278 15134)         |
| 155 | LINESTRING(30269.8 15084,30269.4 15093.4,30258.6 15093,30259 15083.4)       |
| 157 | LINESTRING(30128.2 15011,30113.2 15010.2,30113.6 15000.4,30128.8 15001)     |
+-----+-----------------------------------------------------------------------------+
20 rows in set (0.00 sec)





Now let's use EXPLAIN to check the way this query is executed (the
id column has been removed so the output better fits the page):


mysql> EXPLAIN SELECT fid,AsText(g) FROM geom WHERE
mysql> MBRContains(GeomFromText('Polygon((30000 15000,31000 15000,31000 16000,30000 16000,30000 15000))'),g);
+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| SIMPLE      | geom  | range | g             | g    |      32 | NULL |   50 | Using where |
+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)





Now let's check what would happen without a spatial index:


mysql> EXPLAIN SELECT fid,AsText(g) FROM g IGNORE INDEX (g) WHERE
mysql> MBRContains(GeomFromText('Polygon((30000 15000,31000 15000,31000 16000,30000 16000,30000 15000))'),g);
+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| SIMPLE      | geom  | ALL  | NULL          | NULL |    NULL | NULL | 32376 | Using where |
+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)





Let's execute the SELECT statement, ignoring the spatial key we have:





mysql> SELECT fid,AsText(g) FROM geom IGNORE INDEX (g) WHERE
mysql> MBRContains(GeomFromText('Polygon((30000 15000,31000 15000,31000 16000,30000 16000,30000 15000))'),g);
+-----+-----------------------------------------------------------------------------+
| fid | AsText(g)                                                                   |
+-----+-----------------------------------------------------------------------------+
|   1 | LINESTRING(30250.4 15129.2,30248.8 15138.4,30238.2 15136.4,30240 15127.2)   |
|   2 | LINESTRING(30220.2 15122.8,30217.2 15137.8,30207.6 15136,30210.4 15121)     |
|   3 | LINESTRING(30179 15114.4,30176.6 15129.4,30167 15128,30169 15113)           |
|   4 | LINESTRING(30155.2 15121.4,30140.4 15118.6,30142 15109,30157 15111.6)       |
|   5 | LINESTRING(30192.4 15085,30177.6 15082.2,30179.2 15072.4,30194.2 15075.2)   |
|   6 | LINESTRING(30244 15087,30229 15086.2,30229.4 15076.4,30244.6 15077)         |
|   7 | LINESTRING(30200.6 15059.4,30185.6 15058.6,30186 15048.8,30201.2 15049.4)   |
|  10 | LINESTRING(30179.6 15017.8,30181 15002.8,30190.8 15003.6,30189.6 15019)     |
|  11 | LINESTRING(30154.2 15000.4,30168.6 15004.8,30166 15014.2,30151.2 15009.8)   |
|  13 | LINESTRING(30105 15065.8,30108.4 15050.8,30118 15053,30114.6 15067.8)       |
|  21 | LINESTRING(30350.4 15828.8,30350.6 15845,30333.8 15845,30333.8 15828.8)     |
|  22 | LINESTRING(30350.6 15871.4,30350.6 15887.8,30334 15887.8,30334 15871.4)     |
|  23 | LINESTRING(30350.6 15914.2,30350.6 15930.4,30334 15930.4,30334 15914.2)     |
|  24 | LINESTRING(30290.2 15823,30290.2 15839.4,30273.4 15839.4,30273.4 15823)     |
|  25 | LINESTRING(30291.4 15866.2,30291.6 15882.4,30274.8 15882.4,30274.8 15866.2) |
|  26 | LINESTRING(30291.6 15918.2,30291.6 15934.4,30275 15934.4,30275 15918.2)     |
| 154 | LINESTRING(30276.2 15143.8,30261.4 15141,30263 15131.4,30278 15134)         |
| 155 | LINESTRING(30269.8 15084,30269.4 15093.4,30258.6 15093,30259 15083.4)       |
| 157 | LINESTRING(30128.2 15011,30113.2 15010.2,30113.6 15000.4,30128.8 15001)     |
| 249 | LINESTRING(30337.8 15938.6,30337.8 15946.8,30320.4 15946.8,30320.4 15938.4) |
+-----+-----------------------------------------------------------------------------+
20 rows in set (0.46 sec)





When the index is not used, the execution time for this query rises from
0.00 seconds to 0.46 seconds.





In future releases, spatial indexes may also be used for optimizing
other functions.
See section 19.5.4  Functions for Testing Spatial Relations Between Geometric Objects.







19.7  MySQL Conformance and Compatibility





19.7.1  GIS Features That Are Not Yet Implemented






Additional Metadata Views


OpenGIS specifications propose several additional metadata views.
For example, a system view named GEOMETRY_COLUMNS contains a
description of geometry columns, one row for each geometry column
in the database.


The OpenGIS function Length() on LineString and MultiLineString currently should be called in MySQL as GLength()


The problem is that there is an existing SQL function
Length() which calculates the length of string values,
and sometimes it is not possible to distinguish whether the function is
called in a textual or spatial context. We need either to solve this
somehow, or decide on another function name.






20  Stored Procedures and Functions











Stored procedures and functions are a new feature in MySQL version 5.0.
A stored procedure is a set of SQL statements that can be stored in the server.
Once this has been done, clients don't need to keep reissuing the individual
statements but can refer to the stored procedure instead.





Some situations where stored procedures can be particularly useful:






Stored procedures can provide improved performance because less information
needs to be sent between the server and the client. The tradeoff is that
this does increase the load on the database server system because more of
the work is done on the server side and less is done on the client
(application) side. Consider this if many client machines (such as
Web servers) are serviced by only one or a few database servers.





Stored procedures also allow you to have libraries of functions in the
database server. This is a feature shared by modern application languages
that allow such design internally, for example, by using classes. Using
these client application language features is beneficial for the programmer
even outside the scope of database use.





MySQL follows the SQL:2003 syntax for stored procedures, which is also used
by IBM's DB2.





The MySQL implementation of stored procedures is still in progress. All
syntax described in this chapter is supported and any limitations and
extensions are documented where appropriate.





Stored procedures require the proc table in the mysql database.
This table is created during the MySQL 5.0 installation procedure.
If you are upgrading to MySQL 5.0 from an earlier version, be sure to update
your grant tables to make sure that the proc table exists.
See section 2.5.8  Upgrading the Grant Tables.







20.1  Stored Procedure Syntax




Stored procedures and functions are routines that are created with
CREATE PROCEDURE and CREATE FUNCTION statements.
A routine is either a procedure or a function.
A procedure is invoked using a CALL statement, and can only
pass back values using output variables.  A function can be called from
inside a statement just like any other function (that is, by invoking the
function's name), and can return a scalar value.
Stored routines may call other stored routines.





As of MySQL 5.0.1, a stored procedure or function is associated with a
particular database. This has several implications:








(In MySQL 5.0.0, stored routines are global and not associated with
a database. They inherit the default database from the caller.  If a
USE db_name is executed within the routine, the original default
database is restored upon routine exit.)





MySQL supports the very useful extension that allows the use of regular
SELECT statements (that is, without using cursors or local variables)
inside a stored procedure. The result set of such a query is simply sent
directly to the client.
Multiple SELECT statements generate multiple result sets, so the client
must use a MySQL client library that supports multiple result sets. This means
the client must use a client library from a version of MySQL at least as
recent as 4.1.





This following section describes the syntax used to create, alter, drop, and
query stored procedures and functions.







20.1.1  Maintaining Stored Procedures





20.1.1.1  CREATE PROCEDURE and CREATE FUNCTION











CREATE PROCEDURE sp_name ([parameter[,...]])
    [characteristic ...] routine_body

CREATE FUNCTION sp_name ([parameter[,...]])
    [RETURNS type]
    [characteristic ...] routine_body

parameter:
    [ IN | OUT | INOUT ] param_name type

type:
    Any valid MySQL data type

characteristic:
    LANGUAGE SQL
  | [NOT] DETERMINISTIC
  | SQL SECURITY {DEFINER | INVOKER}
  | COMMENT 'string'

routine_body:
    Valid SQL procedure statements or statements





By default, the routine is associated with the current database. To associate
the routine explicitly with a given database, specify the name as
db_name.sp_name when you create it.





The RETURNS clause may be specified only for a FUNCTION.
It is used to indicate the return type of the function, and the function
body must contain a RETURN value statement.





The parameter list enclosed within parentheses must always be present.
If there are no parameters, an empty parameter list of () should
be used.  Each parameter is an IN parameter by default. To specify
otherwise for a parameter, use the keyword OUT or INOUT before
the parameter name. Specifying IN, OUT, or INOUT is only
valid for a PROCEDURE.





The CREATE FUNCTION statement is used in
earlier versions of MySQL to support UDFs (User Defined Functions).
See section 24.2  Adding New Functions to MySQL.
UDFs continue to be supported, even with the existence of stored functions.
A UDF can be regarded as an external stored function.  However, do
note that stored functions share their namespace with UDFs.





A framework for external stored procedures will be introduced in the near
future. This will allow you to write stored procedures in languages other
than SQL.
Most likely, one of the first languages to be supported will be PHP because the
core PHP engine is small, thread-safe, and can easily be embedded. Because the
framework will be public, it is expected that many other languages will also
be supported.





A function is considered ``deterministic'' if it always returns the same
result for the same input parameters, and ``not deterministic'' otherwise.
Currently, the DETERMINISTIC characteristic is accepted, but not yet
used by the optimizer.





The SQL SECURITY characteristic can be used to specify whether the
routine should be executed using the permissions of the user who creates
the routine or the user who invokes it.
The default value is DEFINER.
This feature is new in SQL:2003.
The creator or invoker must have permission to access the database with which
the routine is associated.





MySQL does not yet use the GRANT EXECUTE privilege.





MySQL stores the sql_mode system variable setting that is in effect
at the time a routine is created, and will always execute the routine with
this setting in force.





The COMMENT clause is a MySQL extension, and may be used to describe
the stored procedure. This information is displayed by the
SHOW CREATE PROCEDURE and SHOW CREATE FUNCTION statements.





MySQL allows routines to contain DDL statements (such as CREATE and
 DROP) and SQL transaction statements (such as COMMIT). This is
not required by the standard and is therefore implementation-specific.





Note:
Currently, stored functions created with CREATE FUNCTION may not contain references to tables.
Please note that this includes some SET statements, but excludes some
SELECT statements.
This limitation will be lifted as soon as possible.





The following is an example of a simple stored procedure that uses
an OUT parameter.
The example uses the @command{mysql} client delimiter command to change
the statement delimiter from ; to // while the procedure is
being defined.
This allows the ; delimiter used in the procedure body to be passed
through to the server rather than being interpreted by @command{mysql} itself.





mysql> delimiter //

mysql> CREATE PROCEDURE simpleproc (OUT param1 INT)
    -> BEGIN
    ->   SELECT COUNT(*) INTO param1 FROM t;
    -> END
    -> //
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;

mysql> CALL simpleproc(@a);
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @a;
+------+
| @a   |
+------+
| 3    |
+------+
1 row in set (0.00 sec)





The following is an example of a function that takes a parameter,
performs an operation using an SQL function, and returns the result:





mysql> delimiter //

mysql> CREATE FUNCTION hello (s CHAR(20)) RETURNS CHAR(50)
    -> RETURN CONCAT('Hello, ',s,'!');
    -> //
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;

mysql> SELECT hello('world');
+----------------+
| hello('world') |
+----------------+
| Hello, world!  |
+----------------+
1 row in set (0.00 sec)






20.1.1.2  ALTER PROCEDURE and ALTER FUNCTION











ALTER {PROCEDURE | FUNCTION} sp_name [characteristic ...]

characteristic:
    NAME new_name
  | SQL SECURITY {DEFINER | INVOKER}
  | COMMENT 'string'





This statement can be used to rename a stored procedure or function, and to
change its characteristics. More than one change may be specified in an
ALTER PROCEDURE or ALTER FUNCTION statement.







20.1.1.3  DROP PROCEDURE and DROP FUNCTION











DROP {PROCEDURE | FUNCTION} [IF EXISTS] sp_name





This statement is used to drop a stored procedure or function. That is, the
specified routine is removed from the server.





The IF EXISTS clause is a MySQL extension.
It prevents an error from occurring if the procedure or function does not
exist. A warning is produced that can be viewed with SHOW WARNINGS.







20.1.1.4  SHOW CREATE PROCEDURE and SHOW CREATE FUNCTION











SHOW CREATE {PROCEDURE | FUNCTION} sp_name





This statement is a MySQL extension. Similar to SHOW CREATE TABLE, it
returns the exact string that can be used to re-create the named routine.





mysql> SHOW CREATE FUNCTION test.hello\G
*************************** 1. row ***************************
       Function: hello
       sql_mode: 
Create Function: CREATE FUNCTION `test`.`hello`(s CHAR(20)) RETURNS CHAR(50)
RETURN CONCAT('Hello, ',s,'!')






20.1.2  SHOW PROCEDURE STATUS and SHOW FUNCTION STATUS











SHOW {PROCEDURE | FUNCTION} STATUS [LIKE 'pattern']





This statement is a MySQL extension. It returns characteristics of routines,
such as the database, name, type, creator, and creation and modification dates. If no
pattern is specified, the information for all stored procedures or all stored
functions is listed, depending on which statement you use.





mysql> SHOW FUNCTION STATUS LIKE 'hello'\G
*************************** 1. row ***************************
           Db: test
         Name: hello
         Type: FUNCTION
      Definer: paul@localhost
     Modified: 0000-00-00 00:00:00
      Created: 2004-08-03 15:29:37
Security_type: DEFINER
      Comment: 






20.1.3  CALL Statement










CALL sp_name([parameter[,...]])





The CALL statement is used to invoke a procedure that was defined
previously with CREATE PROCEDURE.





CALL can return values through its parameters. It also ``returns''
the number of rows affected, which a client program can obtain by calling the
mysql_affected_rows() C API function.







20.1.4  BEGIN ... END Compound Statement











[begin_label:] BEGIN
    statement [statement] ...
END [end_label]





Stored routines may contain multiple statements, using a
BEGIN ... END compound statement.





begin_label and end_label must be the same, if both are
specified.





Please note that the optional [NOT] ATOMIC clause is not yet
supported. This means that no transactional savepoint is set at the start of
the instruction block and the BEGIN clause used in this context has
no effect on the current transaction.





Using multiple statements requires that a client is able to send query
strings containing the ; statement delimiter. This is handled in the
@command{mysql} command-line client with the delimiter command. Changing
the ; end-of-query delimiter (for example, to //) allows
; to be used in a routine body.







20.1.5  DECLARE Statement










The DECLARE statement is used to define various items local to a
routine:
local variables (see section 20.1.6  Variables in Stored Procedures),
conditions and handlers (see section 20.1.7  Conditions and Handlers) and
cursors (see section 20.1.8  Cursors).
SIGNAL and RESIGNAL statements are not currently supported.





DECLARE may be used only inside a BEGIN ... END compound
statement and must be at its start, before any other statements.







20.1.6  Variables in Stored Procedures




You may declare and use variables within a routine.







20.1.6.1  DECLARE Local Variables




DECLARE var_name[,...] type [DEFAULT value]





This statement is used to declare local variables. The scope of a variable is
within the BEGIN ... END block.







20.1.6.2  Variable SET Statement










SET var_name = expr [, var_name = expr] ...





The SET statement in stored procedures is an extended version of
the general SET statement. Referenced variables may be ones declared
inside a routine, or global server variables.





The SET statement in stored procedures is implemented as part of the
pre-existing SET syntax. This allows an extended syntax of
SET a=x, b=y, ... where different variable types (locally declared
variables, server variables, and global and session server variables) can be
mixed. This also allows combinations of local variables and some options
that make sense only for global/system variables; in that case, the options
are accepted but ignored.







20.1.6.3  SELECT ... INTO Statement










SELECT col_name[,...] INTO var_name[,...] table_expr





This SELECT syntax stores selected columns directly into variables.
Therefore, only a single row may be retrieved. This statement is also
extremely useful when used in combination with cursors.





SELECT id,data INTO x,y FROM test.t1 LIMIT 1;






20.1.7  Conditions and Handlers




Certain conditions may require specific handling. These conditions can relate
to errors, as well as general flow control inside a routine.







20.1.7.1  DECLARE Conditions










DECLARE condition_name CONDITION FOR condition_value

condition_value:
    SQLSTATE [VALUE] sqlstate_value
  | mysql_error_code





This statement specifies conditions that will need
specific handling. It associates a name with a specified error
condition. The name can subsequently be used in a
DECLARE HANDLER statement. See section 20.1.7.2  DECLARE Handlers.





In addition to SQLSTATE values, MySQL error codes are also supported.







20.1.7.2  DECLARE Handlers










DECLARE handler_type HANDLER FOR condition_value[,...] sp_statement

handler_type:
    CONTINUE
  | EXIT
  | UNDO

condition_value:
    SQLSTATE [VALUE] sqlstate_value
  | condition_name
  | SQLWARNING
  | NOT FOUND
  | SQLEXCEPTION
  | mysql_error_code





This statement specifies handlers that each may deal with
one or more conditions. If one of these conditions occurs, the specified
statement is executed.





For a CONTINUE handler, execution of the current routine continues
after execution of the handler statement.
For an EXIT handler, execution of the current BEGIN...END
compound statement is terminated.
The UNDO handler type statement is not yet supported.








In addition to SQLSTATE values, MySQL error codes are also supported.






For example:





mysql> CREATE TABLE test.t (s1 int,primary key (s1));
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter //

mysql> CREATE PROCEDURE handlerdemo ()
    -> BEGIN
    ->   DECLARE CONTINUE HANDLER FOR SQLSTATE '23000' SET @x2 = 1;
    ->   SET @x = 1;
    ->   INSERT INTO test.t VALUES (1);
    ->   SET @x = 2;
    ->   INSERT INTO test.t VALUES (1);
    ->   SET @x = 3;
    -> END;
    -> //
Query OK, 0 rows affected (0.00 sec)

mysql> CALL handlerdemo()//
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @x//
    +------+
    | @x   |
    +------+
    | 3    |
    +------+
    1 row in set (0.00 sec)





Notice that @x is 3, which shows that MySQL executed to the end of
the procedure. If the line
DECLARE CONTINUE HANDLER FOR SQLSTATE '23000' SET @x2 = 1;
had not been present, MySQL would have taken the default (EXIT) path
after the second INSERT failed due to the PRIMARY KEY
constraint, and SELECT @x would have returned 2.







20.1.8  Cursors










Simple cursors are supported inside stored procedures and functions.
The syntax is as in embedded SQL.
Cursors are currently asensitive, read-only, and non-scrolling.
Asensitive means that the server may or may not make a copy of its result
table.





For example:





CREATE PROCEDURE curdemo()
BEGIN
  DECLARE done INT DEFAULT 0;
  DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET done = 1;
  DECLARE cur1 CURSOR FOR SELECT id,data FROM test.t1;
  DECLARE cur2 CURSOR FOR SELECT i FROM test.t2;
  DECLARE a CHAR(16);
  DECLARE b,c INT;

  OPEN cur1;
  OPEN cur2;

  REPEAT
    FETCH cur1 INTO a, b;
    FETCH cur2 INTO c;
    IF NOT done THEN
       IF b < c THEN
          INSERT INTO test.t3 VALUES (a,b);
       ELSE
          INSERT INTO test.t3 VALUES (a,c);
       END IF;
    END IF;
  UNTIL done END REPEAT;

  CLOSE cur1;
  CLOSE cur2;
END






20.1.8.1  Declaring Cursors




DECLARE cursor_name CURSOR FOR sql_statement





Multiple cursors may be defined in a routine, but each must have a unique
name.







20.1.8.2  Cursor OPEN Statement










OPEN cursor_name





This statement opens a previously declared cursor.







20.1.8.3  Cursor FETCH Statement










FETCH cursor_name INTO var_name [, var_name] ...





This statement fetches the next row (if a row exists) using the specified
open cursor, and advances the cursor pointer.







20.1.8.4  Cursor CLOSE Statement










CLOSE cursor_name





This statement closes a previously opened cursor.





If not closed explicitly, a cursor is closed at the end of the compound
statement in which it was declared.







20.1.9  Flow Control Constructs




The IF, CASE, LOOP, WHILE, ITERATE, and
LEAVE constructs are fully implemented.





These constructs may each contain either a single statement, or a block of
statements using the BEGIN ... END compound statement. Constructs
may be nested.





FOR loops are not currently supported.







20.1.9.1  IF Statement










IF search_condition THEN statement_list
    [ELSEIF search_condition THEN statement_list]
    ...
    [ELSE statement_list]
END IF





IF implements a basic conditional construct. If the
search_condition evaluates to true, the corresponding SQL statement list
is executed. If no search_condition matches, the statement list in the
ELSE clause is executed. statement_list can consist of one or
more statements.





Please note that there is also an IF() function.
See section 13.2  Control Flow Functions.







20.1.9.2  CASE Statement










CASE case_value
    WHEN when_value THEN statement
    [WHEN when_value THEN statement ...]
    [ELSE statement]
END CASE





Or:





CASE
    WHEN search_condition THEN statement
    [WHEN search_condition THEN statement ...]
    [ELSE statement]
END CASE





The
CASE statement for stored procedures implements a complex conditional
construct. If a
search_condition evaluates to true, the corresponding SQL statement is executed. If no search condition matches, the statement in the ELSE clause is executed.





Note: The syntax of a CASE statement shown here
for use inside a stored procedure differs slightly from that of the SQL
CASE expression described in section 13.2  Control Flow Functions.
The CASE statement cannot have an ELSE NULL clause, and it
is terminated with END CASE instead of END.







20.1.9.3  LOOP Statement










[begin_label:] LOOP
    statement_list
END LOOP [end_label]





LOOP implements a simple loop construct, enabling repeated execution
of a particular statement or statements. The statements within the
loop are repeated until the loop is exited; usually this is accomplished
with a LEAVE statement.





begin_label and end_label must be the same, if both are
specified.







20.1.9.4  LEAVE Statement










LEAVE label





This statement is used to exit any flow control construct.







20.1.9.5  ITERATE Statement










ITERATE label





ITERATE can only appear within LOOP, REPEAT, and
WHILE statements. ITERATE means ``do the loop iteration again.''





For example:





CREATE PROCEDURE doiterate(p1 INT)
BEGIN
  label1: LOOP
    SET p1 = p1 + 1;
    IF p1 < 10 THEN ITERATE label1; END IF;
    LEAVE label1;
  END LOOP label1;
  SET @x = p1;
END






20.1.9.6  REPEAT Statement











[begin_label:] REPEAT
    statement_list
UNTIL search_condition
END REPEAT [end_label]





The statement or statements within a REPEAT statement are repeated
until the search_condition is true.





begin_label and end_label must be the same, if both are
specified.





For example:





mysql> delimiter //

mysql> CREATE PROCEDURE dorepeat(p1 INT)
    -> BEGIN
    ->   SET @x = 0;
    ->   REPEAT SET @x = @x + 1; UNTIL @x > p1 END REPEAT;
    -> END
    -> //
Query OK, 0 rows affected (0.00 sec)

mysql> CALL dorepeat(1000)//
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @x//
+------+
| @x   |
+------+
| 1001 |
+------+
1 row in set (0.00 sec)






20.1.9.7  WHILE Statement










[begin_label:] WHILE search_condition DO
    statement_list
END WHILE [end_label]





The statement or statements within a WHILE statement are repeated as
long as the search_condition is true.





begin_label and end_label must be the same, if both are
specified.





For example:





CREATE PROCEDURE dowhile()
BEGIN
  DECLARE v1 INT DEFAULT 5;

  WHILE v1 > 0 DO
    ...
    SET v1 = v1 - 1;
  END WHILE;
END






21  MySQL APIs















This chapter describes the APIs available for MySQL, where to get
them, and how to use them.  The C API is the most extensively covered,
because it was developed by the MySQL team, and is the basis for most of the
other APIs.







21.1  MySQL Program Development Utilities




This section describes some utilities that you may find useful when
developing MySQL programs.







msql2mysql



A shell script that converts mSQL programs to MySQL. It doesn't
handle every case, but it gives a good start when converting.


mysql_config



A shell script that produces the option values needed when compiling MySQL
programs.







21.1.1  @command{msql2mysql}, Convert mSQL Programs for Use with MySQL




Initially, the MySQL C API was developed to be very similar to that for the
mSQL database system. Because of this, mSQL programs often can be converted
relatively easily for use with MySQL by changing the names of the C API
functions.





The @command{msql2mysql} utility performs the conversion of mSQL C API function
calls to their MySQL equivalents.
@command{msql2mysql} converts the input file in place, so make a copy of the
original before converting it.  For example, use @command{msql2mysql} like
this:






shell> cp client-prog.c client-prog.c.orig
shell> msql2mysql client-prog.c
client-prog.c converted





Then examine `client-prog.c' and make any post-conversion revisions
that may be necessary.





@command{msql2mysql} uses the @command{replace} utility to make the function name
substitutions.
See section 8.13  The @command{replace} String-Replacement Utility.






21.1.2  @command{mysql_config}, Get compile options for compiling clients




@command{mysql_config} provides you with useful information for compiling
your MySQL client and connecting it to MySQL.





@command{mysql_config} supports the following options:







--cflags


Compiler flags to find include files and critical compiler flags and
defines used when compiling the libmysqlclient library.


--include


Compiler options to find MySQL include files. (Note that normally you would use
--cflags instead of this option.)


--libmysqld-libs, --embedded


Libraries and options required to link with the MySQL embedded server.


--libs


Libraries and options required to link with the MySQL client library.


--libs_r


Libraries and options required to link with the thread-safe MySQL client library.


--port


The default TCP/IP port number, defined when configuring MySQL.


--socket


The default Unix socket file, defined when configuring MySQL.


--version


Version number and version for the MySQL distribution.






If you invoke @command{mysql_config} with no options, it displays a list of all
options that it supports, and their values:





shell> mysql_config
Usage: /usr/local/mysql/bin/mysql_config [options]
Options:
  --cflags         [-I/usr/local/mysql/include/mysql -mcpu=pentiumpro]
  --include        [-I/usr/local/mysql/include/mysql]
  --libs           [-L/usr/local/mysql/lib/mysql -lmysqlclient -lz
                    -lcrypt -lnsl -lm -L/usr/lib -lssl -lcrypto]
  --libs_r         [-L/usr/local/mysql/lib/mysql -lmysqlclient_r
                    -lpthread -lz -lcrypt -lnsl -lm -lpthread]
  --socket         [/tmp/mysql.sock]
  --port           [3306]
  --version        [4.0.16]
  --libmysqld-libs [-L/usr/local/mysql/lib/mysql -lmysqld -lpthread -lz
                    -lcrypt -lnsl -lm -lpthread -lrt]





You can use @command{mysql_config} within a command line to include the value
that it displays for a particular option. For example, to compile a MySQL
client program, use @command{mysql_config} as follows:





CFG=/usr/local/mysql/bin/mysql_config
sh -c "gcc -o progname `$CFG --cflags` progname.c `$CFG --libs`"





When you use @command{mysql_config} this way, be sure to invoke it within
backtick (``') characters.  That tells the shell to execute it and
subsitute its output into the surrounding command.







21.2  MySQL C API












The C API code is distributed with MySQL. It is included in the
mysqlclient library and allows C programs to access a database.





Many of the clients in the MySQL source distribution are
written in C.  If you are looking for examples that demonstrate how to
use the C API, take a look at these clients. You can find these in the
clients directory in the MySQL source distribution.





Most of the other client APIs (all except Connector/J) use the mysqlclient
library to communicate with the MySQL server. This means that, for
example, you can take advantage of many of the same environment variables
that are used by other client programs, because they are referenced from the
library.  See section 8  MySQL Client and Utility Programs, for a list of these variables.





The client has a maximum communication buffer size. The size of the buffer
that is allocated initially (16KB) is automatically increased up to the
maximum size (the maximum is 16MB). Because buffer sizes are increased
only as demand warrants, simply increasing the default maximum limit does not
in itself cause more resources to be used. This size check is mostly a check
for erroneous queries and communication packets.





The communication buffer must be large enough to contain a single SQL
statement (for client-to-server traffic) and one row of returned data (for
server-to-client traffic).  Each thread's communication buffer is dynamically
enlarged to handle any query or row up to the maximum limit.  For example, if
you have BLOB values that contain up to 16MB of data, you must have a
communication buffer limit of at least 16MB (in both server and client).  The
client's default maximum is 16MB, but the default maximum in the server is
1MB.  You can increase this by changing the value of the
max_allowed_packet parameter when the server is started.  See section 7.5.2  Tuning Server Parameters.





The MySQL server shrinks each communication buffer to
net_buffer_length bytes after each query.  For clients, the size of
the buffer associated with a connection is not decreased until the connection
is closed, at which time client memory is reclaimed.





For programming with threads, see section 21.2.14  How to Make a Threaded Client.
For creating a standalone application which includes the
"server" and "client" in the same program (and does not
communicate with an external MySQL server), see section 21.2.15  libmysqld, the Embedded MySQL Server Library.







21.2.1  C API Data types






MYSQL



This structure represents a handle to one database connection. It is
used for almost all MySQL functions.


MYSQL_RES



This structure represents the result of a query that returns rows
(SELECT, SHOW, DESCRIBE, EXPLAIN).  The
information returned from a query is called the result set in the
remainder of this section.


MYSQL_ROW



This is a type-safe representation of one row of data. It is currently
implemented as an array of counted byte strings.  (You cannot treat these as
null-terminated strings if field values may contain binary data, because such
values may contain null bytes internally.)  Rows are obtained by calling
mysql_fetch_row().


MYSQL_FIELD



This structure contains information about a field, such as the field's
name, type, and size. Its members are described in more detail here.
You may obtain the MYSQL_FIELD structures for each field by
calling mysql_fetch_field() repeatedly.  Field values are not part of
this structure; they are contained in a MYSQL_ROW structure.


MYSQL_FIELD_OFFSET



This is a type-safe representation of an offset into a MySQL field
list.  (Used by mysql_field_seek().)  Offsets are field numbers
within a row, beginning at zero.


my_ulonglong




The type used for the number of rows and for mysql_affected_rows(),
mysql_num_rows(), and mysql_insert_id(). This type provides a
range of 0 to 1.84e19.

On some systems, attempting to print a value of type my_ulonglong
will not work.  To print such a value, convert it to unsigned long
and use a %lu print format.  Example:


printf ("Number of rows: %lu\n", (unsigned long) mysql_num_rows(result));








The MYSQL_FIELD structure contains the members listed here:







char * name


The name of the field, as a null-terminated string.


char * table


The name of the table containing this field, if it isn't a calculated field.
For calculated fields, the table value is an empty string.


char * def


The default value of this field, as a null-terminated string.  This is set
only if you use mysql_list_fields().


enum enum_field_types type


The type of the field.
The type value may be one of the following:




Type Value  Type Description



FIELD_TYPE_TINY  TINYINT field



FIELD_TYPE_SHORT  SMALLINT field



FIELD_TYPE_LONG  INTEGER field



FIELD_TYPE_INT24  MEDIUMINT field



FIELD_TYPE_LONGLONG  BIGINT field



FIELD_TYPE_DECIMAL  DECIMAL or NUMERIC field



FIELD_TYPE_FLOAT  FLOAT field



FIELD_TYPE_DOUBLE  DOUBLE or REAL field



FIELD_TYPE_TIMESTAMP  TIMESTAMP field



FIELD_TYPE_DATE  DATE field



FIELD_TYPE_TIME  TIME field



FIELD_TYPE_DATETIME  DATETIME field



FIELD_TYPE_YEAR  YEAR field



FIELD_TYPE_STRING  CHAR field



FIELD_TYPE_VAR_STRING  VARCHAR field



FIELD_TYPE_BLOB  BLOB or TEXT field (use max_length to determine the maximum length)



FIELD_TYPE_SET  SET field



FIELD_TYPE_ENUM  ENUM field



FIELD_TYPE_NULL  NULL-type field



FIELD_TYPE_CHAR  Deprecated; use FIELD_TYPE_TINY instead





You can use the IS_NUM() macro to test whether a field has a
numeric type.  Pass the type value to IS_NUM() and it
will evaluate to TRUE if the field is numeric:



if (IS_NUM(field->type))
    printf("Field is numeric\n");




unsigned int length


The width of the field, as specified in the table definition.


unsigned int max_length


The maximum width of the field for the result set (the length of the longest
field value for the rows actually in the result set). If you use
mysql_store_result() or mysql_list_fields(), this contains the
maximum length for the field.  If you use mysql_use_result(), the
value of this variable is zero.


unsigned int flags


Different bit-flags for the field.  The flags value may have zero
or more of the following bits set:




Flag Value  Flag Description



NOT_NULL_FLAG  Field can't be NULL



PRI_KEY_FLAG  Field is part of a primary key



UNIQUE_KEY_FLAG  Field is part of a unique key



MULTIPLE_KEY_FLAG  Field is part of a non-unique key



UNSIGNED_FLAG  Field has the UNSIGNED attribute



ZEROFILL_FLAG  Field has the ZEROFILL attribute



BINARY_FLAG  Field has the BINARY attribute



AUTO_INCREMENT_FLAG  Field has the AUTO_INCREMENT
attribute



ENUM_FLAG  Field is an ENUM (deprecated)



SET_FLAG  Field is a SET (deprecated)



BLOB_FLAG  Field is a BLOB or TEXT (deprecated)



TIMESTAMP_FLAG  Field is a TIMESTAMP (deprecated)





Use of the BLOB_FLAG, ENUM_FLAG, SET_FLAG, and
TIMESTAMP_FLAG flags is deprecated because they indicate the type of
a field rather than an attribute of its type.  It is preferable to test
field->type against FIELD_TYPE_BLOB, FIELD_TYPE_ENUM,
FIELD_TYPE_SET, or FIELD_TYPE_TIMESTAMP instead.

The following example illustrates a typical use of the flags value:



if (field->flags & NOT_NULL_FLAG)
    printf("Field can't be null\n");



You may use the following convenience macros to determine the boolean
status of the flags value:




Flag Status  Description



IS_NOT_NULL(flags)  True if this field is defined as NOT NULL



IS_PRI_KEY(flags)  True if this field is a primary key



IS_BLOB(flags)  True if this field is a BLOB or TEXT (deprecated; test field->type instead)






unsigned int decimals


The number of decimals for numeric fields.






21.2.2  C API Function Overview











The functions available in the C API are summarized here and described in
greater detail in a later section.
See section 21.2.3  C API Function Descriptions.






Function  Description




mysql_affected_rows() 
Returns the number of rows changed/deleted/inserted by the last UPDATE,
DELETE, or INSERT query.




mysql_change_user() 
Changes user and database on an open connection.




mysql_charset_name() 
Returns the name of the default character set for the connection.




mysql_close() 
Closes a server connection.




mysql_connect() 
Connects to a MySQL server. This function is deprecated; use
mysql_real_connect() instead.




mysql_create_db() 
Creates a database. This function is deprecated; use the SQL statement
CREATE DATABASE instead.




mysql_data_seek() 
Seeks to an arbitrary row number in a query result set.




mysql_debug() 
Does a DBUG_PUSH with the given string.




mysql_drop_db() 
Drops a database. This function is deprecated; use the SQL statement
DROP DATABASE instead.




mysql_dump_debug_info() 
Makes the server write debug information to the log.




mysql_eof() 
Determines whether the last row of a result set has been read.
This function is deprecated; mysql_errno() or mysql_error()
may be used instead.




mysql_errno() 
Returns the error number for the most recently invoked MySQL function.




mysql_error() 
Returns the error message for the most recently invoked MySQL function.




mysql_escape_string() 
Escapes special characters in a string for use in an SQL statement.




mysql_fetch_field() 
Returns the type of the next table field.




mysql_fetch_field_direct() 
Returns the type of a table field, given a field number.




mysql_fetch_fields() 
Returns an array of all field structures.




mysql_fetch_lengths() 
Returns the lengths of all columns in the current row.




mysql_fetch_row() 
Fetches the next row from the result set.




mysql_field_seek() 
Puts the column cursor on a specified column.




mysql_field_count() 
Returns the number of result columns for the most recent statement.




mysql_field_tell() 
Returns the position of the field cursor used for the last
mysql_fetch_field().




mysql_free_result() 
Frees memory used by a result set.




mysql_get_client_info() 
Returns client version information as a string.




mysql_get_client_version() 
Returns client version information as an integer.




mysql_get_host_info() 
Returns a string describing the connection.




mysql_get_server_version() 
Returns version number of server as an integer (new in 4.1).




mysql_get_proto_info() 
Returns the protocol version used by the connection.




mysql_get_server_info() 
Returns the server version number.




mysql_info() 
Returns information about the most recently executed query.




mysql_init() 
Gets or initializes a MYSQL structure.




mysql_insert_id() 
Returns the ID generated for an AUTO_INCREMENT column by the previous
query.




mysql_kill() 
Kills a given thread.




mysql_list_dbs() 
Returns database names matching a simple regular expression.




mysql_list_fields() 
Returns field names matching a simple regular expression.




mysql_list_processes() 
Returns a list of the current server threads.




mysql_list_tables() 
Returns table names matching a simple regular expression.




mysql_num_fields() 
Returns the number of columns in a result set.




mysql_num_rows() 
Returns the number of rows in a result set.




mysql_options() 
Sets connect options for mysql_connect().




mysql_ping() 
Checks whether the connection to the server is working, reconnecting
as necessary.




mysql_query() 
Executes an SQL query specified as a null-terminated string.




mysql_real_connect() 
Connects to a MySQL server.




mysql_real_escape_string() 
Escapes special characters in a string for use in an SQL statement, taking
into account the current charset of the connection.




mysql_real_query() 
Executes an SQL query specified as a counted string.




mysql_reload() 
Tells the server to reload the grant tables.




mysql_row_seek() 
Seeks to a row offset in a result set, using value returned from
mysql_row_tell().




mysql_row_tell() 
Returns the row cursor position.




mysql_select_db() 
Selects a database.




mysql_set_server_option() 
Sets an option for the connection (like multi-statements).




mysql_sqlstate() 
Returns the SQLSTATE error code for the last error.




mysql_shutdown() 
Shuts down the database server.




mysql_stat() 
Returns the server status as a string.




mysql_store_result() 
Retrieves a complete result set to the client.




mysql_thread_id() 
Returns the current thread ID.




mysql_thread_safe() 
Returns 1 if the clients are compiled as thread-safe.




mysql_use_result() 
Initiates a row-by-row result set retrieval.




mysql_warning_count() 
Returns the warning count for the previous SQL statement.




mysql_commit() 
Commits the transaction.




mysql_rollback() 
Rolls back the transaction.




mysql_autocommit() 
Toggles autocommit mode on/off.




mysql_more_results() 
Checks whether any more results exist.




mysql_next_result() 
Returns/Initiates the next result in multi-query executions.







To connect to the server, call mysql_init() to initialize a
connection handler, then call mysql_real_connect() with that
handler (along with other information such as the hostname, username,
and password).  Upon connection, mysql_real_connect() sets the
reconnect flag (part of the MYSQL structure) to a value of
1. This flag indicates, in the event that a query cannot be
performed because of a lost connection, to try reconnecting to the
server before giving up.  When you are done with the connection, call
mysql_close() to terminate it.





While a connection is active, the client may send SQL queries to the server
using mysql_query() or mysql_real_query().  The difference
between the two is that mysql_query() expects the query to be
specified as a null-terminated string whereas mysql_real_query()
expects a counted string.  If the string contains binary data (which may
include null bytes), you must use mysql_real_query().





For each non-SELECT query (for example, INSERT, UPDATE,
DELETE), you can find out how many rows were changed (affected)
by calling mysql_affected_rows().





For SELECT queries, you retrieve the selected rows as a result set.
(Note that some statements are SELECT-like in that they return rows.
These include SHOW, DESCRIBE, and EXPLAIN.  They should
be treated the same way as SELECT statements.)





There are two ways for a client to process result sets.  One way is to
retrieve the entire result set all at once by calling
mysql_store_result().  This function acquires from the server all the
rows returned by the query and stores them in the client.  The second way is
for the client to initiate a row-by-row result set retrieval by calling
mysql_use_result().  This function initializes the retrieval, but does
not actually get any rows from the server.





In both cases, you access rows by calling mysql_fetch_row().  With
mysql_store_result(), mysql_fetch_row() accesses rows that have
already been fetched from the server.  With mysql_use_result(),
mysql_fetch_row() actually retrieves the row from the server.
Information about the size of the data in each row is available by
calling mysql_fetch_lengths().





After you are done with a result set, call mysql_free_result()
to free the memory used for it.





The two retrieval mechanisms are complementary.  Client programs should
choose the approach that is most appropriate for their requirements.
In practice, clients tend to use mysql_store_result() more
commonly.





An advantage of mysql_store_result() is that because the rows have all
been fetched to the client, you not only can access rows sequentially, you
can move back and forth in the result set using mysql_data_seek() or
mysql_row_seek() to change the current row position within the result
set.  You can also find out how many rows there are by calling
mysql_num_rows().  On the other hand, the memory requirements for
mysql_store_result() may be very high for large result sets and you
are more likely to encounter out-of-memory conditions.





An advantage of mysql_use_result() is that the client requires less
memory for the result set because it maintains only one row at a time (and
because there is less allocation overhead, mysql_use_result() can be
faster).  Disadvantages are that you must process each row quickly to avoid
tying up the server, you don't have random access to rows within the result
set (you can only access rows sequentially), and you don't know how many rows
are in the result set until you have retrieved them all.  Furthermore, you
must retrieve all the rows even if you determine in mid-retrieval that
you've found the information you were looking for.





The API makes it possible for clients to respond appropriately to
queries (retrieving rows only as necessary) without knowing whether or
not the query is a SELECT.  You can do this by calling
mysql_store_result() after each mysql_query() (or
mysql_real_query()).  If the result set call succeeds, the query
was a SELECT and you can read the rows.  If the result set call
fails, call mysql_field_count() to determine whether a
result was actually to be expected.  If mysql_field_count()
returns zero, the query returned no data (indicating that it was an
INSERT, UPDATE, DELETE, etc.), and was not
expected to return rows.  If mysql_field_count() is non-zero, the
query should have returned rows, but didn't.  This indicates that the
query was a SELECT that failed.  See the description for
mysql_field_count() for an example of how this can be done.





Both mysql_store_result() and mysql_use_result() allow you to
obtain information about the fields that make up the result set (the number
of fields, their names and types, etc.).  You can access field information
sequentially within the row by calling mysql_fetch_field() repeatedly,
or by field number within the row by calling
mysql_fetch_field_direct().  The current field cursor position may be
changed by calling mysql_field_seek().  Setting the field cursor
affects subsequent calls to mysql_fetch_field().  You can also get
information for fields all at once by calling mysql_fetch_fields().





For detecting and reporting errors, MySQL provides access to error
information by means of the mysql_errno() and mysql_error()
functions.  These return the error code or error message for the most
recently invoked function that can succeed or fail, allowing you to determine
when an error occurred and what it was.







21.2.3  C API Function Descriptions




In the descriptions here, a parameter or return value of NULL means
NULL in the sense of the C programming language, not a
MySQL NULL value.





Functions that return a value generally return a pointer or an integer.
Unless specified otherwise, functions returning a pointer return a
non-NULL value to indicate success or a NULL value to indicate
an error, and functions returning an integer return zero to indicate success
or non-zero to indicate an error.  Note that ``non-zero'' means just that.
Unless the function description says otherwise, do not test against a value
other than zero:





if (result)                   /* correct */
    ... error ...

if (result < 0)               /* incorrect */
    ... error ...

if (result == -1)             /* incorrect */
    ... error ...





When a function returns an error, the Errors subsection of the
function description lists the possible types of errors.  You can
find out which of these occurred by calling mysql_errno().
A string representation of the error may be obtained by calling
mysql_error().







21.2.3.1  mysql_affected_rows()










my_ulonglong mysql_affected_rows(MYSQL *mysql)





21.2.3.2  Description




Returns the number of rows changed by the last UPDATE, deleted by
the last DELETE or inserted by the last INSERT
statement. May be called immediately after mysql_query() for
UPDATE, DELETE, or INSERT statements.  For
SELECT statements, mysql_affected_rows() works like
mysql_num_rows().





21.2.3.3  Return Values




An integer greater than zero indicates the number of rows affected or
retrieved.  Zero indicates that no records were updated for an
UPDATE statement, no rows matched the WHERE clause in the
query or that no query has yet been executed.  -1 indicates that the
query returned an error or that, for a SELECT query,
mysql_affected_rows() was called prior to calling
mysql_store_result(). Because mysql_affected_rows()
returns an unsigned value, you can check for -1 by comparing the
return value to (my_ulonglong)-1 (or to (my_ulonglong)~0,
which is equivalent).





21.2.3.4  Errors




None.





21.2.3.5  Example




mysql_query(&mysql,"UPDATE products SET cost=cost*1.25 WHERE group=10");
printf("%ld products updated",(long) mysql_affected_rows(&mysql));





If you specify the flag CLIENT_FOUND_ROWS when connecting to
@command{mysqld}, mysql_affected_rows() will return the number of
rows matched by the WHERE statement for UPDATE statements.





Note that when you use a REPLACE command,
mysql_affected_rows() returns 2 if the new row replaced an
old row.  This is because in this case one row was inserted after the
duplicate was deleted.





If you use INSERT ... ON DUPLICATE KEY UPDATE to insert a row,
mysql_affected_rows() returns 1 if the row is inserted as a new row and
2 if an existing row is updated.







21.2.3.6  mysql_change_user()










my_bool mysql_change_user(MYSQL *mysql, const char *user, const
char *password, const char *db)





21.2.3.7  Description




Changes the user and causes the database specified by db to
become the default (current) database on the connection specified by
mysql.  In subsequent queries, this database is the default for
table references that do not include an explicit database specifier.





This function was introduced in MySQL 3.23.3.





mysql_change_user() fails if the connected user cannot be
authenticated or doesn't have permission to use the database.  In
this case the user and database are not changed





The db parameter may be set to NULL if you don't want to have a
default database.





Starting from MySQL 4.0.6 this command will always ROLLBACK any
active transactions, close all temporary tables, unlock all locked
tables and reset the state as if one had done a new connect.
This will happen even if the user didn't change.





21.2.3.8  Return Values




Zero for success.  Non-zero if an error occurred.





21.2.3.9  Errors




The same that you can get from mysql_real_connect().







CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.

ER_UNKNOWN_COM_ERROR


The MySQL server doesn't implement this command (probably an old server).

ER_ACCESS_DENIED_ERROR


The user or password was wrong.

ER_BAD_DB_ERROR


The database didn't exist.

ER_DBACCESS_DENIED_ERROR


The user did not have access rights to the database.

ER_WRONG_DB_NAME


The database name was too long.





21.2.3.10  Example




if (mysql_change_user(&mysql, "user", "password", "new_database"))
{
   fprintf(stderr, "Failed to change user.  Error: %s\n",
           mysql_error(&mysql));
}






21.2.3.11  mysql_character_set_name()










const char *mysql_character_set_name(MYSQL *mysql)





21.2.3.12  Description




Returns the default character set for the current connection.





21.2.3.13  Return Values




The default character set





21.2.3.14  Errors



None.







21.2.3.15  mysql_close()










void mysql_close(MYSQL *mysql)





21.2.3.16  Description



Closes a previously opened connection.  mysql_close() also deallocates
the connection handle pointed to by mysql if the handle was allocated
automatically by mysql_init() or mysql_connect().





21.2.3.17  Return Values




None.





21.2.3.18  Errors




None.







21.2.3.19  mysql_connect()










MYSQL *mysql_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd)





21.2.3.20  Description




This function is deprecated.  It is preferable to use
mysql_real_connect() instead.





mysql_connect() attempts to establish a connection to a MySQL
database engine running on host.  mysql_connect() must complete
successfully before you can execute any of the other API functions, with the
exception of mysql_get_client_info().





The meanings of the parameters are the same as for the corresponding
parameters for mysql_real_connect() with the difference that the
connection parameter may be NULL. In this case the C API
allocates memory for the connection structure automatically and frees it
when you call mysql_close(). The disadvantage of this approach is
that you can't retrieve an error message if the connection fails.  (To
get error information from mysql_errno() or mysql_error(),
you must provide a valid MYSQL pointer.)





21.2.3.21  Return Values




Same as for mysql_real_connect().





21.2.3.22  Errors




Same as for mysql_real_connect().







21.2.3.23  mysql_create_db()










int mysql_create_db(MYSQL *mysql, const char *db)





21.2.3.24  Description



Creates the database named by the db parameter.





This function is deprecated.  It is preferable to use mysql_query()
to issue an SQL CREATE DATABASE statement instead.





21.2.3.25  Return Values




Zero if the database was created successfully. Non-zero if an error
occurred.





21.2.3.26  Errors





CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.


CR_SERVER_GONE_ERROR


The MySQL server has gone away.


CR_SERVER_LOST


The connection to the server was lost during the query.


CR_UNKNOWN_ERROR


An unknown error occurred.





21.2.3.27  Example




if(mysql_create_db(&mysql, "my_database"))
{
   fprintf(stderr, "Failed to create new database.  Error: %s\n",
           mysql_error(&mysql));
}






21.2.3.28  mysql_data_seek()










void mysql_data_seek(MYSQL_RES *result, my_ulonglong offset)





21.2.3.29  Description



Seeks to an arbitrary row in a query result set.  The offset
value is a row number and should be in the range from 0 to
mysql_num_rows(result)-1.





This function requires that the result set structure contains the
entire result of the query, so mysql_data_seek() may be
used only in conjunction with mysql_store_result(), not with
mysql_use_result().





21.2.3.30  Return Values




None.





21.2.3.31  Errors



None.







21.2.3.32  mysql_debug()










void mysql_debug(const char *debug)





21.2.3.33  Description



Does a DBUG_PUSH with the given string. mysql_debug() uses the
Fred Fish debug library. To use this function, you must compile the client
library to support debugging.
See section D.1  Debugging a MySQL Server. See section D.2  Debugging a MySQL Client.





21.2.3.34  Return Values




None.





21.2.3.35  Errors



None.





21.2.3.36  Example




The call shown here causes the client library to generate a trace file in
`/tmp/client.trace' on the client machine:





mysql_debug("d:t:O,/tmp/client.trace");






21.2.3.37  mysql_drop_db()










int mysql_drop_db(MYSQL *mysql, const char *db)





21.2.3.38  Description



Drops the database named by the db parameter.





This function is deprecated.  It is preferable to use mysql_query()
to issue an SQL DROP DATABASE statement instead.





21.2.3.39  Return Values




Zero if the database was dropped successfully. Non-zero if an error
occurred.





21.2.3.40  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.





21.2.3.41  Example




if(mysql_drop_db(&mysql, "my_database"))
  fprintf(stderr, "Failed to drop the database: Error: %s\n",
          mysql_error(&mysql));






21.2.3.42  mysql_dump_debug_info()










int mysql_dump_debug_info(MYSQL *mysql)





21.2.3.43  Description




Instructs the server to write some debug information to the log. For
this to work, the connected user must have the SUPER privilege.





21.2.3.44  Return Values




Zero if the command was successful. Non-zero if an error occurred.





21.2.3.45  Errors





CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.46  mysql_eof()










my_bool mysql_eof(MYSQL_RES *result)





21.2.3.47  Description




This function is deprecated.  mysql_errno() or mysql_error()
may be used instead.





mysql_eof() determines whether the last row of a result
set has been read.





If you acquire a result set from a successful call to
mysql_store_result(), the client receives the entire set in one
operation.  In this case, a NULL return from
mysql_fetch_row() always means the end of the result set has been
reached and it is unnecessary to call mysql_eof().  When used
with mysql_store_result(), mysql_eof() will always return
true.





On the other hand, if you use mysql_use_result() to initiate a result
set retrieval, the rows of the set are obtained from the server one by one as
you call mysql_fetch_row() repeatedly.  Because an error may occur on
the connection during this process, a NULL return value from
mysql_fetch_row() does not necessarily mean the end of the result set
was reached normally.  In this case, you can use mysql_eof() to
determine what happened.  mysql_eof() returns a non-zero value if the
end of the result set was reached and zero if an error occurred.





Historically, mysql_eof() predates the standard MySQL error
functions mysql_errno() and mysql_error().  Because those error
functions provide the same information, their use is preferred over
mysql_eof(), which is now deprecated.  (In fact, they provide more
information, because mysql_eof() returns only a boolean value whereas
the error functions indicate a reason for the error when one occurs.)





21.2.3.48  Return Values




Zero if no error occurred. Non-zero if the end of the result set has been
reached.





21.2.3.49  Errors



None.





21.2.3.50  Example




The following example shows how you might use mysql_eof():





mysql_query(&mysql,"SELECT * FROM some_table");
result = mysql_use_result(&mysql);
while((row = mysql_fetch_row(result)))
{
    // do something with data
}
if(!mysql_eof(result))  // mysql_fetch_row() failed due to an error
{
    fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
}





However, you can achieve the same effect with the standard MySQL
error functions:





mysql_query(&mysql,"SELECT * FROM some_table");
result = mysql_use_result(&mysql);
while((row = mysql_fetch_row(result)))
{
    // do something with data
}
if(mysql_errno(&mysql))  // mysql_fetch_row() failed due to an error
{
    fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
}






21.2.3.51  mysql_errno()










unsigned int mysql_errno(MYSQL *mysql)





21.2.3.52  Description




For the connection specified by mysql, mysql_errno() returns
the error code for the most recently invoked API function that can succeed
or fail.  A return value of zero means that no error occurred. Client error
message numbers are listed in the MySQL `errmsg.h' header file.
Server error message numbers are listed in `mysqld_error.h'. In the
MySQL source distribution you can find a complete list of
error messages and error numbers in the file `Docs/mysqld_error.txt'.
The server error codes also are listed at section 23  Error Handling in MySQL.





Note that some functions like mysql_fetch_row() don't set
mysql_errno() if they succeed.





A rule of thumb is that all functions that have to ask the server for
information will reset mysql_errno() if they succeed.





21.2.3.53  Return Values




An error code value for the last mysql_xxx() call, if it failed.
zero means no error occurred.





21.2.3.54  Errors



None.







21.2.3.55  mysql_error()










const char *mysql_error(MYSQL *mysql)





21.2.3.56  Description




For the connection specified by mysql, mysql_error()
returns a null-terminated string containing the error message for the
most recently invoked API function that failed. If a function didn't
fail, the return value of mysql_error() may be the previous error
or an empty string to indicate no error.





A rule of thumb is that all functions that have to ask the server for
information will reset mysql_error() if they succeed.





For functions that reset mysql_errno(), the following two tests
are equivalent:





if(mysql_errno(&mysql))
{
    // an error occurred
}

if(mysql_error(&mysql)[0] != '\0')
{
    // an error occurred
}





The language of the client error messages may be changed by
recompiling the MySQL client library. Currently you can choose
error messages in several different languages.
See section 5.8.2  Setting the Error Message Language.





21.2.3.57  Return Values




A null-terminated character string that describes the error.  An empty string
if no error occurred.





21.2.3.58  Errors



None.







21.2.3.59  mysql_escape_string()










You should use mysql_real_escape_string() instead!





This function is identical to mysql_real_escape_string() except
that mysql_real_escape_string() takes a connection handler as
its first argument and escapes the string according to the current
character set.  mysql_escape_string() does not take a connection
argument and does not respect the current charset setting.







21.2.3.60  mysql_fetch_field()










MYSQL_FIELD *mysql_fetch_field(MYSQL_RES *result)





21.2.3.61  Description




Returns the definition of one column of a result set as a MYSQL_FIELD
structure. Call this function repeatedly to retrieve information about all
columns in the result set.  mysql_fetch_field() returns NULL
when no more fields are left.





mysql_fetch_field() is reset to return information about the first
field each time you execute a new SELECT query.  The field returned by
mysql_fetch_field() is also affected by calls to
mysql_field_seek().





If you've called mysql_query() to perform a SELECT on a table
but have not called mysql_store_result(), MySQL returns the
default blob length (8KB) if you call mysql_fetch_field() to ask
for the length of a BLOB field.  (The 8KB size is chosen because
MySQL doesn't know the maximum length for the BLOB. This
should be made configurable sometime.) Once you've retrieved the result set,
field->max_length contains the length of the largest value for this
column in the specific query.





21.2.3.62  Return Values




The MYSQL_FIELD structure for the current column. NULL
if no columns are left.





21.2.3.63  Errors



None.





21.2.3.64  Example




MYSQL_FIELD *field;

while((field = mysql_fetch_field(result)))
{
    printf("field name %s\n", field->name);
}






21.2.3.65  mysql_fetch_fields()










MYSQL_FIELD *mysql_fetch_fields(MYSQL_RES *result)





21.2.3.66  Description




Returns an array of all MYSQL_FIELD structures for a result set.
Each structure provides the field definition for one column of the result
set.





21.2.3.67  Return Values




An array of MYSQL_FIELD structures for all columns of a result set.





21.2.3.68  Errors



None.





21.2.3.69  Example




unsigned int num_fields;
unsigned int i;
MYSQL_FIELD *fields;

num_fields = mysql_num_fields(result);
fields = mysql_fetch_fields(result);
for(i = 0; i < num_fields; i++)
{
   printf("Field %u is %s\n", i, fields[i].name);
}






21.2.3.70  mysql_fetch_field_direct()










MYSQL_FIELD *mysql_fetch_field_direct(MYSQL_RES *result, unsigned int fieldnr)





21.2.3.71  Description




Given a field number fieldnr for a column within a result set, returns
that column's field definition as a MYSQL_FIELD structure. You may use
this function to retrieve the definition for an arbitrary column.  The value
of fieldnr should be in the range from 0 to
mysql_num_fields(result)-1.





21.2.3.72  Return Values




The MYSQL_FIELD structure for the specified column.





21.2.3.73  Errors



None.





21.2.3.74  Example




unsigned int num_fields;
unsigned int i;
MYSQL_FIELD *field;

num_fields = mysql_num_fields(result);
for(i = 0; i < num_fields; i++)
{
    field = mysql_fetch_field_direct(result, i);
    printf("Field %u is %s\n", i, field->name);
}






21.2.3.75  mysql_fetch_lengths()










unsigned long *mysql_fetch_lengths(MYSQL_RES *result)





21.2.3.76  Description




Returns the lengths of the columns of the current row within a result set.
If you plan to copy field values, this length information is also useful for
optimization, because you can avoid calling strlen().  In addition, if
the result set contains binary data, you must use this function to
determine the size of the data, because strlen() returns incorrect
results for any field containing null characters.





The length for empty columns and for columns containing NULL values is
zero.  To see how to distinguish these two cases, see the description for
mysql_fetch_row().





21.2.3.77  Return Values




An array of unsigned long integers representing the size of each column (not
including any terminating null characters).
NULL if an error occurred.





21.2.3.78  Errors



mysql_fetch_lengths() is valid only for the current row of the result
set.  It returns NULL if you call it before calling
mysql_fetch_row() or after retrieving all rows in the result.





21.2.3.79  Example




MYSQL_ROW row;
unsigned long *lengths;
unsigned int num_fields;
unsigned int i;

row = mysql_fetch_row(result);
if (row)
{
    num_fields = mysql_num_fields(result);
    lengths = mysql_fetch_lengths(result);
    for(i = 0; i < num_fields; i++)
    {
         printf("Column %u is %lu bytes in length.\n", i, lengths[i]);
    }
}






21.2.3.80  mysql_fetch_row()










MYSQL_ROW mysql_fetch_row(MYSQL_RES *result)





21.2.3.81  Description




Retrieves the next row of a result set. When used after
mysql_store_result(), mysql_fetch_row() returns NULL
when there are no more rows to retrieve.  When used after
mysql_use_result(), mysql_fetch_row() returns NULL when
there are no more rows to retrieve or if an error occurred.





The number of values in the row is given by mysql_num_fields(result).
If row holds the return value from a call to mysql_fetch_row(),
pointers to the values are accessed as row[0] to
row[mysql_num_fields(result)-1].  NULL values in the row are
indicated by NULL pointers.





The lengths of the field values in the row may be obtained by calling
mysql_fetch_lengths().  Empty fields and fields containing
NULL both have length 0; you can distinguish these by checking
the pointer for the field value.  If the pointer is NULL, the field
is NULL; otherwise, the field is empty.





21.2.3.82  Return Values




A MYSQL_ROW structure for the next row.  NULL if
there are no more rows to retrieve or if an error occurred.





21.2.3.83  Errors




Note that error is not reset between calls to mysql_fetch_row()







CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.





21.2.3.84  Example




MYSQL_ROW row;
unsigned int num_fields;
unsigned int i;

num_fields = mysql_num_fields(result);
while ((row = mysql_fetch_row(result)))
{
   unsigned long *lengths;
   lengths = mysql_fetch_lengths(result);
   for(i = 0; i < num_fields; i++)
   {
       printf("[%.*s] ", (int) lengths[i], row[i] ? row[i] : "NULL");
   }
   printf("\n");
}






21.2.3.85  mysql_field_count()










unsigned int mysql_field_count(MYSQL *mysql)





If you are using a version of MySQL earlier than Version 3.22.24, you
should use unsigned int mysql_num_fields(MYSQL *mysql) instead.





21.2.3.86  Description




Returns the number of columns for the most recent query on the connection.





The normal use of this function is when mysql_store_result()
returned NULL (and thus you have no result set pointer).
In this case, you can call mysql_field_count() to
determine whether mysql_store_result() should have produced a
non-empty result.  This allows the client program to take proper action
without knowing whether the query was a SELECT (or
SELECT-like) statement.  The example shown here illustrates how this
may be done.





See section 21.2.12.1  Why mysql_store_result() Sometimes Returns NULL After mysql_query() Returns Success.





21.2.3.87  Return Values




An unsigned integer representing the number of columns in a result set.





21.2.3.88  Errors



None.





21.2.3.89  Example




MYSQL_RES *result;
unsigned int num_fields;
unsigned int num_rows;

if (mysql_query(&mysql,query_string))
{
    // error
}
else // query succeeded, process any data returned by it
{
    result = mysql_store_result(&mysql);
    if (result)  // there are rows
    {
        num_fields = mysql_num_fields(result);
        // retrieve rows, then call mysql_free_result(result)
    }
    else  // mysql_store_result() returned nothing; should it have?
    {
        if(mysql_field_count(&mysql) == 0)
        {
            // query does not return data
            // (it was not a SELECT)
            num_rows = mysql_affected_rows(&mysql);
        }
        else // mysql_store_result() should have returned data
        {
            fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
        }
    }
}





An alternative is to replace the mysql_field_count(&mysql) call with
mysql_errno(&mysql).  In this case, you are checking directly for an
error from mysql_store_result() rather than inferring from the value
of mysql_field_count() whether the statement was a
SELECT.







21.2.3.90  mysql_field_seek()










MYSQL_FIELD_OFFSET mysql_field_seek(MYSQL_RES *result, MYSQL_FIELD_OFFSET offset)





21.2.3.91  Description




Sets the field cursor to the given offset.   The next call to
mysql_fetch_field() will retrieve the field definition of the column
associated with that offset.





To seek to the beginning of a row, pass an offset value of zero.





21.2.3.92  Return Values




The previous value of the field cursor.





21.2.3.93  Errors



None.







21.2.3.94  mysql_field_tell()










MYSQL_FIELD_OFFSET mysql_field_tell(MYSQL_RES *result)





21.2.3.95  Description




Returns the position of the field cursor used for the last
mysql_fetch_field(). This value can be used as an argument to
mysql_field_seek().





21.2.3.96  Return Values




The current offset of the field cursor.





21.2.3.97  Errors



None.







21.2.3.98  mysql_free_result()










void mysql_free_result(MYSQL_RES *result)





21.2.3.99  Description




Frees the memory allocated for a result set by mysql_store_result(),
mysql_use_result(), mysql_list_dbs(), etc.  When you are done
with a result set, you must free the memory it uses by calling
mysql_free_result().





Do not attempt to access a result set after freeing it.





21.2.3.100  Return Values




None.





21.2.3.101  Errors



None.







21.2.3.102  mysql_get_client_info()










char *mysql_get_client_info(void)





21.2.3.103  Description




Returns a string that represents the client library version.





21.2.3.104  Return Values




A character string that represents the MySQL client library version.





21.2.3.105  Errors



None.







21.2.3.106  mysql_get_client_version()










unsigned long mysql_get_client_version(void)





21.2.3.107  Description




Returns an integer that represents the client library version.
The value has the format XYYZZ where X is the major
version, YY is the release level, and ZZ is the version
number within the release level. For example, a value of 40102
represents a client library version of 4.1.2.





This function was added in MySQL 4.0.16.





21.2.3.108  Return Values




An integer that represents the MySQL client library version.





21.2.3.109  Errors



None.






21.2.3.110  mysql_get_host_info()










char *mysql_get_host_info(MYSQL *mysql)





21.2.3.111  Description




Returns a string describing the type of connection in use, including the
server hostname.





21.2.3.112  Return Values




A character string representing the server hostname and the connection type.





21.2.3.113  Errors



None.







21.2.3.114  mysql_get_proto_info()










unsigned int mysql_get_proto_info(MYSQL *mysql)





21.2.3.115  Description




Returns the protocol version used by current connection.





21.2.3.116  Return Values




An unsigned integer representing the protocol version used by the current
connection.





21.2.3.117  Errors



None.







21.2.3.118  mysql_get_server_info()










char *mysql_get_server_info(MYSQL *mysql)





21.2.3.119  Description




Returns a string that represents the server version number.





21.2.3.120  Return Values




A character string that represents the server version number.





21.2.3.121  Errors



None.







21.2.3.122  mysql_get_server_version()










unsigned long mysql_get_server_version(MYSQL *mysql)





21.2.3.123  Description




Returns the version number of the server as an integer.





This function was added in MySQL 4.1.0.





21.2.3.124  Return Values




A number that represents the MySQL server version in this format:





major_version*10000 + minor_version *100 + sub_version





For example, 4.1.2 is returned as 40102.





This function is useful in client programs for quickly determining
whether some version-specific server capability exists.





21.2.3.125  Errors



None.







21.2.3.126  mysql_info()










char *mysql_info(MYSQL *mysql)





21.2.3.127  Description




Retrieves a string providing information about the most recently executed
query, but only for the statements listed here.  For other statements,
mysql_info() returns NULL.  The format of the string varies
depending on the type of query, as described here.  The numbers are
illustrative only; the string will contain values appropriate for the query.







INSERT INTO ... SELECT ...


String format: Records: 100 Duplicates: 0 Warnings: 0

INSERT INTO ... VALUES (...),(...),(...)...


String format: Records: 3  Duplicates: 0  Warnings: 0

LOAD DATA INFILE ...


String format: Records: 1 Deleted: 0 Skipped: 0 Warnings: 0

ALTER TABLE


String format: Records: 3 Duplicates: 0 Warnings: 0

UPDATE


String format: Rows matched: 40 Changed: 40 Warnings: 0





Note that mysql_info() returns a non-NULL value for
INSERT ... VALUES only for the multiple-row form
of the statement (that is, only if multiple value lists are
specified).





21.2.3.128  Return Values




A character string representing additional information about the most
recently executed query.  NULL if no information is available for the
query.





21.2.3.129  Errors



None.







21.2.3.130  mysql_init()










MYSQL *mysql_init(MYSQL *mysql)





21.2.3.131  Description




Allocates or initializes a MYSQL object suitable for
mysql_real_connect().  If mysql is a NULL pointer, the
function allocates, initializes, and returns a new object.  Otherwise, the
object is initialized and the address of the object is returned.  If
mysql_init() allocates a new object, it will be freed when
mysql_close() is called to close the connection.





21.2.3.132  Return Values




An initialized MYSQL* handle.  NULL if there was
insufficient memory to allocate a new object.





21.2.3.133  Errors



In case of insufficient memory, NULL is returned.







21.2.3.134  mysql_insert_id()










my_ulonglong mysql_insert_id(MYSQL *mysql)





21.2.3.135  Description




Returns the value generated for an AUTO_INCREMENT column by the
previous INSERT or UPDATE statement.  Use this function after
you have performed an INSERT statement into a table that contains an
AUTO_INCREMENT field.





More precisely, mysql_insert_id() is updated under these conditions:








Note that mysql_insert_id() returns 0 if the previous statement
does not use an AUTO_INCREMENT value.  If you need to save
the value for later, be sure to call mysql_insert_id() immediately
after the statement that generates the value.





The value of mysql_insert_id() is affected only by statements issued
within the current client connection. It is not affected by statements issued
by other clients.





See section 13.8.3  Information Functions.





Also note that the value of the SQL LAST_INSERT_ID() function always
contains the most recently generated AUTO_INCREMENT value, and is
not reset between statements because the value of that function is maintained
in the server.  Another difference is that LAST_INSERT_ID() is not
updated if you set an AUTO_INCREMENT column to a specific non-special
value.





The reason for the difference between LAST_INSERT_ID() and
mysql_insert_id() is that LAST_INSERT_ID() is made easy to
use in scripts while mysql_insert_id() tries to provide a little
more exact information of what happens to the AUTO_INCREMENT column.





21.2.3.136  Return Values




Described in the preceding discussion.





21.2.3.137  Errors



None.







21.2.3.138  mysql_kill()










int mysql_kill(MYSQL *mysql, unsigned long pid)





21.2.3.139  Description




Asks the server to kill the thread specified by pid.





21.2.3.140  Return Values




Zero for success.  Non-zero if an error occurred.





21.2.3.141  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.142  mysql_list_dbs()










MYSQL_RES *mysql_list_dbs(MYSQL *mysql, const char *wild)





21.2.3.143  Description




Returns a result set consisting of database names on the server that match
the simple regular expression specified by the wild parameter.
wild may contain the wildcard characters `%' or `_', or may
be a NULL pointer to match all databases.  Calling
mysql_list_dbs() is similar to executing the query SHOW
databases [LIKE wild].





You must free the result set with mysql_free_result().





21.2.3.144  Return Values




A MYSQL_RES result set for success.  NULL if an error occurred.





21.2.3.145  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_OUT_OF_MEMORY


Out of memory.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.146  mysql_list_fields()










MYSQL_RES *mysql_list_fields(MYSQL *mysql, const char *table, const char *wild)





21.2.3.147  Description




Returns a result set consisting of field names in the given table that match
the simple regular expression specified by the wild parameter.
wild may contain the wildcard characters `%' or `_', or may
be a NULL pointer to match all fields.  Calling
mysql_list_fields() is similar to executing the query SHOW
COLUMNS FROM tbl_name [LIKE wild].





Note that it's recommended that you use SHOW COLUMNS FROM tbl_name
instead of mysql_list_fields().





You must free the result set with mysql_free_result().





21.2.3.148  Return Values




A MYSQL_RES result set for success.  NULL if an error occurred.





21.2.3.149  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.150  mysql_list_processes()










MYSQL_RES *mysql_list_processes(MYSQL *mysql)





21.2.3.151  Description




Returns a result set describing the current server threads.  This is the same
kind of information as that reported by @command{mysqladmin processlist} or
a SHOW PROCESSLIST query.





You must free the result set with mysql_free_result().





21.2.3.152  Return Values




A MYSQL_RES result set for success.  NULL if an error occurred.





21.2.3.153  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.154  mysql_list_tables()










MYSQL_RES *mysql_list_tables(MYSQL *mysql, const char *wild)





21.2.3.155  Description




Returns a result set consisting of table names in the current database that
match the simple regular expression specified by the wild parameter.
wild may contain the wildcard characters `%' or `_', or may
be a NULL pointer to match all tables.  Calling
mysql_list_tables() is similar to executing the query SHOW
tables [LIKE wild].





You must free the result set with mysql_free_result().





21.2.3.156  Return Values




A MYSQL_RES result set for success.  NULL if an error occurred.





21.2.3.157  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.158  mysql_num_fields()











unsigned int mysql_num_fields(MYSQL_RES *result)





Or:





unsigned int mysql_num_fields(MYSQL *mysql)





The second form doesn't work on MySQL 3.22.24 or newer.  To pass a
MYSQL* argument, you must use
unsigned int mysql_field_count(MYSQL *mysql) instead.





21.2.3.159  Description




Returns the number of columns in a result set.





Note that you can get the number of columns either from a pointer to a result
set or to a connection handle.  You would use the connection handle if
mysql_store_result() or mysql_use_result() returned
NULL (and thus you have no result set pointer).  In this case, you can
call mysql_field_count() to determine whether
mysql_store_result() should have produced a non-empty result.  This
allows the client program to take proper action without knowing whether or
not the query was a SELECT (or SELECT-like) statement.  The
example shown here illustrates how this may be done.





See section 21.2.12.1  Why mysql_store_result() Sometimes Returns NULL After mysql_query() Returns Success.





21.2.3.160  Return Values




An unsigned integer representing the number of columns in a result set.





21.2.3.161  Errors



None.





21.2.3.162  Example




MYSQL_RES *result;
unsigned int num_fields;
unsigned int num_rows;

if (mysql_query(&mysql,query_string))
{
    // error
}
else // query succeeded, process any data returned by it
{
    result = mysql_store_result(&mysql);
    if (result)  // there are rows
    {
        num_fields = mysql_num_fields(result);
        // retrieve rows, then call mysql_free_result(result)
    }
    else  // mysql_store_result() returned nothing; should it have?
    {
        if (mysql_errno(&mysql))
        {
           fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
        }
        else if (mysql_field_count(&mysql) == 0)
        {
            // query does not return data
            // (it was not a SELECT)
            num_rows = mysql_affected_rows(&mysql);
        }
    }
}





An alternative (if you know that your query should have returned a result set)
is to replace the mysql_errno(&mysql) call with a check whether
mysql_field_count(&mysql) is = 0.  This will happen only if something
went wrong.







21.2.3.163  mysql_num_rows()










my_ulonglong mysql_num_rows(MYSQL_RES *result)





21.2.3.164  Description




Returns the number of rows in the result set.





The use of mysql_num_rows() depends on whether you use
mysql_store_result() or mysql_use_result() to return the result
set.  If you use mysql_store_result(), mysql_num_rows() may be
called immediately.  If you use mysql_use_result(),
mysql_num_rows() will not return the correct value until all the rows
in the result set have been retrieved.





21.2.3.165  Return Values




The number of rows in the result set.





21.2.3.166  Errors



None.







21.2.3.167  mysql_options()










int mysql_options(MYSQL *mysql, enum mysql_option option, const char *arg)





21.2.3.168  Description




Can be used to set extra connect options and affect behavior for a connection.
This function may be called multiple times to set several options.





mysql_options() should be called after mysql_init() and before
mysql_connect() or mysql_real_connect().





The option argument is the option that you want to set; the arg
argument is the value for the option. If the option is an integer, then
arg should point to the value of the integer.





Possible option values:







Option  Argument Type  Function



MYSQL_OPT_CONNECT_TIMEOUT  unsigned int *  Connect timeout in seconds.



MYSQL_OPT_READ_TIMEOUT  unsigned int *  Timeout for reads from server (works currently only on Windows on TCP/IP connections)



MYSQL_OPT_WRITE_TIMEOUT  unsigned int *  Timeout for writes to server (works currently only on Windows on TCP/IP connections)



MYSQL_OPT_COMPRESS  Not used  Use the compressed client/server protocol.



MYSQL_OPT_LOCAL_INFILE  optional pointer to uint  If no pointer is given or if pointer points to an unsigned int != 0 the command LOAD LOCAL INFILE is enabled.



MYSQL_OPT_NAMED_PIPE  Not used  Use named pipes to connect to a MySQL server on NT.



MYSQL_INIT_COMMAND  char *  Command to execute when connecting to the MySQL server. Will automatically be re-executed when reconnecting.



MYSQL_READ_DEFAULT_FILE  char *  Read options from the named option file instead of from `my.cnf'.



MYSQL_READ_DEFAULT_GROUP  char *  Read options from the named group from `my.cnf' or the file specified with MYSQL_READ_DEFAULT_FILE.



MYSQL_OPT_PROTOCOL  unsigned int *  Type of protocol to use. Should be one of the enum values of mysql_protocol_type defined in `mysql.h'.



MYSQL_SHARED_MEMORY_BASE_NAME  char*  Named of shared memory object for communication to server. Should be same as the option -shared-memory-base-name used for the mysqld server you want's to connect to.







MYSQL_OPT_READ_TIMEOUT and MYSQL_OPT_WRITE_TIMEOUT were added
in MySQL 4.1.1.





Note that the group client is always read if you use
MYSQL_READ_DEFAULT_FILE or MYSQL_READ_DEFAULT_GROUP.





The specified group in the option file may contain the following options:






Option  Description



connect-timeout  Connect timeout in seconds. On Linux this timeout is also used for waiting for the first answer from the server.



compress  Use the compressed client/server protocol.



database  Connect to this database if no database was specified in the connect command.



debug  Debug options.



disable-local-infile  Disable use of LOAD DATA LOCAL.



host  Default hostname.



init-command  Command to execute when connecting to MySQL server. Will automatically be re-executed when reconnecting.



interactive-timeout  Same as specifying CLIENT_INTERACTIVE to mysql_real_connect(). See section 21.2.3.179  mysql_real_connect().



local-infile[=(0|1)]  If no argument or argument != 0 then enable use of LOAD DATA LOCAL.



max_allowed_packet  Max size of packet client can read from server.



password  Default password.



pipe  Use named pipes to connect to a MySQL server on NT.



protocol={TCP | SOCKET | PIPE | MEMORY}  The protocol to use when connecting to server (New in 4.1)



port  Default port number.



return-found-rows  Tell mysql_info() to return found rows instead of updated rows when using UPDATE.



shared-memory-base-name=name  Shared memory name to use to connect to server (default is "MySQL"). New in MySQL 4.1.



socket  Default socket file.



user  Default user.







Note that timeout has been replaced by connect-timeout, but
timeout will still work for a while.





For more information about option files, see section 4.3.2  Using Option Files.





21.2.3.169  Return Values




Zero for success.  Non-zero if you used an unknown option.





21.2.3.170  Example




MYSQL mysql;

mysql_init(&mysql);
mysql_options(&mysql,MYSQL_OPT_COMPRESS,0);
mysql_options(&mysql,MYSQL_READ_DEFAULT_GROUP,"odbc");
if (!mysql_real_connect(&mysql,"host","user","passwd","database",0,NULL,0))
{
    fprintf(stderr, "Failed to connect to database: Error: %s\n",
          mysql_error(&mysql));
}





This code requests the client to use the compressed client/server protocol and
read the additional options from the odbc section in the `my.cnf'
file.







21.2.3.171  mysql_ping()










int mysql_ping(MYSQL *mysql)





21.2.3.172  Description




Checks whether the connection to the server is working. If it has gone
down, an automatic reconnection is attempted.





This function can be used by clients that remain idle for a long while,
to check whether the server has closed the connection and reconnect
if necessary.





21.2.3.173  Return Values




Zero if the server is alive. Non-zero if an error occurred.





21.2.3.174  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.175  mysql_query()










int mysql_query(MYSQL *mysql, const char *query)





21.2.3.176  Description



Executes the SQL query pointed to by the null-terminated string query.
The query must consist of a single SQL statement.  You should not add
a terminating semicolon (`;') or \g to the statement.





mysql_query() cannot be used for queries that contain binary data; you
should use mysql_real_query() instead.  (Binary data may contain the
`\0' character, which mysql_query() interprets as the end of the
query string.)





If you want to know whether the query should return a result set, you can
use mysql_field_count() to check for this.
See section 21.2.3.85  mysql_field_count().





21.2.3.177  Return Values




Zero if the query was successful. Non-zero if an error occurred.





21.2.3.178  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.179  mysql_real_connect()










MYSQL *mysql_real_connect(MYSQL *mysql, const char *host,
                 const char *user, const char *passwd, const char *db,
                 unsigned int port, const char *unix_socket,
                 unsigned long client_flag)





21.2.3.180  Description




mysql_real_connect() attempts to establish a connection to a
MySQL database engine running on host.
mysql_real_connect() must complete successfully before you can execute
any of the other API functions, with the exception of
mysql_get_client_info().





The parameters are specified as follows:








21.2.3.181  Return Values




A MYSQL* connection handle if the connection was successful,
NULL if the connection was unsuccessful.  For a successful connection,
the return value is the same as the value of the first parameter.





21.2.3.182  Errors






CR_CONN_HOST_ERROR


Failed to connect to the MySQL server.


CR_CONNECTION_ERROR


Failed to connect to the local MySQL server.


CR_IPSOCK_ERROR


Failed to create an IP socket.


CR_OUT_OF_MEMORY


Out of memory.


CR_SOCKET_CREATE_ERROR


Failed to create a Unix socket.


CR_UNKNOWN_HOST


Failed to find the IP address for the hostname.


CR_VERSION_ERROR


A protocol mismatch resulted from attempting to connect to a server with a
client library that uses a different protocol version.  This can happen if you
use a very old client library to connect to a new server that wasn't started
with the --old-protocol option.


CR_NAMEDPIPEOPEN_ERROR


Failed to create a named pipe on Windows.


CR_NAMEDPIPEWAIT_ERROR


Failed to wait for a named pipe on Windows.


CR_NAMEDPIPESETSTATE_ERROR


Failed to get a pipe handler on Windows.


CR_SERVER_LOST


If connect_timeout > 0 and it took longer than connect_timeout
seconds to connect to the server or if the server died while executing the
init-command.






21.2.3.183  Example




MYSQL mysql;

mysql_init(&mysql);
mysql_options(&mysql,MYSQL_READ_DEFAULT_GROUP,"your_prog_name");
if (!mysql_real_connect(&mysql,"host","user","passwd","database",0,NULL,0))
{
    fprintf(stderr, "Failed to connect to database: Error: %s\n",
          mysql_error(&mysql));
}





By using mysql_options() the MySQL library will read the
[client] and [your_prog_name] sections in the `my.cnf'
file which will ensure that your program will work, even if someone has
set up MySQL in some non-standard way.





Note that upon connection, mysql_real_connect() sets the reconnect
flag (part of the MYSQL structure) to a value of 1. This
flag indicates, in the event that a query cannot be performed because
of a lost connection, to try reconnecting to the server before giving up.







21.2.3.184  mysql_real_escape_string()










unsigned long mysql_real_escape_string(MYSQL *mysql, char *to, const char *from, unsigned long length)





Note that mysql must be a valid, open connection. This is needed
because the escaping depends on the character-set in use by the server.





21.2.3.185  Description




This function is used to create a legal SQL string that you can use in a
SQL statement. See section 10.1.1  Strings.





The string in from is encoded to an escaped SQL string, taking
into account the current character set of the connection. The result is placed
in to and a terminating null byte is appended.  Characters
encoded are NUL (ASCII 0), `\n', `\r', `\',
`'', `"', and Control-Z (see section 10.1  Literal Values).
(Strictly speaking, MySQL requires only that backslash and the quote
character used to quote the string in the query be escaped.  This function
quotes the other characters to make them easier to read in log files.)





The string pointed to by from must be length bytes long.  You
must allocate the to buffer to be at least length*2+1 bytes
long.  (In the worst case, each character may need to be encoded as using two
bytes, and you need room for the terminating null byte.)  When
mysql_real_escape_string() returns, the contents of to will be a
null-terminated string.  The return value is the length of the encoded
string, not including the terminating null character.





21.2.3.186  Example




char query[1000],*end;

end = strmov(query,"INSERT INTO test_table values(");
*end++ = '\'';
end += mysql_real_escape_string(&mysql, end,"What's this",11);
*end++ = '\'';
*end++ = ',';
*end++ = '\'';
end += mysql_real_escape_string(&mysql, end,"binary data: \0\r\n",16);
*end++ = '\'';
*end++ = ')';

if (mysql_real_query(&mysql,query,(unsigned int) (end - query)))
{
   fprintf(stderr, "Failed to insert row, Error: %s\n",
           mysql_error(&mysql));
}





The strmov() function used in the example is included in the
mysqlclient library and works like strcpy() but returns a
pointer to the terminating null of the first parameter.





21.2.3.187  Return Values




The length of the value placed into to, not including the
terminating null character.





21.2.3.188  Errors



None.







21.2.3.189  mysql_real_query()










int mysql_real_query(MYSQL *mysql, const char *query, unsigned long length)





21.2.3.190  Description




Executes the SQL query pointed to by query, which should be a string
length bytes long.  The query must consist of a single SQL statement.
You should not add a terminating semicolon (`;') or \g to the
statement.





You must use mysql_real_query() rather than
mysql_query() for queries that contain binary data, because binary data
may contain the `\0' character.  In addition, mysql_real_query()
is faster than mysql_query() because it does not call strlen() on
the query string.





If you want to know whether the query should return a result set, you can
use mysql_field_count() to check for this.
See section 21.2.3.85  mysql_field_count().





21.2.3.191  Return Values




Zero if the query was successful. Non-zero if an error occurred.





21.2.3.192  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.193  mysql_reload()










int mysql_reload(MYSQL *mysql)





21.2.3.194  Description




Asks the MySQL server to reload the grant tables. The
connected user must have the RELOAD privilege.





This function is deprecated.  It is preferable to use mysql_query()
to issue an SQL FLUSH PRIVILEGES statement instead.





21.2.3.195  Return Values




Zero for success.  Non-zero if an error occurred.





21.2.3.196  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.197  mysql_row_seek()










MYSQL_ROW_OFFSET mysql_row_seek(MYSQL_RES *result, MYSQL_ROW_OFFSET offset)





21.2.3.198  Description



Sets the row cursor to an arbitrary row in a query result set.
The offset value is a row offset that should be a value returned
from mysql_row_tell() or from mysql_row_seek().
This value is not a row number; if you want to seek to a row within a
result set by number, use mysql_data_seek() instead.





This function requires that the result set structure contains the
entire result of the query, so mysql_row_seek() may be used
only in conjunction with mysql_store_result(), not with
mysql_use_result().





21.2.3.199  Return Values




The previous value of the row cursor.  This value may be passed to a
subsequent call to mysql_row_seek().





21.2.3.200  Errors



None.







21.2.3.201  mysql_row_tell()










MYSQL_ROW_OFFSET mysql_row_tell(MYSQL_RES *result)





21.2.3.202  Description




Returns the current position of the row cursor for the last
mysql_fetch_row(). This value can be used as an argument to
mysql_row_seek().





You should use mysql_row_tell() only after mysql_store_result(),
not after mysql_use_result().





21.2.3.203  Return Values




The current offset of the row cursor.





21.2.3.204  Errors



None.






21.2.3.205  mysql_select_db()










int mysql_select_db(MYSQL *mysql, const char *db)





21.2.3.206  Description




Causes the database specified by db to become the default (current)
database on the connection specified by mysql.  In subsequent queries,
this database is the default for table references that do not include an
explicit database specifier.





mysql_select_db() fails unless the connected user can be authenticated
as having permission to use the database.





21.2.3.207  Return Values




Zero for success.  Non-zero if an error occurred.





21.2.3.208  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.209  mysql_set_server_option()










int mysql_set_server_option(MYSQL *mysql, enum enum_mysql_set_option option)





21.2.3.210  Description




Enables or disables an option for the connection. option can have one
of the following values:






MYSQL_OPTION_MULTI_STATEMENTS_ON  Enable multi statement support.



MYSQL_OPTION_MULTI_STATEMENTS_OFF  Disable multi statement support.







This function was added in MySQL 4.1.1.





21.2.3.211  Return Values




Zero for success.  Non-zero if an error occurred.





21.2.3.212  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

ER_UNKNOWN_COM_ERROR


The server didn't support mysql_set_server_option() (which is the
case that the server is older than 4.1.1) or the server didn't support
the option one tried to set.






21.2.3.213  mysql_shutdown()










int mysql_shutdown(MYSQL *mysql, enum enum_shutdown_level shutdown_level)





21.2.3.214  Description




Asks the database server to shut down. The connected user must have
SHUTDOWN privileges.
The shutdown_level argument was added in MySQL 4.1.3 (and 5.0.1). The
MySQL server currently supports only one type (level of gracefulness) of
shutdown; shutdown_level must be equal to
SHUTDOWN_DEFAULT. Later we will add more levels and then the
shutdown_level argument will enable to choose the desired level.
MySQL servers and MySQL clients before and after 4.1.3 are compatible; MySQL
servers newer than 4.1.3 accept the mysql_shutdown(MYSQL *mysql) call,
and MySQL servers older than 4.1.3 accept the new mysql_shutdown()
call. But dynamically linked executables which have been compiled with older
versions of libmysqlclient headers, and call mysql_shutdown(),
need to be used with the old libmysqlclient dynamic library.





The shutdown process is described in section 5.3  The MySQL Server Shutdown Process.





21.2.3.215  Return Values




Zero for success.  Non-zero if an error occurred.





21.2.3.216  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.217  mysql_sqlstate()










const char *mysql_sqlstate(MYSQL *mysql)





21.2.3.218  Description




Returns a null-terminated string containing the SQLSTATE error code for the
last error.  The error code consists of five characters.
'00000' means ``no error.''
The values are specified by ANSI SQL and ODBC.
For a list of possible values, see section 23  Error Handling in MySQL.





Note that not all MySQL errors are yet mapped to SQLSTATE's.
The value 'HY000' (general error) is used
for unmapped errors.





This function was added to MySQL 4.1.1.





21.2.3.219  Return Values




A null-terminated character string containing the SQLSTATE error code.





21.2.3.220  See Also




See section 21.2.3.51  mysql_errno().
See section 21.2.3.55  mysql_error().
See section 21.2.7.111  mysql_stmt_sqlstate().







21.2.3.221  mysql_ssl_set()










int mysql_ssl_set(MYSQL *mysql, const char *key,
                 const char *cert, const char *ca,
                 const char *capath, const char *cipher)





21.2.3.222  Description




mysql_ssl_set() is used for establishing secure connections using
SSL. It must be called before mysql_real_connect().





mysql_ssl_set() does nothing unless OpenSSL support is enabled in
the client library.





mysql is the connection handler returned from mysql_init().
The other parameters are specified as follows:








Any unused SSL parameters may be given as NULL.





21.2.3.223  Return Values




This function always returns 0. If SSL setup is incorrect,
mysql_real_connect() will return an error when you attempt to connect.






21.2.3.224  mysql_stat()










char *mysql_stat(MYSQL *mysql)





21.2.3.225  Description




Returns a character string containing information similar to that provided by
the @command{mysqladmin status} command.  This includes uptime in seconds and
the number of running threads, questions, reloads, and open tables.





21.2.3.226  Return Values




A character string describing the server status. NULL if an
error occurred.





21.2.3.227  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.228  mysql_store_result()










MYSQL_RES *mysql_store_result(MYSQL *mysql)





21.2.3.229  Description




You must call mysql_store_result() or mysql_use_result()
for every query that successfully retrieves data (SELECT,
SHOW, DESCRIBE, EXPLAIN, CHECK TABLE, and so forth).





You don't have to call mysql_store_result() or
mysql_use_result() for other queries, but it will not do any
harm or cause any notable performance if you call mysql_store_result()
in all cases.  You can detect if the query didn't have a result set by
checking if mysql_store_result() returns 0 (more about this later on).





If you want to know whether the query should return a result set, you can
use mysql_field_count() to check for this.
See section 21.2.3.85  mysql_field_count().





mysql_store_result() reads the entire result of a query to the client,
allocates a MYSQL_RES structure, and places the result into this
structure.





mysql_store_result() returns a null pointer if the query didn't return
a result set (if the query was, for example, an INSERT statement).





mysql_store_result() also returns a null pointer if reading of the
result set failed. You can check whether an error occurred by checking if
mysql_error() returns a non-empty string, if
mysql_errno() returns non-zero, or if mysql_field_count()
returns zero.





An empty result set is returned if there are no rows returned.  (An empty
result set differs from a null pointer as a return value.)





Once you have called mysql_store_result() and got a result back
that isn't a null pointer, you may call mysql_num_rows() to find
out how many rows are in the result set.





You can call mysql_fetch_row() to fetch rows from the result set,
or mysql_row_seek() and mysql_row_tell() to obtain or
set the current row position within the result set.





You must call mysql_free_result() once you are done with the result
set.





See section 21.2.12.1  Why mysql_store_result() Sometimes Returns NULL After mysql_query() Returns Success.





21.2.3.230  Return Values




A MYSQL_RES result structure with the results. NULL if
an error occurred.





21.2.3.231  Errors




mysql_store_result() resets mysql_error() and
mysql_errno() if it succeeds.







CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_OUT_OF_MEMORY


Out of memory.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.232  mysql_thread_id()










unsigned long mysql_thread_id(MYSQL *mysql)





21.2.3.233  Description




Returns the thread ID of the current connection.  This value can be used as
an argument to mysql_kill() to kill the thread.





If the connection is lost and you reconnect with mysql_ping(), the
thread ID will change.  This means you should not get the thread ID and store
it for later. You should get it when you need it.





21.2.3.234  Return Values




The thread ID of the current connection.





21.2.3.235  Errors



None.







21.2.3.236  mysql_use_result()










MYSQL_RES *mysql_use_result(MYSQL *mysql)





21.2.3.237  Description




You must call mysql_store_result() or mysql_use_result() for
every query that successfully retrieves data (SELECT, SHOW,
DESCRIBE, EXPLAIN).





mysql_use_result() initiates a result set retrieval but does not
actually read the result set into the client like mysql_store_result()
does.  Instead, each row must be retrieved individually by making calls to
mysql_fetch_row().  This reads the result of a query directly from the
server without storing it in a temporary table or local buffer, which is
somewhat faster and uses much less memory than mysql_store_result().
The client will allocate memory only for the current row and a communication
buffer that may grow up to max_allowed_packet bytes.





On the other hand, you shouldn't use mysql_use_result() if you are
doing a lot of processing for each row on the client side, or if the output
is sent to a screen on which the user may type a ^S (stop scroll).
This will tie up the server and prevent other threads from updating any
tables from which the data is being fetched.





When using mysql_use_result(), you must execute
mysql_fetch_row() until a NULL value is returned, otherwise, the
unfetched rows will be returned as part of the result set for your next
query. The C API will give the error Commands out of sync; you can't
run this command now if you forget to do this!





You may not use mysql_data_seek(), mysql_row_seek(),
mysql_row_tell(), mysql_num_rows(), or
mysql_affected_rows() with a result returned from
mysql_use_result(), nor may you issue other queries until the
mysql_use_result() has finished.  (However, after you have fetched all
the rows, mysql_num_rows() will accurately return the number of rows
fetched.)





You must call mysql_free_result() once you are done with the result
set.





21.2.3.238  Return Values




A MYSQL_RES result structure. NULL if an error occurred.





21.2.3.239  Errors




mysql_use_result() resets mysql_error() and
mysql_errno() if it succeeds.







CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_OUT_OF_MEMORY


Out of memory.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.3.240  mysql_warning_count()










unsigned int mysql_warning_count(MYSQL *mysql)





21.2.3.241  Description




Returns the number of warnings generated during execution of the previous
SQL statement.





This function was added in MySQL 4.1.0.





21.2.3.242  Return Values




The warning count.





21.2.3.243  Errors




None.







21.2.3.244  mysql_commit()










my_bool mysql_commit(MYSQL *mysql)





21.2.3.245  Description




Commits the current transaction.





This function was added in MySQL 4.1.0.





21.2.3.246  Return Values




Zero if successful. Non-zero if an error occurred.





21.2.3.247  Errors




None.







21.2.3.248  mysql_rollback()










my_bool mysql_rollback(MYSQL *mysql)





21.2.3.249  Description




Rolls back the current transaction.





This function was added in MySQL 4.1.0.





21.2.3.250  Return Values




Zero if successful. Non-zero if an error occurred.





21.2.3.251  Errors




None.







21.2.3.252  mysql_autocommit()










my_bool mysql_autocommit(MYSQL *mysql, my_bool mode)





21.2.3.253  Description




Sets autocommit mode on if mode is 1, off if mode is 0.





This function was added in MySQL 4.1.0.





21.2.3.254  Return Values




Zero if successful. Non-zero if an error occurred.





21.2.3.255  Errors




None.







21.2.3.256  mysql_more_results()










my_bool mysql_more_results(MYSQL *mysql)





21.2.3.257  Description




Returns true if more results exist from the currently executed query,
and the application must call mysql_next_result() to fetch the
results.





This function was added in MySQL 4.1.0.





21.2.3.258  Return Values




TRUE (1) if more results exist. FALSE (0) if no more results
exist.





In most cases, you can call mysql_next_result() instead to
test whether more results exist and initiate retrieval if so.





See section 21.2.8  C API Handling of Multiple Query Execution.
See section 21.2.3.260  mysql_next_result().





21.2.3.259  Errors




None.







21.2.3.260  mysql_next_result()










int mysql_next_result(MYSQL *mysql)





21.2.3.261  Description




If more query results exist, mysql_next_result() reads the
next query results and returns the status back to application.





You must call mysql_free_result() for the preceding
query if it returned a result set.





After calling mysql_next_result() the state of the connection
is as if you had called mysql_real_query() or mysql_query()
for the next query.
This means that you can now call mysql_store_result(),
mysql_warning_count(), mysql_affected_rows(), and so forth.





If mysql_next_result() returns an error, no other statements will
be executed and there are no more results to fetch.





See section 21.2.8  C API Handling of Multiple Query Execution.





This function was added in MySQL 4.1.0.





21.2.3.262  Return Values





Return Value  Description



0  Successful and there are more results



-1  Successful and there are no more results



>0  An error occurred







21.2.3.263  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order. For example if you didn't call
mysql_use_result() for a previous result set.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query.

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.4  C API Prepared Statements




As of MySQL 4.1, the client/server protocol provides for the use of
prepared statements. This capability uses the
MYSQL_STMT statement handler data structure returned by the
mysql_stmt_init() initialization function.
Prepared execution is an efficient way to execute a statement more than
once. The statement is first parsed to prepare it for execution. Then
it is executed one or more times at a later time, using the statement
handle returned by the initialization function.





Prepared execution is faster than direct execution for statements
executed more than once, primarly because the query is parsed only
once. In the case of direct execution, the query is parsed every
time it is executed. Prepared execution also can provide a reduction of network
traffic because for each execution of the prepared statement,
it is necessary only to send the data for the parameters.





Another advantage of prepared statements is that it uses a binary protocol
that makes data transfer between client and server more efficient.







21.2.5  C API Prepared Statement Data types




Note:
Some incompatible changes were made in MySQL 4.1.2.
See section 21.2.7  C API Prepared Statement Function Descriptions for details.





Prepared statements mainly use the MYSQL_STMT and
MYSQL_BIND data structures. A third structure, MYSQL_TIME,
is used to transfer temporal data.







MYSQL_STMT



This structure represents a prepared statement.
A statement is created by calling mysql_stmt_init(),
which returns a statement handle, that is, a pointer to a
MYSQL_STMT.
The handle is used for all subsequent statement-related functions until you
close it with mysql_stmt_close().

The MYSQL_STMT structure has no members that are for application
use.

Multiple statement handles can be associated with a single connection.
The limit on the number of handles depends on the available system resources.


MYSQL_BIND



This structure is used both for statement input (data values sent to the server)
and output (result values returned from the server). For input, it is used with
mysql_stmt_bind_param() to bind parameter data values to
buffers for use by mysql_stmt_execute(). For output, it is
used with mysql_stmt_bind_result() to bind result set
buffers for use in fetching rows with mysql_stmt_fetch().

The MYSQL_BIND structure contains the following members for
use by application programs.
Each is used both for input and for output, although sometimes for different
purposes depending on the direction of data transfer.





enum enum_field_types buffer_type


The type of the buffer. The allowable buffer_type values are listed
later in this section. For input, buffer_type indicates what type of
value you are binding to a statement parameter. For output, it indicates what
type of value you expect to receive in a result buffer.


void *buffer


For input, this is a pointer to the buffer in which a statement parameter's data
value is stored. For output, it is
a pointer to the buffer in which to return a result set column value.
For numeric column types, buffer should point to a variable of the proper
C type.  (If you are associating the variable with a column that has the
UNSIGNED attribute, the variable should be an unsigned C type.
Indicate whether the variable is signed or unsigned by using the
is_unsigned member, described later in this list.)
For date and time column types, buffer should point to a
MYSQL_TIME structure.  For character and binary string column types,
buffer should point to a character buffer.


unsigned long buffer_length


The actual size of *buffer in bytes. This indicates the maximum amount
of data that can be stored in the buffer. For character and binary C data,
the buffer_length value specifies the length of *buffer
when used with mysql_stmt_bind_param(),
or the maximum number of data bytes that can be fetched into the buffer
when used with mysql_stmt_bind_result().


unsigned long *length


A pointer to an unsigned long variable that indicates the actual number
of bytes of data stored in *buffer.
length is used for character or binary C data.
For input parameter data binding, length
points to an unsigned long variable that indicates the
length of the parameter value stored in *buffer; this is used by
mysql_stmt_execute().
If length is a null pointer, the protocol assumes
that all character and binary data are null-terminated.
For output value binding, mysql_stmt_fetch() places
the length of the column value that is returned
into the variable that length points to.

length
is ignored for numeric and temporal data types because the length
of the data value is determined by the buffer_type value.


my_bool *is_null


This member points to a my_bool variable that is true if a value is
NULL, false if it is not NULL. For input, set *is_null
to true to
indicate that you are passing a NULL value as a statement parameter. For
output, this value will be set to true after you fetch a row if the result
set column value returned from the statement is NULL.


my_bool is_unsigned


This member is used for integer types.
(These correspond to the MYSQL_TYPE_TINY, MYSQL_TYPE_SHORT,
MYSQL_TYPE_LONG, and MYSQL_TYPE_LONGLONG type codes.)
is_unsigned
should be set to true for unsigned types and false for signed types.




MYSQL_TIME



This structure is used to send and receive DATE, TIME,
DATETIME, and TIMESTAMP data directly to and from the server.
This is done by setting the buffer_type member of a MYSQL_BIND
structure to one of the temporal types, and setting the buffer member
to point to a MYSQL_TIME structure.

The MYSQL_TIME structure contains the following members:





unsigned int year


The year.


unsigned int month


The month of the year.


unsigned int day


The day of the month.


unsigned int hour


The hour of the day.


unsigned int minute


The minute of the hour.


unsigned int second


The second of the minute.


my_bool neg


A boolean flag to indicate whether the time is negative.


unsigned long second_part


The fractional part of the second.
This member currently is unused.




Only those parts of a MYSQL_TIME structure that apply to a given
type of temporal value are used:
The year, month, and day elements are used for
DATE, DATETIME, and TIMESTAMP values.
The hour, minute, and second elements are used for
TIME, DATETIME, and TIMESTAMP values.
See section 21.2.9  C API Handling of Date and Time Values.






The following table shows the allowable values that may be specified in the
buffer_type member of MYSQL_BIND structures.
The table also shows those SQL types that correspond most closely to each
buffer_type value, and, for numeric and temporal types, the
corresponding C type.






buffer_type Value  SQL Type  C Type



MYSQL_TYPE_TINY  TINYINT  char



MYSQL_TYPE_SHORT  SMALLINT  short int



MYSQL_TYPE_LONG  INT  int



MYSQL_TYPE_LONGLONG  BIGINT  long long int



MYSQL_TYPE_FLOAT  FLOAT  float



MYSQL_TYPE_DOUBLE  DOUBLE  double



MYSQL_TYPE_TIME  TIME  MYSQL_TIME



MYSQL_TYPE_DATE  DATE  MYSQL_TIME



MYSQL_TYPE_DATETIME  DATETIME  MYSQL_TIME



MYSQL_TYPE_TIMESTAMP  TIMESTAMP  MYSQL_TIME



MYSQL_TYPE_STRING  CHAR 



MYSQL_TYPE_VAR_STRING  VARCHAR 



MYSQL_TYPE_TINY_BLOB  TINYBLOB/TINYTEXT 



MYSQL_TYPE_BLOB  BLOB/TEXT 



MYSQL_TYPE_MEDIUM_BLOB  MEDIUMBLOB/MEDIUMTEXT 



MYSQL_TYPE_LONG_BLOB  LONGBLOB/LONGTEXT 







Implicit type conversion may be performed in both directions.







21.2.6  C API Prepared Statement Function Overview











Note:
Some incompatible changes were made in MySQL 4.1.2.
See section 21.2.7  C API Prepared Statement Function Descriptions for details.






The functions available for prepared statement processing are summarized here and
described in greater detail in a later section.
See section 21.2.7  C API Prepared Statement Function Descriptions.







Function  Description




mysql_stmt_affected_rows() 
Returns the number of rows changes, deleted, or inserted by prepared
UPDATE, DELETE, or INSERT statement.




mysql_stmt_attr_get() 
Get value of an attribute for a prepared statement.




mysql_stmt_attr_set() 
Sets an attribute for a prepared statement.




mysql_stmt_bind_param() 
Associates application data buffers with the parameter markers in a prepared
SQL statement.




mysql_stmt_bind_result() 
Associates application data buffers with columns in the result set.




mysql_stmt_close() 
Frees memory used by prepared statement.




mysql_stmt_data_seek() 
Seeks to an arbitrary row number in a statement result set.




mysql_stmt_errno() 
Returns the error number for the last statement execution.




mysql_stmt_error() 
Returns the error message for the last statement execution.




mysql_stmt_execute() 
Executes the prepared statement.




mysql_stmt_fetch() 
Fetches the next row of data from the result set and returns data for all bound columns.




mysql_stmt_fetch_column() 
Fetch data for one column of the current row of the result set.




mysql_stmt_field_count() 
Returns the number of result columns for the most recent statement.




mysql_stmt_free_result() 
Free the resources allocated to the statement handle.




mysql_stmt_init() 
Allocates memory for MYSQL_STMT structure and initializes it.




mysql_stmt_insert_id() 
Returns the ID generated for an AUTO_INCREMENT column by prepared
statement.




mysql_stmt_num_rows() 
Returns total rows from the statement buffered result set.




mysql_stmt_param_count() 
Returns the number of parameters in a prepared SQL statement.




mysql_stmt_param_metadata() 
Return parameter metadata in the form of a result set.




mysql_stmt_prepare() 
Prepares an SQL string for execution.




mysql_stmt_reset() 
Reset the statement buffers in the server.




mysql_stmt_result_metadata() 
Returns prepared statement metadata in the form of a result set.




mysql_stmt_row_seek() 
Seeks to a row offset in a statement result set, using value returned from
mysql_stmt_row_tell().




mysql_stmt_row_tell() 
Returns the statement row cursor position.




mysql_stmt_send_long_data() 
Sends long data in chunks to server.




mysql_stmt_sqlstate() 
Returns the SQLSTATE error code for the last statement execution.




mysql_stmt_store_result() 
Retrieves the complete result set to the client.








Call mysql_stmt_init() to create a statement
handle, then mysql_stmt_prepare to prepare it, 
mysql_stmt_bind_param() to supply the parameter
data, and mysql_stmt_execute() to execute the statement. You can
repeat the mysql_stmt_execute() by changing parameter values in the
respective buffers supplied through mysql_stmt_bind_param().





If the statement is a SELECT or any other statement that produces
a result set, mysql_stmt_prepare() will also return the result
set metadata information in the form of a MYSQL_RES result set 
through mysql_stmt_result_metadata().





You can supply the result buffers using mysql_stmt_bind_result(), so
that the mysql_stmt_fetch() will automatically return data to these
buffers. This is row-by-row fetching.





You can also send the text or binary data in chunks to server using
mysql_stmt_send_long_data().
See section 21.2.7.106  mysql_stmt_send_long_data().





When statement execution has been completed, the statement handle must be
closed using mysql_stmt_close() so that all resources associated
with it can be freed.





If you obtained a SELECT statement's result set metadata by calling
mysql_stmt_result_metadata(), you should also free the metadata using
mysql_free_result().





21.2.6.1  Execution Steps




To prepare and execute a statement, an application follows these steps:





    

  1. 

Create a prepared statement handle with msyql_stmt_init().
To prepare the statement on the server, call mysql_stmt_prepare() and
pass it a string containing the SQL statement.

  2. 

If the statement produces a result set, call mysql_stmt_result_metadata()
to obtain the result set metadata.  This metadata is itself in the form of
result set, albeit a separate one from the one that contains the rows returned
by the query. The metadata result set indicates how many columns are in the
result and contains information about each column.

  3. 

Set the values of any parameters using mysql_stmt_bind_param(). All
parameters must be set. Otherwise, statement execution will return an error or
produce unexpected results.

  4. 

Call mysql_stmt_execute() to execute the statement.

  5. 

If the statement produces a result set,
bind the data buffers to use for retrieving the row values by calling
mysql_stmt_bind_result().

  6. 

Fetch the data into the buffers row by row by calling
mysql_stmt_fetch() repeatedly until no more rows are found.

  7. 

Repeat steps 3 through 6 as necessary, by changing the parameter values and
re-executing the statement.






When mysql_stmt_prepare() is called, the MySQL client/server protocol
performs these actions:








When mysql_stmt_execute() is called, the MySQL
client/server protocol performs these actions:








When mysql_stmt_fetch() is called, the MySQL client/server protocol
performs these actions:








If an error occurs,
you can get the statement error code, error message, and SQLSTATE value using
mysql_stmt_errno(), mysql_stmt_error(), and
mysql_stmt_sqlstate(), respectively.







21.2.7  C API Prepared Statement Function Descriptions




To prepare and execute queries, use the functions in the following sections.





In MySQL 4.1.2, the names of several prepared statement functions were
changed:






Old Name  New Name



mysql_bind_param()  mysql_stmt_bind_param()



mysql_bind_result()  mysql_stmt_bind_result()



mysql_prepare()  mysql_stmt_prepare()



mysql_execute()  mysql_stmt_execute()



mysql_fetch()  mysql_stmt_fetch()



mysql_fetch_column()  mysql_stmt_fetch_column()



mysql_param_count()  mysql_stmt_param_count()



mysql_param_result()  mysql_stmt_param_metadata()



mysql_get_metadata()  mysql_stmt_result_metadata()



mysql_send_long_data()  mysql_stmt_send_long_data()







All functions that operate with a MYSQL_STMT structure now begin with
the prefix mysql_stmt_.





Also in 4.1.2, the signature of the mysql_stmt_prepare() function was
changed to int mysql_stmt_prepare(MYSQL_STMT *stmt, const char *query,
unsigned long length).  To create a MYSQL_STMT handle, you should
use the mysql_stmt_init() function.







21.2.7.1  mysql_stmt_affected_rows()










my_ulonglong mysql_stmt_affected_rows(MYSQL_STMT *stmt)





21.2.7.2  Description




Returns the total number of rows changed, deleted, or inserted by the last
executed statement. May be called immediately after mysql_stmt_execute()
for UPDATE, DELETE, or INSERT statements. For
SELECT statements, mysql_stmt_affected_rows() works like
mysql_num_rows().






This function was added in MySQL 4.1.0.





21.2.7.3  Return Values




An integer greater than zero indicates the number of rows affected
or retrieved. Zero indicates that no records were updated for an
UPDATE statement, no rows matched the WHERE clause in
the query, or that no query has yet been executed. -1 indicates that
the query returned an error or that, for a SELECT query,
mysql_stmt_affected_rows() was called prior to calling
mysql_stmt_store_result().
Because mysql_stmt_affected_rows()
returns an unsigned value, you can check for -1 by comparing the
return value to (my_ulonglong)-1 (or to (my_ulonglong)~0,
which is equivalent).





See section 21.2.3.1  mysql_affected_rows() for additional
information on the return value.





21.2.7.4  Errors




None.





21.2.7.5  Example




For the usage of mysql_stmt_affected_rows(), refer to the Example
from section 21.2.7.41  mysql_stmt_execute().







21.2.7.6  mysql_stmt_attr_get()










int mysql_stmt_attr_get(MYSQL_STMT *stmt, enum enum_stmt_attr_type option, void *arg)





21.2.7.7  Description




Can be used to get the current value for a statement attribute.





The option argument is the option that you want to get; the
arg should point to a variable that should contain the option
value. If the option is an integer, then arg should point to the
value of the integer.





See mysql_stmt_attr_set() for a list of options and option
types. See section 21.2.7.10  mysql_stmt_attr_set().





This function was added in MySQL 4.1.2.





21.2.7.8  Return Values




0 if okay.  Non-zero if option is unknown.





21.2.7.9  Errors




None.







21.2.7.10  mysql_stmt_attr_set()










int mysql_stmt_attr_set(MYSQL_STMT *stmt, enum enum_stmt_attr_type option, const void *arg)





21.2.7.11  Description




Can be used to set affect behavior for a statement.
This function may be called multiple times to set several options.





The option argument is the option that you want to set; the arg
argument is the value for the option. If the option is an integer, then
arg should point to the value of the integer.





Possible options values:






Option  Argument Type  Function



STMT_ATTR_UPDATE_MAX_LENGTH  my_bool *  If set to 1: Update metadata MYSQL_FIELD->max_length in mysql_stmt_store_result().







This function was added in MySQL 4.1.2.





21.2.7.12  Return Values




0 if okay.  Non-zero if option is unknown.





21.2.7.13  Errors




None.







21.2.7.14  mysql_stmt_bind_param()










my_bool mysql_stmt_bind_param(MYSQL_STMT *stmt, MYSQL_BIND *bind)





21.2.7.15  Description




mysql_stmt_bind_param() is used to bind data for the parameter markers
in the SQL statement that was passed to mysql_stmt_prepare(). It uses
MYSQL_BIND structures to supply the data. bind is the address
of an array of MYSQL_BIND structures.
The client library expects the array to contain an element for each
`?' parameter marker that is present in the query.





Suppose that you prepare the following statement:





INSERT INTO mytbl VALUES(?,?,?)





When you bind the parameters, the array of MYSQL_BIND structures must
contain three elements, and can be declared like this:





MYSQL_BIND bind[3];





The members of each MYSQL_BIND element that should be set are described
in section 21.2.5  C API Prepared Statement Data types.





This function was added in MySQL 4.1.2.





21.2.7.16  Return Values




Zero if the bind was successful. Non-zero if an error occurred.





21.2.7.17  Errors





CR_INVALID_BUFFER_USE


Indicates if the bind is to supply the long data in chunks and if the
buffer type is non string or binary.

CR_UNSUPPORTED_PARAM_TYPE


The conversion is not supported. Possibly the buffer_type value is
illegal or is not one of the supported types.

CR_OUT_OF_MEMORY


Out of memory.

CR_UNKNOWN_ERROR


An unknown error occurred.





21.2.7.18  Example




For the usage of mysql_stmt_bind_param(), refer to the Example from
section 21.2.7.41  mysql_stmt_execute().







21.2.7.19  mysql_stmt_bind_result()










my_bool mysql_stmt_bind_result(MYSQL_STMT *stmt, MYSQL_BIND *bind)





21.2.7.20  Description




mysql_stmt_bind_result() is used to associate (bind) columns in the
result set to data buffers and length buffers. When
mysql_stmt_fetch() is
called to fetch data, the MySQL client/server protocol places the data for the
bound columns into the specified buffers.





All columns must be bound to buffers prior to calling
mysql_stmt_fetch().
bind is the address of an array of MYSQL_BIND structures.
The client library expects the array to contain
an element for each column of the result set.
If you do not bind columns to MYSQL_BIND structures,
mysql_stmt_fetch() simply ignores
the data fetch. The buffers should be large enough to hold the
data values, because the protocol doesn't return data values in chunks.





A column can be bound or rebound at any time, even after a result set has been
partially retrieved.  The new binding takes effect the next time
mysql_stmt_fetch() is called. Suppose that an application binds
the columns in a result set and calls mysql_stmt_fetch().
The client/server
protocol returns data in the bound buffers. Then suppose the application
binds the columns to a different set of buffers. The protocol does
not place data into the newly bound
buffers until the next call to mysql_stmt_fetch() occurs.





To bind a column, an application calls mysql_stmt_bind_result() and
passes the type, address, and the address of the length buffer.
The members of each MYSQL_BIND element that should be set are described
in section 21.2.5  C API Prepared Statement Data types.





This function was added in MySQL 4.1.2.





21.2.7.21  Return Values




Zero if the bind was successful. Non-zero if an error occurred.





21.2.7.22  Errors






CR_UNSUPPORTED_PARAM_TYPE


The conversion is not supported. Possibly the buffer_type value is
illegal or is not one of the supported types.

CR_OUT_OF_MEMORY


Out of memory.

CR_UNKNOWN_ERROR


An unknown error occurred.





21.2.7.23  Example




For the usage of mysql_stmt_bind_result(), refer to the Example from
section 21.2.7.54  mysql_stmt_fetch().







21.2.7.24  mysql_stmt_close()










my_bool mysql_stmt_close(MYSQL_STMT *)





21.2.7.25  Description




Closes the prepared statement. mysql_stmt_close() also
deallocates the statement handle pointed to by stmt. 





If the current statement has pending or unread results, this function
cancels them so that the next query can be executed.





This function was added in MySQL 4.1.0.





21.2.7.26  Return Values




Zero if the statement was freed successfully. Non-zero if an error occurred.





21.2.7.27  Errors






CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_UNKNOWN_ERROR


An unknown error occurred.





21.2.7.28  Example




For the usage of mysql_stmt_close(), refer to the Example from
section 21.2.7.41  mysql_stmt_execute().







21.2.7.29  mysql_stmt_data_seek()










void mysql_stmt_data_seek(MYSQL_STMT *stmt, my_ulonglong offset)





21.2.7.30  Description



Seeks to an arbitrary row in a statement result set.  The offset
value is a row number and should be in the range from 0 to
mysql_stmt_num_rows(stmt)-1.





This function requires that the statement result set structure
contains the entire result of the last executed query, so
mysql_stmt_data_seek() may be used only in conjunction with
mysql_stmt_store_result().





This function was added in MySQL 4.1.1.






21.2.7.31  Return Values




None.





21.2.7.32  Errors



None.







21.2.7.33  mysql_stmt_errno()










unsigned int mysql_stmt_errno(MYSQL_STMT *stmt)





21.2.7.34  Description




For the statement specified by stmt, mysql_stmt_errno()
returns the error code for the most recently invoked statement API
function that can succeed or fail. A return value of zero means that no
error occurred.  Client error
message numbers are listed in the MySQL `errmsg.h' header file.
Server error message numbers are listed in `mysqld_error.h'. In the
MySQL source distribution you can find a complete list of
error messages and error numbers in the file `Docs/mysqld_error.txt'.
The server error codes also are listed at section 23  Error Handling in MySQL.





This function was added in MySQL 4.1.0.





21.2.7.35  Return Values




An error code value. Zero if no error occurred.





21.2.7.36  Errors




None.







21.2.7.37  mysql_stmt_error()










const char *mysql_stmt_error(MYSQL_STMT *stmt)





21.2.7.38  Description




For the statement specified by stmt, mysql_stmt_error()
returns a null-terminated string containing the error message for the most recently invoked statement API
function that can succeed or fail. An empty string ("") is returned
if no error occurred. This means the following two tests are equivalent:






if (mysql_stmt_errno(stmt))
{
  // an error occurred
}

if (mysql_stmt_error(stmt)[0])
{
  // an error occurred
}





The language of the client error messages may be changed by recompiling
the MySQL client library. Currently you can choose error messages in
several different languages.





This function was added in MySQL 4.1.0.





21.2.7.39  Return Values




A character string that describes the error. An empty string if no error
occurred.





21.2.7.40  Errors




None.







21.2.7.41  mysql_stmt_execute()










int mysql_stmt_execute(MYSQL_STMT *stmt)





21.2.7.42  Description




mysql_stmt_execute() executes the prepared query associated with the
statement handle. The currently bound parameter marker values are sent
to server during this call, and the server replaces the markers with this newly
supplied data.






If the statement is an UPDATE, DELETE, or INSERT,
the total number of
changed, deleted, or inserted rows can be found by calling
mysql_stmt_affected_rows(). If this is a result set query such as
SELECT, you
must call mysql_stmt_fetch() to fetch the data prior to calling any
other functions that result in query processing. For more information on
how to fetch the results, refer to
section 21.2.7.54  mysql_stmt_fetch().





This function was added in MySQL 4.1.2.
 



21.2.7.43  Return Values






Zero if execution was successful. Non-zero if an error occurred.





21.2.7.44  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order. 

CR_OUT_OF_MEMORY


Out of memory. 

CR_SERVER_GONE_ERROR


The MySQL server has gone away. 

CR_SERVER_LOST


The connection to the server was lost during the query. 

CR_UNKNOWN_ERROR


An unknown error occurred. 





21.2.7.45  Example




The following example demonstrates how to create and populate a table using
mysql_stmt_init(),
mysql_stmt_prepare(),
mysql_stmt_param_count(), mysql_stmt_bind_param(),
mysql_stmt_execute(),
and mysql_stmt_affected_rows(). The mysql variable is assumed
to be a valid connection handle.





#define STRING_SIZE 50

#define DROP_SAMPLE_TABLE "DROP TABLE IF EXISTS test_table"
#define CREATE_SAMPLE_TABLE "CREATE TABLE test_table(col1 INT,\
                                                 col2 VARCHAR(40),\
                                                 col3 SMALLINT,\
                                                 col4 TIMESTAMP)"
#define INSERT_SAMPLE "INSERT INTO test_table(col1,col2,col3) VALUES(?,?,?)"

MYSQL_STMT    *stmt;
MYSQL_BIND    bind[3];
my_ulonglong  affected_rows;
int           param_count;
short         small_data;
int           int_data;
char          str_data[STRING_SIZE];
unsigned long str_length;
my_bool       is_null;

if (mysql_query(mysql, DROP_SAMPLE_TABLE))
{
  fprintf(stderr, " DROP TABLE failed\n");
  fprintf(stderr, " %s\n", mysql_error(mysql));
  exit(0);
}

if (mysql_query(mysql, CREATE_SAMPLE_TABLE))
{
  fprintf(stderr, " CREATE TABLE failed\n");
  fprintf(stderr, " %s\n", mysql_error(mysql));
  exit(0);
}

/* Prepare an INSERT query with 3 parameters */
/* (the TIMESTAMP column is not named; the server */
/* will set it to the current date and time) */
stmt = mysql_stmt_init(mysql);
if (!stmt)
{
  fprintf(stderr, " mysql_stmt_init(), out of memory\n");
  exit(0);
}
if (mysql_stmt_prepare(stmt, INSERT_SAMPLE, strlen(INSERT_SAMPLE)))
{
  fprintf(stderr, " mysql_stmt_prepare(), INSERT failed\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}
fprintf(stdout, " prepare, INSERT successful\n");

/* Get the parameter count from the statement */
param_count= mysql_stmt_param_count(stmt);
fprintf(stdout, " total parameters in INSERT: %d\n", param_count);

if (param_count != 3) /* validate parameter count */
{
  fprintf(stderr, " invalid parameter count returned by MySQL\n");
  exit(0);
}

/* Bind the data for all 3 parameters */

memset(bind, 0, sizeof(bind));

/* INTEGER PARAM */
/* This is a number type, so there is no need to specify buffer_length */
bind[0].buffer_type= MYSQL_TYPE_LONG;
bind[0].buffer= (char *)&int_data;
bind[0].is_null= 0;
bind[0].length= 0;

/* STRING PARAM */
bind[1].buffer_type= MYSQL_TYPE_STRING;
bind[1].buffer= (char *)str_data;
bind[1].buffer_length= STRING_SIZE;
bind[1].is_null= 0;
bind[1].length= &str_length;
 
/* SMALLINT PARAM */
bind[2].buffer_type= MYSQL_TYPE_SHORT;
bind[2].buffer= (char *)&small_data;       
bind[2].is_null= &is_null;
bind[2].length= 0;

/* Bind the buffers */
if (mysql_stmt_bind_param(stmt, bind))
{
  fprintf(stderr, " mysql_stmt_bind_param() failed\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}

/* Specify the data values for the first row */
int_data= 10;             /* integer */
strncpy(str_data, "MySQL", STRING_SIZE); /* string  */
str_length= strlen(str_data);

/* INSERT SMALLINT data as NULL */
is_null= 1;

/* Execute the INSERT statement - 1*/
if (mysql_stmt_execute(stmt))
{
  fprintf(stderr, " mysql_stmt_execute(), 1 failed\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}
  
/* Get the total number of affected rows */   
affected_rows= mysql_stmt_affected_rows(stmt);
fprintf(stdout, " total affected rows(insert 1): %lu\n",
                (unsigned long) affected_rows);

if (affected_rows != 1) /* validate affected rows */
{
  fprintf(stderr, " invalid affected rows by MySQL\n");
  exit(0);
}

/* Specify data values for second row, then re-execute the statement */
int_data= 1000;             
strncpy(str_data, "The most popular open source database", STRING_SIZE); 
str_length= strlen(str_data);
small_data= 1000;         /* smallint */
is_null= 0;               /* reset */

/* Execute the INSERT statement - 2*/
if (mysql_stmt_execute(stmt))
{
  fprintf(stderr, " mysql_stmt_execute, 2 failed\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}
  
/* Get the total rows affected */   
affected_rows= mysql_stmt_affected_rows(stmt);
fprintf(stdout, " total affected rows(insert 2): %lu\n",
                (unsigned long) affected_rows);

if (affected_rows != 1) /* validate affected rows */
{
  fprintf(stderr, " invalid affected rows by MySQL\n");
  exit(0);
}

/* Close the statement */
if (mysql_stmt_close(stmt))
{
  fprintf(stderr, " failed while closing the statement\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}





Note: For complete examples on the use of prepared
statement functions, refer to the file `tests/client_test.c'.
This file can be obtained from a MySQL source distribution or from the
BitKeeper source repository.







21.2.7.46  mysql_stmt_free_result()










my_bool mysql_stmt_free_result(MYSQL_STMT *stmt)





21.2.7.47  Description




Releases memory associated with the result set produced by execution of
the prepared statement.





This function was added in MySQL 4.1.1.





21.2.7.48  Return Values




Zero if the result set was freed successfully.  Non-zero if an error occurred.





21.2.7.49  Errors





21.2.7.50  mysql_stmt_insert_id()










my_ulonglong mysql_stmt_insert_id(MYSQL_STMT *stmt)





21.2.7.51  Description




Returns the value generated for an AUTO_INCREMENT column by the
prepared INSERT or UPDATE statement. Use this function after
you have executed prepared INSERT statement into a table which 
contains an AUTO_INCREMENT field.





See section 21.2.3.134  mysql_insert_id() for more 
information.





This function was added in MySQL 4.1.2.





21.2.7.52  Return Values




Value for AUTO_INCREMENT column which was automatically generated or
explicitly set during execution of prepared statement, or value generated
by LAST_INSERT_ID(expr) function. Return value is undefined if statement 
does not set AUTO_INCREMENT value.






21.2.7.53  Errors




None.







21.2.7.54  mysql_stmt_fetch()










int mysql_stmt_fetch(MYSQL_STMT *stmt)





21.2.7.55  Description




mysql_stmt_fetch() returns the next row in the result set. It can
be called only while the result set exists, that is, after a call to
mysql_stmt_execute() that creates a result set or after
mysql_stmt_store_result(), which is called after
mysql_stmt_execute() to buffer the entire result set.





mysql_stmt_fetch() returns row data using the buffers bound by
mysql_stmt_bind_result(). It returns
the data in those buffers for all the columns in the current row
set and the lengths are returned to the length pointer.





All columns must be bound by the application before calling
mysql_stmt_fetch().





If a fetched data value is a NULL value, the *is_null
value of the corresponding MYSQL_BIND structure contains TRUE
(1). Otherwise, the data and its length are returned in the *buffer
and *length elements based on the buffer type specified by the
application. Each numeric and temporal type has a fixed length,
as listed in the following table.
The length of the string types depends on the length of the actual data value,
as indicated by data_length.







Type  Length



MYSQL_TYPE_TINY  1



MYSQL_TYPE_SHORT  2



MYSQL_TYPE_LONG  4



MYSQL_TYPE_LONGLONG  8



MYSQL_TYPE_FLOAT  4



MYSQL_TYPE_DOUBLE  8



MYSQL_TYPE_TIME  sizeof(MYSQL_TIME)



MYSQL_TYPE_DATE  sizeof(MYSQL_TIME)



MYSQL_TYPE_DATETIME  sizeof(MYSQL_TIME)



MYSQL_TYPE_STRING  data length



MYSQL_TYPE_BLOB  data_length







This function was added in MySQL 4.1.2.





21.2.7.56  Return Values





Return Value  Description



0  Successful, the data has been
fetched to application data buffers.



1  Error occurred. Error code and
message can be obtained by calling mysql_stmt_errno() and
mysql_stmt_error().



MYSQL_NO_DATA  No more rows/data exists







21.2.7.57  Errors





CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order. 

CR_OUT_OF_MEMORY


Out of memory. 

CR_SERVER_GONE_ERROR


The MySQL server has gone away. 

CR_SERVER_LOST


The connection to the server was lost during the query. 

CR_UNKNOWN_ERROR


An unknown error occurred. 

CR_UNSUPPORTED_PARAM_TYPE


The buffer type is
MYSQL_TYPE_DATE,
MYSQL_TYPE_TIME,
MYSQL_TYPE_DATETIME,
or
MYSQL_TYPE_TIMESTAMP,
but the data type is not DATE, TIME, DATETIME, or
TIMESTAMP.




All other unsupported conversion errors are returned from
mysql_stmt_bind_result().





21.2.7.58  Example




The following example demonstrates how to fetch data from a table using
 mysql_stmt_result_metadata(),
mysql_stmt_bind_result(), and mysql_stmt_fetch().
(This example expects to retrieve the two rows inserted by the example shown
in section 21.2.7.41  mysql_stmt_execute().)
The mysql variable is assumed to be a valid connection handle.





#define STRING_SIZE 50

#define SELECT_SAMPLE "SELECT col1, col2, col3, col4 FROM test_table"

MYSQL_STMT    *stmt;
MYSQL_BIND    bind[4];
MYSQL_RES     *prepare_meta_result;
MYSQL_TIME    ts;
unsigned long length[4];
int           param_count, column_count, row_count;
short         small_data;
int           int_data;
char          str_data[STRING_SIZE];
my_bool       is_null[4];

/* Prepare a SELECT query to fetch data from test_table */
stmt = mysql_stmt_init(mysql);
if (!stmt)
{
  fprintf(stderr, " mysql_stmt_init(), out of memory\n");
  exit(0);
}
if (mysql_stmt_prepare(stmt, SELECT_SAMPLE, strlen(SELECT_SAMPLE)))
{
  fprintf(stderr, " mysql_stmt_prepare(), SELECT failed\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}
fprintf(stdout, " prepare, SELECT successful\n");

/* Get the parameter count from the statement */
param_count= mysql_stmt_param_count(stmt);
fprintf(stdout, " total parameters in SELECT: %d\n", param_count);

if (param_count != 0) /* validate parameter count */
{
  fprintf(stderr, " invalid parameter count returned by MySQL\n");
  exit(0);
}

/* Fetch result set meta information */
prepare_meta_result = mysql_stmt_result_metadata(stmt);
if (!prepare_meta_result)
{
  fprintf(stderr,
         " mysql_stmt_result_metadata(), returned no meta information\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}

/* Get total columns in the query */
column_count= mysql_num_fields(prepare_meta_result);
fprintf(stdout, " total columns in SELECT statement: %d\n", column_count);

if (column_count != 4) /* validate column count */
{
  fprintf(stderr, " invalid column count returned by MySQL\n");
  exit(0);
}

/* Execute the SELECT query */
if (mysql_stmt_execute(stmt))
{
  fprintf(stderr, " mysql_stmt_execute(), failed\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}

/* Bind the result buffers for all 4 columns before fetching them */

memset(bind, 0, sizeof(bind));

/* INTEGER COLUMN */
bind[0].buffer_type= MYSQL_TYPE_LONG;
bind[0].buffer= (char *)&int_data;
bind[0].is_null= &is_null[0];
bind[0].length= &length[0];

/* STRING COLUMN */
bind[1].buffer_type= MYSQL_TYPE_STRING;
bind[1].buffer= (char *)str_data;
bind[1].buffer_length= STRING_SIZE;
bind[1].is_null= &is_null[1];
bind[1].length= &length[1];
 
/* SMALLINT COLUMN */
bind[2].buffer_type= MYSQL_TYPE_SHORT;
bind[2].buffer= (char *)&small_data;       
bind[2].is_null= &is_null[2];
bind[2].length= &length[2];
 
/* TIMESTAMP COLUMN */
bind[3].buffer_type= MYSQL_TYPE_TIMESTAMP;
bind[3].buffer= (char *)&ts;       
bind[3].is_null= &is_null[3];
bind[3].length= &length[3];

/* Bind the result buffers */
if (mysql_stmt_bind_result(stmt, bind))
{
  fprintf(stderr, " mysql_stmt_bind_result() failed\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}

/* Now buffer all results to client */
if (mysql_stmt_store_result(stmt))
{
  fprintf(stderr, " mysql_stmt_store_result() failed\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}

/* Fetch all rows */
row_count= 0;
fprintf(stdout, "Fetching results ...\n");
while (!mysql_stmt_fetch(stmt))
{
  row_count++;
  fprintf(stdout, "  row %d\n", row_count);

  /* column 1 */
  fprintf(stdout, "   column1 (integer)  : ");
  if (is_null[0])
    fprintf(stdout, " NULL\n");
  else
    fprintf(stdout, " %d(%ld)\n", int_data, length[0]);

  /* column 2 */
  fprintf(stdout, "   column2 (string)   : ");
  if (is_null[1])
    fprintf(stdout, " NULL\n");
  else
    fprintf(stdout, " %s(%ld)\n", str_data, length[1]);

  /* column 3 */
  fprintf(stdout, "   column3 (smallint) : ");
  if (is_null[2])
    fprintf(stdout, " NULL\n");
  else
    fprintf(stdout, " %d(%ld)\n", small_data, length[2]);

  /* column 4 */
  fprintf(stdout, "   column4 (timestamp): ");
  if (is_null[3])
    fprintf(stdout, " NULL\n");
  else
    fprintf(stdout, " %04d-%02d-%02d %02d:%02d:%02d (%ld)\n",
									   ts.year, ts.month, ts.day,
									   ts.hour, ts.minute, ts.second,
									   length[3]);
  fprintf(stdout, "\n");
}

/* Validate rows fetched */
fprintf(stdout, " total rows fetched: %d\n", row_count);
if (row_count != 2)
{
  fprintf(stderr, " MySQL failed to return all rows\n");
  exit(0);
} 

/* Free the prepared result metadata */
mysql_free_result(prepare_meta_result);

/* Close the statement */
if (mysql_stmt_close(stmt))
{
  fprintf(stderr, " failed while closing the statement\n");
  fprintf(stderr, " %s\n", mysql_stmt_error(stmt));
  exit(0);
}






21.2.7.59  mysql_stmt_fetch_column()










int mysql_stmt_fetch_column(MYSQL_STMT *stmt, MYSQL_BIND *bind, unsigned int column, unsigned long offset)






This function was added in MySQL 4.1.2.





21.2.7.60  Description




21.2.7.61  Return Values




21.2.7.62  Errors





21.2.7.63  mysql_stmt_field_count()










unsigned int mysql_stmt_field_count(MYSQL_STMT *stmt)





21.2.7.64  Description




Returns the number of columns for the most recent statement for the statement
handler. This value will be zero for statements such as INSERT or
DELETE that do not produce result sets.





mysql_stmt_field_count() can be called after you have prepared a
statement by invoking mysql_stmt_prepare().





This function was added in MySQL 4.1.3.





21.2.7.65  Return Values




An unsigned integer representing the number of columns in a result set.





21.2.7.66  Errors



None.







21.2.7.67  mysql_stmt_init()










MYSQL_STMT *mysql_stmt_init(MYSQL *mysql)





21.2.7.68  Description




Create a MYSQL_STMT handle.





This function was added in MySQL 4.1.2.





21.2.7.69  Return values




A pointer to a MYSQL_STMT structure in case of success.
NULL if out of memory.





21.2.7.70  Errors






CR_OUT_OF_MEMORY


Out of memory.






21.2.7.71  mysql_stmt_num_rows()










my_ulonglong mysql_stmt_num_rows(MYSQL_STMT *stmt)





21.2.7.72  Description




Returns the number of rows in the result set.





The use of mysql_stmt_num_rows() depends on whether or not you used
mysql_stmt_store_result() to buffer the entire result set in the
statement handle.





If you use mysql_stmt_store_result(), mysql_stmt_num_rows() may be
called immediately.






This function was added in MySQL 4.1.1.





21.2.7.73  Return Values




The number of rows in the result set.





21.2.7.74  Errors



None.







21.2.7.75  mysql_stmt_param_count()










unsigned long mysql_stmt_param_count(MYSQL_STMT *stmt)





21.2.7.76  Description




Returns the number of parameter markers present in the prepared statement.





This function was added in MySQL 4.1.2.





21.2.7.77  Return Values




An unsigned long integer representing the number of parameters in a
statement.





21.2.7.78  Errors




None.





21.2.7.79  Example




For the usage of mysql_stmt_param_count(), refer to the Example from
section 21.2.7.41  mysql_stmt_execute().







21.2.7.80  mysql_stmt_param_metadata()










MYSQL_RES *mysql_stmt_param_metadata(MYSQL_STMT *stmt)





To be added.





This function was added in MySQL 4.1.2.





21.2.7.81  Description




21.2.7.82  Return Values




21.2.7.83  Errors





21.2.7.84  mysql_stmt_prepare()










int mysql_stmt_prepare(MYSQL_STMT *stmt, const char *query, unsigned
long length)





21.2.7.85  Description




Given the statement handle returned by mysql_stmt_init(),
prepares the SQL statement pointed to by the string
query and returns a status value.
The string length should be given by the length argument.
The string must consist of a single SQL statement. You should
not add a terminating semicolon (`;') or \g to the statement.





The application can include one or more parameter markers in the SQL
statement by embedding question mark (`?') characters into the SQL
string at the appropriate positions.





The markers are legal only in certain places in SQL statements. For
example, they are allowed in the VALUES() list of an INSERT
statement (to specify column values for a row), or in a comparison with a
column in a WHERE clause to specify a comparison value.
However, they are not allowed for identifiers (such as table or column
names), in the select list that names the columns to
be returned by a SELECT statement), or to specify both
operands of a binary operator such as the = equal sign.
The latter restriction is necessary because it
would be impossible to determine the parameter type. In general,
parameters are legal only in Data Manipulation Languange (DML)
statements, and not in Data Defination Language (DDL) statements.





The parameter markers must be bound to application variables using
mysql_stmt_bind_param() before executing the statement.





This function was added in MySQL 4.1.2.






21.2.7.86  Return Values




Zero if the statement was prepared successfully.  Non-zero if an error occurred.





21.2.7.87  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_OUT_OF_MEMORY


Out of memory.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query

CR_UNKNOWN_ERROR


An unknown error occurred.





If the prepare operation was unsuccessful (that is,
mysql_stmt_prepare() returns non-zero), the error message can be
obtained by calling mysql_stmt_error().






21.2.7.88  Example




For the usage of mysql_stmt_prepare(), refer to the Example from
section 21.2.7.41  mysql_stmt_execute().







21.2.7.89  mysql_stmt_reset()










my_bool mysql_stmt_reset(MYSQL_STMT *stmt)





21.2.7.90  Description




Reset prepared statement on client and server to state after prepare.
For now this is mainly used to reset data sent with
mysql_stmt_send_long_data().





To re-prepare the statement with another query, use
mysql_stmt_prepare().





This function was added in MySQL 4.1.1.





21.2.7.91  Return Values




Zero if the statement was reset successfully.  Non-zero if an error occurred.





21.2.7.92  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_SERVER_LOST


The connection to the server was lost during the query

CR_UNKNOWN_ERROR


An unknown error occurred.






21.2.7.93  mysql_stmt_result_metadata()










MYSQL_RES *mysql_stmt_result_metadata(MYSQL_STMT *stmt)





21.2.7.94  Description




If a statement passed to mysql_stmt_prepare() is one that produces a result
set,
mysql_stmt_result_metadata() returns the result set
metadata in the form of a pointer to a
MYSQL_RES structure that can be used to process the
meta information such as total number of fields and individual field
information. This result set pointer can be passed as an argument to
any of the field-based API functions that process result set metadata, such
as:








The result set structure should be freed when you are done with it, which
you can do by passing it to mysql_free_result(). This is similar
to the way you free a result set obtained from a call to
mysql_store_result().





The result set returned by mysql_stmt_result_metadata() contains only
metadata. It does not contain any row results. The rows are obtained by using
the statement handle with mysql_stmt_fetch().





This function was added in MySQL 4.1.2.






21.2.7.95  Return Values




A MYSQL_RES result structure. NULL if no meta information exists
for the prepared query.






21.2.7.96  Errors






CR_OUT_OF_MEMORY


Out of memory.

CR_UNKNOWN_ERROR


An unknown error occurred.





21.2.7.97  Example




For the usage of mysql_stmt_result_metadata(), refer to the
Example from section 21.2.7.54  mysql_stmt_fetch().







21.2.7.98  mysql_stmt_row_seek()










MYSQL_ROW_OFFSET mysql_stmt_row_seek(MYSQL_STMT *stmt, MYSQL_ROW_OFFSET offset)





21.2.7.99  Description



Sets the row cursor to an arbitrary row in a statement result set.
The offset value is a row offset that should be a value returned
from mysql_stmt_row_tell() or from mysql_stmt_row_seek().
This value is not a row number; if you want to seek to a row within a
result set by number, use mysql_stmt_data_seek() instead.






This function requires that the result set structure contains the entire
result of the query, so mysql_stmt_row_seek() may be used only
in conjunction with mysql_stmt_store_result().





This function was added in MySQL 4.1.1.





21.2.7.100  Return Values




The previous value of the row cursor.  This value may be passed to a
subsequent call to mysql_stmt_row_seek().





21.2.7.101  Errors



None.







21.2.7.102  mysql_stmt_row_tell()










MYSQL_ROW_OFFSET mysql_stmt_row_tell(MYSQL_STMT *stmt)





21.2.7.103  Description




Returns the current position of the row cursor for the last
mysql_stmt_fetch(). This value can be used as an argument to
mysql_stmt_row_seek().





You should use mysql_stmt_row_tell() only after mysql_stmt_store_result().





This function was added in MySQL 4.1.1.





21.2.7.104  Return Values




The current offset of the row cursor.





21.2.7.105  Errors



None.







21.2.7.106  mysql_stmt_send_long_data()










my_bool mysql_stmt_send_long_data(MYSQL_STMT *stmt, unsigned int
parameter_number, const char *data, unsigned long length)





21.2.7.107  Description




Allows an application to send parameter data to the server in pieces
(or ``chunks'').
This function can be called multiple times to send the parts of a
character or binary data value for a column, which must be one of the
TEXT or BLOB data types.





parameter_number indicates which parameter to associate the data with.
Parameters are numbered beginning with 0.
data is a pointer to a buffer containing data to be sent, and
length indicates the number of bytes in the buffer.





Note: The next mysql_stmt_execute() call will ignore the
bind buffer for all parameters that have been used with
mysql_stmt_send_long_data() since last
mysql_stmt_execute() or mysql_stmt_reset().





If you want to reset/forget the sent data, you can do it with
mysql_stmt_reset(). See section 21.2.7.89  mysql_stmt_reset().





This function was added in MySQL 4.1.2.





21.2.7.108  Return Values




Zero if the data is sent successfully to server. Non-zero if an error
occurred.





21.2.7.109  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order.

CR_SERVER_GONE_ERROR


The MySQL server has gone away.

CR_OUT_OF_MEMORY


Out of memory.

CR_UNKNOWN_ERROR


An unknown error occurred.





21.2.7.110  Example



The following example demonstrates how to send the data for a
TEXT column in chunks.  It inserts the data value
'MySQL - The most popular open source database'
into the text_column column.
The mysql variable is assumed to be a valid connection handle.






#define INSERT_QUERY "INSERT INTO test_long_data(text_column) VALUES(?)"
  
MYSQL_BIND bind[1];
long       length;

smtt = mysql_stmt_init(mysql);
if (!stmt)
{
  fprintf(stderr, " mysql_stmt_init(), out of memory\n");
  exit(0);
}
if (mysql_stmt_prepare(stmt, INSERT_QUERY, strlen(INSERT_QUERY)))
{
  fprintf(stderr, "\n mysql_stmt_prepare(), INSERT failed");
  fprintf(stderr, "\n %s", mysql_stmt_error(stmt));
  exit(0);
}
 memset(bind, 0, sizeof(bind));
 bind[0].buffer_type= MYSQL_TYPE_STRING;
 bind[0].length= &length;
 bind[0].is_null= 0;

/* Bind the buffers */
if (mysql_stmt_bind_param(stmt, bind))
{
  fprintf(stderr, "\n param bind failed");
  fprintf(stderr, "\n %s", mysql_stmt_error(stmt));
  exit(0);
}

 /* Supply data in chunks to server */
 if (!mysql_stmt_send_long_data(stmt,0,"MySQL",5))
{
  fprintf(stderr, "\n send_long_data failed");
  fprintf(stderr, "\n %s", mysql_stmt_error(stmt));
  exit(0);
}

 /* Supply the next piece of data */
 if (mysql_stmt_send_long_data(stmt,0," - The most popular open source database",40))
{
  fprintf(stderr, "\n send_long_data failed");
  fprintf(stderr, "\n %s", mysql_stmt_error(stmt));
  exit(0);
}

 /* Now, execute the query */
 if (mysql_stmt_execute(stmt))
{
  fprintf(stderr, "\n mysql_stmt_execute failed");
  fprintf(stderr, "\n %s", mysql_stmt_error(stmt));
  exit(0);
}






21.2.7.111  mysql_stmt_sqlstate()










const char *mysql_stmt_sqlstate(MYSQL_STMT *stmt)





21.2.7.112  Description




For the statement specified by stmt, mysql_stmt_sqlstate()
returns a null-terminated string containing the SQLSTATE error code for the
most recently invoked prepared statement API function that can succeed or fail.
The error code consists of five characters.
"00000" means ``no error.''
The values are specified by ANSI SQL and ODBC.
For a list of possible values, see section 23  Error Handling in MySQL.





Note that not all MySQL errors are yet mapped to SQLSTATE's.
The value "HY000" (general error) is used
for unmapped errors.





This function was added to MySQL 4.1.1.





21.2.7.113  Return Values




A null-terminated character string containing the SQLSTATE error code.







21.2.7.114  mysql_stmt_store_result()










int mysql_stmt_store_result(MYSQL_STMT *stmt)





21.2.7.115  Description




You must call mysql_stmt_store_result() for every statement that
successfully produces a result set
(SELECT, SHOW, DESCRIBE, EXPLAIN), and only
if you want to buffer the complete result set by the client, so that the
subsequent mysql_stmt_fetch() call returns buffered data.





It is unnecessary to call mysql_stmt_store_result() for other
statements, but if you do, it will not harm or cause any notable performance
problem.
You can detect whether the statement produced a result set by checking
if mysql_stmt_result_metadata() returns NULL.
For more information, refer to section 21.2.7.93  mysql_stmt_result_metadata().





Note:
MySQL doesn't by default calculate MYSQL_FIELD->max_length for
all columns in mysql_stmt_store_result() because calculating this
would slow down mysql_stmt_store_result() considerably and most
applications doesn't need max_length.  If you want
max_length to be updated, you can call
mysql_stmt_attr_set(MYSQL_STMT, STMT_ATTR_UPDATE_MAX_LENGTH, &flag)
to enable this. See section 21.2.7.10  mysql_stmt_attr_set().





This function was added in MySQL 4.1.0.





21.2.7.116  Return Values




Zero if the results are buffered successfully.  Non-zero if an error occurred.





21.2.7.117  Errors






CR_COMMANDS_OUT_OF_SYNC


Commands were executed in an improper order. 

CR_OUT_OF_MEMORY


Out of memory. 

CR_SERVER_GONE_ERROR


The MySQL server has gone away. 

CR_SERVER_LOST


The connection to the server was lost during the query. 

CR_UNKNOWN_ERROR


An unknown error occurred. 






21.2.8  C API Handling of Multiple Query Execution




From version 4.1, MySQL supports the execution of multiple statements
specified in a single query string. To use this capability with a given
connection, you must specify the CLIENT_MULTI_STATEMENTS option in
the flags parameter of mysql_real_connect()
when opening the connection.  You can also set this for an existing connection
by calling mysql_set_server_option(MYSQL_OPTION_MULTI_STATEMENTS_ON)





By default, mysql_query() and mysql_real_query() return
only the first query status and the subsequent queries status can
be processed using mysql_more_results() and
mysql_next_result().






/* Connect to server with option CLIENT_MULTI_STATEMENTS */
mysql_real_connect(..., CLIENT_MULTI_STATEMENTS);

/* Now execute multiple queries */
mysql_query(mysql,"DROP TABLE IF EXISTS test_table;\
                   CREATE TABLE test_table(id INT);\
                   INSERT INTO test_table VALUES(10);\
                   UPDATE test_table SET id=20 WHERE id=10;\
                   SELECT * FROM test_table;\
                   DROP TABLE test_table");
do
{
  /* Process all results */
  ...
  printf("total affected rows: %lld", mysql_affected_rows(mysql));
  ...
  if (!(result= mysql_store_result(mysql)))
  {
     printf(stderr, "Got fatal error processing query\n");
     exit(1);
  }
  process_result_set(result);	/* client function */
  mysql_free_result(result);
} while (!mysql_next_result(mysql));







21.2.9  C API Handling of Date and Time Values




The new binary protocol available in MySQL 4.1 and above allows you to
send and receive date and time values (DATE, TIME,
DATETIME, and TIMESTAMP), using
the MYSQL_TIME structure. The members of this structure are described
in section 21.2.5  C API Prepared Statement Data types.





To send temporal data values, you create a prepared statement with
mysql_stmt_prepare(). Then, before calling
mysql_stmt_execute() to execute the statement, use the following
procedure to set up each temporal parameter:





    


  1. 

In the MYSQL_BIND structure associated with the data value, set the
buffer_type member to the type that indicates what kind of temporal
value you're sending. For
DATE,
TIME,
DATETIME,
or
TIMESTAMP
values, set buffer_type to
MYSQL_TYPE_DATE,
MYSQL_TYPE_TIME,
MYSQL_TYPE_DATETIME,
or
MYSQL_TYPE_TIMESTAMP,
respectively.


  2. 

Set the buffer member of the MYSQL_BIND structure to the address
of the MYSQL_TIME structure in which you will pass the temporal value.


  3. 

Fill in the members of the MYSQL_TIME structure that are appropriate
for the type of temporal value you're passing.





Use mysql_stmt_bind_param() to bind the parameter data to the statement.
Then you can call mysql_stmt_execute().





To retrieve temporal values, the procedure is similar, except that you set
the buffer_type member to the type of value you expect to receive, and
the buffer member to the address of a MYSQL_TIME structure into
which the returned value should be placed.
Use mysql_bind_results() to bind the buffers to the statement after
calling mysql_stmt_execute() and before fetching the results.





Here is a simple example that inserts DATE, TIME, and
TIMESTAMP data.
The mysql variable is assumed to be a valid connection handle.






  MYSQL_TIME  ts;
  MYSQL_BIND  bind[3];
  MYSQL_STMT  *stmt;
  
  strmov(query, "INSERT INTO test_table(date_field, time_field,
                                        timestamp_field) VALUES(?,?,?");

  stmt = mysql_stmt_init(mysql);
  if (!stmt)
  {
    fprintf(stderr, " mysql_stmt_init(), out of memory\n");
    exit(0);
  }
  if (mysql_stmt_prepare(mysql, query, strlen(query)))
  {
    fprintf(stderr, "\n mysql_stmt_prepare(), INSERT failed");
    fprintf(stderr, "\n %s", mysql_stmt_error(stmt));
    exit(0);
  }

  /* set up input buffers for all 3 parameters */
  bind[0].buffer_type= MYSQL_TYPE_DATE;
  bind[0].buffer= (char *)&ts;  
  bind[0].is_null= 0;
  bind[0].length= 0;
  ...
  bind[1]= bind[2]= bind[0];
  ...

  mysql_stmt_bind_param(stmt, bind);

  /* supply the data to be sent in the ts structure */
  ts.year= 2002;
  ts.month= 02;
  ts.day= 03;

  ts.hour= 10;
  ts.minute= 45;
  ts.second= 20;

  mysql_stmt_execute(stmt);
  .. 







21.2.10  C API Threaded Function Descriptions




You need to use the following functions when you want to create a
threaded client. See section 21.2.14  How to Make a Threaded Client.







21.2.10.1  my_init()










void my_init(void)





21.2.10.2  Description




This function needs to be called once in the program before calling any
MySQL function.  This initializes some global variables that MySQL
needs.  If you are using a thread-safe client library, this will also
call mysql_thread_init() for this thread.





This is automatically called by mysql_init(),
mysql_server_init() and mysql_connect().





21.2.10.3  Return Values




None.






21.2.10.4  mysql_thread_init()










my_bool mysql_thread_init(void)





21.2.10.5  Description




This function needs to be called for each created thread to initialize
thread-specific variables.





This is automatically called by my_init() and mysql_connect().





21.2.10.6  Return Values




Zero if successful. Non-zero if an error occurred.






21.2.10.7  mysql_thread_end()










void mysql_thread_end(void)





21.2.10.8  Description




This function needs to be called before calling pthread_exit() to
free memory allocated by mysql_thread_init().





Note that this function is not invoked automatically by the client
library.  It must be called explicitly to avoid a memory leak.





21.2.10.9  Return Values




None.






21.2.10.10  mysql_thread_safe()










unsigned int mysql_thread_safe(void)





21.2.10.11  Description




This function indicates whether the client is compiled as thread-safe.





21.2.10.12  Return Values




1 is the client is thread-safe, 0 otherwise.






21.2.11  C API Embedded Server Function Descriptions




You must use the following functions if you want to allow your
application to be linked against the embedded MySQL server library.
See section 21.2.15  libmysqld, the Embedded MySQL Server Library.





If the program is linked with -lmysqlclient instead of
-lmysqld, these functions do nothing.  This makes it
possible to choose between using the embedded MySQL server and
a standalone server without modifying any code.







21.2.11.1  mysql_server_init()










int mysql_server_init(int argc, char **argv, char **groups)





21.2.11.2  Description




This function must be called once in the program using the
embedded server before calling any other MySQL function.  It starts
the server and initializes any subsystems (mysys, InnoDB, etc.) 
that the server uses.  If this function is not called, the program will
crash.  If you are using the DBUG package that comes with MySQL, you
should call this after you have called MY_INIT().





The argc and argv arguments are analogous to the arguments
to main().  The first element of argv is ignored (it
typically contains the program name).  For convenience, argc may
be 0 (zero) if there are no command-line arguments for the
server.  mysql_server_init() makes a copy of the arguments so
it's safe to destroy argv or groups after the call.





The NULL-terminated list of strings in groups
selects which groups in the option files will be active.
See section 4.3.2  Using Option Files.  For convenience, groups may be
NULL, in which case the [server] and [emedded] groups
will be active.





21.2.11.3  Example




#include <mysql.h>
#include <stdlib.h>

static char *server_args[] = {
  "this_program",       /* this string is not used */
  "--datadir=.",
  "--key_buffer_size=32M"
};
static char *server_groups[] = {
  "embedded",
  "server",
  "this_program_SERVER",
  (char *)NULL
};

int main(void) {
  if (mysql_server_init(sizeof(server_args) / sizeof(char *),
                        server_args, server_groups))
    exit(1);

  /* Use any MySQL API functions here */

  mysql_server_end();

  return EXIT_SUCCESS;
}





21.2.11.4  Return Values




0 if okay, 1 if an error occurred.






21.2.11.5  mysql_server_end()










void mysql_server_end(void)





21.2.11.6  Description




This function must be called once in the program after
all other MySQL functions.  It shuts down the embedded server.





21.2.11.7  Return Values




None.






21.2.12  Common questions and problems when using the C API













21.2.12.1  Why mysql_store_result() Sometimes Returns NULL After mysql_query() Returns Success




It is possible for mysql_store_result() to return NULL
following a successful call to mysql_query().  When this happens, it
means one of the following conditions occurred:








You can always check whether the statement should have produced a
non-empty result by calling mysql_field_count().  If
mysql_field_count() returns zero, the result is empty and the last
query was a statement that does not return values (for example, an
INSERT or a DELETE).  If mysql_field_count() returns a
non-zero value, the statement should have produced a non-empty result.
See the description of the mysql_field_count() function for an
example.





You can test for an error by calling mysql_error() or
mysql_errno().






21.2.12.2  What Results You Can Get from a Query




In addition to the result set returned by a query, you can also get the
following information:









21.2.12.3  How to Get the Unique ID for the Last Inserted Row













If you insert a record into a table that contains an AUTO_INCREMENT
column, you can obtain the value stored into that column by calling the
mysql_insert_id() function.





You can check whether a value was stored into an AUTO_INCREMENT column
by executing the following code. This also checks whether the query was an
INSERT with an AUTO_INCREMENT index:





if (mysql_error(&mysql)[0] == 0 &&
    mysql_num_fields(result) == 0 &&
    mysql_insert_id(&mysql) != 0)
{
    used_id = mysql_insert_id(&mysql);
}





For more information, see section 21.2.3.134  mysql_insert_id().





When a new AUTO_INCREMENT value has been generated,
you can also obtain it by executing a SELECT LAST_INSERT_ID()
statement mysql_query() and retrieving the value from the result set
returned by the statement.





For LAST_INSERT_ID(),
the most recently generated ID is maintained in the server on a
per-connection basis.  It will not be changed by another client. It will not
even be changed if you update another AUTO_INCREMENT column with a
non-magic value (that is, a value that is not NULL and not 0).





If you want to use the ID that was generated for one table and insert
it into a second table, you can use SQL statements like this:





INSERT INTO foo (auto,text)
    VALUES(NULL,'text');              # generate ID by inserting NULL
INSERT INTO foo2 (id,text)
    VALUES(LAST_INSERT_ID(),'text');  # use ID in second table





Note that mysql_insert_id() returns the value stored into an
AUTO_INCREMENT column, whether that value is automatically generated
by storing NULL or 0 or is an explicit value.
LAST_INSERT_ID() returns automatically generated AUTO_INCREMENT
values. If you store an explicit value other than NULL or 0,
it does not affect the value returned by LAST_INSERT_ID().







21.2.12.4  Problems Linking with the C API











When linking with the C API, the following errors may occur on some systems:





gcc -g -o client test.o -L/usr/local/lib/mysql -lmysqlclient -lsocket -lnsl

Undefined        first referenced
 symbol          in file
floor            /usr/local/lib/mysql/libmysqlclient.a(password.o)
ld: fatal: Symbol referencing errors. No output written to client





If this happens on your system, you must include the math library by
adding -lm to the end of the compile/link line.







21.2.13  Building Client Programs













If you compile MySQL clients that you've written yourself or that
you obtain from a third-party, they must be linked using the
-lmysqlclient -lz option on the link command.  You may also need to
specify a -L option to tell the linker where to find the library.  For
example, if the library is installed in `/usr/local/mysql/lib', use
-L/usr/local/mysql/lib -lmysqlclient -lz on the link command.





For clients that use MySQL header files, you may need to specify a
-I option when you compile them (for example,
-I/usr/local/mysql/include), so the compiler can find the header
files.





To make it simpler to compile MySQL programs on Unix, we have provided the
@command{mysql_config} script for you. @xref{mysql_config, , @command{mysql_config}}.





You can use it to compile a MySQL client as follows:





CFG=/usr/local/mysql/bin/mysql_config
sh -c "gcc -o progname `$CFG --cflags` progname.c `$CFG --libs`"





The sh -c is needed to get the shell not to treat the output from
@command{mysql_config} as one word.






21.2.14  How to Make a Threaded Client











The client library is almost thread-safe. The biggest problem is
that the subroutines in `net.c' that read from sockets are not
interrupt safe.  This was done with the thought that you might want to
have your own alarm that can break a long read to a server.  If you
install interrupt handlers for the SIGPIPE interrupt,
the socket handling should be thread-safe.





New in 4.0.16: To not abort the program when a connection terminates,
MySQL blocks SIGPIPE on the first call to
mysql_server_init(), mysql_init() or
mysql_connect().  If you want to have your own SIGPIPE
handler, you should first call mysql_server_init() and then
install your handler. In older versions of MySQL SIGPIPE was blocked,
but only in the thread safe client library, for every call to
mysql_init().





In the older binaries we distribute on our Web site
(http://www.mysql.com/), the client libraries are not normally
compiled with the thread-safe option (the Windows binaries are by
default compiled to be thread-safe). Newer binary distributions should
have both a normal and a thread-safe client library.





To get a threaded client where you can interrupt the client from other
threads and set timeouts when talking with the MySQL server, you should
use the -lmysys, -lmystrings, and -ldbug libraries and
the net_serv.o code that the server uses.





If you don't need interrupts or timeouts, you can just compile a
thread-safe client library (mysqlclient_r) and use this.  See section 21.2  MySQL C API.  In this case, you don't have to worry about the
net_serv.o object file or the other MySQL libraries.





When using a threaded client and you want to use timeouts and
interrupts, you can make great use of the routines in the
`thr_alarm.c' file.  If you are using routines from the
mysys library, the only thing you must remember is to call
my_init() first! See section 21.2.10  C API Threaded Function Descriptions.





All functions except mysql_real_connect() are by default
thread-safe.  The following notes describe how to compile a thread-safe
client library and use it in a thread-safe manner.  (The notes below for
mysql_real_connect() actually apply to mysql_connect() as
well, but because mysql_connect() is deprecated, you should be
using mysql_real_connect() anyway.)





To make mysql_real_connect() thread-safe, you must recompile the
client library with this command:





shell> ./configure --enable-thread-safe-client





This will create a thread-safe client library libmysqlclient_r.
(Assuming that your OS has a thread-safe gethostbyname_r() function.)
This library is thread-safe per connection.  You can let two threads
share the same connection with the following caveats:








You need to know the following if you have a thread that is calling
MySQL functions which did not create the connection to the
MySQL database:





When you call mysql_init() or mysql_connect(), MySQL will
create a thread-specific variable for the thread that is used by the
debug library (among other things).





If you call a MySQL function, before the thread has
called mysql_init() or mysql_connect(), the thread will
not have the necessary thread-specific variables in place and you are
likely to end up with a core dump sooner or later.





The get things to work smoothly you have to do the following:





    

  1. 

Call my_init() at the start of your program if it calls
any other MySQL function before calling mysql_real_connect().

  2. 

Call mysql_thread_init() in the thread handler before calling
any MySQL function.

  3. 

In the thread, call mysql_thread_end() before calling
pthread_exit().  This will free the memory used by MySQL
thread-specific variables.





You may get some errors because of undefined symbols when linking your
client with libmysqlclient_r. In most cases this is because you haven't
included the thread libraries on the link/compile line.






21.2.15  libmysqld, the Embedded MySQL Server Library













21.2.15.1  Overview of the Embedded MySQL Server Library




The embedded MySQL server library makes it possible to run a
full-featured MySQL server inside a client application.  The
main benefits are increased speed and more simple management for
embedded applications.





The embedded server library is based on the client/server version of MySQL,
which is written in C/C++. Consequently, the embedded server also
is written in C/C++. There is no embedded server available in other
languages.





The API is identical for the embedded MySQL version and the
client/server version.  To change an old threaded application to use the
embedded library, you normally only have to add calls to the following
functions:






Function  When to Call



mysql_server_init()  Should be called before any other MySQL function is called, preferably early in the main() function.



mysql_server_end()  Should be called before your program exits.



mysql_thread_init()  Should be called in each thread you create that will access MySQL.



mysql_thread_end()  Should be called before calling pthread_exit()







Then you must link your code with `libmysqld.a' instead of `libmysqlclient.a'.





The mysql_server_xxx() functions are also included in
`libmysqlclient.a' to allow you to change between the embedded and the
client/server version by just linking your application with the right
library. See section 21.2.11.1  mysql_server_init().







21.2.15.2  Compiling Programs with libmysqld




To get a libmysqld library you should configure MySQL with the
--with-embedded-server option.





When you link your program with libmysqld, you must also include
the system-specific pthread libraries and some libraries that
the MySQL server uses.  You can get the full list of libraries by executing
@command{mysql_config --libmysqld-libs}.





The correct flags for compiling and linking a threaded program
must be used, even if you do not directly call any thread
functions in your code.






21.2.15.3  Restrictions when using the Embedded MySQL Server




The embedded server has the following limitations:








Some of these limitations can be changed by editing the `mysql_embed.h'
include file and recompiling MySQL.






21.2.15.4  Using Option Files with the Embedded Server










The following is the recommended way to use option files to make it easy
to switch between a client/server application and one where MySQL is
embedded. See section 4.3.2  Using Option Files.









21.2.15.5  Things left to do in Embedded Server (TODO)














21.2.15.6  A Simple Embedded Server Example




This example program and makefile should work without any
changes on a Linux or FreeBSD system.  For other operating
systems, minor changes will be needed.  This example is
designed to give enough details to understand the problem,
without the clutter that is a necessary part of a real
application.





To try out the example, create an `test_libmysqld' directory
at the same level as the mysql-4.0 source directory.  Save
the `test_libmysqld.c' source and the `GNUmakefile' in the
directory, and run GNU `make' from inside the `test_libmysqld'
directory.





`test_libmysqld.c'


/*
 * A simple example client, using the embedded MySQL server library
 */

#include <mysql.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>

MYSQL *db_connect(const char *dbname);
void db_disconnect(MYSQL *db);
void db_do_query(MYSQL *db, const char *query);

const char *server_groups[] = {
  "test_libmysqld_SERVER", "embedded", "server", NULL
};

int
main(int argc, char **argv)
{
  MYSQL *one, *two;

  /* mysql_server_init() must be called before any other mysql
   * functions.
   *
   * You can use mysql_server_init(0, NULL, NULL), and it will
   * initialize the server using groups = {
   *   "server", "embedded", NULL
   *  }.
   *
   * In your $HOME/.my.cnf file, you probably want to put:

[test_libmysqld_SERVER]
language = /path/to/source/of/mysql/sql/share/english

   * You could, of course, modify argc and argv before passing
   * them to this function.  Or you could create new ones in any
   * way you like.  But all of the arguments in argv (except for
   * argv[0], which is the program name) should be valid options
   * for the MySQL server.
   *
   * If you link this client against the normal mysqlclient
   * library, this function is just a stub that does nothing.
   */
  mysql_server_init(argc, argv, (char **)server_groups);

  one = db_connect("test");
  two = db_connect(NULL);

  db_do_query(one, "SHOW TABLE STATUS");
  db_do_query(two, "SHOW DATABASES");

  mysql_close(two);
  mysql_close(one);

  /* This must be called after all other mysql functions */
  mysql_server_end();

  exit(EXIT_SUCCESS);
}

static void
die(MYSQL *db, char *fmt, ...)
{
  va_list ap;
  va_start(ap, fmt);
  vfprintf(stderr, fmt, ap);
  va_end(ap);
  (void)putc('\n', stderr);
  if (db)
    db_disconnect(db);
  exit(EXIT_FAILURE);
}

MYSQL *
db_connect(const char *dbname)
{
  MYSQL *db = mysql_init(NULL);
  if (!db)
    die(db, "mysql_init failed: no memory");
  /*
   * Notice that the client and server use separate group names.
   * This is critical, because the server will not accept the
   * client's options, and vice versa.
   */
  mysql_options(db, MYSQL_READ_DEFAULT_GROUP, "test_libmysqld_CLIENT");
  if (!mysql_real_connect(db, NULL, NULL, NULL, dbname, 0, NULL, 0))
    die(db, "mysql_real_connect failed: %s", mysql_error(db));

  return db;
}

void
db_disconnect(MYSQL *db)
{
  mysql_close(db);
}

void
db_do_query(MYSQL *db, const char *query)
{
  if (mysql_query(db, query) != 0)
    goto err;

  if (mysql_field_count(db) > 0)
  {
    MYSQL_RES   *res;
    MYSQL_ROW    row, end_row;
    int num_fields;

    if (!(res = mysql_store_result(db)))
      goto err;
    num_fields = mysql_num_fields(res);
    while ((row = mysql_fetch_row(res)))
    {
      (void)fputs(">> ", stdout);
      for (end_row = row + num_fields; row < end_row; ++row)
        (void)printf("%s\t", row ? (char*)*row : "NULL");
      (void)fputc('\n', stdout);
    }
    (void)fputc('\n', stdout);
    mysql_free_result(res);
  }
  else
    (void)printf("Affected rows: %lld\n", mysql_affected_rows(db));

  return;

err:
  die(db, "db_do_query failed: %s [%s]", mysql_error(db), query);
}





`GNUmakefile'


# This assumes the MySQL software is installed in /usr/local/mysql
inc      := /usr/local/mysql/include/mysql
lib      := /usr/local/mysql/lib

# If you have not installed the MySQL software yet, try this instead
#inc      := $(HOME)/mysql-4.0/include
#lib      := $(HOME)/mysql-4.0/libmysqld

CC       := gcc
CPPFLAGS := -I$(inc) -D_THREAD_SAFE -D_REENTRANT
CFLAGS   := -g -W -Wall
LDFLAGS  := -static
# You can change -lmysqld to -lmysqlclient to use the
# client/server library
LDLIBS    = -L$(lib) -lmysqld -lz -lm -lcrypt

ifneq (,$(shell grep FreeBSD /COPYRIGHT 2>/dev/null))
# FreeBSD
LDFLAGS += -pthread
else
# Assume Linux
LDLIBS += -lpthread
endif

# This works for simple one-file test programs
sources := $(wildcard *.c)
objects := $(patsubst %c,%o,$(sources))
targets := $(basename $(sources))

all: $(targets)

clean:
        rm -f $(targets) $(objects) *.core






21.2.15.7  Licensing the Embedded Server




The MySQL source code is covered by the GNU GPL license
(see section G  GNU General Public License).  One result of this is that any program
which includes, by linking with libmysqld, the MySQL
source code must be released as free software (under a license
compatible with the GPL).





We encourage everyone to promote free software by releasing
code under the GPL or a compatible license.  For those who
are not able to do this, another option is to purchase a
commercial license for the MySQL code from MySQL AB.
For details, please see section 1.4.3  MySQL Licenses.







21.3  MySQL PHP API










PHP is a server-side, HTML-embedded scripting language that may be used to
create dynamic Web pages.  It contains support for accessing several
databases, including MySQL.  PHP may be run as a separate program
or compiled as a module for use with the Apache Web server.





The distribution and documentation are available at the PHP Web site
(http://www.php.net/).







21.3.1  Common Problems with MySQL and PHP








21.4  MySQL Perl API














The Perl DBI module provides a generic interface for database access.
You can write a DBI script that works with many different database engines
without change. To use DBI, you must install the DBI module, as well
as a DataBase Driver (DBD) module for each type of server you want to access.
For MySQL, this driver is the DBD::mysql module.





Perl DBI is now the recommended Perl interface. It replaces an older
interface called mysqlperl, which should be considered obsolete.





Installation instructions for Perl DBI support are given in
section 2.7  Perl Installation Notes.





DBI information is available at the command line, online, or in printed form:









21.5  MySQL C++ API











MySQL++ is the MySQL API for C++. More
information can be found at http://www.mysql.com/products/mysql++/.







21.5.1  Borland C++










You can compile the MySQL Windows source with Borland C++ 5.02.
(The Windows source includes only projects for Microsoft VC++, for
Borland C++ you have to do the project files yourself.)





One known problem with Borland C++ is that it uses a different structure
alignment than VC++.  This means that you will run into problems if you
try to use the default libmysql.dll libraries (that was compiled
with VC++) with Borland C++. To avoid this problem, only call
mysql_init() with NULL as an argument, not a pre-allocated
MYSQL structure.







21.6  MySQL Python API










MySQLdb provides MySQL support for Python, compliant with the
Python DB API version 2.0. It can be found at
http://sourceforge.net/projects/mysql-python/.







21.7  MySQL Tcl API










MySQLtcl is a simple API for accessing a MySQL database server from the Tcl
programming language. It can be found at http://www.xdobry.de/mysqltcl/.






21.8  MySQL Eiffel Wrapper











Eiffel MySQL is an interface to the MySQL database server using the Eiffel
programming language, written by Michael Ravits. It can be found at
http://efsa.sourceforge.net/archive/ravits/mysql.htm.







22  MySQL Connectors












This chapter describes MySQL Connectors, drivers that provide connectivity to
the MySQL server for client programs.







22.1  MySQL ODBC Support














MySQL provides support for ODBC by means of MySQL Connector/ODBC, the
family of MyODBC drivers.  This is the reference for the Connector/ODBC
product family of MyODBC drivers that provide ODBC 3.5x compliant access
to the MySQL Database System.  It will teach you how to install MyODBC
and how to use it.  You will also information about common programs that
are known to work with MyODBC and answers to some of the most frequently
asked questions about MyODBC.





This reference applies to MyODBC .  You can find a
manual for an older version of MyODBC in the binary or source distribution
for that version.





This is a reference to the MySQL ODBC drivers, not a general
ODBC reference.  For more information about ODBC, refer to
http://www.microsoft.com/data/.





The application development part of this reference assumes a good working
knowledge of C, general DBMS knowledge, and finally, but not least,
familiarity with MySQL.  For more information about MySQL functionality and
its syntax, refer to http://dev.mysql.com/doc/.





If you have questions that are not aswered in this document, please send
a mail message to [email protected].







22.1.1  Introduction to MyODBC





22.1.1.1  What is ODBC?




ODBC (Open Database Connectivity) provides a way for client programs to
access a wide range of databases or data sources.  ODBC is a standardized
API that allows connections to SQL database servers.  It was developed
according to the specifications of the SQL Access Group and defines a set
of function calls, error codes, and data types that can be used to develop
database-independent applications.  ODBC usually is used when database
independence or simultaneous access to different data sources is required.





For more information about ODBC, refer to
http://www.microsoft.com/data/.







22.1.1.2  What is Connector/ODBC?




Connector/ODBC is the term designating the MySQL AB product family
of MySQL ODBC drivers. These are known as the MyODBC drivers.







22.1.1.3  What is MyODBC 2.50?




MyODBC 2.50 is a 32-bit ODBC driver from MySQL AB that is based on ODBC 2.50
specification level 0 (with level 1 and 2 features). This is one of the most
popular ODBC drivers in the open source market, used by many users to access
the MySQL functionality.







22.1.1.4  What is MyODBC 3.51?




MyODBC 3.51 is a 32-bit ODBC driver, also known as the MySQL ODBC 3.51
driver.  This version is enhanced compared to the existing MyODBC 2.50
driver.  It has support for ODBC 3.5x specification level 1 (complete core
API + level 2 features) in order to continue to provide all functionality of
ODBC for accessing MySQL.







22.1.1.5  Where to Get MyODBC




MySQL AB distributes all its products under the General Public License
(GPL). You can get a copy of the latest version of MyODBC binaries and
sources from the MySQL AB Web site http://dev.mysql.com/downloads/.





For more information about MyODBC, visit
http://www.mysql.com/products/myodbc/.







22.1.1.6  Supported Platforms




MyODBC can be used on all major platforms supported by
MySQL, such as:








If a binary distribution is not available for downloading for a
particular platform, you can build the driver yourself by
downloading the driver sources. You can contribute the binaries to MySQL
by sending a mail message to [email protected], so that it
becomes available for other users.







22.1.1.7  MyODBC Mailing List




MySQL AB provides assistance to the user community by means of its mailing
lists. For MyODBC-related issues, you can get help from experienced
users by using the [email protected] mailing list.





For information about subscribing to MySQL mailing lists or to browse
list archives, visit http://lists.mysql.com/.







22.1.1.8  How to Get MyODBC Support




For the success of any product, support from the product vendor or the
development team plays a key role. If you want any of the following,
you must get a support contract from MySQL AB:








MySQL AB offers different types of support licenses in order to help you
by keeping your requirements and ability to purchase in mind.  For more
information about MySQL support, visit https://order.mysql.com/
or send a mail message to [email protected].





Once you have the support means, you can send queries or bug reports
using the MySQL support wizard from http://support.mysql.com/.
Then the MySQL team will work on your problem immediately until it gets
resolved.







22.1.1.9  How to Report MyODBC Problems or Bugs














If you encounter difficulties or problems with MyODBC, you should start
by making a log file from the ODBC Manager (the log you get when
requesting logs from ODBC ADMIN) and MyODBC. The procedure for doing
this is described in section 22.1.9.7  Getting an ODBC Trace File.





Check the MyODBC trace file to find out what could be wrong.
You should be able to determine what statements were issued by searching for
the string >mysql_real_query in the `myodbc.log' file.





You should also try issuing the statements from the @command{mysql} client
program or from admndemo. This will help you determine whether the
error is in MyODBC or MySQL.





If you find out something is wrong, please only send the relevant rows
(maximum 40 rows) to the myodbc mailing list.
See section 1.7.1.1  The MySQL Mailing Lists.
Please never send the whole MyODBC or ODBC log file!





If you are unable to find out what's wrong, the last option is to
create an archive in @command{tar} or Zip format that contains a MyODBC trace file,
the ODBC log file, and a `README' file that explains the problem.  You can send this
to ftp://ftp.mysql.com/pub/mysql/upload/.  Only we at
MySQL AB will have access to the files you upload, and we will
be very discreet with the data.





If you can create a program that also demonstrates the problem, please include
it in the archive as well.





If the program works with some other SQL server, you should include an ODBC log
file where you do exactly the same thing in the other SQL server.





Remember that the more information you can supply to us, the more
likely it is that we can fix the problem.







22.1.1.10  How to Request a MyODBC Feature or Patch




You must have a support contract from MySQL AB in order to request
a new feature that is not in the TO-DO list, or if you want a patch
prior to its release. For more information on obtaining support, see
section 22.1.1.8  How to Get MyODBC Support.







22.1.1.11  How to Submit a MyODBC Patch




You can send a patch or suggest a better solution for any existing code
or problems by sending a mail message to [email protected].







22.1.2  General Information About ODBC and MyODBC





22.1.2.1  Introduction to ODBC




Open Database Connectivity (ODBC) is a widely accepted application-programming
interface (API) for database access. It is based on the Call-Level Interface
(CLI) specifications from X/Open and ISO/IEC for database APIs and uses
Structured Query Language (SQL) as its database access language. 





A survey of ODBC functions supported by MyODBC is given at
section 22.1.16  MyODBC API Reference.  For general information about ODBC, see
http://www.microsoft.com/data/.







22.1.2.2  MyODBC Architecture




The MyODBC architecture is based on five components, as shown in the
following diagram:






  








Application:


An application is a program that calls the ODBC API to access the data from
the MySQL server. The Application communicates with the Driver Manager
using the standard ODBC calls.  The Application does not care where the
data is stored, how it is stored, or even how the system is configured to
access the data.  It needs to know only the Data Source Name (DSN).

A number of tasks are common to all applications, no matter how they use ODBC.
These tasks are:


    

  • Selecting the MySQL server and connecting to it


  • Submitting SQL statements for execution


  • Retrieving results (if any)


  • Processing errors


  • Committing or rolling back the transaction enclosing the SQL statement


  • Disconnecting from the MySQL server




Because most data access work is done with SQL, the primary tasks for
applications that use ODBC are submitting SQL statements and retrieving
any results generated by those statements.


Driver manager:


The Driver Manager is a library that manages communication
between application and driver or drivers. It performs the following tasks:



    

  • Resolves Data Source Names (DSN)


  • Driver loading and unloading


  • Processes ODBC function calls or passes them to the driver





MyODBC Driver:


The MyODBC driver is a library that implements the functions in the
ODBC API. It processes ODBC function calls, submits SQL requests to MySQL
server, and returns results back to the application. If necessary, the driver
modifies an application's request so that the request conforms to syntax
supported by the MySQL.


ODBC.INI:


`ODBC.INI' is the ODBC configuration file that stores the driver
and database information required to connect to the server. It is used by
the Driver Manager to determine which driver to be loaded using the Data
Source Name.  The driver uses this to read connection parameters based on
the DSN specified.  For more information, section 22.1.9  MyODBC Configuration.


MySQL Server:


The MySQL server is the source of data. MySQL is:



    

  • A database management system (DBMS)


  • A relational database management system (RDBMS)


  • Open Source Software










22.1.2.3  ODBC Driver Managers




An ODBC Driver Manager is a library that manages communication between
the ODBC aware application and driver(s). Its main functionality includes:








	
The following driver managers are commonly used:








MyODBC 3.51 also is shipped with UnixODBC beginning with version 2.1.2.







22.1.2.4  Types of MySQL ODBC Drivers




MySQL AB supports two Open Source ODBC drivers for accessing MySQL
functionality through the ODBC API: MyODBC (MyODBC 2.50) and MySQL ODBC
3.51 Driver (MyODBC 3.51).





Note: From this section onward, we refer both the drivers
generically as MyODBC.  Whenever there is a difference, we use the
original names.







22.1.3  How to Install MyODBC




MyODBC works on Windows 9x, Me, NT, 2000, and XP, and on most Unix platforms.





MyODBC is Open Source. You can find the newest
version at http://dev.mysql.com/downloads/connector/odbc/.
Please note that the 2.50.x versions are LGPL licensed,
whereas the 3.51.x versions are GPL licensed.





If you have problem with MyODBC and your program also works
with OLEDB, you should try the OLEDB driver.





Normally you need to install MyODBC only on Windows machines.
You need MyODBC for Unix only if you have a program like
ColdFusion that is running on a Unix machine and uses ODBC to connect
for database access.





If you want to install MyODBC on a Unix box, you will also need
an ODBC manager.  MyODBC is known to work with most Unix ODBC managers.








Notice that other configuration options are shown on the MySQL screen that
you can try if you run into problems (options such as trace, don't prompt
on connect, and so forth).







22.1.4  Installing MyODBC from a Binary Distribution on Windows




To install MyODBC on Windows, you should download the appropriate
distribution file from http://dev.mysql.com/downloads/connector/odbc/,
unpack it, and execute the @command{MyODBC-VERSION.exe} file.





On Windows, you may get the following error when trying to install the
older MyODBC 2.50 driver:





An error occurred while copying C:\WINDOWS\SYSTEM\MFC30.DLL. Restart
Windows and try installing again (before running any applications
which use ODBC)





The problem is that some other program is using ODBC.  Because of how
Windows is designed, you may not be able in this case to install new
ODBC drivers with Microsoft's ODBC setup program. In most cases, you can
continue by pressing Ignore to copy the rest of the MyODBC files and
the final installation should still work.  If it doesn't, the solution is
to re-boot your computer in ``safe mode.'' Choose safe mode by pressing F8
just before your machine starts Windows during re-booting, install MyODBC,
and re-boot to normal mode.







22.1.5  Installing MyODBC from a Binary Distribution on Unix





22.1.5.1  Installing MyODBC from an RPM Distribution




To install or upgrade MyODBC from an RPM distribution on
Linux, simply download the RPM distribution of the latest version
of MyODBC and follow the instructions below.  Use @command{su root}
to become root, then install the RPM file.





If you are installing for the first time:





shell> su root
shell> rpm -ivh MyODBC-3.51.01.i386-1.rpm





If the driver already exists, upgrade like this:





shell> su root
shell> rpm -Uvh MyODBC-3.51.01.i386-1.rpm





If there is any dependancy error for MySQL client library,
libmysqlclient, simply ignore it by supplying the --nodeps
option, and then make sure the MySQL client shared library is in the path
or set through LD_LIBRARY_PATH.





This installs the driver libraries and related documents to
`/usr/local/lib' and `/usr/share/doc/MyODBC' respectively. Now
proceed onto section 22.1.9.3  Configuring a MyODBC DSN on Unix.





To uninstall the driver, become root and execute
an @command{rpm} command:





shell> su root
shell> rpm -e MyODBC






22.1.5.2  Installing MyODBC from a Binary Tarball Distribution




To install the driver from a tarball distribution (`.tar.gz' file),
download the latest version of the driver for your operating system and
follow these steps:





shell> su root
shell> gunzip MyODBC-3.51.01-i686-pc-linux.tar.gz
shell> tar xvf MyODBC-3.51.01-i686-pc-linux.tar
shell> cd MyODBC-3.51.01-i686-pc-linux





Read the installation instructions in the `INSTALL-BINARY' file
and execute these commands.





shell> cp libmyodbc* /usr/local/lib
shell> cp odbc.ini /usr/local/etc
shell> export ODBCINI=/usr/local/etc/odbc.ini





Then proceed on to how to configure the DSN on
unix to configure the DSN for MyODBC. For more information,
refer to the `INSTALL-BINARY' file that comes with your distribution.







22.1.6  Installing MyODBC from a Source Distribution on Windows





22.1.6.1  Requirements








22.1.6.2  Building MyODBC 3.51




MyODBC 3.51 source distributions include `Makefiles' that
uses @command{nmake}. In the distribution, you can find `Makefile'
for building the release version and `Makefile_debug' for building
debugging versions of the driver libraries and DLLs.





To build the driver, use this procedure:





    


  1. 

Download and extract the sources to a folder, then change location into that
folder.  The following command assumes the folder is named `myodbc3-src':



    C:\> cd myodbc3-src

    


  2. 

Edit `Makefile' to specify the correct path for the MySQL client
libraries and headers files. Then use use the following commands to build
and install the release version:



    C:\> nmake -f Makefile
C:\> nmake -f Makefile install

    

@command{nmake -f Makefile} builds the release version of the driver
and places the binaries in subdirectory called `Release'.

@command{nmake -f Makefile install} installs (copies) the driver DLLs
and libraries(`myodbc3.dll', `myodbc3.lib') to your system
directory.


  3. 

To build the debug version, use `Makefile_Debug' rather than
`Makefile', as shown below:



    C:\> nmake -f Makefile_debug
C:\> nmake -f Makefile_debug install

    


  4. 

You can clean and rebuild the driver by using:



    C:\> nmake -f Makefile clean
C:\> nmake -f Makefile install

    






Note:









22.1.6.3  Testing




After the driver libraries are copied/installed to the system directory, you
can test whether the libraries are properly built by using the samples
provided in the `samples' subdirectory:





C:\> cd samples
C:\> nmake -f Makefile all






22.1.6.4  Building MyODBC 2.50




The MyODBC 2.50 source distribution includes VC workspace files. You can
build the driver using these files (`.dsp' and `.dsw') directly
by loading them from Microsoft Visual Studio 6.0 or higher.







22.1.7  Installing MyODBC from a Source Distribution on Unix





22.1.7.1  Requirements







Once you have all the required files, unpack the source files to a
separate directory and follow the instructions as given below:






22.1.7.2  Typical @command{configure} Options




The @command{configure} script gives you a great deal of control over how
you configure your MyODBC build. Typically you do this using options on the
@command{configure} command line. You can also affect @command{configure}
using certain environment variables. For a list of options and environment
variables supported by @command{configure}, run this command:





shell> ./configure --help





Some of the more commonly used @command{configure} options are described here:





    

  1. 

To compile MyODBC, you need to supply the MySQL client include and library
files path using the --with-mysql-path=DIR option, where
DIR is the directory where the MySQL is installed.

MySQL compile options can be determined by running
DIR/bin/mysql_config.


  2. 

Supply the standard header and library files path for your
ODBC Driver Manager(iodbc or unixobc).



      


    • 

If you are using iodbc and iodbc is not installed in its default
location (`/usr/local'), you might have to use
the --with-iodbc=DIR option, where
DIR is the directory where iodbc is installed.

If the iodbc headers do not reside in `DIR/include',
you can use the --with-iodbc-includes=INCDIR option to
specify their location.

The applies to libraries. If they are not in `DIR/lib',
you can use the --with-iodbc-libs=LIBDIR option.


    • 

If you are using unixODBC, use the --with-unixODBC=DIR
option (case sensitive) to make @command{configure} look for unixODBC
instead of iodbc by default, DIR is the directory where
unixODBC is installed.

If the unixODBC headers and libraries aren't located in
`DIR/include' and `DIR/lib', use
the --with-unixODBC-includes=INCDIR
and --with-unixODBC-libs=LIBDIR options.


    


  3. 

You might want to specify an installation prefix other than
`/usr/local'.  For example, to install the MyODBC drivers in
`/usr/local/odbc/lib', use the
--prefix=/usr/local/odbc option.






The final configuration command will look something like this:





shell> ./configure --prefix=/usr/local \
           --with-iodbc=/usr/local \
           --with-mysql-path=/usr/local/mysql






22.1.7.3  Thread-Safe Client




In order to link the driver with MySQL thread safe client libraries
`libmysqlclient_r.so' or `libmysqlclient_r.a', you must specify the
following @command{configure} option:





--enable-thread-safe





and can be disabled(default) using





--disable-thread-safe





This option enables the building of driver thread-safe library
`libmyodbc3_r.so' from by linking with mysql thread-safe client library
`libmysqlclient_r.so' (The extensions are OS dependent).





In case while configuring with thread-safe option, and gotten into a
configure error; then look at the `config.log' and see if it is due
to the lack of thread-libraries in the system; and supply one with LIBS
options i.e.





LIBS="-lpthread" ./configure ..






22.1.7.4  Shared or Static Options




You can enable or disable the shared and static versions using these options:





--enable-shared[=yes/no]
--disable-shared
--enable-static[=yes/no]
--disable-static






22.1.7.5  Enabling Debugging Information




By default, all the binary distributions are built as non-debugging versions
(configured with --without-debug).





To enable debugging information,
build the driver from source distribution and use the
--with-debug) when you run @command{configure}.







22.1.7.6  Enabling the Documentation




This option is available only for BK clone trees; not for
normal source distributions.





By default, the driver is built with (--without-docs); And
in case if you want the documentation to be taken care in the normal
build, then configure with:


 --with-docs






22.1.7.7  Building and Compilation




To build the driver libraries, you have to just execute @command{make},
which takes care of everything.





shell> make





If any errors occur, correct them and continue
the build process. If you aren't able to build, then send a detailed email
to [email protected] for further assistance.







22.1.7.8  Building Shared Libraries




On most platforms, MySQL doesn't build or support `.so' (shared)
client libraries by default, because building with shared libraries has
caused us problems in the past.





In cases like this, you have to download the MySQL distribution
and configure it with these options:





--without-server --enable-shared





To build shared driver libraries, you must specify the
--enable-shared option for @command{configure}.  By default,
@command{configure} does not enable this option.





If you have configured with the --disable-shared option, you can
build the `.so' file from the static libraries using the following
commands:





shell> cd MyODBC-3.51.01
shell> make
shell> cd driver
shell> CC=/usr/bin/gcc \
       $CC -bundle -flat_namespace -undefined error \
           -o .libs/libmyodbc3-3.51.01.so \
           catalog.o connect.o cursor.o dll.o error.o execute.o \
           handle.o info.o misc.o myodbc3.o options.o prepare.o \
           results.o transact.o utility.o \
           -L/usr/local/mysql/lib/mysql/ \
           -L/usr/local/iodbc/lib/ \
           -lz -lc -lmysqlclient -liodbcinst





Make sure to change -liodbcinst to -lodbcinst if you are
using unixODBC instead of iODBC, and configure the library paths
accordingly.





This builds and places the `libmyodbc3-3.51.01.so' file in the
`.libs' directory.  Copy this file to MyODBC library directory
(`/usr/local/lib' (or the `lib' directory under the installation
directory that you supplied with the --prefix).





shell> cd .libs
shell> cp libmyodbc3-3.51.01.so /usr/local/lib
shell> cd /usr/local/lib
shell> ln -s libmyodbc3-3.51.01.so libmyodbc3.so





To build the thread-safe driver library:





shell> CC=/usr/bin/gcc \
       $CC -bundle -flat_namespace -undefined error
	      -o .libs/libmyodbc3_r-3.51.01.so
	      catalog.o connect.o cursor.o dll.o error.o execute.o
	      handle.o info.o misc.o myodbc3.o options.o prepare.o
	      results.o transact.o utility.o
	      -L/usr/local/mysql/lib/mysql/
	      -L/usr/local/iodbc/lib/
	      -lz -lc -lmysqlclient_r -liodbcinst






22.1.7.9  Installing Driver Libraries




To install the driver libraries, execute the following command:





shell> make install





That command installs one of the following sets of libraries:





For MyODBC 3.51:








For thread-safe MyODBC 3.51:








For MyODBC 2.5.0:








For more information on build process, refer to the `INSTALL' file
that comes with the source distribution. Note that if you are trying to
use the @command{make} from Sun, you may end up with errors. On
the other hand, GNU @command{gmake} should work fine on all
platforms.







22.1.7.10  Testing MyODBC on Unix




To run the basic samples provided in the distribution with the libraries
that you built, just execute:


shell> make test





Make sure the DSN 'myodbc3' is configured first in `odbc.ini' and
environment variable ODBCINI is pointing to the right
`odbc.ini' file; and MySQL server is running. You can find a sample
`odbc.ini' with the driver distribution.





You can even modify the `samples/run-samples' script to pass the desired
DSN, UID, and PASSWORD values as the command line arguments to each sample.







22.1.7.11  Mac OS X Notes




To build the driver on Mac OS X (Darwin), make use of the
following @command{configure} example:





shell> ./configure --prefix=/usr/local
			       --with-unixODBC=/usr/local
			       --with-mysql-path=/usr/local/mysql
			       --disable-shared
			       --enable-gui=no
			       --host=powerpc-apple





The command assumes that the unixODBC and MySQL are installed in the default
locations. If not, configure accordingly.





On Mac OS X, --enable-shared builds `.dylib' files by default.
You can build `.so' files like this:





shell> make
shell> cd driver
shell> CC=/usr/bin/gcc \
       $CC -bundle -flat_namespace -undefined error
           -o .libs/libmyodbc3-3.51.01.so *.o
           -L/usr/local/mysql/lib/
           -L/usr/local/iodbc/lib
           -liodbcinst -lmysqlclient -lz -lc





To build the thread-safe driver library:





shell> CC=/usr/bin/gcc \
       $CC -bundle -flat_namespace -undefined error
       -o .libs/libmyodbc3-3.51.01.so *.o
       -L/usr/local/mysql/lib/
       -L/usr/local/iodbc/lib
       -liodbcinst -lmysqlclienti_r -lz -lc -lpthread





Make sure to change the -liodbcinst to -lodbcinst in case of
using unixODBC instead of iODBC and configure the libraries path
accordingly.





In Apple's version of GCC, both @command{cc} and @command{gcc} are actually
symbolic links to @command{gcc3}.





Now copy this library to the `$prefix/lib' directory and symlink to
`libmyodbc3.so'.





You can cross-check the output shared-library properties using this command:





shell> otool -LD .libs/libmyodbc3-3.51.01.so






22.1.7.12  HP-UX Notes




To build the driver on HP-UX 10.x or 11.x, make use of
the following @command{configure} example:





If using @command{cc}:





shell> CC="cc" \
       CFLAGS="+z" \
       LDFLAGS="-Wl,+b:-Wl,+s" \
       ./configure --prefix=/usr/local
			       --with-unixodbc=/usr/local
			       --with-mysql-path=/usr/local/mysql/lib/mysql
			       --enable-shared
			       --enable-thread-safe





If using @command{gcc}:





shell> CC="gcc" \
       LDFLAGS="-Wl,+b:-Wl,+s" \
       ./configure --prefix=/usr/local
			       --with-unixodbc=/usr/local
			       --with-mysql-path=/usr/local/mysql
			       --enable-shared
			       --enable-thread-safe





Once the driver is built, cross-check its attributes using
@command{chatr .libs/libmyodbc3.sl}
to see whether or not you need to have the MySQL client
libraries path using the SHLIB_PATH environment variable. For
static versions, ignore all shared-library options and run @command{configure}
with the --disable-shared option.







22.1.7.13  AIX Notes:




To build the driver on AIX, make use of
the following @command{configure} example:





shell> ./configure --prefix=/usr/local
			       --with-unixodbc=/usr/local
			       --with-mysql-path=/usr/local/mysql
			       --disable-shared
			       --enable-thread-safe





NOTE: For more information about how to build and set up the
static and shared libraries across the different platforms refer to '

Using static and shared libraries across platforms'.







22.1.8  Installing MyODBC from the BitKeeper Development Source Tree




Note: You should read this section only if you are interested in helping us
test our new code.





To obtain our most recent development source tree, use these instructions:





    

  1. 

Download BitKeeper from
http://www.bitmover.com/cgi-bin/download.cgi.  You will need
BitKeeper 3.0 or newer to access our repository.

  2. 

Follow the instructions to install it.

  3. 

After BitKeeper is installed, first go to the directory you
want to work from, and then use this command if you want to clone
the MyODBC 3.51 branch:



    shell> bk clone bk://mysql.bkbits.net/myodbc3 myodbc-3.51

    

In the preceding example, the source tree will be set up in the
`myodbc-3.51/' or by default `myodbc3/' subdirectory of
your current directory. If you are behind the firewall and can only
initiate HTTP connections, you can also use BitKeeper via
HTTP. If you are required to use a proxy server, simply set the
environment variable http_proxy to point to your proxy:



    shell> export http_proxy="http://your.proxy.server:8080/"

    

Now, simply replace the bk:// with http:// when doing a
clone. Example:



    shell> bk clone http://mysql.bkbits.net/myodbc3 myodbc-3.51

    

The initial download of the source tree may take a while, depending on the
speed of your connection; be patient.

  4. 

You will need GNU autoconf 2.52 (or newer), automake 1.4,
libtool 1.4, and m4 to run the next set of commands.



    shell> cd myodbc-3.51
shell> bk -r edit
shell> aclocal; autoheader; autoconf;  automake;
shell> ./configure  # Add your favorite options here
shell> make

    

For more information on how to build, refer to `INSTALL' file located
in the same directory. On Windows, make use of Windows
Makefiles `WIN-Makefile' and `WIN-Makefile_debug' in
building the driver, for more information, see
section 22.1.6  Installing MyODBC from a Source Distribution on Windows.

  5. 

When the build is done, run @command{make install} to install the
MyODBC 3.51 driver on your system.

  6. 

If you have gotten to the @command{make} stage and the distribution does
not compile, please report it to [email protected].

  7. 

After the initial @command{bk clone} operation to get the source tree, you
should run @command{bk pull} periodically to get the updates.

  8. 

You can examine the change history for the tree with all the diffs by using
@command{bk sccstool}.  If you see some funny diffs or code that you have a
question about, do not hesitate to send e-mail to
[email protected].

Also, if you think you have a better idea on how to do something, send an
e-mail to the same address with a patch. @command{bk diffs} will produce a
patch for you after you have made changes to the source. If you do not
have the time to code your idea, just send a description.

  9. 

BitKeeper has a nice help utility that you can access via
@command{bk helptool}.





You can also browse changesets, comments and source code online by
browsing to http://mysql.bkbits.net:8080/myodbc3.







22.1.9  MyODBC Configuration




This section describes how to configure MyODBC, including
DSN creation and the different arguments that the driver takes as
an input arguments in the connection string. It also describes how
to create an ODBC trace file.







22.1.9.1  What is a Data Source Name?




A "data source" is a place where data comes from.  The data source must
have a persistent identifier, the Data Source Name. Using the Data Source
Name, MySQL can access initialization information. With the initialization
information, MySQL knows where to access the database and what settings
to use when the access starts.





In effect, the data source is the path to the data. In different
contexts this might mean different things, but typically it identifies
a running MySQL server (for example via a network address or service
name), plus the default database for that server at connection time, plus
necessary connection information such as the port. The MySQL drivers (and,
on Windows systems, the ODBC Driver Manager) will use the data source for
connecting. An administrative utility called the Microsoft ODBC Data Source
Administrator may be useful for this purpose.





There are two places where the initialization information might be: in
the Windows registry (on a Windows system), or in a DSN file (on any system).





If the information is in the Windows registry, it is called a "Machine
data source". It might be a "User data source", in which case only one
user can see it. Or it might be a "System data source" in which case
it is accessible to all users on the computer, or indeed to all users
connected to the computer, if the users are connected by Microsoft Windows
NT services. When you run the ODBC Data Administration program, you will
have a choice whether to use "User" or "System" -- there are separate tabs.





If the information is in a DSN file, it is called a "File data source". This
is a text file. Its advantages are: (a) it is an option for any kind of
computer, not just a computer with a Windows operating system; (b) its
contents can be transmitted or copied relatively easily.







22.1.9.2  Configuring a MyODBC DSN on Windows




To add and configure a new MyODBC data source on Windows, use the
ODBC Data Source Administrator. The ODBC Administrator
updates your data source connection information. As you
add data sources, the ODBC Administrator updates the
registry information for you.





To open the ODBC Administrator from the Control Panel:





    


  1. 

Click Start, point to Settings, and then click Control
Panel.


  2. 

On computers running Microsoft Windows 2000 or newer, double-click
Administrative Tools, and then double-click Data Sources
(ODBC). On computers running older versions of Windows, double-click
32-bit ODBC or ODBC.


      
    

The ODBC Data Source Administrator dialog box appears, as
shown here:


      
    

Click Help for detailed information about each tab of the ODBC
Data Source Administrator dialog box.






To add a data source on Windows:





    


  1. 

Open the ODBC Data Source Administrator.


  2. 

In the ODBC Data Source Administrator dialog box, click
Add. The Create New Data Source dialog box appears.


  3. 

Select MySQL ODBC 3.51 Driver, and then click Finish.
The MySQL ODBC 3.51 Driver - DSN Configuration dialog box appears,
as shown here:


      
    


  4. 

In the Data Source Name box, enter the name of the data
source you want to access. It can be any valid name that you choose.


  5. 

In the Description box, enter the description needed for the DSN.


  6. 

For Host or Server Name (or IP) box, enter the name of the MySQL
server host that you want to access. By default, it is localhost.


  7. 

In the Database Name box, enter the name of the MySQL database
that you want to use as the default database.


  8. 

In the User box, enter your MySQL username (your database
user ID).


  9. 

In the Password box, enter your password.


  10. 

In the Port box, enter the port number if it is not the default (3306).


  11. 

In the SQL Command box, you can enter an optional SQL statement that
you want to issue automatically after the connection has been established.

The final dialog looks like this:


      
    

Click OK to add this data source.






Note: Upon clicking OK, the Data Sources dialog
box appears, and the ODBC Administrator updates the registry
information. The username and connect string that you entered become the
default connection values for this data source when you connect to it.





You can also test whether your settings are suitable for connecting to the
server using the button Test Data Source. This feature is available
only for the MyODBC 3.51 driver. A successful test results in the following
window:






  






A failed test results in an error:






  






The DSN configuration dialog also has an Options button. If you
select it, the following options dialog appears displaying that control
driver behavior. Refer to section 22.1.9.4  Connection Parameters for information
about the meaning of these options.






  






Note: The options listed under Driver Trace Options are
disabled (grayed out) unless you are using the debugging version of the
driver DLL.





To modify a data source on Windows:





    


  1. 

Open the ODBC Data Source Administrator. Click the appropriate
DSN tab.

  2. 

Select the MySQL data source that you want to modify and then click
Configure.  The MySQL ODBC 3.51 Driver - DSN Configuration
dialog box appears.

  3. 

Modify the applicable data source fields, and then click OK.






When you have finished modifying the information in this dialog box, the
ODBC Administrator updates the registry information.







22.1.9.3  Configuring a MyODBC DSN on Unix




On Unix, you configure DSN entries directly in the
`odbc.ini' file. Here is a typical `odbc.ini' file that configures
myodbc and myodbc3 as the DSN names for MyODBC 2.50 and MyODBC
3.51, respectively:





;
;  odbc.ini configuration for MyODBC and MyODBC 3.51 drivers
;

[ODBC Data Sources]
myodbc      = MyODBC 2.50 Driver DSN
myodbc3     = MyODBC 3.51 Driver DSN

[myodbc]
Driver       = /usr/local/lib/libmyodbc.so
Description  = MyODBC 2.50 Driver DSN
SERVER       = localhost
PORT         =
USER         = root
Password     =
Database     = test
OPTION       = 3
SOCKET       =

[myodbc3]
Driver       = /usr/local/lib/libmyodbc3.so
Description  = MyODBC 3.51 Driver DSN
SERVER       = localhost
PORT         =
USER         = root
Password     =
Database     = test
OPTION       = 3
SOCKET       =

[Default]
Driver       = /usr/local/lib/libmyodbc3.so
Description  = MyODBC 3.51 Driver DSN
SERVER       = localhost
PORT         =
USER         = root
Password     =
Database     = test
OPTION       = 3
SOCKET       =





Refer to the section 22.1.9.4  Connection Parameters, for the list of
connection parameters that can be supplied.





Note: If you are using unixODBC, you can use the following tools
in order to set up the DSN:








In some cases when using unixODBC, you might get this error:





Data source name not found and no default driver specified





If this happens, make sure the ODBCINI and ODBCSYSINI
environment variables are pointing to the right `odbc.ini' file.  For
example, if your `odbc.ini' file is located in `/usr/local/etc',
set the environment variables like this:





export ODBCINI=/usr/local/etc/odbc.ini
export ODBCSYSINI=/usr/local/etc






22.1.9.4  Connection Parameters




You can specify the following parameters for MyODBC in the [Data
Source Name] section of an ODBC.INI file or through the
InConnectionString argument in the SQLDriverConnect() call.






Parameter  Default Value  Comment



user  ODBC (on Windows)  The username used to connect to MySQL.



server  localhost  The hostname of the MySQL server.



database   The default database.



option  0  Options that specify how MyODBC should work. See below.



port  3306  The TCP/IP port to use if server is not localhost.



stmt   A statement to execute when connecting to MySQL.



password   The password for the user account on server.



socket   The Unix socket file or Windows named pipe to connect to if server is localhost.







The option argument is used to tell MyODBC that the client isn't 100%
ODBC compliant. On Windows, you normally select options by toggling the
checkboxes in the connection screen, but you can also select them in the
option argument. The following options are listed in the order
in which they appear in the MyODBC connect screen:






Value  Description



1  The client can't handle that MyODBC returns the real width of a column.



2  The client can't handle that MySQL returns the true value of affected rows.  If this flag is set, MySQL returns ``found rows'' instead.  You must have MySQL 3.21.14 or newer to get this to work.



4  Make a debug log in `c:\myodbc.log'.  This is the same as putting MYSQL_DEBUG=d:t:O,c::\myodbc.log in `AUTOEXEC.BAT'. (On Unix, the file is `/tmp/myodbc.log'.)



8  Don't set any packet limit for results and parameters.



16  Don't prompt for questions even if driver would like to prompt.



32  Enable or disable the dynamic cursor support. (Not allowed in MyODBC 2.50.)



64  Ignore use of database name in db_name.tbl_name.col_name.



128  Force use of ODBC manager cursors (experimental).



256  Disable the use of extended fetch (experimental).



512  Pad CHAR columns to full column length.



1024  SQLDescribeCol() will return fully qualified column names.



2048  Use the compressed client/server protocol.



4096  Tell server to ignore space after function name and before `(' (needed by PowerBuilder).  This will make all function names keywords.



8192  Connect with named pipes to a @command{mysqld} server running on NT.



16384  Change LONGLONG columns to INT columns (some applications can't handle LONGLONG).



32768  Return 'user' as Table_qualifier and Table_owner from SQLTables (experimental).



65536  Read parameters from the [client] and [odbc] groups from `my.cnf'.



131072  Add some extra safety checks (should not be needed but...).



262144  Disable transactions.



524288  Enable query logging to `c:\myodbc.sql'(`/tmp/myodbc.sql') file. (Enabled only in debug mode.)



1048576  Do not cache the results locally in the driver,
instead read from server (mysql_use_result()). This works only for
forward-only cursors. This option is very important in dealing
with large tables when you don't want the driver to cache the
entire result set.



2097152  Force the use of Forward-only cursor type. In case
of applications setting the default static/dynamic cursor type, and one
wants driver to use non-cache result sets, then this option will ensure
the forward-only cursor behavior.







To select multiple options, add together their values.  For example,
setting option to 12 (4+8) gives you debugging without packet limits.





The default `myodbc3.dll' is compiled for optimal performance.  If you
want to to debug MyODBC 3.51 (for example, to enable tracing), you should
instead use `myodbc3d.dll'.  To install this file, copy
`myodbc3d.dll' over the installed `myodbc3.dll' file. Make sure to
revert back to the release version of the driver DLL once you are done with
the debugging because the debug version may cause performance issues.





For MyODBC 2.50, `myodbc.dll' and `myodbcd.dll' are used instead.





The following table shows some recommended option values for
various configurations:






Configuration  Option Value



Microsoft Access  3



Microsoft Visual Basic  3



Large tables with too many rows  2049



Driver trace generation (Debug mode)  4



Query log generation (Debug mode)  524288



Generate driver trace as well as query log (Debug mode)  524292



Large tables with no-cache results  3145731








22.1.9.5  Connecting Without a Predefined DSN




Yes.  You can connect to the MySQL server using SQLDriverConnect, by
specifying the DRIVER name field. Here are the connection strings
for MyODBC using DSN-Less connection:





For MyODBC 2.50:





ConnectionString = "DRIVER={MySQL};\
                    SERVER=localhost;\
                    DATABASE=test;\
                    USER=venu;\
                    PASSWORD=venu;\
                    OPTION=3;"





For MyODBC 3.51:





ConnectionString = "DRIVER={MySQL ODBC 3.51 Driver};\
                    SERVER=localhost;\
                    DATABASE=test;\
                    USER=venu;\
                    PASSWORD=venu;\
                    OPTION=3;"





If your programming language converts backslash followed by whitespace to
a space, it is preferable to specify the connection string as a single long
string, or to use a concatenation of multiple strings that does not add
spaces in between.  For example:





ConnectionString = "DRIVER={MySQL ODBC 3.51 Driver};"
                    "SERVER=localhost;"
                    "DATABASE=test;"
                    "USER=venu;"
                    "PASSWORD=venu;"
                    "OPTION=3;"





Refer to the section 22.1.9.4  Connection Parameters, for the list of connection parameters that can be supplied.







22.1.9.6  Establishing a Remote Connection to System A from System B




If you want to connect to system A from system B with a username
and password of myuser and mypassword, here is a simple
procedure.





On system A, follow these steps:





    


  1. 

Start the MySQL server.


  2. 

Use
GRANT to set up an account with a username of myuser that can
connect from system B using a password of myuser:



    GRANT ALL ON *.* to 'myuser'@'B' IDENTIFIED BY 'mypassword';

    


  3. 

The GRANT statement grants all privileges to user `myuser' for
connecting
from system B using the password mypassword. To execute this statement,
you should be either root on system A (or another user who has
appropriate privileges). For more information about MySQL privileges, refer to
section 5.6  MySQL User Account Management.






On system B, follow these steps:





    


  1. 

Configure a MyODBC DSN using the following connection parameters:



    DSN            = remote_test
SERVER or HOST = A (or IP address of system A)
DATABASE       = test (The default database or an appropriate one)
USER           = myuser
PASSWORD       = mypassword

    

To set up a DSN-less connection, refer to section 22.1.9.5  Connecting Without a Predefined DSN.


  2. 

Check whether you are able to access system A from system B by
using ping or other means. If you are not able to reach system A, check
your network or Internet connections or contact your system administrator.


  3. 

Now, try to connect using DSN=remote_test. If it fails, trace the
MyODBC log, and take the further steps based on the error message
from the log. If you need further assistance, send a detailed mail message
to [email protected].






You can also find a simple HOWTO at
http://www.phphelp.com/tutorial/using-myodbc-to-connect-to-a-remote-database.html.







22.1.9.7  Getting an ODBC Trace File




If you encounter difficulties or problems with MyODBC, you should start
by making a log file from the ODBC Manager (the log you get when
requesting logs from ODBC ADMIN) and MyODBC.





To get an ODBC trace through Driver Manager, do the following:









22.1.9.8  Applications Tested with MyODBC




MyODBC has been tested with the following applications:








If you know of any other applications that work with MyODBC, please send
mail to [email protected] about them.







22.1.9.9  Programs Known to Work With MyODBC




Most programs should work with MyODBC, but for each of those listed
here, we have tested it ourselves or received confirmation from some user
that it works. Many of the descriptions provide workarounds for problems
that you might encounter.







Program


Comment



Access


To make Access work:


    

  • 

If you are using Access 2000, you should get and install the newest (version
2.6 or higher) Microsoft MDAC (Microsoft Data Access Components) from
http://www.microsoft.com/data/.  This will fix a bug in Access that
when you export data to MySQL, the table and column names aren't specified.
Another way to work around this bug is to upgrade to MyODBC 2.50.33 and
MySQL 3.23.x, which together provide a workaround for the problem.

You should also get and apply the Microsoft Jet 4.0 Service Pack 5 (SP5)
which can be found at
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q239114.
This will fix some cases where columns are marked as #DELETED#
in Access.

Note: If you are using MySQL 3.22, you must to apply the
MDAC patch and use MyODBC 2.50.32 or 2.50.34 and up to work around
this problem.

  • 

For all versions of Access, you should enable the MyODBC Return
matching rows option. For Access 2.0, you should additionally enable
the Simulate ODBC 1.0 option.

  • 

You should have a timestamp in all tables that you want to be able to update.
For maximum portability, don't use a length specification in the column
declaration.  That is, use TIMESTAMP, not TIMESTAMP(n),
n < 14.

  • 

You should have a primary key in the table. If not, new or updated rows
may show up as #DELETED#.

  • 

Use only DOUBLE float fields. Access fails when comparing with
single floats.  The symptom usually is that new or updated rows may show
up as #DELETED# or that you can't find or update rows.

  • 

If you are using MyODBC to link to a table that has a BIGINT
column, the results will be displayed as #DELETED. The
work around solution is:



      

    • 

Have one more dummy column with TIMESTAMP as the data type.

    • 

Select the Change BIGINT columns to INT option in the connection
dialog in ODBC DSN Administrator.

    • 

Delete the table link from Access and re-create it.

    

Old records still will display as #DELETED#, but newly
added/updated records will be displayed properly.

  • 

If you still get the error Another user has changed your data after
adding a TIMESTAMP column, the following trick may help you:

Don't use a table data sheet view. Instead, create a form with the
fields you want, and use that form data sheet view.  You should
set the DefaultValue property for the TIMESTAMP column to
NOW().  It may be a good idea to hide the TIMESTAMP column
from view so your users are not confused.

  • 

In some cases, Access may generate illegal SQL statements that
MySQL can't understand. You can fix this by selecting
"Query|SQLSpecific|Pass-Through" from the Access menu.

  • 

On NT, Access will report BLOB columns as OLE OBJECTS. If
you want to have MEMO columns instead, you should change BLOB
columns to TEXT with ALTER TABLE.

  • 

Access can't always handle DATE columns properly. If you have a problem
with these, change the columns to DATETIME.

  • 

If you have in Access a column defined as BYTE, Access will try
to export this as TINYINT instead of TINYINT UNSIGNED.
This will give you problems if you have values larger than 127 in the column.




ADO



When you are coding with the ADO API and MyODBC, you need to
pay attention to some default properties that aren't supported by the
MySQL server.  For example, using the CursorLocation Property
as adUseServer will return a result of -1 for the
RecordCount Property.  To have the right value, you need to set
this property to adUseClient, as shown in the VB code here:



Dim myconn As New ADODB.Connection
Dim myrs As New Recordset
Dim mySQL As String
Dim myrows As Long

myconn.Open "DSN=MyODBCsample"
mySQL = "SELECT * from user"
myrs.Source = mySQL
Set myrs.ActiveConnection = myconn
myrs.CursorLocation = adUseClient
myrs.Open
myrows = myrs.RecordCount

myrs.Close
myconn.Close



Another workaround is to use a SELECT COUNT(*) statement
for a similar query to get the correct row count.


Active server pages (ASP)


You should select the Return matching rows option.


BDE applications


To get these to work, you should select the
Don't optimize column widths and Return matching rows options.


Borland Builder 4



When you start a query, you can use the Active property or the
Open method.  Note that Active will start by automatically
issuing a SELECT * FROM ... query. That may not be a good thing if
your tables are large.


ColdFusion (On Unix)



The following information is taken from the ColdFusion documentation:

Use the following information to configure ColdFusion Server for Linux
to use the unixODBC driver with MyODBC for MySQL data
sources.  Allaire has verified that MyODBC 2.50.26
works with MySQL 3.22.27 and ColdFusion for Linux. (Any
newer version should also work.) You can download MyODBC at
http://dev.mysql.com/downloads/connector/odbc/.

ColdFusion Version 4.5.1 allows you to us the ColdFusion Administrator
to add the MySQL data source. However, the driver is not
included with ColdFusion Version 4.5.1. Before the MySQL driver
will appear in the ODBC datasources drop-down list, you must build and
copy the MyODBC driver to
`/opt/coldfusion/lib/libmyodbc.so'.

The Contrib directory contains the program `mydsn-xxx.zip' which allows
you to build and remove the DSN registry file for the MyODBC driver
on Coldfusion applications.


DataJunction



You have to change it to output VARCHAR rather than ENUM, as
it exports the latter in a manner that causes MySQL problems.


Excel



Works. A few tips:


    

  • 

If you have problems with dates, try to select them as strings using the
CONCAT() function. For example:


    SELECT CONCAT(rise_time), CONCAT(set_time)
    FROM sunrise_sunset;

    

Values retrieved as strings this way should be correctly recognized
as time values by Excel97.

The purpose of CONCAT() in this example is to fool ODBC into thinking
the column is of ``string type.'' Without the CONCAT(), ODBC knows the
column is of time type, and Excel does not understand that.

Note that this is a bug in Excel, because it automatically converts a
string to a time. This would be great if the source was a text file, but
is unfortunate when the source is an ODBC connection that reports
exact types for each column.




Word




To retrieve data from MySQL to Word/Excel documents, you need to
use the MyODBC driver and the Add-in Microsoft Query help.

For example, create a database with a table containing two columns of text:



    

  • 

Insert rows using the @command{mysql} client command-line tool.

  • 

Create a DSN file using the ODBC manager, for example, `my' for the
database that was just created.

  • 

Open the Word application.

  • 

Create a blank new document.

  • 

In the Database tool bar, press the Insert Database button.

  • 

Press the Get Data button.

  • 

At the right hand of the Get Data screen, press the Ms Query
button.

  • 

In Ms Query, create a new data source using the `my' DSN file.

  • 

Select the new query.

  • 

Select the columns that you want.

  • 

Make a filter if you want.

  • 

Make a Sort if you want.

  • 

Select Return Data to Microsoft Word.

  • 

Click Finish.

  • 

Click Insert Data and select the records.

  • 

Click OK and you see the rows in your Word document.




odbcadmin



Test program for ODBC.


Delphi



You must use BDE Version 3.2 or newer.  Select the Don't optimize column width
option when connecting to MySQL.

Also, here is some potentially useful Delphi code that sets up both an
ODBC entry and a BDE entry for MyODBC. The BDE entry requires a BDE
Alias Editor that is free at a Delphi Super Page near
you. (Thanks to Bryan Brunton [email protected] for this):



fReg:= TRegistry.Create;
  fReg.OpenKey('\Software\ODBC\ODBC.INI\DocumentsFab', True);
  fReg.WriteString('Database', 'Documents');
  fReg.WriteString('Description', ' ');
  fReg.WriteString('Driver', 'C:\WINNT\System32\myodbc.dll');
  fReg.WriteString('Flag', '1');
  fReg.WriteString('Password', '');
  fReg.WriteString('Port', ' ');
  fReg.WriteString('Server', 'xmark');
  fReg.WriteString('User', 'winuser');
  fReg.OpenKey('\Software\ODBC\ODBC.INI\ODBC Data Sources', True);
  fReg.WriteString('DocumentsFab', 'MySQL');
  fReg.CloseKey;
  fReg.Free;

  Memo1.Lines.Add('DATABASE NAME=');
  Memo1.Lines.Add('USER NAME=');
  Memo1.Lines.Add('ODBC DSN=DocumentsFab');
  Memo1.Lines.Add('OPEN MODE=READ/WRITE');
  Memo1.Lines.Add('BATCH COUNT=200');
  Memo1.Lines.Add('LANGDRIVER=');
  Memo1.Lines.Add('MAX ROWS=-1');
  Memo1.Lines.Add('SCHEMA CACHE DIR=');
  Memo1.Lines.Add('SCHEMA CACHE SIZE=8');
  Memo1.Lines.Add('SCHEMA CACHE TIME=-1');
  Memo1.Lines.Add('SQLPASSTHRU MODE=SHARED AUTOCOMMIT');
  Memo1.Lines.Add('SQLQRYMODE=');
  Memo1.Lines.Add('ENABLE SCHEMA CACHE=FALSE');
  Memo1.Lines.Add('ENABLE BCD=FALSE');
  Memo1.Lines.Add('ROWSET SIZE=20');
  Memo1.Lines.Add('BLOBS TO CACHE=64');
  Memo1.Lines.Add('BLOB SIZE=32');

  AliasEditor.Add('DocumentsFab','MySQL',Memo1.Lines);




C++ Builder



Tested with BDE Version 3.0. The only known problem is that when the table
schema changes, query fields are not updated. BDE, however, does not seem
to recognize primary keys, only the index named PRIMARY, though this
has not been a problem.


Vision


You should select the Return matching rows option.


Visual Basic



To be able to update a table, you must define a primary key for the table.

Visual Basic with ADO can't handle big integers. This means that some queries
like SHOW PROCESSLIST will not work properly.  The fix is to use
OPTION=16384 in the ODBC connect string or to select
the Change BIGINT columns to INT option in the MyODBC connect
screen.  You may also want to select the Return matching rows option.


VisualInterDev


If you have a BIGINT in your result, you may get the error
[Microsoft][ODBC Driver Manager] Driver does not support this parameter
Try selecting the Change BIGINT
columns to INT option in the MyODBC connect screen.


Visual Objects


You should select the Don't optimize column widths option.


MS Visio Enterprise 2000


We made database model diagram by connecting from MS Vision Enterprise 
2000 to MySQL via MyODBC (2.50.37 or greater) and using Visio's reverse 
engineer function to retrieve information about the DB (Visio shows all 
the column definitions, primary keys, Indexes and so on). Also we tested 
by designing new tables in Visio and exported them to MySQL via MyODBC.







22.1.10  MyODBC Connection-Related Issues




This section answers MyODBC connection-related questions.







22.1.10.1  While Configuring a MyODBC DSN, a Could Not Load Translator or Setup Library Error Occurs




For more information, refer to
MS KnowledgeBase
Article(Q260558). Also, make sure you have the latest valid
`ctl3d32.dll' in your system directory.







22.1.10.2  While Connecting, an Access denied Error Occurs




Refer to section 5.5.8  Causes of Access denied Errors.







22.1.10.3  INFO: About ODBC Connection Pooling




Refer to this document about connection pooling:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q169470.







22.1.11  MyODBC and Microsoft Access




This section of the document answers questions related to MyODBC
with Microsoft Access.







22.1.11.1  How to Set Up Microsoft Access to Work with MySQL using MyODBC?




The following must be done on your client PC in order to make Microsoft
Access work with MyODBC.





    


  1. 

If you are using Access 2000, you should get and install the newest (version
2.6 or higher) Microsoft MDAC (Microsoft Data Access Components) from
http://www.microsoft.com/data/.  This will fix a bug in Access that
when you export data to MySQL, the table and column names aren't specified.
Another way to work around this bug is to upgrade to MyODBC 2.50.33 and
MySQL 3.23.x, which together provide a workaround for the problem.

You should also get and apply the Microsoft Jet 4.0 Service Pack 5 (SP5)
which can be found at
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q239114.
This will fix some cases where columns are marked as #DELETED#
in Access.

Note: If you are using MySQL 3.22, you must to apply the
MDAC patch and use MyODBC 2.50.32 or 2.50.34 and up to work around
this problem.

  2. 

Install the latest version of MySQL from http://dev.mysql.com/downloads/.

  3. 

Install the latest version of MyODBC 3.51 or 2.50 from
http://dev.mysql.com/downloads/connector/odbc/.

  4. 

For all Access versions, you should enable the Return matching rows
option.

  5. 

Now start working with Access as the front-end for MySQL Server through MyODBC.







22.1.11.2  How to Export a Table or Query from Access to MySQL?




You cannot export a table or query to MySQL unless you have installed
MyODBC.





To export a table from Access to MySQL, follow these instructions:





    


  1. 

When you open an Access database or an Access project, a Database window
will appear.  It displays shortcuts for creating new database objects and
opening existing objects.


      
    


  2. 

Click the name of the table or query you want to export,
and then in the File menu, select Export.


  3. 

In the Export Object Type Object name To dialog box, in the
Save As Type box, select ODBC Databases () as shown here:


      
    


  4. 

In the Export dialog box, enter a name for the file (or use the
suggested name), and then select OK.


  5. 

The Select Data Source dialog box is displayed; it lists the defined data
sources for any ODBC drivers installed on your computer.  Click either the
File Data Source or Machine Data Source tab, and then double-click the
MyODBC or MyODBC 3.51 data source that you want to export to. To define
a new data source for MyODBC, please section 22.1.9.2  Configuring a MyODBC DSN on Windows.






Microsoft Access connects to the MySQL Server through this data source
and exports new tables and or data.







22.1.11.3  How to Import or Link MySQL Database Tables to Access?




You cannot export a table or query to MySQL database unless you have installed
the MyODBC.





To import or link a table(s) from MySQL to Access, follow the instructions:





    


  1. 

Open a database, or switch to the Database window for the open database.

  2. 

To import tables, on the File menu, point to Get External Data,
and then click Import.
To link tables, on the File menu, point to Get External Data, and
then click Link Tables.

  3. 

In the Import (or Link) dialog box, in the Files Of Type
box, select ODBC Databases ().
The Select Data Source dialog box lists the defined data sources The Select Data
Source dialog box is displayed; it lists the defined data sources for any ODBC
drivers installed on your computer.
Click either the File Data Source or Machine Data Source tab, and then double-click
the MyODBC or MyODBC 3.51 data source that you want to export to. To
define a new data source for the MyODBC or MyODBC 3.51 driver, please
section 22.1.9.2  Configuring a MyODBC DSN on Windows.

  4. 

If the ODBC data source that you selected requires you to log on, enter your login
ID and password (additional information might also be required), and then click
OK.

  5. 

Microsoft Access connects to the MySQL server through ODBC data source 
and displays the list of tables that you can import or link.

  6. 

Click each table that you want to import or link, and then click
OK. If you're linking a table and it doesn't have an index that uniquely
identifies each record, then Microsoft Access displays a list of the fields in
the linked table. Click a field or a combination of fields that will uniquely
identify each record, and then click OK.







22.1.11.4  The Structure or Location of a Linked Table has been Changed. Can I See Those Changes Locally in Linked Tables?




Yes. Use the following procedure to view or to refresh links when the
structure or location of a linked table has changed. The Linked Table Manager
lists the paths to all currently linked tables.





To wiew or refresh links:





    


  1. 

Open the database that contains links to tables.

  2. 

On the Tools menu, point to Add-ins, and then click Linked Table Manager.

  3. 

Select the check box for the tables whose links you want to refresh.

  4. 

Click OK to refresh the links.






Microsoft Access confirms a successful refresh or, if the table wasn't
found, displays the Select New Location of <table name> dialog box
in which you can specify its the table's new location.If several selected
tables have moved to the new location that you specify, the Linked Table
Manager searches that location for all selected tables, and updates all
links in one step.





To vhange the path for a set of linked tables:





    


  1. 

Open the database that contains links to tables.

  2. 

On the Tools menu, point to Add-ins, and then click Linked Table Manager.

  3. 

Select the Always Prompt For A New Location check box.

  4. 

Select the check box for the tables whose links you want to change, and then click OK.

  5. 

In the Select New Location of <table name> dialog box, specify the new location, click
Open, and then click OK.







22.1.11.5  When I Insert or Update a Record in Linked Tables, I Get #DELETED#




If the inserted or updated records are shown as #DELETED# in the access, then:









22.1.11.6  How Do I Handle Write Conflicts or Row Location Errors?




If you see the following errors, 
select the Return Matching Rows option in the DSN configuration
dialog, or specify OPTION=2, as the connection parameter:





Write Conflict. Another user has changed your data.

Row cannot be located for updating. Some values may have been changed
since it was last read.






22.1.11.7  Whenever I Export a Table from Access 97, a Strange Syntax Error Occurs




This is a strange issue from Access 97, and doesn't appear with Access 2000 or 2002.
You can overcome this by upgrading the MyODBC driver to at least MyODBC 3.51.02.







22.1.11.8  Access Returns Another user has modified the record that you have modified While Editing Records




With some programs, this error may occur:
Another user has modified the record that you have modified. In most
cases, this can be solved by doing one of the following things:








If these strategies don't help, you should start by making a log file from
the ODBC manager (the log you get when requesting logs from ODBCADMIN)
and a MyODBC log to help you figure out why things go wrong.
For instructions, see section 22.1.9.7  Getting an ODBC Trace File.







22.1.11.9  How to Trap ODBC Login Error Messages in Access?




Read ``How to Trap ODBC Login Error Messages in Access'' at
http://support.microsoft.com/support/kb/articles/Q124/9/01.asp?LN=EN-US&SD=gn&FR=0%3CP%3E.







22.1.11.10  How Do I Optimize Access for Performance with MyODBC?








22.1.11.11  I Have Very Long Tables. What is the Best Configuration for MyODBC to Access These Tables?




If you have very large (long) tables in Access, it might take a very long
time to open them. Or you might run low on virtual memory and eventually get
an ODBC Query Failed error and the table will not open.  To deal with
this, select the following options:








These add up to a value of 10 (OPTION=10).







22.1.11.12  How to Set the QueryTimeout Value for ODBC Connections?




Read ``Set the QueryTimeout Value for ODBC Connections'' at
http://support.microsoft.com/default.aspx?scid=kb%3Ben-us%3B153756.







22.1.11.13  INFO: Tools to Export/Import from/to Access to/from MySQL




Refer to converters section for list of available tools.







22.1.12  MyODBC and Microsoft VBA and ASP




This section answers questions related to using MyODBC
with Microsoft Visual Basic(ADO, DAO & RDO) and ASP.







22.1.12.1  Why Does SELECT COUNT(*) FROM tbl_name Return an Error?




It's because the COUNT(*) expression is returning a BIGINT,
and ADO can't make sense of a number this big.  Select the Change
BIGINT columns to INT option (option value 16384).







22.1.12.2  Whenever I Use the AppendChunk() or GetChunk() ADO Methods, I Get an Error Multiple-step operation generated errors. Check each status value.




The GetChunk() and AppendChunk() methods from ADO doesn't work as
expected when the cursor location is specified as adUseServer. On the other
hand, you can overcome this error by using adUseClient.





A simple example can be found from,  http://www.dwam.net/iishelp/ado/docs/adomth02_4.htm







22.1.12.3  How to Find the Total Number of Rows Affected by a Particular SQL Statement in ADO?




You can make use of RecordsAffected property in the ADO execute method. For more
information on the usage of execute method, refer to
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/ado270/htm/mdmthcnnexecute.asp.







22.1.12.4  How Do I Handle Blob Data in Visual Basic?




Here is an excellent article from Mike Hillyer
([email protected]); explaining how to insert
and/or fetch data from blob columns through MyODBC from ADO:
MySQL
BLOB columns and Visual Basic 6.







22.1.12.5  How Do I Map Visual Basic Data Types to MySQL Types?




Here is yet another good article from Mike Hillyer
([email protected]):
 How
to map Visual basic data type to MySQL types.







22.1.12.6  SAMPLES: VB with ADO, DAO and RDO




A simple examples for the usage of ADO, DAO and RDO with VB can be found her:








If you find any other good example or HOW-TO on ADO/DAO/RDO, then please send the details to [email protected]






22.1.12.7  ASP and MySQL with MyODBC




For more information about how to access MySQL via ASP using MyODBC,
refer to the following articles:








A Frequently Asked Questions list for ASP can be found at
http://support.microsoft.com/default.aspx?scid=/Support/ActiveServer/faq/data/adofaq.asp.







22.1.12.8  INFO: Frequently Asked Questions on ActiveX Data Objects (ADO)




For information, see

ActiveX Data Objects(ADO) Freqently Asked Questions.







22.1.13  MyODBC and Third-Party ODBC Tools




This section answers questions related to MyODBC
with various ODBC-related tools; such as Microsoft Word, Excel and
ColdFusion.







22.1.13.1  How to Retrieve Data from MySQL into MS-Word/Excel Documents?




To retrieve data from MySQL to Word/Excel documents, you need to
use the MyODBC driver and the Add-in Microsoft Query help.





For example, create a database with a table containing two columns of text:









22.1.13.2  Exporting Tables from MS DTS to MySQL Using MyODBC Results in a Syntax Error




This is an issue similar to that of Access 97 when your table consists
of TEXT or VARCHAR data types. You can fix this error by
upgrading your MyODBC driver to version 3.51.02 or higher.






22.1.13.3  HOWTO: Configure MySQL+MyODBC+unixODBC+ColdFusion on Solaris




Refer to

     MySQL ColdFusion unixODBC MyODBC and Solaris - how to succeed







22.1.14  MyODBC General Functionality




This section of the document answers questions related to MyODBC
general functionality.







22.1.14.1  How to Get the Value of an AUTO_INCREMENT Column in ODBC










A common problem is how to get the value of an automatically generated ID
from an INSERT statement. With ODBC, you can do something like this (assuming
that auto is an AUTO_INCREMENT field):





INSERT INTO tbl (auto,text) VALUES(NULL,'text');
SELECT LAST_INSERT_ID();





Or, if you are just going to insert the ID into another table, you can do this:





INSERT INTO tbl (auto,text) VALUES(NULL,'text');
INSERT INTO tbl2 (id,text) VALUES(LAST_INSERT_ID(),'text');





See section 21.2.12.3  How to Get the Unique ID for the Last Inserted Row.





For the benefit of some ODBC applications (at least Delphi and Access),
the following query can be used to find a newly inserted row:


SELECT * FROM tbl WHERE auto IS NULL;






22.1.14.2  Does MyODBC Support Dynamic Cursor Type?




Yes. MyODBC 3.51 supports Dynamic cursor type along with
Forward-only and static.





Due to the performance issues, the driver does not support this feature by
default. You can enable this by specifying the connection option flag as
OPTION=32 or by checking the Enable Dynamic Cursor option
from the DSN configuration.







22.1.14.3  What Causes Transactions are not enabled Errors?




The driver returns this error when an application issues any transactional
call but the underlying MySQL server either does not support transactions
or they are not enabled.





To avoid this problem, you must use a server that has either or both of the
InnoDB or BDB storage engines enabled, and use tables of those
types.  MySQL servers from version 4.0 and up support InnoDB by
default.  MySQL-Max servers also support BDB on platforms where
BDB is available.





Also, if your server supports transactional table types (InnoDB
and BDB)
make sure the disable transactions option is not set from the DSN
configuration.







22.1.14.4  What Causes Cursor not found Errors?




This is becuase the application is using old MyODBC 2.50 version, and
it did not set the cursor name explicitly through SQLSetCursorName. The fix
is to upgrade to MyODBC 3.51 version.







22.1.14.5  Can I Use MyODBC 2.50 Applications with MyODBC 3.51?




Yes. If you find something is not working with MyODBC 3.51 that works with
MyODBC 2.50, then send a mail message to [email protected]






22.1.14.6  Can I Access MySQL from .NET Environment Using MyODBC?




Yes. You can make use of odbc.net to connect to MySQL through
MyODBC. Here are the few basic samples to connect to MySQL from
VC.NET and VB.NET.






Here is yet another excellent article
"Exploring
MySQL on .NET environment" by Venu (MyODBC developer)
that covers about all MySQL .NET interfaces along with
some useful examples.





Caution: Using ODBC.NET with MyODBC, while fetching empty string
(0 length), it starts giving the SQL_NO_DATA exception. You can get the
patch for this from http://support.microsoft.com/default.aspx?scid=kb;EN-US;q319243.







22.1.14.7  Why Does MyODBC Perform Poorly, and Also Make a Lot of Disk Activity for Relatively Small Queries?




MyODBC is a lot faster than any other ODBC driver. Slowness might be
due to not using the following options.









22.1.15  Basic MyODBC Application Steps




Interacting with a MySQL server from MyODBC applications involves the
following operations:








Most applications use some variation of these steps. The basic application 
steps are shown in the following diagram:






  








22.1.16  MyODBC API Reference




This section summarizes ODBC routines, categorized by functionality.





For the complete ODBC API reference, please refer to the ODBC Programer's
Reference at
http://msdn.microsoft.com/library/en-us/odbc/htm/odbcabout_this_manual.asp.





An application can call SQLGetInfo function to obtain conformance 
information about MyODBC. To obtain information about support for a specific 
function in the driver, an application can call SQLGetFunctions.
  
Note: For backward compatibility, the MyODBC 3.51 driver supports all
deprecated functions.
  
The following tables list MyODBC API calls grouped by task:





Connecting to a data source:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   




SQLAllocHandle  No  Yes  ISO 92  Obtains an environment, connection, statement, or descriptor handle.



SQLConnect  Yes  Yes  ISO 92  Connects to a specific driver by data source name, user ID, and password.



SQLDriverConnect  Yes  Yes  ODBC  Connects to a specific driver by connection string or requests that the Driver Manager and driver display connection dialog boxes for the user.



SQLAllocEnv  Yes  Yes  Deprecated  Obtains an environment handle allocated from driver.



SQLAllocConnect  Yes  Yes  Deprecated  Obtains a connection handle







Obtaining information about a driver and data source:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   




SQLDataSources  No  No  ISO 92  Returns the list of available data sources, handled by the Driver Manager



SQLDrivers  No  No  ODBC  Returns the list of installed drivers and their attributes, handles by Driver Manager



SQLGetInfo  Yes  Yes  ISO 92  Returns information about a specific driver and data source.



SQLGetFunctions  Yes  Yes  ISO 92  Returns supported driver functions.



SQLGetTypeInfo  Yes  Yes  ISO 92  Returns information about supported data types.







Setting and retrieving driver attributes:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   




SQLSetConnectAttr  No  Yes  ISO 92  Sets a connection attribute.



SQLGetConnectAttr  No  Yes  ISO 92  Returns the value of a connection attribute.



SQLSetConnectOption  Yes  Yes  Deprecated  Sets a connection option



SQLGetConnectOption  Yes  Yes  Deprecated  Returns the value of a connection option



SQLSetEnvAttr  No  Yes  ISO 92  Sets an environment attribute.



SQLGetEnvAttr  No  Yes  ISO 92  Returns the value of an environment attribute.



SQLSetStmtAttr  No  Yes  ISO 92  Sets a statement attribute.



SQLGetStmtAttr  No  Yes  ISO 92  Returns the value of a statement attribute.



SQLSetStmtOption  Yes  Yes  Deprecated  Sets a statement option



SQLGetStmtOption  Yes  Yes  Deprecated  Returns the value of a statement option







Preparing SQL requests:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   




SQLAllocStmt  Yes  Yes  Deprecated  Allocates a statement handle



SQLPrepare  Yes  Yes  ISO 92  Prepares an SQL statement for later execution.



SQLBindParameter  Yes  Yes  ODBC  Assigns storage for a parameter in an SQL statement.



SQLGetCursorName  Yes  Yes  ISO 92  Returns the cursor name associated with a statement handle.



SQLSetCursorName  Yes  Yes  ISO 92  Specifies a cursor name.



SQLSetScrollOptions  Yes  Yes  ODBC  Sets options that control cursor behavior.







Submitting requests:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   
 



SQLExecute  Yes  Yes  ISO 92  Executes a prepared statement.



SQLExecDirect  Yes  Yes  ISO 92  Executes a statement



SQLNativeSql  Yes  Yes  ODBC  Returns the text of an SQL statement as translated by the driver.



SQLDescribeParam  Yes  Yes  ODBC  Returns the description for a specific parameter in a statement.



SQLNumParams  Yes  Yes  ISO 92  Returns the number of parameters in a statement.



SQLParamData  Yes  Yes  ISO 92  Used in conjunction with SQLPutData to supply parameter data at execution time. (Useful for long data values.)



SQLPutData  Yes  Yes  ISO 92  Sends part or all of a data value for a parameter. (Useful for long data values.)







Retrieving results and information about results:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   




SQLRowCount  Yes  Yes  ISO 92  Returns the number of rows affected by an insert, update, or delete request.



SQLNumResultCols  Yes  Yes  ISO 92  Returns the number of columns in the result set.



SQLDescribeCol  Yes  Yes  ISO 92  Describes a column in the result set.



SQLColAttribute  No  Yes  ISO 92  Describes attributes of a column in the result set.



SQLColAttributes  Yes  Yes  Deprecated  Describes attributes of a column in the result set.



SQLFetch  Yes  Yes  ISO 92  Returns multiple result rows.



SQLFetchScroll  No  Yes  ISO 92  Returns scrollable result rows.



SQLExtendedFetch  Yes  Yes  Deprecated  Returns scrollable result rows.



SQLSetPos  Yes  Yes  ODBC  Positions a cursor within a fetched block of data and allows an application to refresh data in the rowset or to update or delete data in the result set.



SQLBulkOperations  No  Yes  ODBC  Performs bulk insertions and bulk bookmark operations, including update, delete, and fetch by bookmark.







Retrieving error or diagnostic information:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   




SQLError  Yes  Yes  Deprecated  Returns additional error or status information



SQLGetDiagField  Yes  Yes  ISO 92  Returns additional diagnostic information (a single field of the diagnostic data structure).



SQLGetDiagRec  Yes  Yes  ISO 92  Returns additional diagnostic information (multiple fields of the diagnostic data structure).







Obtaining information about the data source's system tables (catalog
functions) item:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   




SQLColumnPrivileges  Yes  Yes  ODBC  Returns a list of columns and associated privileges for one or more tables.



SQLColumns  Yes  Yes  X/Open  Returns the list of column names in specified tables.



SQLForeignKeys  Yes  Yes  ODBC  Returns a list of column names that make up foreign keys, if they exist for a specified table.



SQLPrimaryKeys  Yes  Yes  ODBC  Returns the list of column names that make up the primary key for a table.



SQLSpecialColumns  Yes  Yes  X/Open  Returns information about the optimal set of columns that uniquely identifies a row in a specified table, or the columns that are automatically updated when any value in the row is updated by a transaction.



SQLStatistics  Yes  Yes  ISO 92  Returns statistics about a single table and the list of indexes associated with the table.



SQLTablePrivileges  Yes  Yes  ODBC  Returns a list of tables and the privileges associated with each table.



SQLTables  Yes  Yes  X/Open  Returns the list of table names stored in a specific data source.







Performing transactions:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   




SQLTransact  Yes  Yes  Deprecated  Commits or rolls back a transaction



SQLEndTran  No  Yes  ISO 92  Commits or rolls back a transaction.







Terminating a statement:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   




SQLFreeStmt  Yes  Yes  ISO 92  Ends statement processing, discards pending results, and, optionally, frees all resources associated with the statement handle.



SQLCloseCursor  Yes  Yes  ISO 92  Closes a cursor that has been opened on a statement handle.



SQLCancel  Yes  Yes  ISO 92  Cancels an SQL statement.







Terminating a connection:






Function name  MyODBC  MyODBC  Conformance  Purpose



 2.50  3.51   




SQLDisconnect  Yes  Yes  ISO 92  Closes the connection.



SQLFreeHandle  No  Yes  ISO 92  Releases an environment, connection, statement, or descriptor handle.



SQLFreeConnect  Yes  Yes  Deprecated  Releases connection handle



SQLFreeEnv  Yes  Yes  Deprecated  Releases an environment handle








22.1.17  MyODBC Data Types




The following table illustrates how driver maps the server data types to default 
SQL and C data types:






Native Value  SQL Type  C Type



bit  SQL_BIT  SQL_C_BIT



tinyint  SQL_TINYINT  SQL_C_STINYINT



tinyint unsigned  SQL_TINYINT  SQL_C_UTINYINT



bigint  SQL_BIGINT  SQL_C_SBIGINT



bigint unsigned  SQL_BIGINT  SQL_C_UBIGINT



long varbinary  SQL_LONGVARBINARY  SQL_C_BINARY



blob  SQL_VARBINARY  SQL_C_BINARY



longblob  SQL_VARBINARY  SQL_C_BINARY



tinyblob  SQL_BINARY  SQL_C_BINARY



mediumblob  SQL_LONGVARBINARY  SQL_C_BINARY



long varchar  SQL_LONGVARCHAR  SQL_C_CHAR



text  SQL_LONGVARCHAR  SQL_C_CHAR



mediumtext  SQL_LONGVARCHAR  SQL_C_CHAR



char  SQL_CHAR  SQL_C_CHAR



numeric  SQL_NUMERIC  SQL_C_CHAR



decimal  SQL_DECIMAL  SQL_C_CHAR



integer  SQL_INTEGER  SQL_C_SLONG



integer unsigned  SQL_INTEGER  SQL_C_ULONG



int  SQL_INTEGER  SQL_C_SLONG



int unsigned  SQL_INTEGER  SQL_C_ULONG



mediumint  SQL_INTEGER  SQL_C_SLONG



mediumint unsigned  SQL_INTEGER  SQL_C_ULONG



smallint  SQL_SMALLINT  SQL_C_SSHORT



smallint unsigned  SQL_SMALLINT  SQL_C_USHORT



real  SQL_FLOAT  SQL_C_DOUBLE



double  SQL_FLOAT  SQL_C_DOUBLE



float  SQL_REAL  SQL_C_FLOAT



double precision  SQL_DOUBLE  SQL_C_DOUBLE



date  SQL_DATE  SQL_C_DATE



time  SQL_TIME  SQL_C_TIME



year  SQL_SMALLINT  SQL_C_SHORT



datetime  SQL_TIMESTAMP  SQL_C_TIMESTAMP



timestamp  SQL_TIMESTAMP  SQL_C_TIMESTAMP



text  SQL_VARCHAR  SQL_C_CHAR



varchar  SQL_VARCHAR  SQL_C_CHAR



enum  SQL_VARCHAR  SQL_C_CHAR



set  SQL_VARCHAR  SQL_C_CHAR



bit  SQL_CHAR  SQL_C_CHAR



bool  SQL_CHAR  SQL_C_CHAR








22.1.18  MyODBC Error Codes




The following tables lists the error codes returned by the driver apart 
from the server errors.






Native Code  SQLSTATE 2  SQLSTATE 3  Error Message



500  01000  01000  General warning



501  01004  01004  String data, right truncated



502  01S02  01S02  Option value changed



503  01S03  01S03  No rows updated/deleted



504  01S04  01S04  More than one row updated/deleted



505  01S06  01S06  Attempt to fetch before the result set returned the first row set



506  07001  07002  SQLBindParameter not used for all parameters



507  07005  07005  Prepared statement not a cursor-specification



508  07009  07009  Invalid descriptor index



509  08002  08002  Connection name in use



510  08003  08003  Connection does not exist



511  24000  24000  Invalid cursor state



512  25000  25000  Invalid transaction state



513  25S01  25S01  Transaction state unknown



514  34000  34000  Invalid cursor name



515  S1000  HY000  General driver defined error



516  S1001  HY001  Memory allocation error



517  S1002  HY002  Invalid column number



518  S1003  HY003  Invalid application buffer type



519  S1004  HY004  Invalid SQL data type



520  S1009  HY009  Invalid use of null pointer



521  S1010  HY010  Function sequence error



522  S1011  HY011  Attribute can not be set now



523  S1012  HY012  Invalid transaction operation code



524  S1013  HY013  Memory management error



525  S1015  HY015  No cursor name available



526  S1024  HY024  Invalid attribute value



527  S1090  HY090  Invalid string or buffer length



528  S1091  HY091  Invalid descriptor field identifier



529  S1092  HY092  Invalid attribute/option identifier



530  S1093  HY093  Invalid parameter number



531  S1095  HY095  Function type out of range



532  S1106  HY106  Fetch type out of range



533  S1117  HY117  Row value out of range



534  S1109  HY109  Invalid cursor position



535  S1C00  HYC00  Optional feature not implemented



0  21S01  21S01  Column count does not match value count



0  23000  23000  Integrity constraint violation



0  42000  42000  Syntax error or access violation



0  42S02  42S02  Base table or view not found



0  42S12  42S12  Index not found



0  42S21  42S21  Column already exists



0  42S22  42S22  Column not found



0  08S01  08S01  Communication link failure








22.1.19  MyODBC With VB: ADO, DAO and RDO




This section contains simple examples of the use of MySQL ODBC 3.51 Driver
with ADO, DAO and RDO.







22.1.19.1  ADO: rs.addNew, rs.delete, and rs.update




The following ADO (ActiveX Data Objects) example creates a table
my_ado and demonstrates the use of rs.addNew, rs.delete,
and rs.update.





Private Sub myodbc_ado_Click()
   
  Dim conn As ADODB.Connection
  Dim rs As ADODB.Recordset
  Dim fld As ADODB.Field
  Dim sql As String

  'connect to MySQL server using MySQL ODBC 3.51 Driver
  Set conn = New ADODB.Connection
  conn.ConnectionString = "DRIVER={MySQL ODBC 3.51 Driver};"_
                        & "SERVER=localhost;"_ 
                        & " DATABASE=test;"_
                        & "UID=venu;PWD=venu; OPTION=3"

  conn.Open

  'create table
  conn.Execute "DROP TABLE IF EXISTS my_ado"
  conn.Execute "CREATE TABLE my_ado(id int not null primary key, name varchar(20)," _
                                 & "txt text, dt date, tm time, ts timestamp)"

  'direct insert
  conn.Execute "INSERT INTO my_ado(id,name,txt) values(1,100,'venu')"
  conn.Execute "INSERT INTO my_ado(id,name,txt) values(2,200,'MySQL')"
  conn.Execute "INSERT INTO my_ado(id,name,txt) values(3,300,'Delete')"

  Set rs = New ADODB.Recordset
  rs.CursorLocation = adUseServer

  'fetch the initial table ..
  rs.Open "SELECT * FROM my_ado", conn
    Debug.Print rs.RecordCount
    rs.MoveFirst
    Debug.Print String(50, "-") & "Initial my_ado Result Set " & String(50, "-")
    For Each fld In rs.Fields
      Debug.Print fld.Name,
      Next
      Debug.Print

      Do Until rs.EOF
      For Each fld In rs.Fields
      Debug.Print fld.Value,
      Next
      rs.MoveNext
      Debug.Print
    Loop
  rs.Close

  'rs insert
  rs.Open "select * from my_ado", conn, adOpenDynamic, adLockOptimistic
  rs.AddNew
  rs!Name = "Monty"
  rs!txt = "Insert row"
  rs.Update
  rs.Close

  'rs update
  rs.Open "SELECT * FROM my_ado"
  rs!Name = "update"
  rs!txt = "updated-row"
  rs.Update
  rs.Close

  'rs update second time..
  rs.Open "SELECT * FROM my_ado"
  rs!Name = "update"
  rs!txt = "updated-second-time"
  rs.Update
  rs.Close

  'rs delete
  rs.Open "SELECT * FROM my_ado"
  rs.MoveNext
  rs.MoveNext
  rs.Delete
  rs.Close

  'fetch the updated table ..
  rs.Open "SELECT * FROM my_ado", conn
    Debug.Print rs.RecordCount
    rs.MoveFirst
    Debug.Print String(50, "-") & "Updated my_ado Result Set " & String(50, "-")
    For Each fld In rs.Fields
      Debug.Print fld.Name,
      Next
      Debug.Print

      Do Until rs.EOF
      For Each fld In rs.Fields
      Debug.Print fld.Value,
      Next
      rs.MoveNext
      Debug.Print
    Loop
  rs.Close
  conn.Close
End Sub






22.1.19.2  DAO: rs.addNew, rs.update, and Scrolling




The following DAO (Data Access Objects) example creates a table my_dao
and demonstrates the use of rs.addNew, rs.update, and result
set scrolling.





Private Sub myodbc_dao_Click()

  Dim ws As Workspace
  Dim conn As Connection
  Dim queryDef As queryDef
  Dim str As String

  'connect to MySQL using MySQL ODBC 3.51 Driver
  Set ws = DBEngine.CreateWorkspace("", "venu", "venu", dbUseODBC)
  str = "odbc;DRIVER={MySQL ODBC 3.51 Driver};"_
                        & "SERVER=localhost;"_ 
                        & " DATABASE=test;"_
                        & "UID=venu;PWD=venu; OPTION=3"
  Set conn = ws.OpenConnection("test", dbDriverNoPrompt, False, str)

  'Create table my_dao
  Set queryDef = conn.CreateQueryDef("", "drop table if exists my_dao")
  queryDef.Execute

  Set queryDef = conn.CreateQueryDef("", "create table my_dao(Id INT AUTO_INCREMENT PRIMARY KEY, " _
                                                           & "Ts TIMESTAMP(14) NOT NULL, Name varchar(20), Id2 INT)")
  queryDef.Execute

  'Insert new records using rs.addNew
  Set rs = conn.OpenRecordset("my_dao")
  Dim i As Integer

    For i = 10 To 15
    rs.AddNew
    rs!Name = "insert record" & i
    rs!Id2 = i
    rs.Update
    Next i
             rs.Close

  'rs update..
  Set rs = conn.OpenRecordset("my_dao")
  rs.Edit
  rs!Name = "updated-string"
  rs.Update
  rs.Close

  'fetch the table back...
  Set rs = conn.OpenRecordset("my_dao", dbOpenDynamic)
  str = "Results:"
  rs.MoveFirst
  While Not rs.EOF
  str = " " & rs!Id & " , " & rs!Name & ", " & rs!Ts & ", " & rs!Id2
  Debug.Print "DATA:" & str
  rs.MoveNext
  Wend

  'rs Scrolling
  rs.MoveFirst
  str = " FIRST ROW: " & rs!Id & " , " & rs!Name & ", " & rs!Ts & ", " & rs!Id2
  Debug.Print str

  rs.MoveLast
  str = " LAST ROW: " & rs!Id & " , " & rs!Name & ", " & rs!Ts & ", " & rs!Id2
  Debug.Print str

  rs.MovePrevious
  str = " LAST-1 ROW: " & rs!Id & " , " & rs!Name & ", " & rs!Ts & ", " & rs!Id2
  Debug.Print str

  'free all resources
  rs.Close
  queryDef.Close
  conn.Close
  ws.Close
   
End Sub






22.1.19.3  RDO: rs.addNew and rs.update




The following RDO (Remote Data Objects) example creates
a table my_rdo and demonstrates the use of rs.addNew
and rs.update.





Dim rs As rdoResultset
    Dim cn As New rdoConnection
    Dim cl As rdoColumn
    Dim SQL As String

    'cn.Connect = "DSN=test;"
    cn.Connect = "DRIVER={MySQL ODBC 3.51 Driver};"_
                        & "SERVER=localhost;"_ 
                        & " DATABASE=test;"_
                        & "UID=venu;PWD=venu; OPTION=3"

    cn.CursorDriver = rdUseOdbc
    cn.EstablishConnection rdDriverPrompt

    'drop table my_rdo
    SQL = "drop table if exists my_rdo"
    cn.Execute SQL, rdExecDirect

    'create table my_rdo
    SQL = "create table my_rdo(id int, name varchar(20))"
    cn.Execute SQL, rdExecDirect

    'insert - direct
    SQL = "insert into my_rdo values (100,'venu')"
    cn.Execute SQL, rdExecDirect

    SQL = "insert into my_rdo values (200,'MySQL')"
    cn.Execute SQL, rdExecDirect

    'rs insert
    SQL = "select * from my_rdo"
    Set rs = cn.OpenResultset(SQL, rdOpenStatic, rdConcurRowVer, rdExecDirect)
    rs.AddNew
    rs!id = 300
    rs!Name = "Insert1"
    rs.Update
    rs.Close

    'rs insert
    SQL = "select * from my_rdo"
    Set rs = cn.OpenResultset(SQL, rdOpenStatic, rdConcurRowVer, rdExecDirect)
    rs.AddNew
    rs!id = 400
    rs!Name = "Insert 2"
    rs.Update
    rs.Close

    'rs update
    SQL = "select * from my_rdo"
    Set rs = cn.OpenResultset(SQL, rdOpenStatic, rdConcurRowVer, rdExecDirect)
    rs.Edit
    rs!id = 999
    rs!Name = "updated"
    rs.Update
    rs.Close 

    'fetch back...
    SQL = "select * from my_rdo"
    Set rs = cn.OpenResultset(SQL, rdOpenStatic, rdConcurRowVer, rdExecDirect)
    Do Until rs.EOF
    For Each cl In rs.rdoColumns
                Debug.Print cl.Value,
      Next
      rs.MoveNext
      Debug.Print
               Loop
    Debug.Print "Row count="; rs.RowCount

    'close
    rs.Close
    cn.Close
   
End Sub






22.1.20  MyODBC with Microsoft .NET




This section contains simple examples that demonstrate the use 
of MyODBC drivers with ODBC.NET.







22.1.20.1  ODBC.NET: CSHARP(C#)




The following sample creates a table my_odbc_net and 
demonstrates the use in C#.






 /**
 * @sample    : mycon.cs
 * @purpose   : Demo sample for ODBC.NET using MyODBC
 * @author    : Venu, [email protected]
 *
 * (C) Copyright MySQL AB, 1995-2004
 *
 **/

 /* build command
  * 
  *  csc /t:exe 
  *      /out:mycon.exe mycon.cs 
  *      /r:Microsoft.Data.Odbc.dll 
  */ 

 using Console = System.Console;
 using Microsoft.Data.Odbc;

 namespace myodbc3
 {
  class mycon
  {
    static void Main(string[] args)
    {
      try
      { 
        //Connection string for MyODBC 2.50
        /*string MyConString = "DRIVER={MySQL};" + 
                             "SERVER=localhost;" +
                             "DATABASE=test;" +
                             "UID=venu;" +
                             "PASSWORD=venu;" +
                             "OPTION=3";
        */
        //Connection string for MyODBC 3.51
        string MyConString = "DRIVER={MySQL ODBC 3.51 Driver};" + 
                             "SERVER=localhost;" +
                             "DATABASE=test;" +
                             "UID=venu;" +
                             "PASSWORD=venu;" +
                             "OPTION=3";
                    
        //Connect to MySQL using MyODBC
        OdbcConnection MyConnection = new OdbcConnection(MyConString);    
        MyConnection.Open();
  
        Console.WriteLine("\n !!! success, connected successfully !!!\n");    

        //Display connection information
        Console.WriteLine("Connection Information:");   
        Console.WriteLine("\tConnection String:" + MyConnection.ConnectionString);    
        Console.WriteLine("\tConnection Timeout:" + MyConnection.ConnectionTimeout);    
        Console.WriteLine("\tDatabase:" + MyConnection.Database);   
        Console.WriteLine("\tDataSource:" + MyConnection.DataSource);
        Console.WriteLine("\tDriver:" + MyConnection.Driver);
        Console.WriteLine("\tServerVersion:" + MyConnection.ServerVersion);

        //Create a sample table
        OdbcCommand MyCommand = new OdbcCommand("DROP TABLE IF EXISTS my_odbc_net",MyConnection);
        MyCommand.ExecuteNonQuery();
        MyCommand.CommandText = "CREATE TABLE my_odbc_net(id int, name varchar(20), idb bigint)";
        MyCommand.ExecuteNonQuery();

        //Insert
        MyCommand.CommandText = "INSERT INTO my_odbc_net VALUES(10,'venu', 300)";        
        Console.WriteLine("INSERT, Total rows affected:" + MyCommand.ExecuteNonQuery());;

        //Insert
        MyCommand.CommandText = "INSERT INTO my_odbc_net VALUES(20,'mysql',400)";        
        Console.WriteLine("INSERT, Total rows affected:" + MyCommand.ExecuteNonQuery());

        //Insert
        MyCommand.CommandText = "INSERT INTO my_odbc_net VALUES(20,'mysql',500)";        
        Console.WriteLine("INSERT, Total rows affected:" + MyCommand.ExecuteNonQuery());

        //Update
        MyCommand.CommandText = "UPDATE my_odbc_net SET id=999 WHERE id=20";        
        Console.WriteLine("Update, Total rows affected:" + MyCommand.ExecuteNonQuery());

        //COUNT(*)        
        MyCommand.CommandText = "SELECT COUNT(*) as TRows FROM my_odbc_net";        
        Console.WriteLine("Total Rows:" + MyCommand.ExecuteScalar());

        //Fetch
        MyCommand.CommandText = "SELECT * FROM my_odbc_net";                
        OdbcDataReader MyDataReader;
        MyDataReader =  MyCommand.ExecuteReader();
        while (MyDataReader.Read())
        {
         if(string.Compare(MyConnection.Driver,"myodbc3.dll") == 0) {
           Console.WriteLine("Data:" + MyDataReader.GetInt32(0) + " " +
                                       MyDataReader.GetString(1) + " " +
                                       MyDataReader.GetInt64(2)); //Supported only by MyODBC 3.51
         }
         else {            
           Console.WriteLine("Data:" + MyDataReader.GetInt32(0) + " " +
                                       MyDataReader.GetString(1) + " " +                                         
                                       MyDataReader.GetInt32(2)); //BIGINTs not supported by MyODBC
         }
        }

        //Close all resources
        MyDataReader.Close();
        MyConnection.Close();
      }
      catch (OdbcException MyOdbcException)//Catch any ODBC exception ..
      {
        for (int i=0; i < MyOdbcException.Errors.Count; i++)
        {
          Console.Write("ERROR #" + i + "\n" +
            "Message: " + MyOdbcException.Errors[i].Message + "\n" +
            "Native: " + MyOdbcException.Errors[i].NativeError.ToString() + "\n" +
            "Source: " + MyOdbcException.Errors[i].Source + "\n" +
            "SQL: " + MyOdbcException.Errors[i].SQLState + "\n");
        }
      }
    }
  }
 }






22.1.20.2  ODBC.NET: VB




The following sample creates a table my_vb_net and 
demonstrates the use in VB.





' @sample    : myvb.vb
' @purpose   : Demo sample for ODBC.NET using MyODBC
' @author    : Venu, [email protected]
'
' (C) Copyright MySQL AB, 1995-2004
'
'

' 
' build command
'
' vbc /target:exe 
'     /out:myvb.exe 
'     /r:Microsoft.Data.Odbc.dll 
'     /r:System.dll 
'     /r:System.Data.dll 
'
 
Imports Microsoft.Data.Odbc
Imports System

Module myvb
    Sub Main()
        Try

            'MyODBC 3.51 connection string
            Dim MyConString As String = "DRIVER={MySQL ODBC 3.51 Driver};" & _
                           "SERVER=localhost;" & _
                           "DATABASE=test;" & _
                           "UID=venu;" & _
                           "PASSWORD=venu;" & _
                           "OPTION=3;"

            'Connection
            Dim MyConnection As New OdbcConnection(MyConString)
            MyConnection.Open()

            Console.WriteLine ("Connection State::" & MyConnection.State.ToString)
            
            'Drop
            Console.WriteLine ("Dropping table")
            Dim MyCommand As New OdbcCommand()
            MyCommand.Connection = MyConnection
            MyCommand.CommandText = "DROP TABLE IF EXISTS my_vb_net"
            MyCommand.ExecuteNonQuery()
            
            'Create
            Console.WriteLine ("Creating....")
            MyCommand.CommandText = "CREATE TABLE my_vb_net(id int, name varchar(30))"
            MyCommand.ExecuteNonQuery()
            
            'Insert
            MyCommand.CommandText = "INSERT INTO my_vb_net VALUES(10,'venu')"    
            Console.WriteLine("INSERT, Total rows affected:" & MyCommand.ExecuteNonQuery())

            'Insert
            MyCommand.CommandText = "INSERT INTO my_vb_net VALUES(20,'mysql')"
            Console.WriteLine("INSERT, Total rows affected:" & MyCommand.ExecuteNonQuery())

            'Insert
            MyCommand.CommandText = "INSERT INTO my_vb_net VALUES(20,'mysql')"
            Console.WriteLine("INSERT, Total rows affected:" & MyCommand.ExecuteNonQuery())

            'Insert
            MyCommand.CommandText = "INSERT INTO my_vb_net(id) VALUES(30)"
            Console.WriteLine("INSERT, Total rows affected:" & MyCommand.ExecuteNonQuery())

            'Update
            MyCommand.CommandText = "UPDATE my_vb_net SET id=999 WHERE id=20"
            Console.WriteLine("Update, Total rows affected:" & MyCommand.ExecuteNonQuery())

            'COUNT(*)        
            MyCommand.CommandText = "SELECT COUNT(*) as TRows FROM my_vb_net"
            Console.WriteLine("Total Rows:" & MyCommand.ExecuteScalar())

            'Select              
            Console.WriteLine ("Select * FROM my_vb_net")
            MyCommand.CommandText = "SELECT * FROM my_vb_net"
            Dim MyDataReader As OdbcDataReader
            MyDataReader = MyCommand.ExecuteReader
            While MyDataReader.Read
                If MyDataReader("name") Is DBNull.Value Then
                    Console.WriteLine ("id = " & CStr(MyDataReader("id")) & "  name = " & _
                      "NULL")
                Else
                    Console.WriteLine ("id = " & CStr(MyDataReader("id")) & "  name = " & _
                                          CStr(MyDataReader("name")))
                End If
            End While
        
        'Catch ODBC Exception
        Catch MyOdbcException As OdbcException 
            Dim i As Integer
            Console.WriteLine (MyOdbcException.ToString)

        'Catch program exception
        Catch MyException As Exception
            Console.WriteLine (MyException.ToString)
    End Try
    End Sub
End Module






22.1.21  Credits




These are the developers that have worked on the MyODBC 
and MyODBC 3.51 Drivers from MySQL AB.









22.2  MySQL Java Connectivity (JDBC)












There are two supported JDBC drivers for MySQL:








For more information, consult any general JDBC documentation, plus each
driver's own documentation for MySQL-specific features.





Documentation for MySQL Connector/J is available online at the
MySQL AB Web site at http://dev.mysql.com/doc/.







23  Error Handling in MySQL




This chapter lists the errors that may appear when you call MySQL from
any host language.  The first list displays server error messages.
The second list displays client program messages.





Server error information comes from the following files:








Because updates are frequent, it is possible that
these files will contain additional error information not listed here.









Client error information comes from the following files:








Because updates are frequent, it is possible that
these files will contain additional error information not listed here.










24  Extending MySQL





24.1  MySQL Internals











This chapter describes a lot of things that you need to know when
working on the MySQL code. If you plan to contribute to MySQL
development, want to have access to the bleeding-edge in-between
versions code, or just want to keep track of development, follow the
instructions in section 2.3.3  Installing from the Development Source Tree.
If you are interested in MySQL internals, you should also subscribe
to our internals mailing list. This list is relatively low
traffic. For details on how to subscribe, please see
section 1.7.1.1  The MySQL Mailing Lists.
All developers at MySQL AB are on the internals list and we
help other people who are working on the MySQL code. Feel free to
use this list both to ask questions about the code and to send
patches that you would like to contribute to the MySQL project!







24.1.1  MySQL Threads




The MySQL server creates the following threads:








@command{mysqladmin processlist} only shows the connection, INSERT DELAYED,
and replication threads.







24.1.2  MySQL Test Suite











Until recently, our main full-coverage test suite was based on proprietary
customer data and for that reason has not been publicly available. The only
publicly available part of our testing process consisted of the crash-me
test, a Perl DBI/DBD benchmark found in the sql-bench directory, and
miscellaneous tests located in tests directory. The lack of a
standardized publicly available test suite has made it difficult for our users,
as well developers, to do regression tests on the MySQL code. To address
this problem, we have created a new test system that is included in
Unix source distributions and binary distributions starting with Version
3.23.29.  The tests can be run under Unix, or on Windows in the Cygwin
environment if the server has been compiled under Cygwin. They cannot be
run in a native Windows environment currently.





The current set of test cases doesn't test everything in MySQL, but it
should catch most obvious bugs in the SQL processing code, OS/library
issues, and is quite thorough in testing replication.  Our eventual goal
is to have the tests cover 100% of the code.  We welcome contributions
to our test suite.  You may especially want to contribute tests that
examine the functionality critical to your system, because this will ensure
that all future MySQL releases will work well with your
applications.







24.1.2.1  Running the MySQL Test Suite




The test system consist of a test language interpreter
(@command{mysqltest}), a shell script to run all
tests(@command{mysql-test-run}), the actual test cases written in a special
test language, and their expected results.  To run the test suite on
your system after a build, type @command{make test} or
mysql-test/mysql-test-run from the source root.  If you have
installed a binary distribution, cd to the install root
(eg. /usr/local/mysql), and do scripts/mysql-test-run.
All tests should succeed.  If not, you should try to find out why and
report the problem if this is a bug in MySQL.
See section 24.1.2.3  Reporting Bugs in the MySQL Test Suite.





From MySQL 4.1 on,
if you have a copy of @command{mysqld} running on the machine where you want to
run the test suite you do not have to stop it, as long as it is not using
ports 9306 and 9307.  If one of those ports is taken, you should
edit @command{mysql-test-run} and change the values of the master and/or slave
port to one that is available.





Before MySQL 4.1, @command{mysql-test-run} does not try to run its own server
by default but tries to use your currently running server. To override this
and cause @command{mysql-test-run} to start its own server, run it with with
the --local option.





You can run one individual test case with
mysql-test/mysql-test-run test_name.





If one test fails, you should test running @command{mysql-test-run} with
the --force option to check whether any other tests fail.







24.1.2.2  Extending the MySQL Test Suite




You can use the @command{mysqltest} language to write your own test cases.
Unfortunately, we have not yet written full documentation for it.
You can, however, look at our current test cases and use
them as an example.  The following points should help you get started:









24.1.2.3  Reporting Bugs in the MySQL Test Suite




If your MySQL version doesn't pass the test suite you should
do the following:









24.2  Adding New Functions to MySQL














There are two ways to add new functions to MySQL:








Each method has advantages and disadvantages:








Whichever method you use to add new functions, they may be used just like
native functions such as ABS() or SOUNDEX().







24.2.1  CREATE FUNCTION/DROP FUNCTION Syntax
















CREATE [AGGREGATE] FUNCTION function_name RETURNS {STRING|REAL|INTEGER}
       SONAME shared_library_name

DROP FUNCTION function_name





A user-defined function (UDF) is a way to extend MySQL with a new
function that works like native (built in) MySQL function such as
ABS() and CONCAT().





AGGREGATE is a new option for MySQL 3.23.  An
AGGREGATE function works exactly like a native MySQL
GROUP function like SUM or COUNT().





CREATE FUNCTION saves the function's name, type, and shared library
name in the mysql.func system table.  You must have the
INSERT and DELETE privileges for the mysql database
to create and drop functions.





All active functions are reloaded each time the server starts, unless
you start @command{mysqld} with the --skip-grant-tables option.  In
this case, UDF initialization is skipped and UDFs are unavailable.
(An active function is one that has been loaded with CREATE FUNCTION
and not removed with DROP FUNCTION.)





For instructions on writing user-defined functions, see section 24.2.2  Adding a New User-defined Function.
For the UDF mechanism to work, functions must be written in C or
C++, your operating system must support dynamic loading and you must have
compiled @command{mysqld} dynamically (not statically).





Note that to make AGGREGATE work, you must have a
mysql.func table that contains the column type.  If you do not
have this table, you should run the script
@command{mysql_fix_privilege_tables} to create it.







24.2.2  Adding a New User-defined Function













For the UDF mechanism to work, functions must be written in C or C++ and
your operating system must support dynamic loading.  The MySQL source
distribution includes a file `sql/udf_example.cc' that defines 5 new
functions.  Consult this file to see how UDF calling conventions work.





For @command{mysqld} to be able to use UDF functions, you should configure MySQL
with --with-mysqld-ldflags=-rdynamic The reason is that to on
many platforms (including Linux) you can load a dynamic library (with
dlopen()) from a static linked program, which you would get if
you are using --with-mysqld-ldflags=-all-static If you want to
use an UDF that needs to access symbols from @command{mysqld} (like the
metaphone example in `sql/udf_example.cc' that uses
default_charset_info), you must link the program with
-rdynamic (see man dlopen).





If you are using a precompiled version of the server, use MySQL-Max,
which supports dynamic loading.





For each function that you want to use in SQL statements, you should define
corresponding C (or C++) functions.  In the following discussion, the name
``xxx'' is used for an example function name.  To distinguish between SQL and
C/C++ usage, XXX() (uppercase) indicates an SQL function call, and
xxx() (lowercase) indicates a C/C++ function call.





The C/C++ functions that you write to implement the interface for
XXX() are:







xxx() (required)


The main function.  This is where the function result is computed.
The correspondence between the SQL type and return type of your C/C++
function is shown here:




SQL Type  C/C++ Type



STRING  char *



INTEGER  long long



REAL  double






xxx_init() (optional)


The initialization function for xxx().  It can be used to:



    

  • 

Check the number of arguments to XXX().

  • 

Check that the arguments are of a required type or, alternatively,
tell MySQL to coerce arguments to the types you want when
the main function is called.

  • 

Allocate any memory required by the main function.

  • 

Specify the maximum length of the result.

  • 

Specify (for REAL functions) the maximum number of decimals.

  • 

Specify whether the result can be NULL.




xxx_deinit() (optional)


The deinitialization function for xxx().  It should deallocate any
memory allocated by the initialization function.





When an SQL statement invokes XXX(), MySQL calls the
initialization function xxx_init() to let it perform any required
setup, such as argument checking or memory allocation.  If xxx_init()
returns an error, the SQL statement is aborted with an error message and the
main and deinitialization functions are not called.  Otherwise, the main
function xxx() is called once for each row.  After all rows have been
processed, the deinitialization function xxx_deinit() is called so it
can perform any required cleanup.





For aggregate functions (like SUM()), you must also provide the
following functions:







xxx_reset() (required)


Reset sum and insert the argument as the initial value for a new group.

xxx_add() (required)


Add the argument to the old sum.





When using aggregate UDFs, MySQL works the following way:





    

  1. 

Call xxx_init() to let the aggregate function allocate the memory it
will need to store results.

  2. 

Sort the table according to the GROUP BY expression.

  3. 

For the first row in a new group, call the xxx_reset() function.

  4. 

For each new row that belongs in the same group, call the
xxx_add() function.

  5. 

When the group changes or after the last row has been processed,
call xxx() to get the result for the aggregate.

  6. 

Repeat 3-5 until all rows has been processed

  7. 

Call xxx_deinit() to let the UDF free any memory it has allocated.





All functions must be thread-safe (not just the main function,
but the initialization and deinitialization functions as well). This means
that you are not allowed to allocate any global or static variables that
change!  If you need memory, you should allocate it in xxx_init()
and free it in xxx_deinit().







24.2.2.1  UDF Calling Sequences for simple functions










The main function should be declared as shown here.  Note that the return
type and parameters differ, depending on whether you will declare the SQL
function XXX() to return STRING, INTEGER, or REAL
in the CREATE FUNCTION statement:





For STRING functions:





char *xxx(UDF_INIT *initid, UDF_ARGS *args,
          char *result, unsigned long *length,
          char *is_null, char *error);





For INTEGER functions:





long long xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);





For REAL functions:





double xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);





The initialization and deinitialization functions are declared like this:





my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message);

void xxx_deinit(UDF_INIT *initid);





The initid parameter is passed to all three functions.  It points to a
UDF_INIT structure that is used to communicate information between
functions.  The UDF_INIT structure members follow.  The
initialization function should fill in any members that it wishes to change.
(To use the default for a member, leave it unchanged.):







my_bool maybe_null


xxx_init() should set maybe_null to 1 if xxx()
can return NULL.  The default value is 1 if any of the
arguments are declared maybe_null.


unsigned int decimals


The number of decimals.  The default value is the maximum number of decimals in
the arguments passed to the main function.  (For example, if the function is
passed 1.34, 1.345, and 1.3, the default would be 3,
because 1.345 has 3 decimals.


unsigned int max_length


The maximum length of the string result.  The default value differs depending
on the result type of the function.  For string functions, the default is the
length of the longest argument.  For integer functions, the default is 21
digits.  For real functions, the default is 13 plus the number of decimals
indicated by initid->decimals.  (For numeric functions, the length
includes any sign or decimal point characters.)

If you want to return a blob, you can set this to 65KB or 16MB; this
memory is not allocated but used to decide which column type to use if
there is a need to temporary store the data.


char *ptr


A pointer that the function can use for its own purposes.  For example,
functions can use initid->ptr to communicate allocated memory
between functions.  In xxx_init(), allocate the memory and assign it
to this pointer:



initid->ptr = allocated_memory;



In xxx() and xxx_deinit(), refer to initid->ptr to use
or deallocate the memory.






24.2.2.2  UDF Calling Sequences for aggregate functions










Here follows a description of the different functions you need to define
when you want to create an aggregate UDF function.





Note that the following function is NOT needed or used by MySQL 4.1.1.
You can keep still have define this function if you want to have your
code work with both MySQL 4.0 and MySQL 4.1.1





char *xxx_reset(UDF_INIT *initid, UDF_ARGS *args,
                char *is_null, char *error);





This function is called when MySQL finds the first row in a new group.
In the function you should reset any internal summary variables and then set
the given argument as the first argument in the group.





In many cases this is implemented internally by reseting all variables
(for example by calling xxx_clear() and then calling
xxx_add().





The following function is only required by MySQL 4.1.1 and above:





char *xxx_clear(UDF_INIT *initid, char *is_null, char *error);





This function is called when MySQL needs to reset the summary results.
This will be called at the beginning for each new group but can also be
called to reset the values for a query where there was no matching rows.
is_null will be set to point to CHAR(0) before calling
xxx_clear().





You can use the error pointer to store a byte if something went
wrong .





char *xxx_add(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);





This function is called for all rows that belongs to the same group,
except for the first row. In this you should add the value in UDF_ARGS
to your internal summary variable.





The xxx() function should be declared identical as when you
define a simple UDF function. See section 24.2.2.1  UDF Calling Sequences for simple functions.





This function is called when all rows in the group has been processed.
You should normally never access the args variable here but
return your value based on your internal summary variables.





All argument processing in xxx_reset() and xxx_add()
should be done identically as for normal UDFs. See section 24.2.2.3  Argument Processing.





The return value handling in xxx() should be done identically as
for a normal UDF. See section 24.2.2.4  Return Values and Error Handling.





The pointer argument to is_null and error is the same for
all calls to xxx_reset(), xxx_clear(), xxx_add() and
xxx().
You can use this to remember that you got an error or if the xxx()
function should return NULL. Note that you should not store a string
into *error!  This is just a 1 byte flag!





is_null is reset for each group (before calling xxx_clear()).
error is never reset.





If is_null or error are set after xxx(), then MySQL
will return NULL as the result for the group function.






24.2.2.3  Argument Processing











The args parameter points to a UDF_ARGS structure that has the
members listed here:







unsigned int arg_count


The number of arguments.  Check this value in the initialization function
if you want your function to be called with a particular number of arguments.
For example:



if (args->arg_count != 2)
{
    strcpy(message,"XXX() requires two arguments");
    return 1;
}




enum Item_result *arg_type


The types for each argument.  The possible type values are
STRING_RESULT, INT_RESULT, and REAL_RESULT.

To make sure that arguments are of a given type and return an
error if they are not, check the arg_type array in the initialization
function.  For example:



if (args->arg_type[0] != STRING_RESULT ||
    args->arg_type[1] != INT_RESULT)
{
    strcpy(message,"XXX() requires a string and an integer");
    return 1;
}



As an alternative to requiring your function's arguments to be of particular
types, you can use the initialization function to set the arg_type
elements to the types you want.  This causes MySQL to coerce
arguments to those types for each call to xxx().  For example, to
specify coercion of the first two arguments to string and integer, do this in
xxx_init():



args->arg_type[0] = STRING_RESULT;
args->arg_type[1] = INT_RESULT;




char **args


args->args communicates information to the initialization function
about the general nature of the arguments your function was called with.  For a
constant argument i, args->args[i] points to the argument
value.  (See below for instructions on how to access the value properly.)
For a non-constant argument, args->args[i] is 0.
A constant argument is an expression that uses only constants, such as
3 or 4*7-2 or SIN(3.14).  A non-constant argument is an
expression that refers to values that may change from row to row, such as
column names or functions that are called with non-constant arguments.

For each invocation of the main function, args->args contains the
actual arguments that are passed for the row currently being processed.

Functions can refer to an argument i as follows:



    

  • 

An argument of type STRING_RESULT is given as a string pointer plus a
length, to allow handling of binary data or data of arbitrary length.  The
string contents are available as args->args[i] and the string length
is args->lengths[i].  You should not assume that strings are
null-terminated.


  • 

For an argument of type INT_RESULT, you must cast
args->args[i] to a long long value:



    long long int_val;
int_val = *((long long*) args->args[i]);

    


  • 

For an argument of type REAL_RESULT, you must cast
args->args[i] to a double value:



    double    real_val;
real_val = *((double*) args->args[i]);

    





unsigned long *lengths


For the initialization function, the lengths array indicates the
maximum string length for each argument. You should not change these.
For each invocation of the main function, lengths contains the
actual lengths of any string arguments that are passed for the row
currently being processed.  For arguments of types INT_RESULT or
REAL_RESULT, lengths still contains the maximum length of
the argument (as for the initialization function).






24.2.2.4  Return Values and Error Handling













The initialization function should return 0 if no error occurred and
1 otherwise.  If an error occurs, xxx_init() should store a
null-terminated error message in the message parameter.  The message
will be returned to the client.  The message buffer is
MYSQL_ERRMSG_SIZE characters long, but you should try to keep the
message to less than 80 characters so that it fits the width of a standard
terminal screen.





The return value of the main function xxx() is the function value, for
long long and double functions.  A string functions should
return a pointer to the result and store the length of the string in the
length arguments.





Set these to the contents and length of the return value.  For example:





memcpy(result, "result string", 13);
*length = 13;





The result buffer that is passed to the calc function is 255 byte
big. If your result fits in this, you don't have to worry about memory
allocation for results.





If your string function needs to return a string longer than 255 bytes,
you must allocate the space for it with malloc() in your
xxx_init() function or your xxx() function and free it in
your xxx_deinit() function.  You can store the allocated memory
in the ptr slot in the UDF_INIT structure for reuse by
future xxx() calls.  See section 24.2.2.1  UDF Calling Sequences for simple functions.





To indicate a return value of NULL in the main function, set
is_null to 1:





*is_null = 1;





To indicate an error return in the main function, set the error
parameter to 1:





*error = 1;





If xxx() sets *error to 1 for any row, the function
value is NULL for the current row and for any subsequent rows
processed by the statement in which XXX() was invoked.  (xxx()
will not even be called for subsequent rows.) Note: In
MySQL versions prior to 3.22.10, you should set both *error
and *is_null:





*error = 1;
*is_null = 1;






24.2.2.5  Compiling and Installing User-defined Functions












Files implementing UDFs must be compiled and installed on the host where
the server runs.  This process is described below for the example UDF
file `udf_example.cc' that is included in the MySQL source distribution.





The immediately following instructions are for Unix. Instructions for
Windows are given later in this section.





The `udf_example.cc' file contains the following functions:








A dynamically loadable file should be compiled as a sharable object file,
using a command something like this:





shell> gcc -shared -o udf_example.so myfunc.cc





You can easily find out the correct compiler options for your system by
running this command in the `sql' directory of your MySQL
source tree:





shell> make udf_example.o





You should run a compile command similar to the one that @command{make} displays,
except that you should remove the -c option near the end of the line
and add -o udf_example.so to the end of the line.  (On some systems,
you may need to leave  the -c on the command.)





Once you compile a shared object containing UDFs, you must install it and
tell MySQL about it. Compiling a shared object from `udf_example.cc'
produces a file named something like `udf_example.so' (the exact name
may vary from platform to platform). Copy this file to some directory
searched by the dynamic linker ld, such as `/usr/lib' or add the
directory in which you placed the shared object to the linker configuration
file (for example, `/etc/ld.so.conf').





On many systems, you can also set the LD_LIBRARY or
LD_LIBRARY_PATH environment variable to point at the directory where
you have your UDF function files. The dlopen manual page tells you
which variable you should use on your system. You should set this in
@command{mysql.server} or @command{mysqld_safe} startup scripts and restart
@command{mysqld}.





On some systems, the @command{ldconfig} program that configures the dynamic
linker will not recognize shared objects unless their name begins with
lib. In this case you should rename a file such as
`udf_example.so' to `libudf_example.so'.





On Windows, you can compile user-defined functions by using the following
procedure:





    


  1. 

You will need to obtain the BitKeeper source repository for MySQL 4.0 or
higher.
See section 2.3.3  Installing from the Development Source Tree.


  2. 

In the source repository, look in the `VC++Files/examples/udf_example'
directory. You will find files named `udf_example.def',
`udf_example.dsp', and `udf_example.dsw' there.


  3. 

In the source repository, look in the `sql' directory. Copy
the `udf_example.cc' from this directory to the
`VC++Files/examples/udf_example' directory and rename the file to
`udf_example.cpp'.


  4. 

Open the `udf_example.dsw' file with Visual Studio VC++ and use it
to compile the UDFs as a normal project.






After the library is installed, notify @command{mysqld} about the new
functions with these commands:





mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE FUNCTION myfunc_double RETURNS REAL SONAME 'udf_example.so';
mysql> CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME 'udf_example.so';
mysql> CREATE FUNCTION lookup RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE FUNCTION reverse_lookup
    ->        RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE AGGREGATE FUNCTION avgcost
    ->        RETURNS REAL SONAME 'udf_example.so';





Functions can be deleted using DROP FUNCTION:





mysql> DROP FUNCTION metaphon;
mysql> DROP FUNCTION myfunc_double;
mysql> DROP FUNCTION myfunc_int;
mysql> DROP FUNCTION lookup;
mysql> DROP FUNCTION reverse_lookup;
mysql> DROP FUNCTION avgcost;





The CREATE FUNCTION and DROP FUNCTION statements update the
system table func in the mysql database.  The function's name,
type and shared library name are saved in the table.  You must have the
INSERT and DELETE privileges for the mysql database
to create and drop functions.





You should not use CREATE FUNCTION to add a function that has already
been created.  If you need to reinstall a function, you should remove it with
DROP FUNCTION and then reinstall it with CREATE FUNCTION.  You
would need to do this, for example, if you recompile a new version of your
function, so that @command{mysqld} gets the new version.  Otherwise, the server
will continue to use the old version.





Active functions are reloaded each time the server starts, unless you start
@command{mysqld} with the --skip-grant-tables option.  In this case, UDF
initialization is skipped and UDFs are unavailable.  (An active function is
one that has been loaded with CREATE FUNCTION and not removed with
DROP FUNCTION.)







24.2.3  Adding a New Native Function












The procedure for adding a new native function is described here.  Note
that you cannot add native functions to a binary distribution because
the procedure involves modifying MySQL source code.  You must
compile MySQL yourself from a source distribution.  Also note
that if you migrate to another version of MySQL (for example,
when a new version is released), you will need to repeat the procedure
with the new version.





To add a new native MySQL function, follow these steps:





    

  1. 

Add one line to `lex.h' that defines the function name in the
sql_functions[] array.

  2. 

If the function prototype is simple (just takes zero, one, two or three
arguments), you should in lex.h specify SYM(FUNC_ARG#) (where # is the
number of arguments) as the second argument in the
sql_functions[] array and add a function that creates a function
object in `item_create.cc'.  Take a look at "ABS" and
create_funcs_abs() for an example of this.

If the function prototype is complicated (for example takes a variable number
of arguments), you should add two lines to `sql_yacc.yy'. One
indicates the preprocessor symbol that @command{yacc} should define (this
should be added at the beginning of the file). Then define the function
parameters and add an ``item'' with these parameters to the
simple_expr parsing rule.  For an example, check all occurrences
of ATAN in `sql_yacc.yy' to see how this is done.

  3. 

In `item_func.h', declare a class inheriting from Item_num_func or
Item_str_func, depending on whether your function returns a number or a
string.

  4. 

In `item_func.cc', add one of the following declarations, depending
on whether you are defining a numeric or string function:


    double   Item_func_newname::val()
longlong Item_func_newname::val_int()
String  *Item_func_newname::Str(String *str)

    

If you inherit your object from any of the standard items (like
Item_num_func), you probably only have to define one of these
functions and let the parent object take care of the other functions.
For example, the Item_str_func class defines a val() function
that executes atof() on the value returned by ::str().


  5. 

You should probably also define the following object function:


    void Item_func_newname::fix_length_and_dec()

    

This function should at least calculate max_length based on the
given arguments. max_length is the maximum number of characters
the function may return.  This function should also set maybe_null
= 0 if the main function can't return a NULL value.  The
function can check whether any of the function arguments can return
NULL by checking the arguments' maybe_null variable. You
can take a look at Item_func_mod::fix_length_and_dec for a
typical example of how to do this.





All functions must be thread-safe (in other words, don't use any global or
static variables in the functions without protecting them with mutexes).





If you want to return NULL, from ::val(), ::val_int()
or ::str() you should set null_value to 1 and return 0.





For ::str() object functions, there are some additional
considerations to be aware of:









24.3  Adding New Procedures to MySQL












In MySQL, you can define a procedure in C++ that can access and
modify the data in a query before it is sent to the client.  The modification
can be done on a row-by-row or GROUP BY level.





We have created an example procedure in MySQL 3.23 to
show you what can be done.





Additionally we recommend you to take a look at mylua.
With this you can use the LUA language to load a procedure at
runtime into @command{mysqld}.







24.3.1  Procedure Analyse




analyse([max elements,[max memory]])





This procedure is defined in the `sql/sql_analyse.cc'.  This
examines the result from your query and returns an analysis of the
results:








SELECT ... FROM ... WHERE ... PROCEDURE ANALYSE([max elements,[max memory]])






24.3.2  Writing a Procedure




For the moment, the only documentation for this is the source.





You can find all information about procedures by examining the following files:









A  Problems and Common Errors












This appendix lists some common problems and error messages that you may
encounter.  It describes how to determine the causes of the
problems and what to do to solve them.







A.1  How to Determine What Is Causing a Problem




When you run into a problem, the first thing you should do is to find out
which program or piece of equipment is causing it:








If after you have examined all other possibilities and you have
concluded that the MySQL server or a MySQL client
is causing the problem, it's time to create a bug report for our
mailing list or our support team.  In the bug report, try to give a
very detailed description of how the system is behaving and what you think is
happening. You should also state why you think that MySQL
is causing the problem.  Take into consideration all the situations in
this chapter. State any problems exactly how they appear when you
examine your system. Use the ``copy and paste'' method for any output
and error messages from programs and log files.





Try to describe in detail which program is not working and all
symptoms you see.  We have in the past received many bug reports that
state only ``the system doesn't work.''  This doesn't provide us with any
information about what could be the problem.





If a program fails, it's always useful to know the following information:








When sending a bug report, you should follow the outline
described in section 1.7.1.2  Asking Questions or Reporting Bugs.







A.2  Common Errors When Using MySQL Programs











This section lists some errors that users frequently encounter when running
MySQL programs. Although the problems show up when you try to run client
programs, the solutions to many of the problems involves changing the
configuration of the MySQL server.







A.2.1  Access denied












An Access denied error can have many causes.  Often the problem is
related to the MySQL accounts that the server allows client programs to
use when connecting.
See section 5.5.8  Causes of Access denied Errors.
See section 5.5.2  How the Privilege System Works.







A.2.2  Can't connect to [local] MySQL server




A MySQL client on Unix can connect to the @command{mysqld} server in two
different ways: By using a Unix socket file to connect through a file in the
filesystem (default `/tmp/mysql.sock'), or by using TCP/IP, which
connects through a port number.  A Unix socket file connection is faster
than TCP/IP, but can be used only when connecting to a server on the same
computer.  A Unix socket file is used if you don't specify a hostname or if
you specify the special hostname localhost.





If the MySQL server is running on Windows 9x or Me, you can connect
only via TCP/IP. If the server is running on Windows NT, 2000, or XP
and is started with the --enable-named-pipe option, you can also
connect with named pipes if you run the client on the host where the
server is running. The name of the named pipe is MySQL by default.
If you don't give a hostname when connecting to @command{mysqld}, a MySQL
client first will try to connect to the named pipe. If that doesn't work,
it will connect to the TCP/IP port.  You can force the use of named pipes
on Windows by using . as the hostname.





The error (2002) Can't connect to ... normally means that there is no
MySQL server running on the system or that you are using an incorrect Unix
socket filename or TCP/IP port number when trying to connect to the
server.





Start by checking whether there is a process named @command{mysqld} running on
your server host.
(Use @command{ps xa | grep mysqld} on Unix or the Task Manager on Windows.)
If there is no such process, you should start the server.
See section 2.4.2.3  Starting and Troubleshooting the MySQL Server.





If a @command{mysqld} process is running, you can check it by
trying the following commands. The port number or Unix socket filename
might be different in your setup. host_ip represents the IP number of
the machine where the server is running.





shell> mysqladmin version
shell> mysqladmin variables
shell> mysqladmin -h `hostname` version variables
shell> mysqladmin -h `hostname` --port=3306 version
shell> mysqladmin -h host_ip version
shell> mysqladmin --protocol=socket --socket=/tmp/mysql.sock version





Note the use of backticks rather than forward quotes with the hostname
command; these cause the output of hostname (that is, the current
hostname) to be substituted into the @command{mysqladmin} command.
If you have no hostname command or are running on Windows, you can
manually type the hostname of your machine (without backticks) following the
-h option.
You can also try -h 127.0.0.1 to connect with TCP/IP to the local host.





Here are some reasons the Can't connect to local MySQL server
error might occur:








If you get the error message Can't connect to MySQL server on
some_host, you can try the following things to find out what the
problem is:









A.2.3  Client does not support authentication protocol




MySQL 4.1 and up uses an authentication protocol based on a password hashing
algorithm that is incompatible with that used by older clients.
If you upgrade the server to 4.1, attempts to connect to it with an
older client may fail with the following message:





shell> mysql
Client does not support authentication protocol requested
by server; consider upgrading MySQL client





To solve this problem, you should use one of the following approaches:








For additional background on password hashing and authentication, see
section 5.5.9  Password Hashing in MySQL 4.1.







A.2.4  Password Fails When Entered Interactively




MySQL client programs prompt for a password when invoked with a
--password or -p option that has no following password value:





shell> mysql -u user_name -p
Enter password:





On some systems, you may find that your password works when specified in an
option file or on the command line, but not when you enter it interactively
at the Enter password: prompt. This occurs when the library provided
by the system to read passwords limits password values to a small number of
characters (typically eight).  That is a problem with the system library, not
with MySQL. To work around it, change your MySQL password to
a value that is eight or fewer characters long, or put your password in an
option file.







A.2.5  Host 'host_name' is blocked




If you get the following error, it means that @command{mysqld} has received many
connect requests from the host 'host_name' that have been
interrupted in the middle:





Host 'host_name' is blocked because of many connection errors.
Unblock with 'mysqladmin flush-hosts'





The number of interrupted connect requests allowed is determined by the
value of the max_connect_errors system variable.  After
max_connect_errors failed requests, @command{mysqld} assumes that
something is wrong (for example, that someone is trying to break in), and
blocks the host from further connections until you execute a
@command{mysqladmin flush-hosts} command or issue a FLUSH HOSTS
statement.
See section 5.2.3  Server System Variables.





By default, @command{mysqld} blocks a host after 10 connection errors.
You can adjust the value by starting the server like this:





shell> mysqld_safe --max_connect_errors=10000 &





If you get this error message for a given host, you should first
verify that there isn't anything wrong with TCP/IP connections from that
host.  If you are having network problems, it will do you no good to
increase the value of the max_connect_errors variable.







A.2.6  Too many connections




If you get a Too many connections error when you try to connect to the
@command{mysqld} server, this means that that all available connections already
are used by other clients.





The number of connections allowed is controlled by the max_connections
system variable. Its default value is 100.  If you need to support more
connections, you should restart @command{mysqld} with a larger value for this
variable.





@command{mysqld} actually allows max_connections+1 clients to
connect.  The extra connection is reserved for use by accounts that have the
SUPER privilege.  By granting the SUPER privilege to
administrators and not to normal users (who should not need it), an
administrator can connect to the server and use SHOW PROCESSLIST to
diagnose problems even if the maximum number of unprivileged clients
already are connected.
See section 14.5.3.16  SHOW PROCESSLIST Syntax.





The maximum number of connections MySQL can support depends on the quality of
the thread library on a given platform. Linux or Solaris should be
able to support 500-1000 simultaneous connections, depending on how much
RAM you have and what your clients are doing. Static Linux binaries provided
by MySQL AB can support up to 4000 connections.







A.2.7  Out of memory




If you issue a query using the @command{mysql} client program and receive an
error like the following one, it means that @command{mysql} does not
have enough memory to store the entire query result:





mysql: Out of memory at line 42, 'malloc.c'
mysql: needed 8136 byte (8k), memory in use: 12481367 bytes (12189k)
ERROR 2008: MySQL client ran out of memory





To remedy the problem, first check whether your query is correct. Is it
reasonable that it should return so many rows? If not, correct the query and
try again. Otherwise, you can invoke @command{mysql} with the --quick
option. This causes it to use the mysql_use_result() C API function
to retrieve the
result set, which places less of a load on the client (but more on the
server).







A.2.8  MySQL server has gone away




This section also covers the related Lost connection to server
during query error.





The most common reason for the MySQL server has gone away error
is that the server timed out and closed the connection.  In this case,
you normally get one of the following error codes (which one you get is
operating system-dependent):






Error Code  Description



CR_SERVER_GONE_ERROR  The client couldn't send a question to the
server.



CR_SERVER_LOST  The client didn't get an error when writing
to the server, but it didn't get a full answer (or any answer) to the question.







By default, the
server closes the connection after eight hours if nothing has happened. You
can change the time limit by setting the wait_timeout variable when
you start @command{mysqld}.
See section 5.2.3  Server System Variables.





If you have a script, you just have to issue the query again for the client
to do an automatic reconnection.






You will also get an error if someone has killed the running thread with a
KILL statement or a @command{mysqladmin kill} command.





Another common reason the MySQL server has gone away error occurs
within an application program is that you tried to run a query after
closing the connection to the server.  This indicates a logic error in the
application that should be corrected.





You can check whether the MySQL server died and restarted by executing
@command{mysqladmin version} and examining the server's uptime.  If the
client connection was broken because @command{mysqld} crashed and restarted,
you should concentrate on finding the reason for the crash.  Start by
checking whether issuing the query again kills the server again.
See section A.4.2  What to Do If MySQL Keeps Crashing.





You can also get these errors if you send a query to the server that is
incorrect or too large. If @command{mysqld} receives a packet that is too large
or out of order, it assumes that something has gone wrong with the client and
closes the connection.  If you need big queries (for example, if you are
working with big BLOB columns), you can increase the query limit by
setting the server's max_allowed_packet variable, which has a default
value of 1MB.  You may also need to increase the maximum packet size on the
client end.  More information on setting the packet size is given in
section A.2.9  Packet too large.





You will also get a lost connection if you are sending a packet 16MB or larger
if your client is older than 4.0.8 and your server is 4.0.8 and above, or
the other way around.





If you want to create a bug report regarding this problem, be sure that
you include the following information:








See section 1.7.1.2  Asking Questions or Reporting Bugs.







A.2.9  Packet too large




A communication packet is a single SQL statement sent to the MySQL server
or a single row that is sent to the client.





In MySQL 3.23, the largest possible packet is 16MB, due to limits in the
client/server protocol. In MySQL 4.0.1 and up, the limit is 1GB.






When a MySQL client or the @command{mysqld} server receives a packet bigger
than max_allowed_packet bytes, it issues a Packet too
large error and closes the connection.  With some clients, you may also
get a Lost connection to MySQL server during query error if the
communication packet is too large.





Both the client and the server have their own
max_allowed_packet variable, so if you want to handle big packets,
you must increase this variable both in the client and in the server.





If you are using the @command{mysql} client program, its default
max_allowed_packet variable is 16MB. That is also the maximum value
before MySQL 4.0. To set a larger value from 4.0 on, start @command{mysql} like
this:





mysql> mysql --max_allowed_packet=32M





That sets the packet size to 32MB.





The server's default max_allowed_packet value is 1MB. You can increase
this if the server needs to handle big queries (for example, if you are
working with big BLOB columns).  For example, to set the variable to
16MB, start the server like this:





mysql> mysqld --max_allowed_packet=16M





Before MySQL 4.0, use this syntax instead:





mysql> mysqld --set-variable=max_allowed_packet=16M





You can also use an option file to set max_allowed_packet.  For
example, to set the size for the server to 16MB, add the following lines in
an option file:





[mysqld]
max_allowed_packet=16M





Before MySQL 4.0, use this syntax instead:





[mysqld]
set-variable = max_allowed_packet=16M





It's safe to increase the value of this variable because the extra memory is
allocated only when needed.  For example, @command{mysqld} allocates more
memory only when you issue a long query or when @command{mysqld} must return a
large result row.  The small default value of the variable is a
precaution to catch incorrect packets between the client and server and also
to ensure that you don't run out of memory by using large packets
accidentally.





You can also get strange problems with large packets if you are using large
BLOB values but have not given @command{mysqld} access to enough memory
to handle the query.  If you suspect this is the case, try adding
@command{ulimit -d 256000} to the beginning of the @command{mysqld_safe} script
and restarting @command{mysqld}.







A.2.10  Communication Errors and Aborted Connections












The server error log can be a useful source of information about connection
problems.
See section 5.9.1  The Error Log.
Starting with MySQL 3.23.40, if you start the server with the
--warnings option (or --log-warnings from MySQL 4.0.3 on), you
might find messages like this in your error log:





010301 14:38:23  Aborted connection 854 to db: 'users' user: 'josh'





If Aborted connections messages appear in the error log, the cause
can be any of the following:








When any of these things happen, the server increments the
Aborted_clients status variable.





The server increments the Aborted_connects status variable when the
following things happen:








If these kinds of things happen, it might indicate that someone is
trying to break into your server!





Other reasons for problems with aborted clients or aborted connections:









A.2.11  The table is full










There are several ways a full-table error can occur:









A.2.12  Can't create/write to file










If you get an error of the following type for some queries, it means
that MySQL cannot create a temporary file for the result set in the
temporary directory:





Can't create/write to file '\\sqla3fe_0.ism'.





The preceding error is a
typical message for Windows; the Unix message is similar.
The fix is to start @command{mysqld} with the --tmpdir option or to
add the option to the [mysqld] section of your option file.
For example, to specify a directory of `C:\temp', use these lines:





[mysqld]
tmpdir=C:/temp





The `C:\temp' directory must already exist. See section 4.3.2  Using Option Files.





Check also the error code that you get with @command{perror}. One reason
the server cannot write to a table is that the filesystem is full:





shell> perror 28
Error code  28:  No space left on device






A.2.13  Commands out of sync










If you get Commands out of sync; you can't run this command now
in your client code, you are calling client functions in the wrong order.





This can happen, for example, if you are using mysql_use_result() and
try to execute a new query before you have called mysql_free_result().
It can also happen if you try to execute two queries that return data without
calling mysql_use_result() or mysql_store_result() in between.







A.2.14  Ignoring user




If you get the following error, it means that when @command{mysqld} was started
or when it reloaded the grant tables, it found an account in the user
table that had an invalid password.





Found wrong password for user 'some_user'@'some_host'; ignoring user





As a result, the account is simply ignored by the permission system.





The following list indicates possible causes of and fixes for this problem:









A.2.15  Table 'tbl_name' doesn't exist




If you get either of the following errors, it usually means that no table
exists in the current database with the given name:





Table 'tbl_name' doesn't exist
Can't find file: 'tbl_name' (errno: 2)





In some cases, it may be that the table does exist but that you are
referring to it incorrectly:








You can check which tables are in the current database with
SHOW TABLES. See section 14.5.3  SET and SHOW Syntax.







A.2.16  Can't initialize character set










You might see an error like this if you have character set problems:





MySQL Connection Failed: Can't initialize character set charset_name





This error can have any of the following causes:









A.2.17  File Not Found




If you get ERROR '...' not found (errno: 23), Can't open file:
... (errno: 24), or any other error with errno 23 or errno 24
from MySQL, it means that you haven't allocated enough file descriptors for
the MySQL server.  You can use the @command{perror} utility to get a
description of what the error number means:





shell> perror 23
Error code  23:  File table overflow
shell> perror 24
Error code  24:  Too many open files
shell> perror 11
Error code  11:  Resource temporarily unavailable





The problem here is that @command{mysqld} is trying to keep open too many
files simultaneously.  You can either tell @command{mysqld} not to open so
many files at once or increase the number of file descriptors
available to @command{mysqld}.





To tell @command{mysqld} to keep open fewer files at a time, you can make the
table cache smaller by reducing the value of the table_cache system
variable (the default value is 64). Reducing the value of
max_connections also will reduce the number of open files (the
default value is 100).






To change the number of file descriptors available to @command{mysqld}, you can
use the --open-files-limit option to @command{mysqld_safe} or (as of
MySQL 3.23.30) set the open_files_limit system variable.
See section 5.2.3  Server System Variables.
The easiest way to set these values is to add an option to your option file.
See section 4.3.2  Using Option Files.  If you have an old version of @command{mysqld} that
doesn't support setting the open files limit, you can edit the
@command{mysqld_safe} script.  There is a commented-out line @command{ulimit -n
256} in the script.  You can remove the `#' character to uncomment
this line, and change the number 256 to set the number of file
descriptors to be made available to @command{mysqld}.





--open-files-limit and @command{ulimit} can increase the number of file
descriptors, but only up to the limit imposed by the operating system. There
is also a ``hard'' limit that can be overridden only if you start
@command{mysqld_safe} or @command{mysqld} as root (just remember that you
also need to start the server with the --user option in this case so
that it does not continue to run as root after it starts up).
If you need to increase the operating system limit on the number of file
descriptors available to each process, consult the documentation for your
system.





Note: If you run the @command{tcsh} shell, @command{ulimit} will not work!
@command{tcsh} will also report incorrect values when you ask for the current
limits.  In this case, you should start @command{mysqld_safe} using @command{sh}.







A.3  Installation-Related Issues





A.3.1  Problems Linking to the MySQL Client Library












When you are linking an application program to use the MySQL client library,
you might get undefined reference errors for symbols that start with mysql_,
such as those shown here:





/tmp/ccFKsdPa.o: In function `main':
/tmp/ccFKsdPa.o(.text+0xb): undefined reference to `mysql_init'
/tmp/ccFKsdPa.o(.text+0x31): undefined reference to `mysql_real_connect'
/tmp/ccFKsdPa.o(.text+0x57): undefined reference to `mysql_real_connect'
/tmp/ccFKsdPa.o(.text+0x69): undefined reference to `mysql_error'
/tmp/ccFKsdPa.o(.text+0x9a): undefined reference to `mysql_close'





You should be able to solve this problem by adding -Ldir_path
-lmysqlclient at the end of your link command, where dir_path
represents the pathname of the directory where the client library is
located. To determine the correct directory, try this command:





shell> mysql_config --libs





The output from @command{mysql_config} might indicate other libraries that
should be specified on the link command as well.





If you get undefined reference errors for the uncompress
or compress function, add -lz to the end of your
link command and try again.





If you get undefined reference errors for a function that should
exist on your system, such as connect, check the manual page for the
function in question to determine which libraries you should add to the link
command.





You might get undefined reference errors such as the following for
functions that don't exist on your system:





mf_format.o(.text+0x201): undefined reference to `__lxstat'





This usually means that your MySQL client library was compiled on a system
that is not 100% compatible with yours.  In this case, you should download
the latest MySQL source distribution and compile MySQL yourself.
See section 2.3  MySQL Installation Using a Source Distribution.





You might get undefined reference errors at runtime when you try to execute a
MySQL program. If these errors specify symbols that start with mysql_
or indicate that the mysqlclient library can't be found, it means
that your system can't find the shared `libmysqlclient.so' library.
The fix for this is to tell your system to search for shared libraries
where the library is located. Use whichever of the following methods is
appropriate for your system:








Another way to solve this problem is by linking your program statically with
the -static option, or by removing the dynamic MySQL libraries
before linking your code. Before trying the second method, you should be
sure that no other programs are using the dynamic libraries.







A.3.2  How to Run MySQL as a Normal User











On Windows, you can run the server as a Windows service using normal user
accounts beginning with MySQL 4.0.17 and 4.1.2.  (Older MySQL versions
required you to have administrator rights.  This was a bug introduced in
MySQL 3.23.54.)





On Unix, the MySQL server @command{mysqld} can be started and run by any user.
However, you should avoid running the server as the Unix root user
for security reasons.  In order to change @command{mysqld} to run as a normal
unprivileged Unix user user_name, you must do the following:





    

  1. 

Stop the server if it's running (use @command{mysqladmin shutdown}).


  2. 

Change the database directories and files so that user_name has
privileges to read and write files in them (you might need to do this as
the Unix root user):



    shell> chown -R user_name /path/to/mysql/datadir

    

If you do not do this, the server will not be able to access databases or tables when it
runs as user_name.

If directories or files within the MySQL data directory are symbolic links,
you'll also need to follow those links and change the directories and files
they point to.  chown -R might not follow symbolic links for you.


  3. 

Start the server as user user_name. If you are using MySQL 3.22 or
later, another alternative is to start @command{mysqld} as the Unix root
user and use the --user=user_name option.  @command{mysqld} will start up,
then switch
to run as the Unix user user_name before accepting any connections.


  4. 

To start the server as the given user automatically at system
startup time, specify the username by adding a user option to
the [mysqld] group of the `/etc/my.cnf' option file or the
`my.cnf' option file in the server's data directory. For example:



    [mysqld]
user=user_name

    






If your Unix machine itself isn't secured, you should assign passwords
to the MySQL root accounts in the grant tables.  Otherwise, any
user with a login account on that machine can run the @command{mysql} client with a
--user=root option and perform any operation.  (It is a good idea to
assign passwords to MySQL accounts in any case, but especially so when
other login accounts exist on the server host.)
See section 2.4  Post-Installation Setup and Testing.







A.3.3  Problems with File Permissions














If you have problems with file permissions, the UMASK environment
variable might be set incorrectly when @command{mysqld} starts.  For example,
MySQL might issue the following error message when you create a table:





ERROR: Can't find file: 'path/with/filename.frm' (Errcode: 13)





The default UMASK value is 0660. You can change this behavior by
starting @command{mysqld_safe} as follows:





shell> UMASK=384  # = 600 in octal
shell> export UMASK
shell> mysqld_safe &












By default, MySQL creates database and RAID directories
with an access permission value of 0700.  You can modify this
behavior by setting the UMASK_DIR variable. If you set its value, new
directories are created with the combined UMASK and UMASK_DIR
values. For example, if you want to give group access to all new
directories, you can do this:





shell> UMASK_DIR=504  # = 770 in octal
shell> export UMASK_DIR
shell> mysqld_safe &





In MySQL 3.23.25 and above, MySQL assumes that the
value for UMASK and UMASK_DIR is in octal if it starts
with a zero.





See section E  Environment Variables.







A.4  Administration-Related Issues





A.4.1  How to Reset the Root Password













If you have never set a root password for MySQL, the server will
not require a password at all for connecting as root. However, it is
recommended to set a password for each account. See section 5.4.1  General Security Guidelines.





If you set a root password previously, but have forgotten what it
was, you can set a new password. The following procedure is for Windows
systems. The procedure for Unix systems is given later in this section.





The procedure under Windows:





    

  1. 

Log on to your system as Administrator.

  2. 

Stop the MySQL server if it is running. For a server that is running as a
Windows service, go to the Services manager:


    Start Menu -> Control Panel -> Administrative Tools -> Services

    

Then find the MySQL service in the list, and stop it.

If your server is not running as a service, you may need to use the Task
Manager to force it to stop.

  3. 

Open a console window to get to the DOS command prompt:


    Start Menu -> Run -> cmd

    


  4. 

We are assuming that you installed MySQL to `C:\mysql'. If you installed
MySQL to another location, adjust the following commands accordingly.

At the DOS command prompt, execute this command:


    C:\> C:\mysql\bin\mysqld-nt --skip-grant-tables

    

This starts the server in a special mode that does not check the grant
tables to control access.

  5. 

Keeping the first console window open, open a second console window and
execute the following commands (type each on a single line):


    C:\> C:\mysql\bin\mysqladmin -u root
         flush-privileges password "newpwd"
C:\> C:\mysql\bin\mysqladmin -u root -p shutdown

    

Replace ``newpwd'' with the actual root password that you want
to use.
The second command will prompt you to enter the new password for access.
Enter the password that you assigned in the first command.

  6. 

Stop the MySQL server, then restart it in normal mode again.
If you run the server as a service, start it from the Windows Services
window.  If you start the server manually, use whatever command you
normally use.

  7. 

You should now be able to connect using the new password.





In a Unix environment, the procedure for resetting the root password
is as follows:





    

  1. 

Log on to your system as either the Unix root user or as the
same user that the @command{mysqld} server runs as.

  2. 

Locate the `.pid' file that contains the server's process ID.
The exact location and name of this file depend on your distribution,
hostname, and configuration. Common locations are `/var/lib/mysql/',
`/var/run/mysqld/', and `/usr/local/mysql/data/'. Generally, the
filename has the extension of `.pid' and begins with either
`mysqld' or your system's hostname.

Now you can stop the MySQL server by sending a normal kill
(not kill -9) to the @command{mysqld} process, using the pathname of the
`.pid' file in the following command:


    shell> kill `cat /mysql-data-directory/host_name.pid`

    

Note the use of backticks rather than forward quotes with the cat
command; these cause the output of cat
to be substituted into the kill command.

  3. 

Restart the MySQL server with the special --skip-grant-tables option:


    shell> mysqld_safe --skip-grant-tables &

    


  4. 

Set a new password for the root@localhost MySQL account:


    shell> mysqladmin -u root flush-privileges password "newpwd"

    

Replace ``newpwd'' with the actual root password that you want
to use.

  5. 

You should now be able to connect using the new password.





Alternatively, on any platform, you can set the new password using the @command{mysql} client:


    

  1. 

Stop @command{mysqld} and restart it with the --skip-grant-tables
option as described earlier.

  2. 

Connect to the @command{mysqld} server with this command:


    shell> mysql -u root

    


  3. 

Issue the following statements in the @command{mysql} client:


    mysql> UPDATE mysql.user SET Password=PASSWORD('newpwd')
    ->                   WHERE User='root';
mysql> FLUSH PRIVILEGES;

    

Replace ``newpwd'' with the actual root password that you want
to use.

  4. 

You should now be able to connect using the new password.






A.4.2  What to Do If MySQL Keeps Crashing










Each MySQL version is tested on many platforms before it is
released.  This doesn't mean that there are no bugs in
MySQL, but if there are bugs, they should be very few and can be
hard to find.  If you have a problem, it will always help if you try to
find out exactly what crashes your system, because you will have a much better
chance of getting the problem fixed quickly.





First, you should try to find out whether the problem is that the @command{mysqld} server dies or whether your problem has to do with your
client.  You can check how long your @command{mysqld} server has been up by
executing @command{mysqladmin version}.  If @command{mysqld} has died and
restarted, you may find the reason by looking in the server's error log.
See section 5.9.1  The Error Log.





On some systems, you can find in the error log a stack trace of where
@command{mysqld} died that you can resolve with the resolve_stack_dump
program.  See section D.1.4  Using a Stack Trace. Note that the variable values written in
the error log may not always be 100% correct.





Many server crashes are caused by corrupted data files or index files.  MySQL
will update the files on disk with the write() system call after every
SQL statement and before the client is notified about the result. (This is
not true if you are running with --delay-key-write, in which case
data files are written but not index files.) This means that data file
contents are safe even
if @command{mysqld} crashes, because the operating system will ensure that the
unflushed data is written to disk.  You can force MySQL to flush everything
to disk after every SQL statement by starting @command{mysqld} with the
--flush option.





The preceding means that normally you should not get corrupted tables unless
one of the following happens:








Because it is very difficult to know why something is crashing, first try to
check whether things that work for others crash for you.  Please try
the following things:









A.4.3  How MySQL Handles a Full Disk











When a disk-full condition occurs, MySQL does the following:








To alleviate the problem, you can take the following actions:








Exceptions to the preceding behavior are when you use REPAIR TABLE or
OPTIMIZE TABLE or when the indexes are created in a batch after
LOAD DATA INFILE or after an ALTER TABLE statement.
All of these statements may create large temporary files that, if left to
themselves, would cause big problems for the rest of the system.  If the disk
becomes full while MySQL is doing any of these operations,
it will remove the big temporary files and mark the table as crashed.
The exception is that for ALTER TABLE, the old table will be left
unchanged.







A.4.4  Where MySQL Stores Temporary Files




MySQL uses the value of the TMPDIR environment variable as the
pathname of the directory in which to store temporary files.  If you don't
have TMPDIR set, MySQL uses the system default, which is normally
`/tmp', `/var/tmp', or `/usr/tmp'.  If the filesystem
containing your temporary file directory is too small, you can use the
--tmpdir option to @command{mysqld} to specify a directory in a
filesystem where you have enough space.





Starting from MySQL 4.1, the --tmpdir option can be set to a list
of several paths that are used in round-robin fashion. Paths should be
separated by colon characters (`:') on Unix and semicolon characters
(`;') on Windows, NetWare, and OS/2.  Note: To spread the load
effectively, these paths should be located on different
physical disks, not different partitions of the same disk.





If the MySQL server is acting as a replication slave, you should not set
--tmpdir to point to a directory on a memory-based filesystem or to a
directory that is cleared when the server host restarts.  A replication
slave needs some of its temporary files to survive a machine restart so that
it can replicate temporary tables or LOAD DATA INFILE operations. If
files in the temporary file directory are lost when the server restarts,
replication will fail.





MySQL creates all temporary files as hidden files. This ensures
that the temporary files will be removed if @command{mysqld} is terminated.  The
disadvantage of using hidden files is that you will not see a big temporary
file that fills up the filesystem in which the temporary file directory is
located.





When sorting (ORDER BY or GROUP BY), MySQL normally
uses one or two temporary files. The maximum disk space required is determined
by the following expression:





(length of what is sorted + sizeof(row pointer))
* number of matched rows
* 2





The row pointer size is usually four bytes, but may grow in the future for
really big tables.





For some SELECT queries, MySQL also creates temporary SQL
tables. These are not hidden and have names of the form `SQL_*'.





ALTER TABLE creates a temporary table in the same directory as
the original table.






A.4.5  How to Protect or Change the MySQL Socket File `/tmp/mysql.sock'











The default location for the Unix socket file that the server uses for
communication with local clients is `/tmp/mysql.sock'.  This might
cause problems, because on some versions of Unix, anyone can delete files
in the `/tmp' directory.





On most versions of Unix, you can protect your `/tmp' directory so that
files can be deleted only by their owners or the superuser (root).
To do this, set the sticky bit on the `/tmp' directory by
logging in as root and using the following command:





shell> chmod +t /tmp





You can check whether the sticky bit is set by executing ls -ld
/tmp.  If the last permission character is t, the bit is set.











Another approach is to change the place where the server creates the Unix
socket file. If you do this, you should also let client programs know the
new location of the file.  You can specify the file location in several
ways:








You can test whether the new socket location works by attempting to connect to
the server with this command:





shell> mysqladmin --socket=/path/to/socket version






A.4.6  Time Zone Problems


















If you have a problem with SELECT NOW() returning values in GMT and
not your local time, you have to tell the server your current time zone.
The same applies if UNIX_TIMESTAMP() returns the wrong value.
This should be done for the environment in which the server runs; for
example, in @command{mysqld_safe} or @command{mysql.server}.
See section E  Environment Variables.





You can set the time zone for the server with the
--timezone=timezone_name option to @command{mysqld_safe}.  You can
also set it by setting the TZ environment variable before you
start @command{mysqld}.





The allowable values for --timezone or TZ are
system-dependent. Consult your operating system documentation to see
what values are acceptable.






A.5  Query-Related Issues





A.5.1  Case Sensitivity in Searches













By default, MySQL searches are not case sensitive (although there are
some character sets that are never case insensitive, such as czech).
This means that if you search with col_name LIKE 'a%', you will get all
column values that start with A or a. If you want to make this
search case sensitive, make sure that one of the operands is a binary string.
You can do this with the BINARY operator. Write the condition as either
BINARY col_name LIKE 'a%' or col_name LIKE BINARY 'a%'.





If you want a column always to be treated in case-sensitive fashion,
declare it as BINARY. See section 14.2.6  CREATE TABLE Syntax.





Simple comparison operations (>=, >, =, <, <=, sorting, and grouping)
are based on each character's ``sort value.'' Characters with the same
sort value (such as `E', `e', and `'e') are treated as the
same character.





If you are using Chinese data in the so-called big5 encoding, you
want to make all character columns BINARY. This works because the
sorting order of big5 encoding characters is based on the order of
ASCII codes. As of MySQL 4.1, you can explicitly declare that a column should
use the big5 character set:





CREATE TABLE t (name CHAR(40) CHARACTER SET big5);






A.5.2  Problems Using DATE Columns

















The format of a DATE value is 'YYYY-MM-DD'. According to
standard SQL, no other format is allowed. You should use this format in
UPDATE expressions and in the WHERE clause of SELECT
statements.  For example:





mysql> SELECT * FROM tbl_name WHERE date >= '2003-05-05';





As a convenience, MySQL automatically converts a date to a number if
the date is used in a numeric context (and vice versa). It is also smart
enough to allow a ``relaxed'' string form when updating and in a WHERE
clause that compares a date to a TIMESTAMP, DATE, or
DATETIME column.  (``Relaxed form'' means that any punctuation character
may be used as the separator between parts. For example, '2004-08-15'
and '2004#08#15' are equivalent.) MySQL can also convert a
string containing no separators (such as '20040815'), provided it
makes sense as a date.





The special date '0000-00-00' can be stored and retrieved as
'0000-00-00'. When using a '0000-00-00' date through
MyODBC, it is automatically converted to NULL in
MyODBC 2.50.12 and above, because ODBC can't handle this kind of
date.





Because MySQL performs the conversions described above, the following
statements work:





mysql> INSERT INTO tbl_name (idate) VALUES (19970505);
mysql> INSERT INTO tbl_name (idate) VALUES ('19970505');
mysql> INSERT INTO tbl_name (idate) VALUES ('97-05-05');
mysql> INSERT INTO tbl_name (idate) VALUES ('1997.05.05');
mysql> INSERT INTO tbl_name (idate) VALUES ('1997 05 05');
mysql> INSERT INTO tbl_name (idate) VALUES ('0000-00-00');

mysql> SELECT idate FROM tbl_name WHERE idate >= '1997-05-05';
mysql> SELECT idate FROM tbl_name WHERE idate >= 19970505;
mysql> SELECT MOD(idate,100) FROM tbl_name WHERE idate >= 19970505;
mysql> SELECT idate FROM tbl_name WHERE idate >= '19970505';





However, the following will not work:





mysql> SELECT idate FROM tbl_name WHERE STRCMP(idate,'20030505')=0;





STRCMP() is a string function, so it converts idate to a
string in 'YYYY-MM-DD' format and performs a string comparison.
It does not convert '20030505' to the date '2003-05-05'
and perform a date comparison.





If you are using the ALLOW_INVALID_DATES SQL mode, MySQL allows you to
store dates that are given only limited checking: MySQL ensures only that the
day is in the range from 1 to 31 and the month is in the range from 1 to 12.





This makes MySQL very convenient for Web applications where you obtain
year, month, and day in three different fields and you want to store exactly
what the user inserted (without date validation).





If you are not using the NO_ZERO_IN_DATE SQL mode, the day or month
part can be zero.  This is convenient if you want to store a birthdate
in a DATE column and you know only part of the date.





If you are not using the NO_ZERO_DATE SQL mode, MySQL also allows
you to store '0000-00-00' as a ``dummy date.'' This is in some cases
more convenient than using NULL values.





If the date cannot be converted to any reasonable value, a 0 is
stored in the DATE column, which will be retrieved as
'0000-00-00'.  This is both a speed and a convenience issue. We believe
that the database server's responsibility is to retrieve the same date you
stored (even if the data was not logically correct in all cases).  We think
it is up to the application and not the server to check the dates.





If you want MySQL to check all dates and accept only legal dates
(unless overriden by IGNORE), you should set sql_mode to
"NO_ZERO_IN_DATE,NO_ZERO_DATE".





Date handling in MySQL 5.0.1 and earlier works like MySQL 5.0.2 with the
ALLOW_INVALID_DATES SQL mode enabled.







A.5.3  Problems with NULL Values
















The concept of the NULL value is a common source of confusion for
newcomers to SQL, who often think that NULL is the same thing as an
empty string ''.  This is not the case. For example, the following
statements are completely different:





mysql> INSERT INTO my_table (phone) VALUES (NULL);
mysql> INSERT INTO my_table (phone) VALUES ('');





Both statements insert a value into the phone column, but the first
inserts a NULL value and the second inserts an empty string.  The
meaning of the first can be regarded as ``phone number is not known'' and the
meaning of the second can be regarded as ``the person is known to have no
phone, and thus no phone number.''





To help with NULL handling, you can use the IS NULL and
IS NOT NULL operators and the IFNULL() function.





In SQL, the NULL value is never true in comparison to any
other value, even NULL.  An expression that contains NULL
always produces a NULL value unless otherwise indicated in
the documentation for the operators and functions involved in the
expression. All columns in the following example return NULL:





mysql> SELECT NULL, 1+NULL, CONCAT('Invisible',NULL);





If you want to search for column values that are NULL, you
cannot use an expr = NULL test. The following statement returns no
rows, because expr = NULL is never true for any expression:





mysql> SELECT * FROM my_table WHERE phone = NULL;





To look for NULL values, you must use the IS NULL test.
The following statements show how to find the NULL phone number and the
empty phone number:





mysql> SELECT * FROM my_table WHERE phone IS NULL;
mysql> SELECT * FROM my_table WHERE phone = '';





You can add an index on a column that can have NULL
values if you are using MySQL 3.23.2 or newer and are using the
MyISAM, InnoDB, or BDB storage engine.
As of MySQL 4.0.2, the MEMORY storage engine also supports NULL
values in indexes.  Otherwise, you must declare an indexed column NOT
NULL and you cannot insert NULL into the column.






When reading data with LOAD DATA INFILE, empty or missing columns
are updated with ''. If you want a NULL value in a column,
you should use \N in the data file. The literal word ``NULL''
may also be used under some circumstances.
See section 14.1.5  LOAD DATA INFILE Syntax.





When using DISTINCT, GROUP BY, or ORDER BY, all
NULL values are regarded as equal.





When using ORDER BY, NULL values are presented first, or
last if you specify DESC to sort in descending order. Exception:
In MySQL 4.0.2 through 4.0.10, NULL values sort first
regardless of sort order.





Aggregate (summary) functions such as COUNT(), MIN(), and
SUM() ignore NULL values. The exception to this is
COUNT(*), which counts rows and not individual column values.
For example, the following statement produces two counts.
The first is a count of the number of rows in the table, and the second
is a count of the number of non-NULL values in the age
column:





mysql> SELECT COUNT(*), COUNT(age) FROM person;














For some column types, MySQL handles NULL values specially.  If you
insert NULL into a TIMESTAMP column, the
current date and time is inserted.  If you insert NULL into an
integer column that has the AUTO_INCREMENT attribute, the next
number in the sequence is inserted.







A.5.4  Problems with Column Aliases










You can use an alias to refer to a column in GROUP BY,
ORDER BY, or HAVING clauses.  Aliases can also be used
to give columns better names:





SELECT SQRT(a*b) AS root FROM tbl_name GROUP BY root HAVING root > 0;
SELECT id, COUNT(*) AS cnt FROM tbl_name GROUP BY id HAVING cnt > 0;
SELECT id AS 'Customer identity' FROM tbl_name;





Standard SQL doesn't allow you to refer to a column alias in a
WHERE clause.  This is because when the WHERE code is
executed, the column value may not yet be determined. For example, the
following query is illegal:





SELECT id, COUNT(*) AS cnt FROM tbl_name WHERE cnt > 0 GROUP BY id;





The WHERE statement is executed to determine which rows should
be included in the GROUP BY part, whereas HAVING is used to
decide which rows from the result set should be used.







A.5.5  Rollback Failure for Non-Transactional Tables










If you receive the following message when trying to perform a
ROLLBACK, it means that one or more of the tables you used in the
transaction do not support transactions:





Warning: Some non-transactional changed tables couldn't be rolled back





These non-transactional tables will not be affected by the ROLLBACK
statement.





If you were not deliberately mixing transactional and non-transactional
tables within the transaction, the most likely cause for this message is
that a table you thought was transactional actually is not. This can happen
if you try to create a table using a transactional storage engine that is
not supported by your @command{mysqld} server (or that was disabled with a
startup option).  If @command{mysqld} doesn't support a storage engine, it will
instead create the table as a MyISAM table, which is
non-transactional.





You can check the table type for a table by using either of these statements:





SHOW TABLE STATUS LIKE 'tbl_name';
SHOW CREATE TABLE tbl_name;





See section 14.5.3.18  SHOW TABLE STATUS Syntax and
section 14.5.3.6  SHOW CREATE TABLE Syntax.





You can check which storage engines your @command{mysqld} server supports by
using this statement:





SHOW ENGINES;





Before MySQL 4.1.2, SHOW ENGINES is unavailable. Use the following
statement instead and check the value of the variable that is associated
with the storage engine in which you are interested:





SHOW VARIABLES LIKE 'have_%';





For example, to determine whether the InnoDB storage engine is
available, check the value of the have_innodb variable.





See section 14.5.3.9  SHOW ENGINES Syntax and
section 14.5.3.20  SHOW VARIABLES Syntax.







A.5.6  Deleting Rows from Related Tables












MySQL does not support subqueries prior to Version 4.1, or the use of more
than one table in the DELETE statement prior to Version 4.0. If your
version of MySQL does not support subqueries or multiple-table DELETE
statements, you can use the following approach to delete rows from two
related tables:





    

  1. 

SELECT the rows based on some WHERE condition in the main table.

  2. 

DELETE the rows in the main table based on the same condition.

  3. 

DELETE FROM related_table WHERE related_column IN (selected_rows).





If the total length of the DELETE statement for related_table is
more than 1MB (the default value of the max_allowed_packet system
variable), you should split it into smaller parts and execute multiple
DELETE statements.  You will probably get the fastest DELETE
by specifying only 100 to 1,000 related_column values per statement if the
related_column is indexed.  If the related_column isn't
indexed, the speed is independent of the number of arguments in the
IN clause.







A.5.7  Solving Problems with No Matching Rows











If you have a complicated query that uses many tables but that doesn't
return any rows, you should use the following procedure to find out what
is wrong:





    

  1. 

Test the query with EXPLAIN to check whether you can find something
that is obviously wrong.
See section 7.2.1  EXPLAIN Syntax (Get Information About a SELECT).


  2. 

Select only those columns that are used in the WHERE clause.


  3. 

Remove one table at a time from the query until it returns some rows.
If the tables are large, it's a good idea to use LIMIT 10 with the query.


  4. 

Issue a SELECT for the column that should have matched a row against
the table that was last removed from the query.


  5. 

If you are comparing FLOAT or DOUBLE columns with numbers that
have decimals, you can't use equality (=) comparisons. This problem
is common in most computer languages because not all floating-point values
can be stored with exact precision.  In some cases, changing the
FLOAT to a DOUBLE will fix this.
See section A.5.8  Problems with Floating-Point Comparisons.


  6. 

If you still can't figure out what's wrong, create a minimal test that can
be run with mysql test < query.sql that shows your problems.  You can
create a test file by dumping the tables with @command{mysqldump --quick
db_name tbl_name_1 ... tbl_name_n > query.sql}. Open the file in an editor,
remove some insert lines (if there are more than needed to demonstrate
the problem), and add your SELECT statement at the end of the file.

Verify that the test file demonstrates the problem by executing these
commands:



    shell> mysqladmin create test2
shell> mysql test2 < query.sql

    

Post the test file using @command{mysqlbug} to the general MySQL mailing list.
See section 1.7.1.1  The MySQL Mailing Lists.






A.5.8  Problems with Floating-Point Comparisons




Floating-point numbers sometimes cause confusion because they
are not stored as exact values inside computer architecture. What you
can see on the screen usually is not the exact value of the number.
The column types FLOAT, DOUBLE, and DECIMAL are such.
DECIMAL columns store values with exact precision because they are
represented as strings, but calculations on DECIMAL values may be done
using floating-point operations.





The following example demonstrate the problem. It shows that even for the
DECIMAL column type, calculations that are done using floating-point
operations are subject to floating-point error.





mysql> CREATE TABLE t1 (i INT, d1 DECIMAL(9,2), d2 DECIMAL(9,2));
mysql> INSERT INTO t1 VALUES (1, 101.40, 21.40), (1, -80.00, 0.00),
    -> (2, 0.00, 0.00), (2, -13.20, 0.00), (2, 59.60, 46.40),
    -> (2, 30.40, 30.40), (3, 37.00, 7.40), (3, -29.60, 0.00),
    -> (4, 60.00, 15.40), (4, -10.60, 0.00), (4, -34.00, 0.00),
    -> (5, 33.00, 0.00), (5, -25.80, 0.00), (5, 0.00, 7.20),
    -> (6, 0.00, 0.00), (6, -51.40, 0.00);

mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b
    -> FROM t1 GROUP BY i HAVING a <> b;
+------+--------+-------+
| i    | a      | b     |
+------+--------+-------+
|    1 |  21.40 | 21.40 |
|    2 |  76.80 | 76.80 |
|    3 |   7.40 |  7.40 |
|    4 |  15.40 | 15.40 |
|    5 |   7.20 |  7.20 |
|    6 | -51.40 |  0.00 |
+------+--------+-------+





The result is correct. Although the first five records look like they
shouldn't pass the comparison test (the values of a and b do
not appear to be different), they may do so because the difference between
the numbers shows up around the tenth decimal or so, depending on computer
architecture.





The problem cannot be solved by using ROUND() or similar functions,
because the result is still a floating-point number:





mysql> SELECT i, ROUND(SUM(d1), 2) AS a, ROUND(SUM(d2), 2) AS b
    -> FROM t1 GROUP BY i HAVING a <> b;
+------+--------+-------+
| i    | a      | b     |
+------+--------+-------+
|    1 |  21.40 | 21.40 |
|    2 |  76.80 | 76.80 |
|    3 |   7.40 |  7.40 |
|    4 |  15.40 | 15.40 |
|    5 |   7.20 |  7.20 |
|    6 | -51.40 |  0.00 |
+------+--------+-------+





This is what the numbers in column a look like when displayed with more
decimal places:





mysql> SELECT i, ROUND(SUM(d1), 2)*1.0000000000000000 AS a,
    -> ROUND(SUM(d2), 2) AS b FROM t1 GROUP BY i HAVING a <> b;
+------+----------------------+-------+
| i    | a                    | b     |
+------+----------------------+-------+
|    1 |  21.3999999999999986 | 21.40 |
|    2 |  76.7999999999999972 | 76.80 |
|    3 |   7.4000000000000004 |  7.40 |
|    4 |  15.4000000000000004 | 15.40 |
|    5 |   7.2000000000000002 |  7.20 |
|    6 | -51.3999999999999986 |  0.00 |
+------+----------------------+-------+





Depending on your computer architecture, you may or may not see similar
results.  Different CPUs may evaluate floating-point numbers differently.
For example, on some machines you may get the ``correct'' results by
multiplying both arguments by 1, as the following example shows.





Warning: Never use this method in your applications. It is not
an example of a trustworthy method!





mysql> SELECT i, ROUND(SUM(d1), 2)*1 AS a, ROUND(SUM(d2), 2)*1 AS b
    -> FROM t1 GROUP BY i HAVING a <> b;
+------+--------+------+
| i    | a      | b    |
+------+--------+------+
|    6 | -51.40 | 0.00 |
+------+--------+------+





The reason that the preceding example seems to work is that on the particular
machine where the test was done, CPU floating-point arithmetic happens to
round the numbers to the same value. However, there is no rule that any CPU
should do so, so this method cannot be trusted.





The correct way to do floating-point number comparison is to first
decide on an acceptable tolerance for differences between the numbers and
then do the comparison against the tolerance value. For example, if we
agree that floating-point numbers should be regarded the same if they are
same within a precision of one in ten thousand (0.0001), the comparison
should be written to find differences larger than the tolerance value:





mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b FROM t1
    -> GROUP BY i HAVING ABS(a - b) > 0.0001;
+------+--------+------+
| i    | a      | b    |
+------+--------+------+
|    6 | -51.40 | 0.00 |
+------+--------+------+
1 row in set (0.00 sec)





Conversely, to get rows where the numbers are the same, the test should
find differences within the tolerance value:





mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b FROM t1
    -> GROUP BY i HAVING ABS(a - b) <= 0.0001;
+------+-------+-------+
| i    | a     | b     |
+------+-------+-------+
|    1 | 21.40 | 21.40 |
|    2 | 76.80 | 76.80 |
|    3 |  7.40 |  7.40 |
|    4 | 15.40 | 15.40 |
|    5 |  7.20 |  7.20 |
+------+-------+-------+






A.6  Optimizer-Related Issues




MySQL uses a cost-based optimizer to determine the best way to resolve a
query.  In many cases, MySQL can calculate the best possible query plan,
but sometimes MySQL doesn't have enough information about the data
at hand and has to make ``educated'' guesses about the data.





For the cases when MySQL does not do the "right" thing, tools that you
have available to help MySQL are:









A.7  Table Definition-Related Issues





A.7.1  Problems with ALTER TABLE












ALTER TABLE changes a table to the current character set.
If you get a duplicate-key error during ALTER TABLE, the cause
is either that the new character sets maps two keys to the same value
or that the table is corrupted. In the latter case, you should run
REPAIR TABLE on the table.





If ALTER TABLE dies with the following error, the problem may be that
MySQL crashed during an earlier ALTER TABLE operation and there is an
old table named `A-xxx' or `B-xxx' lying around:





Error on rename of './database/name.frm'
to './database/B-xxx.frm' (Errcode: 17)





In this case, go to the MySQL data directory and delete all files that have
names starting with A- or B-.  (You may want to move them
elsewhere instead of deleting them.)





ALTER TABLE works in the following way:








If something goes wrong with the renaming operation, MySQL tries to
undo the changes.  If something goes seriously wrong (although this shouldn't
happen), MySQL may leave the old table as `B-xxx'. A
simple rename of the table files at the system level should get your data back.





If you use ALTER TABLE on a transactional table or if you are using
Windows or OS/2, ALTER TABLE will UNLOCK the table if you had
done a LOCK TABLE on it. This is because InnoDB and these
operating systems cannot drop a table that is in use.







A.7.2  How to Change the Order of Columns in a Table













First, consider whether you really need to change the column order in a
table.  The whole point of SQL is to abstract the application from the data
storage format.  You should always specify the order in which you wish to
retrieve your data.  The first of the following statements returns columns
in the order col_name1, col_name2, col_name3, whereas
the second returns them in the order col_name1, col_name3,
col_name2:





mysql> SELECT col_name1, col_name2, col_name3 FROM tbl_name;
mysql> SELECT col_name1, col_name3, col_name2 FROM tbl_name;





If you decide to change the order of table columns anyway, you can do so as
follows:





    

  1. 

Create a new table with the columns in the new order. 

  2. 

Execute this statement:



    mysql> INSERT INTO new_table
    -> SELECT columns-in-new-order FROM old_table;

    


  3. 

Drop or rename old_table.

  4. 

Rename the new table to the original name:



    mysql> ALTER TABLE new_table RENAME old_table;

    






SELECT * is quite suitable for testing queries. However, in an
application, you should never rely on using SELECT * and
retrieving the columns based on their position. The order and position
in which columns are returned will not remain the same if you add, move,
or delete columns. A simple change to your table structure will cause
your application to fail.







A.7.3  TEMPORARY TABLE Problems










The following list indicates limitations on the use of TEMPORARY
tables:









B  Credits










This appendix lists the developers, contributors, and supporters that have
helped to make MySQL what it is today.







B.1  Developers at MySQL AB




These are the developers that are or have been employed by MySQL AB
to work on the MySQL database software, roughly in the order they
started to work with us. Following each developer is a small list of the
tasks that the developer is responsible for, or the accomplishments they
have made. All developers are involved in support.








Michael (Monty) Widenius




    

  • 

Lead developer and main author of the MySQL server (@command{mysqld}).

  • 

New functions for the string library.

  • 

Most of the mysys library.

  • 

The ISAM and MyISAM libraries (B-tree index file
handlers with index compression and different record formats).

  • 

The HEAP library. A memory table system with our superior full dynamic
hashing. In use since 1981 and published around 1984.

  • 

The @command{replace} program (take a look at it, it's COOL!).

  • 

Connector/ODBC (MyODBC), the ODBC driver for Windows.

  • 

Fixing bugs in MIT-pthreads to get it to work for MySQL Server. And
also Unireg, a curses-based application tool with many utilities.

  • 

Porting of mSQL tools like msqlperl, DBD/DBI, and
DB2mysql.

  • 

Most of crash-me and the foundation for the MySQL benchmarks.




David Axmark




    

  • 

Initial main writer of the Reference Manual,
including enhancements to @command{texi2html}.

  • 

Automatic Web site updating from the manual.

  • 

Initial Autoconf, Automake, and Libtool support.

  • 

Licensing.

  • 

Parts of all the text files.  (Nowadays only the `README' is
left. The rest ended up in the manual.)

  • 

Lots of testing of new features.

  • 

Our in-house Free Software legal expert.

  • 

Mailing list maintainer (who never has the time to do it right...).

  • 

Our original portability code (more than 10 years old now). Nowadays
only some parts of mysys are left.

  • 

Someone for Monty to call in the middle of the night when he just got
that new feature to work.

  • 

Chief "Open Sourcerer" (MySQL community relations).




Jani Tolonen




    

  • 

@command{mysqlimport}

  • 

A lot of extensions to the command-line clients.

  • 

PROCEDURE ANALYSE()




Sinisa Milivojevic




    

  • 

Compression (with zlib) in the client/server protocol.

  • 

Perfect hashing for the lexical analyzer phase.

  • 

Multi-row INSERT

  • 

@command{mysqldump} -e option

  • 

LOAD DATA LOCAL INFILE

  • 

SQL_CALC_FOUND_ROWS SELECT option

  • 

--max-user-connections=... option

  • 

net_read and net_write_timeout

  • 

GRANT/REVOKE and SHOW GRANTS FOR

  • 

New client/server protocol for 4.0

  • 

UNION in 4.0

  • 

Multiple-table DELETE/UPDATE

  • 

Derived tables in 4.1

  • 

User resources management

  • 

Initial developer of the MySQL++ C++ API and the MySQLGUI client.




Tonu Samuel (past developer)




    

  • 

VIO interface (the foundation for the encrypted client/server protocol).

  • 

MySQL Filesystem (a way to use MySQL databases as files
and directories).

  • 

The CASE expression.

  • 

The MD5() and COALESCE() functions.

  • 

RAID support for MyISAM tables.




Sasha Pachev




    

  • 

Initial implementation of replication (up to version 4.0).

  • 

SHOW CREATE TABLE.

  • 

mysql-bench




Matt Wagner




    

  • 

MySQL test suite.

  • 

Webmaster (until 2002).

  • 

Coordinator of development.




Miguel Solorzano




    

  • 

Win32 development and release builds.

  • 

Windows NT server code.

  • 

WinMySQLAdmin




Timothy Smith (past developer)




    

  • 

Dynamic character sets support.

  • 

configure, RPMs and other parts of the build system.

  • 

Initial developer of libmysqld, the embedded server.




Sergei Golubchik




    

  • 

Full-text search.

  • 

Added keys to the MERGE library.




Jeremy Cole




    

  • 

Proofreading and editing this fine manual.

  • 

ALTER TABLE ... ORDER BY ....

  • 

UPDATE ... ORDER BY ....

  • 

DELETE ... ORDER BY ....




Indrek Siitan




    

  • 

Designing/programming of our Web interface.

  • 

Author of our newsletter management system.




Jorge del Conde




    

  • 

@command{MySQLCC} (@command{MySQL Control Center})

  • 

Win32 development

  • 

Initial implementation of the Web site portals.




Venu Anuganti




    

  • 

MyODBC 3.51

  • 

New client/server protocol for 4.1 (for prepared statements).




Arjen Lentz




    

  • 

Maintainer of the MySQL Reference Manual.

  • 

Preparing the O'Reilly printed edition of the manual.




Alexander (Bar) Barkov, Alexey (Holyfoot) Botchkov, and Ramil Kalimullin




    

  • 

Spatial data (GIS) and R-Trees implementation for 4.1

  • 

Unicode and character sets for 4.1; documentation for same




Oleksandr (Sanja) Byelkin




    

  • 

Query cache in 4.0

  • 

Implementation of subqueries (4.1).




Aleksey (Walrus) Kishkin and Alexey (Ranger) Stroganov




    

  • 

Benchmarks design and analysis.

  • 

Maintenance of the MySQL test suite.




Zak Greant




    

  • 

Open Source advocate, MySQL community relations.




Carsten Pedersen




    

  • 

The MySQL Certification program.




Lenz Grimmer




    

  • 

Production (build and release) engineering.




Peter Zaitsev




    

  • 

SHA1(), AES_ENCRYPT() and AES_DECRYPT() functions.

  • 

Debugging, cleaning up various features.




Alexander (Salle) Keremidarski




    

  • 

Support.

  • 

Debugging.




Per-Erik Martin




    

  • 

Lead developer for stored procedures (5.0) and triggers.




Jim Winstead




    

  • 

Lead Web developer.




Mark Matthews




    

  • 

Connector/J driver (Java).




Peter Gulutzan




    

  • 

SQL standards compliance.

  • 

Documentation of existing MySQL code/algorithms.

  • 

Character set documentation.




Guilhem Bichot




    

  • 

Replication, from MySQL version 4.0.

  • 

Fixed handling of exponents for DECIMAL.

  • 

Author of mysql_tableinfo.




Antony T. Curtis




    

  • 

Porting of the MySQL Database software to OS/2.




Mikael Ronstrom




    

  • 

Much of the initial work on NDB Cluster until 2000. Roughly half the
code base at that time. Transaction protocol, node recovery, system
restart and restart code and parts of the API functionality.

  • 

Lead Architect, developer, debugger of NDB Cluster 1994-2004

  • 

Lots of optimisations




Jonas Oreland




    

  • 

On-line Backup

  • 

The automatic test environment of MySQL Cluster

  • 

Portability Library for NDB Cluster

  • 

Lots of other things




Pekka Nouisiainen




    

  • 

Ordered index implementation of MySQL Cluster

  • 

BLOB support in MySQL Cluster

  • 

Charset support in MySQL Cluster




Martin Skold




    

  • 

Unique index implementation of MySQL Cluster

  • 

Integration of NDB Cluster into MySQL




Magnus Svensson




    

  • 

The test framework for MySQL Cluster

  • 

Integration of NDB Cluster into MySQL




Tomas Ulin




    

  • 

Lots of work on configuration changes for simple installation and use of
MySQL Cluster









B.2  Contributors to MySQL










While MySQL AB owns all copyrights in the MySQL server
and the MySQL manual, we wish to recognize those who have made
contributions of one kind or another to the MySQL distribution.
Contributors are listed here, in somewhat random order:







Gianmassimo Vigazzola [email protected] or [email protected]


The initial port to Win32/NT.

Per Eric Olsson


For more or less constructive criticism and real testing of the dynamic
record format.

Irena Pancirov [email protected]


Win32 port with Borland compiler.  mysqlshutdown.exe and
mysqlwatch.exe

David J. Hughes


For the effort to make a shareware SQL database. At TcX, the predecessor
of MySQL AB, we started with mSQL, but found that it couldn't
satisfy our purposes so instead we wrote an SQL interface to our
application builder Unireg.  @command{mysqladmin} and @command{mysql} client are
programs that were largely influenced by their mSQL counterparts.
We have put a lot of effort into making the MySQL syntax a superset of
mSQL. Many of the API's ideas are borrowed from mSQL to
make it easy to port free mSQL programs to the MySQL API.
The MySQL software doesn't contain any code from mSQL.
Two files in the distribution (`client/insert_test.c' and
`client/select_test.c') are based on the corresponding (non-copyrighted)
files in the mSQL distribution, but are modified as examples showing
the changes necessary to convert code from mSQL to MySQL Server.
(mSQL is copyrighted David J. Hughes.)

Patrick Lynch


For helping us acquire http://www.mysql.com/.

Fred Lindberg


For setting up qmail to handle the MySQL mailing list and for the
incredible help we got in managing the MySQL mailing lists.

Igor Romanenko [email protected]


@command{mysqldump} (previously msqldump, but ported and enhanced by
Monty).

Yuri Dario


For keeping up and extending the MySQL OS/2 port.

Tim Bunce


Author of @command{mysqlhotcopy}.

Zarko Mocnik [email protected]


Sorting for Slovenian language.

"TAMITO" [email protected]


The _MB character set macros and the ujis and sjis character sets.

Joshua Chamas [email protected]


Base for concurrent insert, extended date syntax, debugging on NT, and
answering on the MySQL mailing list.

Yves Carlier [email protected]


@command{mysqlaccess}, a program to show the access rights for a user.

Rhys Jones [email protected] (And GWE Technologies Limited)


For one of the early JDBC drivers.

Dr Xiaokun Kelvin ZHU [email protected]


Further development of one of the early JDBC drivers and other MySQL-related
Java tools.

James Cooper [email protected]


For setting up a searchable mailing list archive at his site.

Rick Mehalick [email protected]


For xmysql, a graphical X client for MySQL Server.

Doug Sisk [email protected]


For providing RPM packages of MySQL for Red Hat Linux.

Diemand Alexander V. [email protected]


For providing RPM packages of MySQL for Red Hat Linux-Alpha.

Antoni Pamies Olive [email protected]


For providing RPM versions of a lot of MySQL clients for Intel
and SPARC.

Jay Bloodworth [email protected]


For providing RPM versions for MySQL 3.21.

David Sacerdote [email protected]


Ideas for secure checking of DNS hostnames.

Wei-Jou Chen [email protected]


Some support for Chinese(BIG5) characters.

Wei He [email protected]


A lot of functionality for the Chinese(GBK) character set.

Jan Pazdziora [email protected]


Czech sorting order.

Zeev Suraski [email protected]


FROM_UNIXTIME() time formatting, ENCRYPT() functions, and
@command{bison} advisor.
Active mailing list member.

Luuk de Boer [email protected]


Ported (and extended) the benchmark suite to DBI/DBD. Have
been of great help with crash-me and running benchmarks. Some new
date functions. The @command{mysql_setpermission} script.

Alexis Mikhailov [email protected]


User-defined functions (UDFs); CREATE FUNCTION and
DROP FUNCTION.

Andreas F. Bobak [email protected]


The AGGREGATE extension to UDF functions.

Ross Wakelin [email protected]


Help to set up InstallShield for MySQL-Win32.

Jethro Wright III [email protected]


The `libmysql.dll' library.

James Pereria [email protected]


Mysqlmanager, a Win32 GUI tool for administering MySQL Servers.

Curt Sampson [email protected]


Porting of MIT-pthreads to NetBSD/Alpha and NetBSD 1.3/i386.

Martin Ramsch [email protected]


Examples in the MySQL Tutorial.

Steve Harvey


For making @command{mysqlaccess} more secure.

Konark IA-64 Centre of Persistent Systems Private Limited


http://www.pspl.co.in/konark/.  Help with the Win64 port of the
MySQL server.

Albert Chin-A-Young.


Configure updates for Tru64, large file support and better TCP wrappers
support.

John Birrell


Emulation of pthread_mutex() for OS/2.

Benjamin Pflugmann


Extended MERGE tables to handle INSERTS. Active member
on the MySQL mailing lists.

Jocelyn Fournier


Excellent spotting and reporting innumerable bugs
(especially in the MySQL 4.1 subquery code).

Marc Liyanage


Maintaining the Mac OS X packages and providing invaluable feedback
on how to create Mac OS X PKGs.

Robert Rutherford


Providing invaluable information and feedback about the QNX port.

Previous developers of NDB Cluster


Lots of people were involved in various ways summer students, master thesis
students, employees. In total more than 100 people so too many to mention here.
Notable name is Ataullah Dabaghi who up until 1999 contributed around a
third of the code base. A special thanks also to developers of the AXE system
which provided much of the architecturial foundations for NDB Cluster with
blocks, signals and crash tracing functionality. Also credit should be given
to those who believed in the ideas enough to allocate of their budgets for its
development from 1992 to present time.





Other contributors, bugfinders, and testers: James H. Thompson, Maurizio
Menghini, Wojciech Tryc, Luca Berra, Zarko Mocnik, Wim Bonis, Elmar
Haneke, jehamby@lightside, [email protected],
[email protected], Ted Deppner [email protected],
Mike Simons, Jaakko Hyvatti.





And lots of bug report/patches from the folks on the mailing list.





A big tribute goes to those that help us answer questions on the
MySQL mailing lists:







Daniel Koch [email protected]


Irix setup.

Luuk de Boer [email protected]


Benchmark questions.

Tim Sailer [email protected]


DBD::mysql questions.

Boyd Lynn Gerber [email protected]


SCO-related questions.

Richard Mehalick [email protected]


xmysql-related questions and basic installation questions.

Zeev Suraski [email protected]


Apache module configuration questions (log & auth), PHP-related
questions, SQL syntax-related questions and other general questions.

Francesc Guasch [email protected]


General questions.

Jonathan J Smith [email protected]


Questions pertaining to OS-specifics with Linux, SQL syntax, and other
things that might need some work.

David Sklar [email protected]


Using MySQL from PHP and Perl.

Alistair MacDonald [email protected]


Not yet specified, but is flexible and can handle Linux and maybe HP-UX.
Will try to get user to use @command{mysqlbug}.

John Lyon [email protected]


Questions about installing MySQL on Linux systems, using either
`.rpm' files or compiling from source.

Lorvid Ltd. [email protected]


Simple billing/license/support/copyright issues.

Patrick Sherrill [email protected]


ODBC and VisualC++ interface questions.

Randy Harmon [email protected]


DBD, Linux, some SQL syntax questions.






B.3  Documenters and translators











The following people has helped us with writing the MySQL documentation and
translating the documentation or error messages in MySQL.







Paul DuBois


Ongoing help with making this manual correct and understandable.
That includes rewriting Monty's and David's attempts at English into
English as other people know it.

Kim Aldale


Helped to rewrite Monty's and David's early attempts at English into English.

Michael J. Miller Jr. [email protected]


For the first MySQL manual. And a lot of spelling/language
fixes for the FAQ (that turned into the MySQL manual a long
time ago).

Yan Cailin


First translator of the MySQL Reference Manual into simplified
Chinese in early 2000 on which the Big5 and HK coded
(http://mysql.hitstar.com/) versions were
based.  Personal home page at
linuxdb.yeah.net.

Jay Flaherty [email protected]


Big parts of the Perl DBI/DBD section in the manual.

Paul Southworth [email protected], Ray Loyzaga [email protected]


Proof-reading of the Reference Manual.

Therrien Gilbert [email protected], Jean-Marc Pouyot [email protected]


French error messages.

Petr Snajdr, [email protected]


Czech error messages.

Jaroslaw Lewandowski [email protected]


Polish error messages.

Miguel Angel Fernandez Roiz


Spanish error messages.

Roy-Magne Mo [email protected]


Norwegian error messages and testing of MySQL 3.21.xx.

Timur I. Bakeyev [email protected]


Russian error messages.

[email protected] & Filippo Grassilli [email protected]


Italian error messages.

Dirk Munzinger [email protected]


German error messages.

Billik Stefan [email protected]


Slovak error messages.

Stefan Saroiu [email protected]


Romanian error messages.

Peter Feher


Hungarian error messages.

Roberto M. Serqueira


Portuguese error messages.

Carsten H. Pedersen


Danish error messages.

Arjen G. Lentz


Dutch error messages, completing earlier partial translation
(also work on consistency and spelling).






B.4  Libraries used by and included with MySQL










The following is a list of the creators of the libraries we have
included with the MySQL server source to make it easy to compile and
install MySQL. We are very thankfully to all individuals that have
created these and it has made our life much easier.







Fred Fish


For his excellent C debugging and trace library. Monty has made a number
of smaller improvements to the library (speed and additional options).

Richard A. O'Keefe


For his public domain string library.

Henry Spencer


For his regex library, used in WHERE column REGEXP regexp.

Chris Provenzano


Portable user level pthreads. From the copyright: This product includes
software developed by Chris Provenzano, the University of California,
Berkeley, and contributors.  We are currently using version 1_60_beta6
patched by Monty (see `mit-pthreads/Changes-mysql').

Jean-loup Gailly and Mark Adler


For the zlib library (used on MySQL on Windows).

Bjorn Benson


For his safe_malloc (memory checker) package which is used in when you
configure MySQL with --debug.

Free Software Foundation


The readline library (used by the @command{mysql} command-line client).

The NetBSD foundation


The libedit package (optionally used by the @command{mysql} command-line
client).






B.5  Packages that support MySQL











The following is a list of creators/maintainers of some of the most
important API/packages/applications that a lot of people use with MySQL.





We can't list every possible package here because the list would then be
way to hard to maintain. For other packages, please refer to the
software portal at http://solutions.mysql.com/software/.







Tim Bunce, Alligator Descartes


For the DBD (Perl) interface.

Andreas Koenig [email protected]


For the Perl interface for MySQL Server.

Jochen Wiedmann [email protected]


For maintaining the Perl DBD::mysql module.

Eugene Chan [email protected]


For porting PHP for MySQL Server.

Georg Richter


MySQL 4.1 testing and bug hunting.
New PHP 5.0 mysqli extension (API) for use with MySQL 4.1 and up.

Giovanni Maruzzelli [email protected]


For porting iODBC (Unix ODBC).

Xavier Leroy [email protected]


The author of LinuxThreads (used by the MySQL Server on Linux).






B.6  Tools that were used to create MySQL










The following is a list of some of the tools we have used to create MySQL.
We use this to express our thanks to those that has created them as without
these we could not have made MySQL what is is today.







Free Software Foundation


From whom we got an excellent compiler (@command{gcc}), an excellent
debugger (@command{gdb} and the libc library (from which we have
borrowed `strto.c' to get some code working in Linux).

Free Software Foundation & The XEmacs development team


For a really great editor/environment used by almost everybody at
MySQL AB.

Julian Seward


Author of valgrind, an excellent memory checker tool that has helped
us find a lot of otherwise hard to find bugs in MySQL.

Dorothea L@"utkehaus and Andreas Zeller


For DDD (The Data Display Debugger) which is an excellent
graphical frontend to @command{gdb}).






B.7  Supporters of MySQL










While MySQL AB owns all copyrights in the MySQL server
and the MySQL manual, we wish to recognize the following companies,
which helped us finance the development of the MySQL server,
such as by paying us for developing a new feature or giving us hardware
for development of the MySQL server.







VA Linux / Andover.net


Funded replication.

NuSphere


Editing of the MySQL manual.

Stork Design studio


The MySQL Web site in use between 1998-2000.

Intel


Contributed to development on Windows and Linux platforms.

Compaq


Contributed to Development on Linux/Alpha.

SWSoft


Development on the embedded @command{mysqld} version.

FutureQuest


--skip-show-database






C  MySQL Change History












This appendix lists the changes from version to version in the MySQL
source code.





We are now working actively on MySQL 4.1 and 5.0, and will provide
only critical bugfixes for MySQL 4.0 and MySQL 3.23.  We update this section
as we add new features, so that everybody can follow the development.





Our TODO section contains what further plans we have for MySQL 4.1 and 5.0.
See section 1.6  MySQL and the Future (the TODO).





Note that we tend to update the manual at the same time we make changes to
MySQL.  If you find a recent version of MySQL listed here that you can't
find on our download page (http://dev.mysql.com/downloads/),
it means that the version has not yet been released.





The date mentioned with a release version is the date of the last BitKeeper
ChangeSet on which the release was based, not the date when
the packages were made available. The binaries are usually made
available a few days after the date of the tagged ChangeSet, because building
and testing all packages takes some time.







C.1  Changes in release 5.0.x (Development)










The following changelog shows what has already been done in the 5.0 tree:








For a full list of changes, please refer to the changelog sections for
each individual 5.0.x release.







C.1.1  Changes in release 5.0.2 (not released yet)




Functionality added or changed:






Bugs fixed:







C.1.2  Changes in release 5.0.1 (27 Jul 2004)




Note: This build passes our test suite and fixes a lot of reported
bugs found in the previous 5.0.0 release. However, please be aware that this is
not a ``standard MYSQL build'' in the sense that there are still some
open critical bugs in our bugs database at http://bugs.mysql.com/
that affect this release as well. We are actively fixing these and will
make a new release where these are fixed as soon as possible. However,
this binary should be a good candidate for testing new MySQL 5.0 features
for future products.





Functionality added or changed:






Bugs fixed:







C.1.3  Changes in release 5.0.0 (22 Dec 2003: Alpha)




Functionality added or changed:






Bugs fixed:







C.2  Changes in release 4.1.x (Gamma)










Version 4.1 of the MySQL server includes many enhancements and new features.
Binaries for this version are available for download at
http://dev.mysql.com/downloads/mysql-4.1.html.








For a full list of changes, please refer to the changelog sections for
each individual 4.1.x release.







C.2.1  Changes in release 4.1.8 (not released yet)




Functionality added or changed:






Bugs fixed:







C.2.2  Changes in release 4.1.7 (to be released soon)




Functionality added or changed:






Bugs fixed:







C.2.3  Changes in release 4.1.6 (10 Oct 2004)




Functionality added or changed:






Bugs fixed:







C.2.4  Changes in release 4.1.5 (16 Sep 2004)




Functionality added or changed:






Bugs fixed:







C.2.5  Changes in release 4.1.4 (26 Aug 2004: Gamma)




Note: To fix a compile problem on systems that do not have
automake 1.7 installed, an updated 4.1.4a source tarball has been
published.  In addition to resolving this automake dependency (Bug
#5319), it also fixes some reported libedit compile errors when using
a non-gcc compiler (Bug #5353).





Functionality added or changed:






Bugs fixed:







C.2.6  Changes in release 4.1.3 (28 Jun 2004: Beta)




Note: The initial release of MySQL 4.1.3 for Windows accidentally was
not compiled with support for the Spatial Extensions (OpenGIS). This was fixed
by rebuilding from the same 4.1 code snapshot with the missing option and
releasing those packages as version 4.1.3a.





To enable compiling the newly released PHP 5 against MySQL 4.1.3 on Windows, the
Windows packages had to be rebuilt once more to add a few missing symbols to
the MySQL client library. These packages were released as MySQL 4.1.3b.





Functionality added or changed:






Bugs fixed:







C.2.7  Changes in release 4.1.2 (28 May 2004)




Functionality added or changed:






Bugs fixed:







C.2.8  Changes in release 4.1.1 (01 Dec 2003)




This release includes all fixes in MySQL 4.0.16 and most of the fixes
in MySQL 4.0.17.





Functionality added or changed:






Bugs fixed:







C.2.9  Changes in release 4.1.0 (03 Apr 2003: Alpha)




Functionality added or changed:







C.3  Changes in release 4.0.x (Production)










Version 4.0 of the MySQL server includes many enhancements and new features:








For a full list of changes, please refer to the changelog sections for
each individual 4.0.x release.







C.3.1  Changes in release 4.0.22 (not released yet)




Functionality added or changed:






Bugs fixed:







C.3.2  Changes in release 4.0.21 (06 Sep 2004)




Functionality added or changed:






Bugs fixed:







C.3.3  Changes in release 4.0.20 (17 May 2004)




Note: The windows packages had to be repackaged and re-released
several times to resolve packaging issues (such as missing files). This did not
affect the binaries included (they have not been recompiled), therefore the
installation packages are of version 4.0.20d, while the binaries included still
identify themselves as version 4.0.20b.





Functionality added or changed:






Bugs fixed:







C.3.4  Changes in release 4.0.19 (04 May 2004)




Note: The MySQL 4.0.19 binaries were uploaded to the download
mirrors on May, 10th. However, a potential crashing bug was found just
before the 4.0.19 release was publicly announced and published from the
4.0 download pages at http://dev.mysql.com/.





A fix for the bug was pushed into the MySQL source tree shortly after it
could be reproduced and is included in MySQL 4.0.20. Users upgrading from
MySQL 4.0.18 should upgrade directly to MySQL 4.0.20 or later.





See (Bug #3596) for details (it was reported against MySQL-4.1, but was
confirmed to affect 4.0.19 as well).





Functionality added or changed:






Bugs fixed:







C.3.5  Changes in release 4.0.18 (12 Feb 2004)




Functionality added or changed:






Bugs fixed:







C.3.6  Changes in release 4.0.17 (14 Dec 2003)




Functionality added or changed:






Bugs fixed:







C.3.7  Changes in release 4.0.16 (17 Oct 2003)




Functionality added or changed:






Bugs fixed:







C.3.8  Changes in release 4.0.15 (03 Sep 2003)




IMPORTANT:





If you are using this release on Windows, you should upgrade at least your
clients (any program that uses libmysql.lib) to 4.0.16 or above.  This
is because the 4.0.15 release had a bug in the Windows client library that
causes Windows clients using the library to die with a Lost connection
to MySQL server during query error for queries that take more than 30
seconds.  This problem is specific to Windows; clients on other platforms are
unaffected.





Functionality added or changed:






Bugs fixed:







C.3.9  Changes in release 4.0.14 (18 Jul 2003)




Functionality added or changed:






Bugs fixed:







C.3.10  Changes in release 4.0.13 (16 May 2003)




Functionality added or changed:






Bugs fixed:







C.3.11  Changes in release 4.0.12 (15 Mar 2003: Production)




Functionality added or changed:






Bugs fixed:







C.3.12  Changes in release 4.0.11 (20 Feb 2003)




Functionality added or changed:






Bugs fixed:







C.3.13  Changes in release 4.0.10 (29 Jan 2003)




Functionality added or changed:






Bugs fixed:







C.3.14  Changes in release 4.0.9 (09 Jan 2003)




Functionality added or changed:






Bugs fixed:







C.3.15  Changes in release 4.0.8 (07 Jan 2003)




Functionality added or changed:






Bugs fixed:







C.3.16  Changes in release 4.0.7 (20 Dec 2002)




Functionality added or changed:






Bugs fixed:







C.3.17  Changes in release 4.0.6 (14 Dec 2002: Gamma)




Functionality added or changed:








Bugs fixed:









C.3.18  Changes in release 4.0.5 (13 Nov 2002)




Functionality added or changed:








Bugs fixed:









C.3.19  Changes in release 4.0.4 (29 Sep 2002)








C.3.20  Changes in release 4.0.3 (26 Aug 2002: Beta)








C.3.21  Changes in release 4.0.2 (01 Jul 2002)








C.3.22  Changes in release 4.0.1 (23 Dec 2001)








C.3.23  Changes in release 4.0.0 (Oct 2001: Alpha)








C.4  Changes in release 3.23.x (Recent; still supported)










Please note that since release 4.0 is now production level, only critical
fixes are done in the 3.23 release series. You are recommended to upgrade
when possible, to take advantage of all speed and feature improvements in
4.0. See section 2.5.3  Upgrading from Version 3.23 to 4.0.





The 3.23 release has several major features that are not
present in previous versions.  We have added three new
table types:







MyISAM


A new ISAM library which is tuned for SQL and supports large files.

InnoDB


A transaction-safe storage engine that supports row level locking, and many
Oracle-like features.

BerkeleyDB or BDB


Uses the Berkeley DB library from Sleepycat Software to implement
transaction-safe tables.





Note that only MyISAM is available in the standard binary distribution.





The 3.23 release also includes support for database replication between
a master and many slaves, full-text indexing, and much more.





All new features are being developed in the 4.x version.  Only
bugfixes and minor enhancements to existing features will be
added to 3.23.





The replication code and BerkeleyDB code is still not as tested and as
the rest of the code, so we will probably need to do a couple of future
releases of 3.23 with small fixes for this part of the code. As long as
you don't use these features, you should be quite safe with
MySQL 3.23!





Note that the preceding remarks don't mean that replication or Berkeley DB don't
work.  We have done a lot of testing of all code, including replication
and BDB without finding any problems.  It only means that not as many
users use this code as the rest of the code and because of this we are
not yet 100% confident in this code.







C.4.1  Changes in release 3.23.59 (not released yet)








C.4.2  Changes in release 3.23.58 (11 Sep 2003)








C.4.3  Changes in release 3.23.57 (06 Jun 2003)








C.4.4  Changes in release 3.23.56 (13 Mar 2003)







C.4.5  Changes in release 3.23.55 (23 Jan 2003)







C.4.6  Changes in release 3.23.54 (05 Dec 2002)







C.4.7  Changes in release 3.23.53 (09 Oct 2002)







C.4.8  Changes in release 3.23.52 (14 Aug 2002)







C.4.9  Changes in release 3.23.51 (31 May 2002)








C.4.10  Changes in release 3.23.50 (21 Apr 2002)








C.4.11  Changes in release 3.23.49 (14 Feb 2002)








C.4.12  Changes in release 3.23.48 (07 Feb 2002)








C.4.13  Changes in release 3.23.47 (27 Dec 2001)








C.4.14  Changes in release 3.23.46 (29 Nov 2001)








C.4.15  Changes in release 3.23.45 (22 Nov 2001)








C.4.16  Changes in release 3.23.44 (31 Oct 2001)







The following changes are for InnoDB tables:









C.4.17  Changes in release 3.23.43 (04 Oct 2001)








C.4.18  Changes in release 3.23.42 (08 Sep 2001)








C.4.19  Changes in release 3.23.41 (11 Aug 2001)








C.4.20  Changes in release 3.23.40 (18 Jul 2001)








C.4.21  Changes in release 3.23.39 (12 Jun 2001)








C.4.22  Changes in release 3.23.38 (09 May 2001)








C.4.23  Changes in release 3.23.37 (17 Apr 2001)








C.4.24  Changes in release 3.23.36 (27 Mar 2001)








C.4.25  Changes in release 3.23.35 (15 Mar 2001)








C.4.26  Changes in release 3.23.34a (11 Mar 2001)








C.4.27  Changes in release 3.23.34 (10 Mar 2001)








C.4.28  Changes in release 3.23.33 (09 Feb 2001)








C.4.29  Changes in release 3.23.32 (22 Jan 2001)








C.4.30  Changes in release 3.23.31 (17 Jan 2001: Production)








C.4.31  Changes in release 3.23.30 (04 Jan 2001)








C.4.32  Changes in release 3.23.29 (16 Dec 2000)








C.4.33  Changes in release 3.23.28 (22 Nov 2000: Gamma)








C.4.34  Changes in release 3.23.27 (24 Oct 2000)








C.4.35  Changes in release 3.23.26 (18 Oct 2000)








C.4.36  Changes in release 3.23.25 (29 Sep 2000)








C.4.37  Changes in release 3.23.24 (08 Sep 2000)








C.4.38  Changes in release 3.23.23 (01 Sep 2000)








C.4.39  Changes in release 3.23.22 (31 Jul 2000)








C.4.40  Changes in release 3.23.21 (04 Jul 2000)








C.4.41  Changes in release 3.23.20 (28 Jun 2000: Beta)








C.4.42  Changes in release 3.23.19








C.4.43  Changes in release 3.23.18 (11 Jun 2000)








C.4.44  Changes in release 3.23.17 (07 Jun 2000)








C.4.45  Changes in release 3.23.16 (16 May 2000)








C.4.46  Changes in release 3.23.15 (08 May 2000)








C.4.47  Changes in release 3.23.14 (09 Apr 2000)








C.4.48  Changes in release 3.23.13 (14 Mar 2000)








C.4.49  Changes in release 3.23.12 (07 Mar 2000)








C.4.50  Changes in release 3.23.11 (16 Feb 2000)








C.4.51  Changes in release 3.23.10 (30 Jan 2000)








C.4.52  Changes in release 3.23.9 (29 Jan 2000)








C.4.53  Changes in release 3.23.8 (02 Jan 2000)








C.4.54  Changes in release 3.23.7 (10 Dec 1999)








C.4.55  Changes in release 3.23.6 (15 Nov 1999)








C.4.56  Changes in release 3.23.5 (20 Oct 1999)








C.4.57  Changes in release 3.23.4 (28 Sep 1999)








C.4.58  Changes in release 3.23.3 (13 Sep 1999)








C.4.59  Changes in release 3.23.2 (09 Aug 1999)








C.4.60  Changes in release 3.23.1 (08 Jul 1999)








C.4.61  Changes in release 3.23.0 (05 Jul 1999: Alpha)








C.5  Changes in release 3.22.x (Old; discontinued)










The 3.22 version has faster and safer connect code than version 3.21, as well
as a lot of new nice enhancements.
As there aren't really any major changes, upgrading from 3.21 to 3.22 should
be very easy and painless.  See section 2.5.5  Upgrading from Version 3.21 to 3.22.







C.5.1  Changes in release 3.22.35








C.5.2  Changes in release 3.22.34








C.5.3  Changes in release 3.22.33








C.5.4  Changes in release 3.22.32 (14 Feb 2000)








C.5.5  Changes in release 3.22.31








C.5.6  Changes in release 3.22.30 (11 Jan 2000)








C.5.7  Changes in release 3.22.29 (02 Jan 2000)








C.5.8  Changes in release 3.22.28 (20 Oct 1999)








C.5.9  Changes in release 3.22.27 (05 Oct 1999)








C.5.10  Changes in release 3.22.26 (16 Sep 1999)








C.5.11  Changes in release 3.22.25 (07 Jun 1999)








C.5.12  Changes in release 3.22.24 (05 Jul 1999)








C.5.13  Changes in release 3.22.23 (08 Jun 1999)








C.5.14  Changes in release 3.22.22 (30 Apr 1999)








C.5.15  Changes in release 3.22.21 (04 Apr 1999)








C.5.16  Changes in release 3.22.20 (18 Mar 1999)








C.5.17  Changes in release 3.22.19 (01 Mar 1999)








C.5.18  Changes in release 3.22.18 (26 Feb 1999)








C.5.19  Changes in release 3.22.17 (22 Feb 1999: Production)








C.5.20  Changes in release 3.22.16 (05 Feb 1999)








C.5.21  Changes in release 3.22.15 (27 Jan 1999)








C.5.22  Changes in release 3.22.14 (01 Jan 1999: Gamma)








C.5.23  Changes in release 3.22.13 (16 Dec 1998)








C.5.24  Changes in release 3.22.12 (09 Dec 1998)








C.5.25  Changes in release 3.22.11 (24 Nov 1998)








C.5.26  Changes in release 3.22.10 (04 Nov 1998)








C.5.27  Changes in release 3.22.9 (19 Oct 1998)








C.5.28  Changes in release 3.22.8 (06 Oct 1998)








C.5.29  Changes in release 3.22.7 (21 Sep 1998: Beta)








C.5.30  Changes in release 3.22.6 (31 Aug 1998)








C.5.31  Changes in release 3.22.5 (20 Aug 1998: Alpha)








C.5.32  Changes in release 3.22.4 (06 Jul 1998: Beta)








C.5.33  Changes in release 3.22.3 (30 Jun 1998)








C.5.34  Changes in release 3.22.2








C.5.35  Changes in release 3.22.1 (Jun 1998)








C.5.36  Changes in release 3.22.0 (18 May 1998: Alpha)








C.6  Changes in release 3.21.x










MySQL 3.21 is quite old now, and should be avoided if possible.  This
information is kept here for historical purposes only.







C.6.1  Changes in release 3.21.33 (08 Jul 1998)








C.6.2  Changes in release 3.21.32 (30 Jun 1998)








C.6.3  Changes in release 3.21.31 (10 Jun 1998)








C.6.4  Changes in release 3.21.30








C.6.5  Changes in release 3.21.29








C.6.6  Changes in release 3.21.28








C.6.7  Changes in release 3.21.27








C.6.8  Changes in release 3.21.26








C.6.9  Changes in release 3.21.25








C.6.10  Changes in release 3.21.24








C.6.11  Changes in release 3.21.23








C.6.12  Changes in release 3.21.22








C.6.13  Changes in release 3.21.21a








C.6.14  Changes in release 3.21.21








C.6.15  Changes in release 3.21.20








C.6.16  Changes in release 3.21.19








C.6.17  Changes in release 3.21.18








C.6.18  Changes in release 3.21.17








C.6.19  Changes in release 3.21.16








C.6.20  Changes in release 3.21.15








C.6.21  Changes in release 3.21.14b








C.6.22  Changes in release 3.21.14a








C.6.23  Changes in release 3.21.13








C.6.24  Changes in release 3.21.12








C.6.25  Changes in release 3.21.11








C.6.26  Changes in release 3.21.10








C.6.27  Changes in release 3.21.9








C.6.28  Changes in release 3.21.8








C.6.29  Changes in release 3.21.7








C.6.30  Changes in release 3.21.6








C.6.31  Changes in release 3.21.5








C.6.32  Changes in release 3.21.4








C.6.33  Changes in release 3.21.3








C.6.34  Changes in release 3.21.2








C.6.35  Changes in release 3.21.0








C.7  Changes in release 3.20.x










MySQL 3.20 is quite old now, and should be avoided if possible.  This
information is kept here for historical purposes only.





Changes from 3.20.18 to 3.20.32b are not documented here because the
3.21 release branched here. And the relevant changes are also
documented as changes to the 3.21 version.







C.7.1  Changes in release 3.20.18








C.7.2  Changes in release 3.20.17








C.7.3  Changes in release 3.20.16








C.7.4  Changes in release 3.20.15








C.7.5  Changes in release 3.20.14








C.7.6  Changes in release 3.20.13








C.7.7  Changes in release 3.20.11








C.7.8  Changes in release 3.20.10








C.7.9  Changes in release 3.20.9








C.7.10  Changes in release 3.20.8








C.7.11  Changes in release 3.20.7








C.7.12  Changes in release 3.20.6








C.7.13  Changes in release 3.20.3








C.7.14  Changes in release 3.20.0








C.8  Changes in release 3.19.x










MySQL 3.19 is quite old now, and should be avoided if possible.  This
information is kept here for historical purposes only.







C.8.1  Changes in release 3.19.5








C.8.2  Changes in release 3.19.4








C.8.3  Changes in release 3.19.3








C.9  InnoDB Change History




Starting from 4.0.22 and 4.1.5, all InnoDB changes are included in
the MySQL Change History, and this manual section is no longer separately
maintained.







C.9.1  MySQL/InnoDB-4.0.21, September 10, 2004




Functionality added or changed:








Bugs fixed:









C.9.2  MySQL/InnoDB-4.1.4, August 31, 2004




Functionality added or changed:








Bugs fixed:









C.9.3  MySQL/InnoDB-4.1.3, June 28, 2004




Functionality added or changed:






Bugs fixed:









C.9.4  MySQL/InnoDB-4.1.2, May 30, 2004




NOTE: CRITICAL BUG in 4.1.2 if you specify innodb_file_per_table in
`my.cnf' on Unix. In crash recovery InnoDB will skip the crash recovery
for all `.ibd' files and those tables become CORRUPT! The symptom is
a message
Unable to lock ...ibd with lock 1, error: 9: fcntl: Bad file descriptor
in the `.err' log in crash recovery.





Functionality added or changed:








Bugs fixed:









C.9.5  MySQL/InnoDB-4.0.20, May 18, 2004




Bugs fixed:









C.9.6  MySQL/InnoDB-4.0.19, May 4, 2004




Functionality added or changed:








Bugs fixed:









C.9.7  MySQL/InnoDB-4.0.18, February 13, 2004








C.9.8  MySQL/InnoDB-5.0.0, December 24, 2003








C.9.9  MySQL/InnoDB-4.0.17, December 17, 2003








C.9.10  MySQL/InnoDB-4.1.1, December 4, 2003








C.9.11  MySQL/InnoDB-4.0.16, October 22, 2003








C.9.12  MySQL/InnoDB-3.23.58, September 15, 2003








C.9.13  MySQL/InnoDB-4.0.15, September 10, 2003








C.9.14  MySQL/InnoDB-4.0.14, July 22, 2003








C.9.15  MySQL/InnoDB-3.23.57, June 20, 2003








C.9.16  MySQL/InnoDB-4.0.13, May 20, 2003








C.9.17  MySQL/InnoDB-4.1.0, April 3, 2003








C.9.18  MySQL/InnoDB-3.23.56, March 17, 2003








C.9.19  MySQL/InnoDB-4.0.12, March 18, 2003








C.9.20  MySQL/InnoDB-4.0.11, February 25, 2003








C.9.21  MySQL/InnoDB-4.0.10, February 4, 2003








C.9.22  MySQL/InnoDB-3.23.55, January 24, 2003








C.9.23  MySQL/InnoDB-4.0.9, January 14, 2003








C.9.24  MySQL/InnoDB-4.0.8, January 7, 2003








C.9.25  MySQL/InnoDB-4.0.7, December 26, 2002








C.9.26  MySQL/InnoDB-4.0.6, December 19, 2002








C.9.27  MySQL/InnoDB-3.23.54, December 12, 2002








C.9.28  MySQL/InnoDB-4.0.5, November 18, 2002








C.9.29  MySQL/InnoDB-3.23.53, October 9, 2002








C.9.30  MySQL/InnoDB-4.0.4, October 2, 2002








C.9.31  MySQL/InnoDB-4.0.3, August 28, 2002








C.9.32  MySQL/InnoDB-3.23.52, August 16, 2002








C.9.33  MySQL/InnoDB-4.0.2, July 10, 2002








C.9.34  MySQL/InnoDB-3.23.51, June 12, 2002








C.9.35  MySQL/InnoDB-3.23.50, April 23, 2002








C.9.36  MySQL/InnoDB-3.23.49, February 17, 2002








C.9.37  MySQL/InnoDB-3.23.48, February 9, 2002








C.9.38  MySQL/InnoDB-3.23.47, December 28, 2001








C.9.39  MySQL/InnoDB-4.0.1, December 23, 2001








C.9.40  MySQL/InnoDB-3.23.46, November 30, 2001








C.9.41  MySQL/InnoDB-3.23.45, November 23, 2001








C.9.42  MySQL/InnoDB-3.23.44, November 2, 2001








C.9.43  MySQL/InnoDB-3.23.43, October 4, 2001








C.9.44  MySQL/InnoDB-3.23.42, September 9, 2001








C.9.45  MySQL/InnoDB-3.23.41, August 13, 2001








C.9.46  MySQL/InnoDB-3.23.40, July 16, 2001








C.9.47  MySQL/InnoDB-3.23.39, June 13, 2001








C.9.48  MySQL/InnoDB-3.23.38, May 12, 2001








C.10  MySQL Cluster Change History





C.10.1  MySQL Cluster-4.1.7, (not released yet)




Functionality added or changed:






Bugs fixed:







C.10.2  MySQL Cluster-4.1.6, 10 Oct 2004




Functionality added or changed:






Bugs fixed:







C.10.3  MySQL Cluster-4.1.5, 16 Sep 2004




Functionality added or changed:






Bugs fixed:







C.10.4  MySQL Cluster-4.1.4, 31 Aug 2004




Functionality added or changed:






Bugs fixed:







C.10.5  MySQL Cluster-5.0.1, 27 Jul 2004




Functionality added or changed:






Bugs fixed:







C.10.6  MySQL Cluster-4.1.3, 28 Jun 2004




Functionality added or changed:






Bugs fixed:







D  Porting to Other Systems










This appendix will help you port MySQL to other operating systems.
Do check the list of currently supported operating systems first.
See section 2.1.1  Operating Systems Supported by MySQL.
If you have created a new port of MySQL, please let us know so that
we can list it here and on our Web site (http://www.mysql.com/),
recommending it to other users.





Note: If you create a new port of MySQL, you are free to copy and
distribute it under the GPL license, but it does not make you a
copyright holder of MySQL.





A working POSIX thread library is needed for the server. On Solaris 2.5
we use Sun PThreads (the native thread support in 2.4 and earlier
versions is not good enough), on Linux we use LinuxThreads by Xavier
Leroy, [email protected].





The hard part of porting to a new Unix variant without good native
thread support is probably to port MIT-pthreads. See
`mit-pthreads/README' and Programming POSIX Threads
(http://www.humanfactor.com/pthreads/).





Up to MySQL 4.0.2, the MySQL distribution included a patched version of
Chris Provenzano's Pthreads from MIT (see the MIT Pthreads Web page at
http://www.mit.edu/afs/sipb/project/pthreads/ and a programming
introduction at http://www.mit.edu:8001/people/proven/IAP_2000/).
These can be used for some operating systems that do not have POSIX threads.
See section 2.3.5  MIT-pthreads Notes.





It is also possible to use another user level thread package named
FSU Pthreads (see http://moss.csc.ncsu.edu/~mueller/pthreads/).
This implementation is being used for the SCO port.





See the `thr_lock.c' and `thr_alarm.c' programs in the `mysys'
directory for some tests/examples of these problems.





Both the server and the client need a working C++ compiler. We use @command{gcc}
on many platforms. Other compilers that are known to work are SPARCworks,
Sun Forte, Irix @command{cc}, HP-UX @command{aCC}, IBM AIX @command{xlC_r}), Intel
@command{ecc/icc} and Compaq @command{cxx}).





To compile only the client use @command{./configure --without-server}.





There is currently no support for only compiling the server, nor is it
likly to be added unless someone has a good reason for it.





If you want/need to change any `Makefile' or the configure script you
will also need GNU Automake and Autoconf.
See section 2.3.3  Installing from the Development Source Tree.





All steps needed to remake everything from the most basic files.





/bin/rm */.deps/*.P
/bin/rm -f config.cache
aclocal
autoheader
aclocal
automake
autoconf
./configure --with-debug=full --prefix='your installation directory'

# The makefiles generated above need GNU make 3.75 or newer.
# (called gmake below)
gmake clean all install init-db





If you run into problems with a new port, you may have to do some debugging
of MySQL!
See section D.1  Debugging a MySQL Server.





Note: Before you start debugging @command{mysqld}, first get the test
programs mysys/thr_alarm and mysys/thr_lock to work.  This
will ensure that your thread installation has even a remote chance to work!







D.1  Debugging a MySQL Server












If you are using some functionality that is very new in MySQL,
you can try to run @command{mysqld} with the --skip-new (which will disable all
new, potentially unsafe functionality) or with --safe-mode which
disables a lot of optimization that may cause problems.
See section A.4.2  What to Do If MySQL Keeps Crashing.





If @command{mysqld} doesn't want to start, you should verify that you don't have
any `my.cnf' files that interfere with your setup!
You can check your `my.cnf' arguments with @command{mysqld --print-defaults}
and avoid using them by starting with @command{mysqld --no-defaults ...}.





If @command{mysqld} starts to eat up CPU or memory or if it ``hangs,'' you
can use @command{mysqladmin processlist status} to find out if someone is
executing a query that takes a long time.  It may be a good idea to
run @command{mysqladmin -i10 processlist status} in some window if you are
experiencing performance problems or problems when new clients can't connect.





The command @command{mysqladmin debug} will dump some information about
locks in use, used memory and query usage to the MySQL log file. This
may help solve some problems.  This command also provides some useful
information even if you haven't compiled MySQL for debugging!





If the problem is that some tables are getting slower and slower you
should try to optimize the table with OPTIMIZE TABLE or
@command{myisamchk}.  See section 5  Database Administration. You should also
check the slow queries with EXPLAIN.





You should also read the OS-specific section in this manual for
problems that may be unique to your environment.
See section 2.6  Operating System-Specific Notes.







D.1.1  Compiling MySQL for Debugging




If you have some very specific problem, you can always try to debug
MySQL.  To do this you must configure MySQL with the
--with-debug or the --with-debug=full option.  You can check
whether MySQL was compiled with debugging by doing:
@command{mysqld --help}.  If the --debug flag is listed with the
options then you have debugging enabled.  @command{mysqladmin ver} also
lists the @command{mysqld} version as @command{mysql ... --debug} in this case.





If you are using @command{gcc} or @command{egcs}, the recommended @command{configure} line is:





CC=gcc CFLAGS="-O2" CXX=gcc CXXFLAGS="-O2 -felide-constructors \
   -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql \
   --with-debug --with-extra-charsets=complex





This will avoid problems with the libstdc++ library and with C++
exceptions (many compilers have problems with C++ exceptions in threaded
code) and compile a MySQL version with support for all character sets.





If you suspect a memory overrun error, you can configure MySQL
with --with-debug=full, which will install a memory allocation
(SAFEMALLOC) checker. However, running with SAFEMALLOC is
quite slow, so if you get performance problems you should start
@command{mysqld} with the --skip-safemalloc option. This will
disable the memory overrun checks for each call to malloc() and
free().





If @command{mysqld} stops crashing when you compile it with
--with-debug, you probably have found a compiler bug or a timing
bug within MySQL.  In this case, you can try to add -g to
the CFLAGS and CXXFLAGS variables above and not use
--with-debug. If @command{mysqld} now dies, you can at least attach
to it with @command{gdb} or use @command{gdb} on the core file to find out
what happened.





When you configure MySQL for debugging you automatically enable a
lot of extra safety check functions that monitor the health of @command{mysqld}.
If they find something ``unexpected,'' an entry will be written to
stderr, which @command{safe_mysqld} directs to the error log!  This also
means that if you are having some unexpected problems with MySQL and
are using a source distribution, the first thing you should do is to
configure MySQL for debugging!  (The second thing is to
send mail to a MySQL mailing list and ask for help.
See section 1.7.1.1  The MySQL Mailing Lists.
Please use the
@command{mysqlbug} script for all bug reports or questions regarding the
MySQL version you are using!





In the Windows MySQL distribution, mysqld.exe is by
default compiled with support for trace files.







D.1.2  Creating Trace Files




If the @command{mysqld} server doesn't start or if you can cause it
to crash quickly, you can try to create a trace file to find the problem.





To do this, you must have a @command{mysqld} that has been compiled with
debugging support.
You can check this by executing mysqld -V.  If the version number
ends with -debug, it's compiled with support for trace files.





Start the @command{mysqld} server with a trace log in `/tmp/mysqld.trace'
on Unix or `C:\mysqld.trace' on Windows:





shell> mysqld --debug





On Windows, you should also use the --standalone flag to not start
@command{mysqld} as a service.  In a console window, use this command:





C:\> mysqld --debug --standalone





After this, you can use the mysql.exe command-line tool in a
second console window to reproduce the problem. You can stop the
@command{mysqld} server with @command{mysqladmin shutdown}.





Note that the trace file will become very big!
If you want to generate a smaller trace file, you can use debugging options
something like this:





@command{mysqld --debug=d,info,error,query,general,where:O,/tmp/mysqld.trace}





This only prints information with the most interesting tags to the trace file.





If you make a bug report about this, please only send the lines from the
trace file to the appropriate mailing list where something seems to go
wrong! If you can't locate the wrong place, you can ftp the trace file,
together with a full bug report, to
ftp://ftp.mysql.com/pub/mysql/upload/ so that a MySQL
developer can take a look a this.





The trace file is made with the DBUG package by Fred Fish.
See section D.3  The DBUG Package.







D.1.3  Debugging @command{mysqld} under @command{gdb}










On most systems you can also start @command{mysqld} from @command{gdb} to get
more information if @command{mysqld} crashes.





With some older @command{gdb} versions on Linux you must use @code{run
--one-thread} if you want to be able to debug @command{mysqld} threads.  In
this case, you can only have one thread active at a time.  We recommend you
to upgrade to gdb 5.1 ASAP as thread debugging works much better with this
version!





When running @command{mysqld} under @command{gdb}, you should disable the stack trace
with --skip-stack-trace to be able to catch segfaults within @command{gdb}.





In MySQL 4.0.14 and above you should use the --gdb option to
mysqld.  This will install an interrupt handler for SIGINT (needed
to stop @command{mysqld} with ^C to set breakpoints) and disable stack
tracing and core file handling.





It's very hard to debug MySQL under @command{gdb} if you do a lot of
new connections the whole time as @command{gdb} doesn't free the memory for
old threads.  You can avoid this problem by starting @command{mysqld} with
-O thread_cache_size= 'max_connections +1'.  In most cases just
using -O thread_cache_size=5' will help a lot!





If you want to get a core dump on Linux if @command{mysqld} dies with a
SIGSEGV signal, you can start @command{mysqld} with the --core-file option.
This core file can be used to make a backtrace that may help you
find out why @command{mysqld} died:





shell> gdb mysqld core
gdb>   backtrace full
gdb>   exit





See section A.4.2  What to Do If MySQL Keeps Crashing.





If you are using @command{gdb} 4.17.x or above on Linux, you should install a
`.gdb' file, with the following information, in your current
directory:





set print sevenbit off
handle SIGUSR1 nostop noprint
handle SIGUSR2 nostop noprint
handle SIGWAITING nostop noprint
handle SIGLWP nostop noprint
handle SIGPIPE nostop
handle SIGALRM nostop
handle SIGHUP nostop
handle SIGTERM nostop noprint





If you have problems debugging threads with @command{gdb}, you should download
gdb 5.x and try this instead. The new @command{gdb} version has very improved
thread handling!





Here is an example how to debug mysqld:





shell> gdb /usr/local/libexec/mysqld
gdb> run
...
backtrace full # Do this when mysqld crashes





Include the above output in a mail generated with @command{mysqlbug} and
mail this to the general MySQL mailing list.
See section 1.7.1.1  The MySQL Mailing Lists.





If @command{mysqld} hangs you can try to use some system tools like
strace or /usr/proc/bin/pstack to examine where
@command{mysqld} has hung.





strace /tmp/log libexec/mysqld









If you are using the Perl DBI interface, you can turn on
debugging information by using the trace method or by
setting the DBI_TRACE environment variable.







D.1.4  Using a Stack Trace




On some operating systems, the error log will contain a stack trace if
@command{mysqld} dies unexpectedly. You can use this to find out where (and
maybe why) @command{mysqld} died. See section 5.9.1  The Error Log.  To get a stack trace,
you must not compile @command{mysqld} with the -fomit-frame-pointer
option to gcc. See section D.1.1  Compiling MySQL for Debugging.





If the error file contains something like the following:





mysqld got signal 11;
The manual section 'Debugging a MySQL server' tells you how to use a
stack trace and/or the core file to produce a readable backtrace that may
help in finding out why mysqld died
Attempting backtrace. You can use the following information to find out
where mysqld died.  If you see no messages after this, something went
terribly wrong...
stack range sanity check, ok, backtrace follows
0x40077552
0x81281a0
0x8128f47
0x8127be0
0x8127995
0x8104947
0x80ff28f
0x810131b
0x80ee4bc
0x80c3c91
0x80c6b43
0x80c1fd9
0x80c1686





you can find where @command{mysqld} died by doing the following:





    

  1. 

Copy the preceding numbers to a file, for example `mysqld.stack'.


  2. 

Make a symbol file for the @command{mysqld} server:


    nm -n libexec/mysqld > /tmp/mysqld.sym

    

Note that most MySQL binary distributions (except for the "debug" packages,
where this information is included inside of the binaries themselves)
already ship with the above file, named mysqld.sym.gz.
In this case, you can simply unpack it by doing:



    gunzip < bin/mysqld.sym.gz > /tmp/mysqld.sym

    


  3. 

Execute resolve_stack_dump -s /tmp/mysqld.sym -n mysqld.stack.

This will print out where @command{mysqld} died. If this doesn't help you
find out why @command{mysqld} died, you should make a bug report and include
the output from the above command with the bug report.

Note however that in most cases it will not help us to just have a stack
trace to find the reason for the problem.  To be able to locate the bug
or provide a workaround, we would in most cases need to know the query
that killed @command{mysqld} and preferable a test case so that we can
repeat the problem!  See section 1.7.1.3  How to Report Bugs or Problems.






D.1.5  Using Log Files to Find Cause of Errors in @command{mysqld}




Note that before starting @command{mysqld} with --log you should
check all your tables with @command{myisamchk}.
See section 5  Database Administration.





If @command{mysqld} dies or hangs, you should start @command{mysqld} with
--log.  When @command{mysqld} dies again, you can examine the end of
the log file for the query that killed @command{mysqld}.





If you are using --log without a file name, the log is stored in
the database directory as `host_name.log' In most cases it is the last
query in the log file that killed @command{mysqld}, but if possible you
should verify this by restarting @command{mysqld} and executing the found
query from the @command{mysql} command-line tools. If this works, you
should also test all complicated queries that didn't complete.





You can also try the command EXPLAIN on all SELECT
statements that takes a long time to ensure that @command{mysqld} is using
indexes properly. See section 7.2.1  EXPLAIN Syntax (Get Information About a SELECT).





You can find the queries that take a long time to execute by starting
@command{mysqld} with --log-slow-queries. See section 5.9.5  The Slow Query Log.





If you find the text mysqld restarted in the error log file
(normally named `hostname.err') you probably have found a query
that causes @command{mysqld} to fail.  If this happens, you should check all
your tables with @command{myisamchk} (see section 5  Database Administration),
and test the queries in the MySQL log files to see if one doesn't
work.  If you find such a query, try first upgrading to the newest
MySQL version.  If this doesn't help and you can't find anything
in the mysql mail archive, you should report the bug to a MySQL
mailing list.
The mailing lists are described at
http://lists.mysql.com/, which also has links to online
list archives.





If you have started @command{mysqld} with myisam-recover,
MySQL will automatically check and try to repair MyISAM
tables if they are marked as 'not closed properly' or 'crashed'.  If
this happens, MySQL will write an entry in the
hostname.err file 'Warning: Checking table ...' which is
followed by Warning: Repairing table if the table needs to be
repaired.  If you get a lot of these errors, without @command{mysqld} having
died unexpectedly just before, then something is wrong and needs to
be investigated further. See section 5.2.1  @command{mysqld} Command-Line Options.





It's not a good sign if @command{mysqld} did died unexpectedly,
but in this case one shouldn't investigate the Checking table...
messages but instead try to find out why @command{mysqld} died.







D.1.6  Making a Test Case If You Experience Table Corruption




If you get corrupted tables or if @command{mysqld} always fails after some
update commands, you can test whether this bug is reproducible by doing the
following:








You can also use the script mysql_find_rows to just execute some of the
update statements if you want to narrow down the problem.







D.2  Debugging a MySQL Client











To be able to debug a MySQL client with the integrated debug package,
you should configure MySQL with --with-debug or
--with-debug=full.
@xref{configure options,  , @command{configure} options}.







Before running a client, you should set the MYSQL_DEBUG environment
variable:





shell> MYSQL_DEBUG=d:t:O,/tmp/client.trace
shell> export MYSQL_DEBUG





This causes clients to generate a trace file in `/tmp/client.trace'.





If you have problems with your own client code, you should attempt to
connect to the server and run your query using a client that is known to
work.  Do this by running @command{mysql} in debugging mode (assuming that you
have compiled MySQL with debugging on):





shell> mysql --debug=d:t:O,/tmp/client.trace





This will provide useful information in case you mail a bug report.
See section 1.7.1.3  How to Report Bugs or Problems.





If your client crashes at some 'legal' looking code, you should check
that your `mysql.h' include file matches your MySQL library file.
A very common mistake is to use an old `mysql.h' file from an old
MySQL installation with new MySQL library.







D.3  The DBUG Package










The MySQL server and most MySQL clients are compiled with the DBUG package
originally created by Fred Fish.  When you have configured MySQL for
debugging, this package makes it possible to get a trace file of what the
program is debugging.
See section D.1.2  Creating Trace Files.





You use the debug package by invoking a program with the
--debug="..." or the -#... option.





Most MySQL programs have a default debug string that will be
used if you don't specify an option to --debug.  The default
trace file is usually /tmp/program_name.trace on Unix and
\program_name.trace on Windows.





The debug control string is a sequence of colon-separated fields
as follows:





<field_1>:<field_2>:...:<field_N>





Each field consists of a mandatory flag character followed by
an optional `,' and comma-separated list of modifiers:





flag[,modifier,modifier,...,modifier]





The currently recognized flag characters are:







Flag  Description




d  Enable output from DBUG_<N> macros for the current state. May be followed by a list of keywords which selects output only for the DBUG macros with that keyword. An empty list of keywords implies output for all macros.




D  Delay after each debugger output line. The argument is the number of tenths of seconds to delay, subject to machine capabilities. For example, -#D,20 specifies a delay of two seconds.




f  Limit debugging and/or tracing, and profiling to the list of named functions.  Note that a null list disables all functions.   The appropriate d or t flags must still be given; this flag only limits their actions if they are enabled.




F  Identify the source file name for each line of debug or trace output.




i  Identify the process with the PID or thread ID for each line of debug or trace output.




g  Enable profiling.  Create a file called `dbugmon.out' containing information that can be used to profile the program.  May be followed by a list of keywords that select profiling only for the functions in that list.  A null list implies that all functions are considered.




L  Identify the source file line number for each line of debug or trace output.




n  Print the current function nesting depth for each line of debug or trace output.




N  Number each line of debug output.




o  Redirect the debugger output stream to the specified file.  The default output is stderr.




O  Like o, but the file is really flushed between each write. When needed, the file is closed and reopened between each write.




p  Limit debugger actions to specified processes. A process must be identified with the DBUG_PROCESS macro and match one in the list for debugger actions to occur.




P  Print the current process name for each line of debug or trace output.




r  When pushing a new state, do not inherit the previous state's function nesting level. Useful when the output is to start at the left margin.




S  Do function _sanity(_file_,_line_) at each debugged function until _sanity() returns something that differs from 0. (Mostly used with safemalloc to find memory leaks)




t  Enable function call/exit trace lines. May be followed by a list (containing only one modifier) giving a numeric maximum trace level, beyond which no output will occur for either debugging or tracing macros.  The default is a compile time option.








Some examples of debug control strings that might appear on a shell command
line (the -# is typically used to introduce a control string to an
application program) are:





-#d:t
-#d:f,main,subr1:F:L:t,20
-#d,input,output,files:n
-#d:t:i:O,\\mysqld.trace





In MySQL, common tags to print (with the d option) are
enter, exit, error, warning, info, and
loop.







D.4  Comments about RTS Threads











I have tried to use the RTS thread packages with MySQL but
stumbled on the following problems:





They use old versions of many POSIX calls and it is very tedious to
make wrappers for all functions. I am inclined to think that it would
be easier to change the thread libraries to the newest POSIX
specification.





Some wrappers are already written. See `mysys/my_pthread.c' for more info.





At least the following should be changed:





pthread_get_specific should use one argument.
sigwait should take two arguments.
A lot of functions (at least pthread_cond_wait,
pthread_cond_timedwait())
should return the error code on error. Now they return -1 and set errno.





Another problem is that user-level threads use the ALRM signal and this
aborts a lot of functions (read, write, open...).
MySQL should do a retry on interrupt on all of these but it is
not that easy to verify it.





The biggest unsolved problem is the following:





To get thread-level alarms I changed `mysys/thr_alarm.c' to wait between
alarms with pthread_cond_timedwait(), but this aborts with error
EINTR.  I tried to debug the thread library as to why this happens,
but couldn't find any easy solution.





If someone wants to try MySQL with RTS threads I suggest the
following:









D.5  Differences Between Thread Packages










MySQL is very dependent on the thread package used. So when
choosing a good platform for MySQL, the thread package is very
important.





There are at least three types of thread packages:








In some systems kernel threads are managed by integrating user
level threads in the system libraries. In such cases, the thread
switching can only be done by the thread library and the kernel isn't
really ``thread aware.''







E  Environment Variables










This appendix lists all the environment variables that are used directly or
indirectly by MySQL. Most of these can also be found in other
places in this manual.





Note that any options on the command line take precedence over
values specified in option files and environment variables, and
values in option files take precedence over values in environment
variables.





In many cases, it's preferable to use an option file instead of environment
variables to modify the behavior of MySQL. See section 4.3.2  Using Option Files.





















































Variable  Description



CXX  The name of your C++ compiler (for running @command{configure}).



CC  The name of your C compiler (for running @command{configure}).



CFLAGS  Flags for your C compiler (for running @command{configure}).



CXXFLAGS  Flags for your C++ compiler (for running @command{configure}).



DBI_USER  The default username for Perl DBI.



DBI_TRACE  Trace options for Perl DBI.



HOME  The default path for the @command{mysql} history file is `$HOME/.mysql_history'.



LD_RUN_PATH  Used to specify where your `libmysqlclient.so' is located.



MYSQL_DEBUG  Debug trace options when debugging.



MYSQL_HISTFILE  The path to the @command{mysql} history file. If this variable is set, its value overrides the default of `$HOME/.mysql_history'.



MYSQL_HOST  The default hostname used by the @command{mysql} command-line client.



MYSQL_PS1  The command prompt to use in the @command{mysql} command-line client.



MYSQL_PWD  The default password when connecting to @command{mysqld}. Note that use of this is insecure!  See section 5.6.6  Keeping Your Password Secure.



MYSQL_TCP_PORT  The default TCP/IP port number.



MYSQL_UNIX_PORT  The default Unix socket filename; used for connections to localhost.



PATH  Used by the shell to find MySQL programs.



TMPDIR  The directory where temporary files are created.



TZ  This should be set to your local time zone. See section A.4.6  Time Zone Problems.



UMASK_DIR  The user-directory creation mask when creating directories. Note that this is ANDed with UMASK!



UMASK  The user-file creation mask when creating files.



USER  The default username on Windows and NetWare to use when
connecting to @command{mysqld}.








F  MySQL Regular Expressions












A regular expression is a powerful way of specifying a pattern for a complex
search.





MySQL uses Henry Spencer's implementation of regular expressions, which is
aimed at conformance with POSIX 1003.2.
See section B  Credits.
MySQL uses the extended version to support pattern-matching operations
performed with the REGEXP operator in SQL statements.
See section 3.3.4.7  Pattern Matching.





This appendix is a summary, with examples, of the special characters and
constructs that can be used in MySQL for REGEXP operations.
It does not contain all the details that can be found in
Henry Spencer's regex(7) manual page. That manual page is
included in MySQL source distributions, in the `regex.7' file under the
`regex' directory.





A regular expression describes a set of strings. The simplest regular
expression is one that has no special characters in it. For example, the
regular expression hello matches hello and nothing else.





Non-trivial regular expressions use certain special constructs so that
they can match more than one string. For example, the regular expression
hello|word matches either the string hello or the string
word.





As a more complex example, the regular expression B[an]*s matches any
of the strings Bananas, Baaaaas, Bs, and any other
string starting with a B, ending with an s, and containing any
number of a or n characters in between.





A regular expression for the REGEXP operator may use any of the
following special characters and constructs:







^


Match the beginning of a string.


mysql> SELECT 'fo\nfo' REGEXP '^fo$';                   -> 0
mysql> SELECT 'fofo' REGEXP '^fo';                      -> 1




$


Match the end of a string.


mysql> SELECT 'fo\no' REGEXP '^fo\no$';                 -> 1
mysql> SELECT 'fo\no' REGEXP '^fo$';                    -> 0




.


Match any character (including carriage return and newline).


mysql> SELECT 'fofo' REGEXP '^f.*$';                    -> 1
mysql> SELECT 'fo\r\nfo' REGEXP '^f.*$';                -> 1




a*


Match any sequence of zero or more a characters.


mysql> SELECT 'Ban' REGEXP '^Ba*n';                     -> 1
mysql> SELECT 'Baaan' REGEXP '^Ba*n';                   -> 1
mysql> SELECT 'Bn' REGEXP '^Ba*n';                      -> 1




a+


Match any sequence of one or more a characters.


mysql> SELECT 'Ban' REGEXP '^Ba+n';                     -> 1
mysql> SELECT 'Bn' REGEXP '^Ba+n';                      -> 0




a?


Match either zero or one a character.


mysql> SELECT 'Bn' REGEXP '^Ba?n';                      -> 1
mysql> SELECT 'Ban' REGEXP '^Ba?n';                     -> 1
mysql> SELECT 'Baan' REGEXP '^Ba?n';                    -> 0




de|abc


Match either of the sequences de or abc.


mysql> SELECT 'pi' REGEXP 'pi|apa';                     -> 1
mysql> SELECT 'axe' REGEXP 'pi|apa';                    -> 0
mysql> SELECT 'apa' REGEXP 'pi|apa';                    -> 1
mysql> SELECT 'apa' REGEXP '^(pi|apa)$';                -> 1
mysql> SELECT 'pi' REGEXP '^(pi|apa)$';                 -> 1
mysql> SELECT 'pix' REGEXP '^(pi|apa)$';                -> 0




(abc)*


Match zero or more instances of the sequence abc.


mysql> SELECT 'pi' REGEXP '^(pi)*$';                    -> 1
mysql> SELECT 'pip' REGEXP '^(pi)*$';                   -> 0
mysql> SELECT 'pipi' REGEXP '^(pi)*$';                  -> 1




{1}



{2,3}


{n} or {m,n} notation provides
a more general way of writing regular expressions that match many
occurrences of the previous atom (or ``piece'') of the pattern.
m and n are integers.





a*


Can be written as a{0,}.

a+


Can be written as a{1,}.

a?


Can be written as a{0,1}.



To be more precise, a{n} matches exactly n instances of
a.
a{n,} matches n or more instances of a.
a{m,n} matches m through n instances of a,
inclusive.

m and n must be in the range from 0 to
RE_DUP_MAX (default 255), inclusive.  If both m and n
are given, m must be less than or equal to n.



mysql> SELECT 'abcde' REGEXP 'a[bcd]{2}e';              -> 0
mysql> SELECT 'abcde' REGEXP 'a[bcd]{3}e';              -> 1
mysql> SELECT 'abcde' REGEXP 'a[bcd]{1,10}e';           -> 1




[a-dX]



[^a-dX]


Matches any character that is (or is not, if ^ is used) either a,
b, c, d or X.  A - character between two other
characters forms a range that matches all characters from the first
character to the second. For example, [0-9] matches any decimal
digit.  To include a literal ] character, it must immediately follow
the opening bracket [.  To include a literal - character, it
must be written first or last.  Any character that does not have a defined
special meaning inside a [] pair matches only itself.



mysql> SELECT 'aXbc' REGEXP '[a-dXYZ]';                 -> 1
mysql> SELECT 'aXbc' REGEXP '^[a-dXYZ]$';               -> 0
mysql> SELECT 'aXbc' REGEXP '^[a-dXYZ]+$';              -> 1
mysql> SELECT 'aXbc' REGEXP '^[^a-dXYZ]+$';             -> 0
mysql> SELECT 'gheis' REGEXP '^[^a-dXYZ]+$';            -> 1
mysql> SELECT 'gheisa' REGEXP '^[^a-dXYZ]+$';           -> 0




[.characters.]


Within a bracket expression (written using [ and ]),
matches the sequence of characters of that collating element. characters
is either a single character or a character name like newline.
You can find the full list of character names in the `regexp/cname.h'
file.



mysql> SELECT '~' REGEXP '[[.~.]]';                     -> 1
mysql> SELECT '~' REGEXP '[[.tilde.]]';                 -> 1




[=character_class=]


Within a bracket expression (written using [ and ]),
[=character_class=] represents
an equivalence class. It matches all characters with the same collation
value, including itself.
For example, if o and (+) are the members of an
equivalence class, then [[=o=]], [[=(+)=]], and
[o(+)] are all synonymous.
An equivalence class may not be used as an endpoint of a range.


[:character_class:]


Within a bracket expression (written using [ and ]),
[:character_class:] represents a character class that matches all
characters belonging to that class. The standard class names are:




alnum  Alphanumeric characters



alpha  Alphabetic characters



blank  Whitespace characters



cntrl  Control characters



digit  Digit characters



graph  Graphic characters



lower  Lowercase alphabetic characters



print  Graphic or space characters



punct  Punctuation characters



space  Space, tab, newline, and carriage return



upper  Uppercase alphabetic characters



xdigit  Hexadecimal digit characters





These stand for the character classes defined in the ctype(3) manual
page. A particular locale may provide other class names.
A character class may not be used as an endpoint of a range.



mysql> SELECT 'justalnums' REGEXP '[[:alnum:]]+';       -> 1
mysql> SELECT '!!' REGEXP '[[:alnum:]]+';               -> 0




[[:<:]]



[[:>:]]


These markers stand for word boundaries. They match the beginning and end of
words, respectively.  A word is a sequence of word characters that is not
preceded by or followed by word characters. A word character is an
alphanumeric character in the alnum class or an underscore
(_).



mysql> SELECT 'a word a' REGEXP '[[:<:]]word[[:>:]]';   -> 1
mysql> SELECT 'a xword a' REGEXP '[[:<:]]word[[:>:]]';  -> 0








To use a literal instance of a special character in a regular expression,
precede it by two backslash (\) characters. The MySQL parser
interprets one of the backslashes, and the regular expression library
interprets the other. For example, to match the string 1+2 that
contains the special + character, only the last of the following
regular expressions is the correct one:





mysql> SELECT '1+2' REGEXP '1+2';                       -> 0
mysql> SELECT '1+2' REGEXP '1\+2';                      -> 0
mysql> SELECT '1+2' REGEXP '1\\+2';                     -> 1






G  GNU General Public License











Version 2, June 1991





Copyright © 1989, 1991 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA  02111-1307, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.





24.4  Preamble




  The licenses for most software are designed to take away your
freedom to share and change it.  By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software--to make sure the software is free for all its users.  This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it.  (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.)  You can apply it to
your programs, too.





  When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.





  To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.





  For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have.  You must make sure that they, too, receive or can get the
source code.  And you must show them these terms so they know their
rights.





  We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.





  Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.





  Finally, any free program is threatened constantly by software
patents.  We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary.  To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.





  The precise terms and conditions for copying, distribution and
modification follow.





24.5  TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION




    

  1. 

This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License.  The ``Program'', below,
refers to any such program or work, and a ``work based on the Program''
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language.  (Hereinafter, translation is included without limitation in
the term ``modification''.)  Each licensee is addressed as ``you''.

Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope.  The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.


  2. 

You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.

You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.


  3. 

You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:



      

    1. 

You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.


    2. 

You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
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    3. 

If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
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the Program is not required to print an announcement.)

    

These requirements apply to the modified work as a whole.  If
identifiable sections of that work are not derived from the Program,
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Thus, it is not the intent of this section to claim rights or contest
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In addition, mere aggregation of another work not based on the Program
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  4. 

You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms of
Sections 1 and 2 above provided that you also do one of the following:



      

    1. 

Accompany it with the complete corresponding machine-readable
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    2. 

Accompany it with a written offer, valid for at least three
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customarily used for software interchange; or,


    3. 

Accompany it with the information you received as to the offer
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The source code for a work means the preferred form of the work for
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  5. 

You may not copy, modify, sublicense, or distribute the Program
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However, parties who have received copies, or rights, from you under
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  6. 

You are not required to accept this License, since you have not
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  7. 

Each time you redistribute the Program (or any work based on the
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  8. 

If, as a consequence of a court judgment or allegation of patent
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If any portion of this section is held invalid or unenforceable under
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It is not the purpose of this section to induce you to infringe any
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This section is intended to make thoroughly clear what is believed to
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  9. 

If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces, the
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may add an explicit geographical distribution limitation excluding
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countries not thus excluded.  In such case, this License incorporates
the limitation as if written in the body of this License.


  10. 

The Free Software Foundation may publish revised and/or new versions
of the General Public License from time to time.  Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.

Each version is given a distinguishing version number.  If the Program
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either of that version or of any later version published by the Free
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Foundation.


  11. 

If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to the author
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Software Foundation, write to the Free Software Foundation; we sometimes
make exceptions for this.  Our decision will be guided by the two goals
of preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.




    24.6  NO WARRANTY
    


  12. 

BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
REPAIR OR CORRECTION.


  13. 

IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.





24.7  END OF TERMS AND CONDITIONS




24.8  How to Apply These Terms to Your New Programs




  If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these terms.





  To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the ``copyright'' line and a pointer to where the full notice is found.





one line to give the program's name and a brief idea of what it does.
Copyright (C) yyyy  name of author

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.





Also add information on how to contact you by electronic and paper mail.





If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:





Gnomovision version 69, Copyright (C) 19yy name of author
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.





The hypothetical commands `show w' and `show c' should show
the appropriate parts of the General Public License.  Of course, the
commands you use may be called something other than `show w' and
`show c'; they could even be mouse-clicks or menu items--whatever
suits your program.





You should also get your employer (if you work as a programmer) or your
school, if any, to sign a ``copyright disclaimer'' for the program, if
necessary.  Here is a sample; alter the names:





Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.

signature of Ty Coon, 1 April 1989
Ty Coon, President of Vice





This General Public License does not permit incorporating your program into
proprietary programs.  If your program is a subroutine library, you may
consider it more useful to permit linking proprietary applications with the
library.  If this is what you want to do, use the GNU Library General
Public License instead of this License.







H  MySQL FLOSS License Exception











Version 0.2, July 2004





The MySQL AB Exception for Free/Libre and Open Source Software-only
Applications Using MySQL Client Libraries (the ``FLOSS Exception'').





24.9  Exception Intent




We want specified Free/Libre and Open Source Software (``FLOSS'') applications
to be able to use specified GPL-licensed MySQL client libraries (the
``Program'') despite the fact that not all FLOSS licenses are compatible with
version 2 of the GNU General Public License (the ``GPL''). 





24.10  Legal Terms and Conditions




As a special exception to the terms and conditions of version 2.0 of the
GPL:





    

  1. 

You are free to distribute a Derivative Work that is formed entirely from
the Program and one or more works (each, a ``FLOSS Work'') licensed under
one or more of the licenses listed below in section 1, as long as:



      

    1. 

You obey the GPL in all respects for the Program and the Derivative
Work, except for identifiable sections of the Derivative Work which
are not derived from the Program, and which can reasonably be
considered independent and separate works in themselves, 


    2. 

all identifiable sections of the Derivative Work which are not
derived from the Program, and which can reasonably be considered
independent and separate works in themselves,


      


      i

      
are distributed subject to one of the FLOSS licenses listed below, and

      ii

      
the object code or executable form of those sections are
accompanied by the complete corresponding machine-readable
source code for those sections on the same medium and under the
same FLOSS license as the corresponding object code or
executable forms of those sections, and

      


    3. 

any works which are aggregated with the Program or with a Derivative
Work on a volume of a storage or distribution medium in accordance
with the GPL, can reasonably be considered independent and separate
works in themselves which are not derivatives of either the Program,
a Derivative Work or a FLOSS Work.

    

If the above conditions are not met, then the Program may only be copied,
modified, distributed or used under the terms and conditions of the GPL or
another valid licensing option from MySQL AB.


  2. FLOSS License List


    

    License name  Version(s)/Copyright Date

    

    Academic Free License  2.0

    

    Apache Software License  1.0/1.1/2.0

    

    Apple Public Source License  2.0

    

    Artistic license  From Perl 5.8.0

    

    BSD license  "July 22 1999"

    

    Common Public License  1.0

    

    GNU Library or "Lesser" General Public License (LGPL)  2.0/2.1

    

    Jabber Open Source License  1.0

    

    MIT license  -

    

    Mozilla Public License (MPL)  1.0/1.1

    

    Open Software License  2.0

    

    PHP License  3.0

    

    Python license (CNRI Python License)  -

    

    Python Software Foundation License  2.1.1

    

    Sleepycat License  "1999"

    

    W3C License  "2001"

    

    X11 License  "2001"

    

    Zlib/libpng License  -

    

    Zope Public License  2.0

    

    

Due to the many variants of some of the above licenses, we require that any
version follow the 2003 version of the Free Software Foundation's Free
Software Definition (http://www.gnu.org/philosophy/free-sw.html) or version
1.9 of the Open Source Definition by the Open Source Initiative
(http://www.opensource.org/docs/definition.php).


  3. Definitions



      

    1. 

Terms used, but not defined, herein shall have the meaning provided in
the GPL.

    2. 

Derivative Work means a derivative work under copyright law.

    


  4. Applicability

This FLOSS Exception applies to all Programs that contain a notice placed by
MySQL AB saying that the Program may be distributed under the terms of this
FLOSS Exception.  If you create or distribute a work which is a Derivative
Work of both the Program and any other work licensed under the GPL, then
this FLOSS Exception is not available for that work; thus, you must remove
the FLOSS Exception notice from that work and comply with the GPL in all
respects, including by retaining all GPL notices.  You may choose to
redistribute a copy of the Program exclusively under the terms of the GPL by
removing the FLOSS Exception notice from that copy of the Program, provided
that the copy has never been modified by you or any third party.






SQL Command, Type, and Function Index





 



  • ! (logical NOT)

  • != (not equal)

  • "

  • % (modulo)

  • % (wildcard character)

  • & (bitwise AND)

  • && (logical AND)

  • () (parentheses)

  • (Control-Z) \Z

  • * (multiplication)

  • + (addition)

  • - (subtraction)

  • - (unary minus)

  • --password option

  • -p option

  • .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file

  • .mysql_history file

  • .pid (process ID) file

  • / (division)

  • `/etc/passwd'

  • /etc/passwd

  • < (less than)

  • <<

  • << (left shift)

  • <= (less than or equal)

  • <=> (equal to)

  • <> (not equal)

  • = (equal)

  • > (greater than)

  • >= (greater than or equal)

  • >> (right shift)

  • \" (double quote)

  • \' (single quote)

  • \0 (ASCII 0)

  • \\ (escape)

  • \b (backspace)

  • \n (linefeed)

  • \n (newline)

  • \r (carriage return)

  • \t (tab)

  • \Z (Control-Z) ASCII 26

  • ^ (bitwise XOR)

  • _ (wildcard character)

  • `

    • 

    A
    


  • ABS()

  • ACOS()

  • ADDDATE()

  • addition (+)

  • ADDTIME()

  • AES_DECRYPT()

  • AES_ENCRYPT()

  • alias

  • ALTER COLUMN

  • ALTER DATABASE

  • ALTER FUNCTION

  • ALTER PROCEDURE

  • ALTER SCHEMA

  • ALTER TABLE, ALTER TABLE, ALTER TABLE

  • ALTER VIEW

  • ANALYZE TABLE

  • AND, bitwise

  • AND, logical

  • Area(), Area()

  • arithmetic functions

  • AS, AS

  • AsBinary()

  • ASCII()

  • ASIN()

  • AsText()

  • ATAN()

  • ATAN2()

  • AVG()

    • 

    B
    


  • backspace (\b)

  • BACKUP TABLE

  • BdMPolyFromText()

  • BdMPolyFromWKB()

  • BdPolyFromText()

  • BdPolyFromWKB()

  • BEGIN, BEGIN

  • BENCHMARK()

  • BETWEEN ... AND

  • BIGINT

  • BIN()

  • BINARY

  • BIT

  • BIT_AND()

  • BIT_COUNT

  • BIT_COUNT()

  • BIT_LENGTH()

  • BIT_OR

  • BIT_OR()

  • BIT_XOR()

  • BLOB, BLOB

  • BOOL

  • BOOLEAN

  • Boundary()

  • Buffer()

    • 

    C
    


  • C:\my.cnf file

  • CACHE INDEX

  • CALL

  • carriage return (\r)

  • CASE, CASE

  • CAST

  • casts

  • CC environment variable

  • CC environment variable, CC environment variable, CC environment variable

  • CEILING()

  • Centroid()

  • CFLAGS environment variable, CFLAGS environment variable, CFLAGS environment variable

  • CHANGE MASTER TO

  • CHAR, CHAR

  • CHAR VARYING

  • CHAR()

  • CHAR_LENGTH()

  • CHARACTER

  • CHARACTER VARYING

  • CHARACTER_LENGTH()

  • CHARSET()

  • CHECK TABLE

  • CHECKSUM TABLE

  • CLOSE

  • COALESCE()

  • COERCIBILITY()

  • COLLATION()

  • command-line options

  • Comment syntax

  • COMMIT, COMMIT

  • comparison operators

  • COMPRESS()

  • CONCAT()

  • CONCAT_WS()

  • configure option, --with-charset

  • configure option, --with-collation

  • configure option, --with-extra-charsets

  • CONNECTION_ID()

  • constraints

  • Contains()

  • control flow functions

  • CONV()

  • CONVERT

  • CONVERT TO

  • CONVERT_TZ()

  • ConvexHull()

  • COS()

  • COT()

  • COUNT()

  • COUNT(DISTINCT)

  • CRC32()

  • CREATE DATABASE

  • CREATE FUNCTION, CREATE FUNCTION

  • CREATE INDEX

  • CREATE PROCEDURE

  • CREATE SCHEMA

  • CREATE TABLE

  • CREATE VIEW

  • CROSS JOIN

  • Crosses()

  • CURDATE()

  • CURRENT_DATE

  • CURRENT_TIME

  • CURRENT_TIMESTAMP

  • CURRENT_USER()

  • CURTIME()

  • CXX environment variable, CXX environment variable, CXX environment variable, CXX environment variable, CXX environment variable

  • CXX environment variable

  • CXXFLAGS environment variable, CXXFLAGS environment variable, CXXFLAGS environment variable

    • 

    D
    


  • Database information, obtaining

  • DATABASE()

  • DATE, DATE, DATE

  • date and time functions

  • DATE()

  • DATE_ADD()

  • DATE_FORMAT()

  • DATE_SUB()

  • DATEDIFF()

  • DATETIME, DATETIME

  • DAY()

  • DAYNAME()

  • DAYOFMONTH()

  • DAYOFWEEK()

  • DAYOFYEAR()

  • DBI->quote

  • DBI->trace

  • DBI_TRACE environment variable, DBI_TRACE environment variable

  • DBI_USER environment variable

  • DEALLOCATE PREPARE

  • DEC

  • DECIMAL

  • DECLARE

  • DECODE()

  • DEFAULT()

  • DEGREES()

  • DELAYED

  • DELETE

  • DES_DECRYPT()

  • DES_ENCRYPT()

  • DESC

  • DESCRIBE, DESCRIBE

  • Difference()

  • Dimension()

  • DISCARD TABLESPACE, DISCARD TABLESPACE

  • Disjoint()

  • Distance()

  • DISTINCT, DISTINCT, DISTINCT

  • DIV

  • division (/)

  • DO

  • DOUBLE

  • DOUBLE PRECISION

  • double quote (\")

  • DOUBLE(M,D)

  • DROP DATABASE

  • DROP FOREIGN KEY, DROP FOREIGN KEY

  • DROP FUNCTION, DROP FUNCTION

  • DROP INDEX, DROP INDEX

  • DROP PRIMARY KEY

  • DROP PROCEDURE

  • DROP SCHEMA

  • DROP TABLE

  • DROP USER

  • DROP VIEW

  • DUMPFILE

    • 

    E
    


  • ELT()

  • ENCODE()

  • ENCRYPT()

  • encryption functions

  • END

  • EndPoint()

  • ENUM, ENUM

  • Envelope()

  • Environment variable, CC

  • environment variable, CC, environment variable, CC

  • environment variable, CC

  • Environment variable, CFLAGS

  • environment variable, CFLAGS, environment variable, CFLAGS

  • environment variable, CXX

  • Environment variable, CXX, Environment variable, CXX

  • environment variable, CXX, environment variable, CXX

  • environment variable, CXXFLAGS, environment variable, CXXFLAGS

  • Environment variable, CXXFLAGS

  • Environment variable, DBI_TRACE, Environment variable, DBI_TRACE

  • Environment variable, DBI_USER

  • Environment variable, HOME

  • environment variable, HOME

  • Environment variable, LD_LIBRARY_PATH

  • Environment variable, LD_RUN_PATH, Environment variable, LD_RUN_PATH

  • environment variable, LD_RUN_PATH, environment variable, LD_RUN_PATH

  • Environment variable, MYSQL_DEBUG, Environment variable, MYSQL_DEBUG

  • environment variable, MYSQL_DEBUG

  • environment variable, MYSQL_HISTFILE

  • Environment variable, MYSQL_HISTFILE

  • environment variable, MYSQL_HOST

  • Environment variable, MYSQL_HOST

  • Environment variable, MYSQL_PS1

  • environment variable, MYSQL_PWD, environment variable, MYSQL_PWD

  • Environment variable, MYSQL_PWD

  • environment variable, MYSQL_TCP_PORT

  • environment variable, MYSQL_TCP_PORT, environment variable, MYSQL_TCP_PORT

  • Environment variable, MYSQL_TCP_PORT

  • environment variable, MYSQL_UNIX_PORT, environment variable, MYSQL_UNIX_PORT

  • Environment variable, MYSQL_UNIX_PORT, Environment variable, MYSQL_UNIX_PORT

  • environment variable, MYSQL_UNIX_PORT

  • Environment variable, PATH

  • environment variable, PATH

  • Environment variable, TMPDIR, Environment variable, TMPDIR

  • Environment variable, TZ, Environment variable, TZ

  • Environment variable, UMASK, Environment variable, UMASK

  • Environment variable, UMASK_DIR, Environment variable, UMASK_DIR

  • Environment variable, USER

  • environment variable, USER

  • Environment variables, CXX

  • equal (=)

  • Equals()

  • escape (\\)

  • EXECUTE

  • EXP()

  • EXPLAIN

  • EXPORT_SET()

  • ExteriorRing()

  • EXTRACT()

    • 

    F
    


  • FALSE

  • FETCH

  • FIELD()

  • FILE

  • FIND_IN_SET()

  • FIXED

  • FLOAT, FLOAT

  • FLOAT(M,D)

  • FLOAT(p), FLOAT(p), FLOAT(p)

  • FLOOR()

  • FLUSH

  • FORCE INDEX, FORCE INDEX, FORCE INDEX

  • FORCE KEY, FORCE KEY

  • FORMAT()

  • FOUND_ROWS()

  • FROM

  • FROM_DAYS()

  • FROM_UNIXTIME()

  • functions, arithmetic

  • functions, bit

  • functions, control flow

  • functions, date and time

  • functions, encryption

  • functions, GROUP BY

  • functions, information

  • functions, mathematical

  • functions, miscellaneous

  • functions, string

  • functions, string comparison

  • Functions, user-defined

    • 

    G
    


  • GeomCollFromText()

  • GeomCollFromWKB()

  • GEOMETRY

  • GEOMETRYCOLLECTION

  • GeometryCollection()

  • GeometryCollectionFromText()

  • GeometryCollectionFromWKB()

  • GeometryFromText()

  • GeometryFromWKB()

  • GeometryN()

  • GeometryType()

  • GeomFromText(), GeomFromText()

  • GeomFromWKB(), GeomFromWKB()

  • GET_FORMAT()

  • GET_LOCK()

  • GLength(), GLength()

  • GRANT

  • GRANT statement, GRANT statement

  • GRANTS

  • greater than (>)

  • greater than or equal (>=)

  • GREATEST()

  • GROUP BY

  • GROUP BY functions

  • GROUP_CONCAT()

    • 

    H
    


  • HANDLER

  • HEX()

  • hexadecimal values

  • HOME environment variable

  • HOME environment variable

  • host.frm, problems finding

  • HOUR()

    • 

    I
    


  • identifiers, quoting

  • IF

  • IF()

  • IFNULL()

  • IGNORE INDEX, IGNORE INDEX

  • IGNORE KEY, IGNORE KEY

  • IMPORT TABLESPACE, IMPORT TABLESPACE

  • IN

  • INET_ATON()

  • INET_NTOA()

  • information functions

  • INNER JOIN

  • INSERT, INSERT

  • INSERT ... SELECT

  • INSERT DELAYED

  • INSERT statement, grant privileges

  • INSERT()

  • INSTR()

  • INT

  • INTEGER

  • InteriorRingN()

  • Intersection()

  • Intersects()

  • INTERVAL()

  • IS NOT NULL

  • IS NULL, IS NULL

  • IS NULL, and indexes

  • IS_FREE_LOCK()

  • IS_USED_LOCK()

  • IsClosed(), IsClosed()

  • IsEmpty()

  • ISNULL()

  • ISOLATION LEVEL

  • IsRing()

  • IsSimple()

  • ITERATE

    • 

    J
    


  • JOIN

    • 

    K
    


  • KILL

    • 

    L
    


  • LAST_DAY()

  • LAST_INSERT_ID(), LAST_INSERT_ID()

  • LAST_INSERT_ID([expr])

  • LCASE()

  • LD_LIBRARY_PATH environment variable

  • LD_RUN_PATH environment variable, LD_RUN_PATH environment variable, LD_RUN_PATH environment variable, LD_RUN_PATH environment variable

  • LEAST()

  • LEAVE

  • LEFT JOIN, LEFT JOIN

  • LEFT OUTER JOIN

  • LEFT()

  • LENGTH()

  • less than (<)

  • less than or equal (<=)

  • LIKE

  • LIKE, and indexes

  • LIKE, and wildcards

  • LIMIT, LIMIT

  • linefeed (\n)

  • LineFromText()

  • LineFromWKB()

  • LINESTRING

  • LineString()

  • LineStringFromText()

  • LineStringFromWKB()

  • LN()

  • LOAD DATA FROM MASTER

  • LOAD DATA INFILE, LOAD DATA INFILE

  • LOAD TABLE FROM MASTER

  • LOAD_FILE()

  • LOCALTIME

  • LOCALTIMESTAMP

  • LOCATE()

  • LOCK TABLES

  • LOG()

  • LOG10()

  • LOG2()

  • logical operators

  • LONG

  • LONGBLOB

  • LONGTEXT

  • LOOP

  • LOWER()

  • LPAD()

  • LTRIM()

    • 

    M
    


  • MAKE_SET()

  • MAKEDATE()

  • MAKETIME()

  • MASTER_POS_WAIT(), MASTER_POS_WAIT()

  • MATCH ... AGAINST()

  • mathematical functions

  • MAX()

  • MBRContains()

  • MBRDisjoint()

  • MBREqual()

  • MBRIntersects()

  • MBROverlaps()

  • MBRTouches()

  • MBRWithin()

  • MD5()

  • MEDIUMBLOB

  • MEDIUMINT

  • MEDIUMTEXT

  • MICROSECOND()

  • MID()

  • MIN()

  • minus, unary (-)

  • MINUTE()

  • miscellaneous functions

  • MLineFromText()

  • MLineFromWKB()

  • MOD (modulo)

  • MOD()

  • modulo (%)

  • modulo (MOD)

  • MONTH()

  • MONTHNAME()

  • MPointFromText()

  • MPointFromWKB()

  • MPolyFromText()

  • MPolyFromWKB()

  • MULTILINESTRING

  • MultiLineString()

  • MultiLineStringFromText()

  • MultiLineStringFromWKB()

  • multiplication (*)

  • MULTIPOINT

  • MultiPoint()

  • MultiPointFromText()

  • MultiPointFromWKB()

  • MULTIPOLYGON

  • MultiPolygon()

  • MultiPolygonFromText()

  • MultiPolygonFromWKB()

  • my_init()

  • my_ulonglong C type

  • my_ulonglong values, printing

  • MySQL C type

  • MYSQL C type

  • mysql_affected_rows()

  • mysql_autocommit().

  • MYSQL_BIND C type

  • mysql_change_user()

  • mysql_character_set_name()

  • mysql_close()

  • mysql_commit().

  • mysql_connect()

  • mysql_create_db()

  • mysql_data_seek()

  • MYSQL_DEBUG environment variable

  • MYSQL_DEBUG environment variable, MYSQL_DEBUG environment variable

  • mysql_debug()

  • mysql_drop_db()

  • mysql_dump_debug_info()

  • mysql_eof()

  • mysql_errno()

  • mysql_error()

  • mysql_escape_string()

  • mysql_fetch_field()

  • mysql_fetch_field_direct()

  • mysql_fetch_fields()

  • mysql_fetch_lengths()

  • mysql_fetch_row()

  • MYSQL_FIELD C type

  • mysql_field_count(), mysql_field_count()

  • MYSQL_FIELD_OFFSET C type

  • mysql_field_seek()

  • mysql_field_tell()

  • mysql_free_result()

  • mysql_get_client_info()

  • mysql_get_client_version()

  • mysql_get_host_info()

  • mysql_get_proto_info()

  • mysql_get_server_info()

  • mysql_get_server_version()

  • MYSQL_HISTFILE environment variable

  • MYSQL_HISTFILE environment variable

  • MYSQL_HOST environment variable

  • MYSQL_HOST environment variable

  • mysql_info()

  • mysql_info(), mysql_info(), mysql_info(), mysql_info()

  • mysql_init()

  • mysql_insert_id(), mysql_insert_id()

  • mysql_insert_id()

  • mysql_kill()

  • mysql_list_dbs()

  • mysql_list_fields()

  • mysql_list_processes()

  • mysql_list_tables()

  • mysql_more_results().

  • mysql_next_result().

  • mysql_num_fields()

  • mysql_num_rows()

  • mysql_options()

  • mysql_ping()

  • MYSQL_PS1 environment variable

  • MYSQL_PWD environment variable, MYSQL_PWD environment variable

  • MYSQL_PWD environment variable

  • mysql_query(), mysql_query()

  • mysql_real_connect()

  • mysql_real_escape_string()

  • mysql_real_escape_string()

  • mysql_real_query()

  • mysql_reload()

  • MYSQL_RES C type

  • mysql_rollback().

  • MYSQL_ROW C type

  • mysql_row_seek()

  • mysql_row_tell()

  • mysql_select_db()

  • mysql_server_end()

  • mysql_server_init()

  • mysql_set_sever_option()

  • mysql_shutdown()

  • mysql_sqlstate()

  • mysql_ssl_set()

  • mysql_stat()

  • MYSQL_STMT C type

  • mysql_stmt_affected_rows()

  • mysql_stmt_attr_get()

  • mysql_stmt_attr_set()

  • mysql_stmt_bind_param()

  • mysql_stmt_bind_result()

  • mysql_stmt_close()

  • mysql_stmt_data_seek()

  • mysql_stmt_errno()

  • mysql_stmt_error().

  • mysql_stmt_execute()

  • mysql_stmt_fetch()

  • mysql_stmt_fetch_column()

  • mysql_stmt_field_count()

  • mysql_stmt_free_result()

  • mysql_stmt_init()

  • mysql_stmt_insert_id()

  • mysql_stmt_num_rows()

  • mysql_stmt_param_count()

  • mysql_stmt_param_metadata()

  • mysql_stmt_prepare()

  • mysql_stmt_reset()

  • mysql_stmt_result_metadata.

  • mysql_stmt_row_seek()

  • mysql_stmt_row_tell()

  • mysql_stmt_send_long_data().

  • mysql_stmt_sqlstate()

  • mysql_stmt_store_result()

  • mysql_store_result(), mysql_store_result()

  • MYSQL_TCP_PORT environment variable, MYSQL_TCP_PORT environment variable

  • MYSQL_TCP_PORT environment variable, MYSQL_TCP_PORT environment variable

  • mysql_thread_end()

  • mysql_thread_id()

  • mysql_thread_init()

  • mysql_thread_safe()

  • MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable

  • MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable

  • mysql_use_result()

  • mysql_warning_count().

    • 

    N
    


  • NATIONAL CHAR

  • NATURAL LEFT JOIN

  • NATURAL LEFT OUTER JOIN

  • NATURAL RIGHT JOIN

  • NATURAL RIGHT OUTER JOIN

  • NCHAR

  • newline (\n)

  • NOT BETWEEN

  • not equal (!=)

  • not equal (<>)

  • NOT IN

  • NOT LIKE

  • NOT REGEXP

  • NOT, logical

  • NOW()

  • NUL

  • NULL, NULL

  • NULL value

  • NULLIF()

  • NUMERIC

  • NumGeometries()

  • NumInteriorRings()

  • NumPoints()

    • 

    O
    


  • OCT()

  • OCTET_LENGTH()

  • OLAP

  • OLD_PASSWORD()

  • OPEN

  • operators, logical

  • OPTIMIZE TABLE

  • OR, OR

  • OR, bitwise

  • OR, logical

  • ORD()

  • ORDER BY, ORDER BY, ORDER BY

  • Overlaps()

    • 

    P
    


  • parentheses ( and )

  • PASSWORD(), PASSWORD(), PASSWORD(), PASSWORD()

  • PATH environment variable, PATH environment variable

  • PERIOD_ADD()

  • PERIOD_DIFF()

  • PI()

  • POINT

  • Point()

  • PointFromText()

  • PointFromWKB()

  • PointN()

  • PointOnSurface()

  • PolyFromText()

  • PolyFromWKB()

  • POLYGON

  • Polygon()

  • PolygonFromText()

  • PolygonFromWKB()

  • POSITION()

  • POW()

  • POWER()

  • PREPARE

  • PRIMARY KEY, PRIMARY KEY

  • PROCESSLIST

  • PURGE MASTER LOGS

    • 

    Q
    


  • QUARTER()

  • QUOTE()

  • quoting of identifiers

    • 

    R
    


  • RADIANS()

  • RAND()

  • REAL

  • ref_or_null

  • REGEXP

  • Related()

  • RELEASE_LOCK()

  • RENAME TABLE

  • REPAIR TABLE

  • REPEAT

  • REPEAT()

  • REPLACE

  • REPLACE ... SELECT

  • REPLACE()

  • REQUIRE GRANT option, REQUIRE GRANT option

  • RESET MASTER

  • RESET SLAVE

  • RESTORE TABLE

  • return (\r)

  • REVERSE()

  • REVOKE

  • RIGHT JOIN

  • RIGHT OUTER JOIN

  • RIGHT()

  • RLIKE

  • ROLLBACK, ROLLBACK

  • ROLLBACK TO SAVEPOINT

  • ROLLUP

  • ROUND()

  • ROW_COUNT()

  • RPAD()

  • RTRIM()

    • 

    S
    


  • SAVEPOINT

  • SEC_TO_TIME()

  • SECOND()

  • SELECT

  • SELECT INTO

  • SELECT INTO TABLE

  • SELECT speed

  • SELECT, optimizing

  • SESSION_USER()

  • SET, SET, SET, SET

  • SET GLOBAL SQL_SLAVE_SKIP_COUNTER

  • SET OPTION

  • SET PASSWORD

  • SET PASSWORD statement

  • SET SQL_LOG_BIN

  • SET TRANSACTION

  • SHA()

  • SHA1()

  • SHOW BINARY LOGS

  • SHOW BINLOG EVENTS, SHOW BINLOG EVENTS

  • SHOW CHARACTER SET

  • SHOW COLLATION

  • SHOW COLUMNS, SHOW COLUMNS

  • SHOW CREATE DATABASE, SHOW CREATE DATABASE

  • SHOW CREATE FUNCTION

  • SHOW CREATE PROCEDURE

  • SHOW CREATE SCHEMA, SHOW CREATE SCHEMA

  • SHOW CREATE TABLE, SHOW CREATE TABLE

  • SHOW CREATE VIEW

  • SHOW DATABASES, SHOW DATABASES

  • SHOW ENGINES, SHOW ENGINES, SHOW ENGINES

  • SHOW ERRORS, SHOW ERRORS

  • SHOW FIELDS

  • SHOW FUNCTION STATUS

  • SHOW GRANTS, SHOW GRANTS

  • SHOW INDEX, SHOW INDEX

  • SHOW INNODB STATUS

  • SHOW KEYS, SHOW KEYS

  • SHOW MASTER LOGS, SHOW MASTER LOGS

  • SHOW MASTER STATUS, SHOW MASTER STATUS

  • SHOW PRIVILEGES, SHOW PRIVILEGES

  • SHOW PROCEDURE STATUS

  • SHOW PROCESSLIST, SHOW PROCESSLIST

  • SHOW SCHEMAS, SHOW SCHEMAS

  • SHOW SLAVE HOSTS, SHOW SLAVE HOSTS

  • SHOW SLAVE STATUS, SHOW SLAVE STATUS

  • SHOW STATUS

  • SHOW STORAGE ENGINES

  • SHOW TABLE STATUS

  • SHOW TABLE TYPES, SHOW TABLE TYPES

  • SHOW TABLES, SHOW TABLES

  • SHOW VARIABLES

  • SHOW WARNINGS, SHOW WARNINGS

  • SIGN()

  • SIN()

  • single quote (\')

  • SMALLINT

  • SOUNDEX()

  • SOUNDS LIKE

  • SPACE()

  • SQL_CACHE

  • SQL_NO_CACHE

  • SQRT()

  • SRID()

  • START SLAVE

  • START TRANSACTION

  • StartPoint()

  • statements, GRANT

  • statements, INSERT

  • STD()

  • STDDEV()

  • STOP SLAVE

  • STR_TO_DATE()

  • STRAIGHT_JOIN

  • STRCMP()

  • string comparison functions

  • string functions

  • SUBDATE()

  • SUBSTRING()

  • SUBSTRING_INDEX()

  • SUBTIME()

  • subtraction (-)

  • SUM()

  • SymDifference()

  • SYSDATE()

  • SYSTEM_USER()

    • 

    T
    


  • tab (\t)

  • table_cache

  • TAN()

  • TEXT, TEXT

  • threads

  • TIME, TIME

  • TIME()

  • TIME_FORMAT()

  • TIME_TO_SEC()

  • TIMEDIFF()

  • TIMESTAMP, TIMESTAMP

  • TIMESTAMP()

  • TIMESTAMPADD()

  • TIMESTAMPDIFF()

  • TINYBLOB

  • TINYINT

  • TINYTEXT

  • TMPDIR environment variable, TMPDIR environment variable

  • TO_DAYS()

  • Touches()

  • trace DBI method

  • TRIM()

  • TRUE

  • TRUNCATE

  • TRUNCATE()

  • Types

  • TZ environment variable, TZ environment variable

    • 

    U
    


  • UCASE()

  • UDF functions

  • ulimit

  • UMASK environment variable, UMASK environment variable

  • UMASK_DIR environment variable, UMASK_DIR environment variable

  • unary minus (-)

  • UNCOMPRESS()

  • UNCOMPRESSED_LENGTH()

  • UNHEX()

  • UNION, UNION

  • Union()

  • UNIQUE

  • UNIX_TIMESTAMP()

  • UNLOCK TABLES

  • UNTIL

  • UPDATE

  • UPPER()

  • USE

  • USE INDEX, USE INDEX

  • USE KEY, USE KEY

  • USER environment variable

  • USER environment variable

  • USER()

  • User-defined functions

  • UTC_DATE()

  • UTC_TIME()

  • UTC_TIMESTAMP()

  • UUID()

    • 

    V
    


  • VARCHAR, VARCHAR

  • VARCHARACTER

  • VARIANCE()

  • VERSION()

    • 

    W
    


  • WEEK()

  • WEEKDAY()

  • WEEKOFYEAR()

  • WHERE

  • WHILE

  • Wildcard character (%)

  • Wildcard character (_)

  • Within()

  • without-server option

    • 

    X
    


  • X()

  • XOR, bitwise

  • XOR, logical

    • 

    Y
    


  • Y()

  • YEAR, YEAR

  • YEAR()

    • 

     
    


  • | (bitwise OR)

  • || (logical OR)

  • ~

    • 


    




    Concept Index
    


    

     
    


  • --with-raid link errors

  • @command{configure} script

  • @command{configure}, running after prior invocation

  • @command{gcc}

  • @command{make_binary_distribution}

  • @command{msql2mysql}

  • @command{myisamchk}, @command{myisamchk}

  • @command{myisamchk}, example output

  • @command{myisamchk}, options

  • @command{myisampack}, @command{myisampack}, @command{myisampack}, @command{myisampack}

  • @command{mysql.server}

  • @command{mysql_config}

  • @command{mysql_fix_privilege_tables}, @command{mysql_fix_privilege_tables}

  • @command{mysql_install_db}

  • @command{mysql_install_db} script

  • @command{mysqlaccess}

  • @command{mysqladmin}, @command{mysqladmin}, @command{mysqladmin}, @command{mysqladmin}, @command{mysqladmin}, @command{mysqladmin}, @command{mysqladmin}, @command{mysqladmin}

  • @command{mysqladmin} command-line options

  • @command{mysqladmin}, @command{mysqld_multi} option

  • @command{mysqlbinlog}, @command{mysqlbinlog}

  • @command{mysqlbug}

  • @command{mysqlbug} script

  • @command{mysqlbug} script, location

  • @command{mysqlcc}, @command{mysqlcc}

  • @command{mysqlcc} command-line options

  • @command{mysqlcheck}

  • @command{mysqld-max}, @command{mysqld-max}

  • @command{mysqld_multi}, @command{mysqld_multi}

  • @command{mysqld_safe}, @command{mysqld_safe}

  • @command{mysqldump}, @command{mysqldump}, @command{mysqldump}

  • @command{mysqld}

  • @command{mysqld} options

  • @command{mysqld} server, buffer sizes

  • @command{mysqld}, @command{mysqld_multi} option

  • @command{mysqld}, starting

  • @command{mysqlhotcopy}

  • @command{mysqlimport}, @command{mysqlimport}, @command{mysqlimport}, @command{mysqlimport}

  • @command{mysqlshow}

  • @command{mysql}, @command{mysql}

  • @command{mysql} command-line options

  • @command{pack_isam}

  • @command{perror}

  • @command{replace}

  • @command{safe_mysqld}

    • 

    A
    


  • aborted clients

  • aborted connection

  • access control

  • access denied errors

  • access privileges

  • Access program

  • account privileges, adding

  • accounts, anonymous user

  • accounts, root

  • ACID

  • ACID

  • ACLs

  • ActiveState Perl

  • adding, character sets

  • adding, native functions

  • adding, new account privileges

  • adding, new functions

  • adding, new user privileges

  • adding, new users, adding, new users

  • adding, procedures

  • adding, user-defined functions

  • administration, server

  • ADO program

  • age, calculating

  • alias names, case sensitivity

  • aliases, for expressions

  • aliases, for tables

  • aliases, in GROUP BY clauses

  • aliases, in ORDER BY clauses

  • aliases, names

  • aliases, on expressions

  • ALLOW_INVALID_DATES SQL mode

  • altering, database

  • altering, schema

  • anonymous user, anonymous user, anonymous user, anonymous user

  • ANSI mode, running

  • ANSI SQL mode, ANSI SQL mode

  • ANSI_QUOTES SQL mode

  • answering questions, etiquette

  • Apache

  • API's, list of

  • APIs

  • APIs, Perl

  • argument processing

  • arithmetic expressions

  • attackers, security against

  • AUTO-INCREMENT, ODBC

  • AUTO_INCREMENT

  • AUTO_INCREMENT, and NULL values

    • 

    B
    


  • backing up, databases, backing up, databases

  • backslash, escape character

  • backups

  • backups, database

  • batch mode

  • batch, @command{mysql} option

  • BDB storage engine, BDB storage engine

  • BDB table type, BDB table type

  • BDB tables

  • benchmark suite

  • benchmarks

  • BerkeleyDB storage engine, BerkeleyDB storage engine

  • BerkeleyDB table type, BerkeleyDB table type

  • Big5 Chinese character encoding

  • binary distributions

  • binary distributions, installing

  • binary distributions, on Linux

  • binary log

  • bit_functions, example

  • BitKeeper tree

  • BLOB columns, default values

  • BLOB columns, indexing, BLOB columns, indexing

  • BLOB, inserting binary data

  • BLOB, size

  • blocking_queries, @command{mysqlcc} option

  • Borland Builder 4 program

  • Borland C++ compiler

  • brackets, square

  • buffer sizes, @command{mysqld} server

  • buffer sizes, client

  • bug reports, criteria for

  • bugs database

  • bugs, known

  • bugs, reporting

  • bugs.mysql.com

  • building, client programs

    • 

    C
    


  • C API, data types

  • C API, functions

  • C API, linking problems

  • C Prepared statement API, functions

  • C++ APIs

  • C++ Builder

  • C++ compiler cannot create executables

  • C++ compiler, @command{gcc}

  • caches, clearing

  • calculating, dates

  • calling sequences for aggregate functions, UDF

  • calling sequences for simple functions, UDF

  • can't create/write to file

  • case sensitivity, in identifiers

  • case sensitivity, in names

  • case sensitivity, in searches

  • case sensitivity, in string comparisons

  • case-sensitivity, in access checking

  • case-sensitivity, of database names

  • case-sensitivity, of table names

  • cast functions

  • cast operators

  • casts

  • cc1plus problems

  • certification

  • ChangeLog

  • changes to privileges

  • changes, log

  • changes, version 3.19

  • changes, version 3.20

  • changes, version 3.21

  • changes, version 3.22

  • changes, version 3.23

  • changes, version 4.0

  • changes, version 4.1

  • changes, version 5.0

  • changing socket location, changing socket location, changing socket location

  • changing, column

  • changing, column order

  • changing, field

  • changing, table, changing, table, changing, table

  • Character sets

  • character sets, character sets

  • character sets, adding

  • character-sets-dir, @command{mysql} option

  • characters, multi-byte

  • check options, myisamchk

  • checking, tables for errors

  • checksum errors

  • Chinese

  • choosing types

  • choosing, a MySQL version

  • clearing, caches

  • client programs, building

  • client tools

  • clients, debugging

  • clients, threaded

  • closing, tables

  • ColdFusion program

  • collating, strings

  • column comments

  • column names, case sensitivity

  • column, changing

  • columns, changing

  • columns, displaying

  • columns, indexes

  • columns, names

  • columns, other types

  • columns, selecting

  • columns, storage requirements

  • columns, types

  • command syntax

  • command-line history, mysql

  • command-line options, @command{mysqladmin}

  • command-line options, @command{mysqlcc}

  • command-line options, @command{mysql}

  • command-line tool

  • commands out of sync

  • commands, for binary distribution

  • commands, replication masters

  • commands, replication slaves

  • comments, adding

  • comments, starting

  • commercial support, types

  • compatibility, between MySQL versions, compatibility, between MySQL versions, compatibility, between MySQL versions, compatibility, between MySQL versions, compatibility, between MySQL versions

  • compatibility, with mSQL

  • compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC

  • compatibility, with Oracle, compatibility, with Oracle, compatibility, with Oracle

  • compatibility, with PostgreSQL

  • compatibility, with standard SQL

  • compatibility, with Sybase

  • compiler, C++ @command{gcc}

  • compiling, on Windows

  • compiling, optimizing

  • compiling, problems

  • compiling, speed

  • compiling, statically

  • compiling, user-defined functions

  • compliance, Y2K

  • compress, @command{mysqlcc} option

  • compress, @command{mysql} option

  • compressed tables, compressed tables

  • concurrent inserts

  • Conditions

  • config-file, @command{mysqld_multi} option

  • config.cache

  • config.cache file

  • configuration files

  • configuration options

  • configure option, --with-low-memory

  • connect_timeout variable, connect_timeout variable, connect_timeout variable

  • connecting, remotely with SSH

  • connecting, to the server, connecting, to the server

  • connecting, verification

  • connection, aborted

  • connection_name, @command{mysqlcc} option

  • Connector/J

  • Connector/JDBC

  • Connector/ODBC, Connector/ODBC

  • Connector/ODBC, reporting problems

  • Connectors, MySQL

  • constant table, constant table

  • consulting

  • contact information

  • contributing companies, list of

  • contributors, list of

  • control access

  • conventions, typographical

  • copying databases

  • copying tables

  • copyrights

  • costs, support

  • counting, table rows

  • crash

  • crash, recovery

  • crash, repeated

  • crash-me

  • crash-me program, crash-me program

  • creating, bug reports

  • creating, database

  • creating, databases

  • creating, default startup options

  • creating, schema

  • creating, tables

  • Cursors

  • customer support, mailing address

  • customers, of MySQL

    • 

    D
    


  • data types

  • data types, C API

  • data, character sets

  • data, importing

  • data, loading into tables

  • data, retrieving

  • data, size

  • database design

  • database names, case sensitivity

  • database names, case-sensitivity

  • database, @command{mysqlcc} option

  • database, @command{mysql} option

  • database, altering

  • database, creating

  • database, deleting

  • databases, backups

  • databases, copying

  • databases, creating

  • databases, defined

  • databases, displaying

  • databases, dumping, databases, dumping

  • databases, information about

  • databases, names

  • databases, replicating

  • databases, selecting

  • databases, symbolic links

  • databases, using

  • DataJunction

  • Date and Time types

  • date calculations

  • DATE columns, problems

  • date functions, Y2K compliance

  • date types

  • date types, Y2K issues

  • date values, problems

  • db table, sorting

  • DB2 SQL mode

  • DBI interface

  • DBI/DBD interface

  • DBUG package

  • debug, @command{mysql} option

  • debug-info, @command{mysql} option

  • debugging, client

  • debugging, server

  • decimal point

  • decode_bits @command{myisamchk} variable

  • default hostname

  • default installation location

  • default options

  • default values, default values, default values

  • default values, BLOB and TEXT columns

  • default values, suppression

  • default, privileges

  • default-character-set, @command{mysql} option

  • defaults, embedded

  • delayed_insert_limit

  • deleting, database

  • deleting, foreign key, deleting, foreign key

  • deleting, function

  • deleting, index, deleting, index

  • deleting, primary key

  • deleting, rows

  • deleting, schema

  • deleting, table

  • deleting, user, deleting, user

  • deleting, users, deleting, users

  • deletion, mysql.sock

  • Delphi program

  • derived tables

  • design, choices

  • design, issues

  • design, limitations

  • developers, list of

  • development source tree

  • digits

  • directory structure, default

  • disconnecting, from the server

  • disk full

  • disk issues

  • disks, splitting data across

  • display size

  • display width

  • displaying, database information

  • displaying, information, Cardinality

  • displaying, information, Collation

  • displaying, information, SHOW, displaying, information, SHOW, displaying, information, SHOW, displaying, information, SHOW

  • displaying, table status

  • DNS

  • Documenters, list of

  • downgrading

  • downloading

  • dropping, user, dropping, user

  • dumping, databases, dumping, databases

  • dynamic table characteristics

    • 

    E
    


  • Eiffel Wrapper

  • email lists

  • embedded MySQL server library

  • employment with MySQL

  • employment, contact information

  • entering, queries

  • ENUM, size

  • environment variables, environment variables, environment variables

  • environment variables, list of

  • environment varible, PATH

  • Errcode

  • errno

  • error messages, can't find file

  • error messages, displaying

  • error messages, languages

  • ERROR_FOR_DIVISION_BY_ZERO SQL mode

  • errors, access denied

  • errors, checking tables for

  • errors, common

  • errors, directory checksum

  • errors, handling for UDFs

  • errors, known

  • errors, linking

  • errors, list of

  • errors, reporting, errors, reporting, errors, reporting

  • escape characters

  • estimating, query performance

  • example, @command{mysqld_multi} option

  • examples, @command{myisamchk} output

  • examples, compressed tables

  • examples, queries

  • Excel

  • execute, @command{mysql} option

  • expression aliases, expression aliases

  • expressions, extended

  • extensions, to standard SQL

  • extracting, dates

    • 

    F
    


  • fatal signal 11

  • features of MySQL

  • field, changing

  • files, binary log

  • files, config.cache

  • files, error messages

  • files, log, files, log

  • files, `my.cnf'

  • files, not found message

  • files, permissions

  • files, query log

  • files, repairing

  • files, script

  • files, size limits

  • files, slow query log

  • files, text

  • files, tmp

  • files, update log

  • filesort optimization

  • floating-point number

  • floats

  • flush tables

  • force, @command{mysql} option

  • foreign key, constraint

  • foreign key, deleting, foreign key, deleting

  • foreign keys, foreign keys, foreign keys

  • free licensing

  • FreeBSD troubleshooting

  • ft_max_word_len @command{myisamchk} variable

  • ft_min_word_len @command{myisamchk} variable

  • ft_stopword_file @command{myisamchk} variable

  • full disk

  • full-text search

  • FULLTEXT

  • function, deleting

  • functions

  • functions for SELECT and WHERE clauses

  • functions, C API

  • functions, C Prepared statement API

  • functions, cast

  • functions, grouping

  • functions, native, adding

  • functions, new

  • functions, user-defined

  • functions, user-defined, adding

    • 

    G
    


  • gdb, using

  • general information

  • General Public License

  • General Public License, MySQL

  • geographic feature

  • geometry

  • geospatial feature

  • getting MySQL

  • GIS, GIS

  • global privileges

  • goals of MySQL

  • GPL, General Public License

  • GPL, GNU General Public License

  • GPL, MySQL

  • GPL, MySQL FLOSS License Exception

  • grant tables

  • grant tables, re-creating

  • grant tables, sorting, grant tables, sorting

  • grant tables, upgrading

  • granting, privileges

  • graphical tool

  • GROUP BY, aliases in

  • GROUP BY, extensions to standard SQL, GROUP BY, extensions to standard SQL

  • grouping, expressions

  • GUI tool

    • 

    H
    


  • Handlers

  • handling, errors

  • HEAP storage engine, HEAP storage engine

  • HEAP table type, HEAP table type

  • help, @command{mysqlcc} option

  • help, @command{mysqld_multi} option

  • help, @command{mysql} option

  • hints, hints, hints, hints, hints

  • history of MySQL

  • history_size, @command{mysqlcc} option

  • host table

  • host table, sorting

  • host, @command{mysqlcc} option

  • host, @command{mysql} option

  • hostname caching

  • hostname, default

  • html, @command{mysql} option

    • 

    I
    


  • ID, unique

  • identifiers

  • identifiers, case sensitivity

  • ignore-space, @command{mysql} option

  • IGNORE_SPACE SQL mode

  • importing, data

  • increasing with replication, speed

  • increasing, performance

  • index, deleting, index, deleting

  • indexes

  • indexes, and BLOB columns, indexes, and BLOB columns

  • indexes, and IS NULL

  • indexes, and LIKE

  • indexes, and NULL values

  • indexes, and TEXT columns, indexes, and TEXT columns

  • indexes, assigning to key cache

  • indexes, block size

  • indexes, columns

  • indexes, leftmost prefix of

  • indexes, multi-column

  • indexes, multiple-part

  • indexes, names

  • indexes, use of

  • InnoDB

  • InnoDB storage engine, InnoDB storage engine

  • InnoDB table type, InnoDB table type

  • InnoDB tables

  • INSERT DELAYED

  • inserting, speed of

  • installation layouts

  • installation overview

  • installing, binary distribution

  • installing, Linux RPM packages

  • installing, Mac OS X PKG packages

  • installing, overview

  • installing, Perl

  • installing, Perl on Windows

  • installing, source distribution

  • installing, user-defined functions

  • integers

  • internal compiler errors

  • internal locking

  • internals

  • Internet Relay Chat

  • Internet Service Providers

  • introducer, string literal, introducer, string literal

  • IRC

  • ISAM storage engine, ISAM storage engine

  • ISAM table type, ISAM table type

  • ISP services

    • 

    J
    


  • Java connectivity

  • JDBC

  • jobs at MySQL

    • 

    K
    


  • key cache, assigning indexes to

  • Key cache, MyISAM

  • key space, MyISAM

  • key_buffer_size @command{myisamchk} variable

  • keys

  • keys, foreign, keys, foreign

  • keys, multi-column

  • keys, searching on two

  • keywords

  • known errors

    • 

    L
    


  • language support

  • last row, unique ID

  • layout of installation

  • leftmost prefix of indexes

  • legal names

  • libmysqld

  • libraries, list of

  • library, mysqlclient

  • License

  • licenses

  • licensing costs

  • licensing policy

  • licensing terms

  • licensing, contact information

  • licensing, examples

  • licensing, free

  • limitations, design

  • limitations, replication

  • limits, file-size

  • linking

  • linking, errors

  • linking, problems

  • linking, speed

  • links, symbolic

  • Linux, binary distribution

  • Linux, source distribution

  • literals

  • loading, tables

  • local-infile, @command{mysqlcc} option

  • local-infile, @command{mysql} option

  • locking

  • locking methods

  • locking, page-level

  • locking, row-level, locking, row-level

  • locking, table-level

  • log files

  • Log files

  • log files, maintaining

  • log files, names

  • log, @command{mysqld_multi} option

  • log, changes

  • logos

    • 

    M
    


  • Mac OS X, installation

  • mailing address, for customer support

  • mailing list address

  • mailing lists

  • mailing lists, archive location

  • mailing lists, guidelines

  • main features of MySQL

  • maintaining, log files

  • maintaining, tables

  • manual, available formats

  • manual, online location

  • manual, typographical conventions

  • master/slave setup

  • matching, patterns

  • max_allowed_packet variable, max_allowed_packet variable

  • max_join_size variable, max_join_size variable

  • MAXDB SQL mode

  • maximum memory used

  • MBR

  • MEMORY storage engine, MEMORY storage engine

  • MEMORY table type, MEMORY table type

  • memory usage, myisamchk

  • memory use, memory use

  • MERGE storage engine, MERGE storage engine

  • MERGE table type, MERGE table type

  • MERGE tables, defined

  • messages, languages

  • methods, locking

  • Minimum Bounding Rectangle

  • mirror sites

  • MIT-pthreads

  • modes, batch

  • modules, list of

  • monitor, terminal

  • mSQL compatibility

  • MSSQL SQL mode

  • multi mysqld

  • multi-byte character sets

  • multi-byte characters

  • multi-column indexes

  • multiple servers

  • multiple-part index

  • My, derivation

  • `my.cnf' file

  • MyISAM key cache

  • MyISAM storage engine, MyISAM storage engine

  • MyISAM table type, MyISAM table type

  • MyISAM, compressed tables, MyISAM, compressed tables

  • MyISAM, size

  • myisam_block_size @command{myisamchk} variable

  • MyODBC

  • MyODBC, reporting problems

  • MySQL AB, defined

  • MySQL binary distribution

  • MySQL certification

  • mysql commands, list of

  • MySQL consulting

  • MySQL Dolphin name

  • MySQL history

  • mysql history file

  • MySQL mailing lists

  • MySQL name

  • mysql prompt command

  • MySQL source distribution

  • mysql status command

  • MySQL storage engines

  • MySQL table types

  • MySQL training

  • MySQL version

  • MySQL++

  • MySQL, defined

  • MySQL, introduction

  • MySQL, pronunciation

  • mysql.sock, changing location of

  • mysql.sock, protection

  • MYSQL323 SQL mode

  • MYSQL40 SQL mode

  • mysqlclient library

  • mysqld options

  • mysqltest, MySQL Test Suite

    • 

    N
    


  • named pipes, named pipes

  • named-commands, @command{mysql} option

  • names

  • names, case sensitivity

  • names, variables

  • naming, releases of MySQL

  • native functions, adding

  • native thread support

  • negative values

  • nested queries

  • nested query

  • nesting queries

  • net etiquette, net etiquette

  • net_buffer_length variable, net_buffer_length variable

  • netmask notation, in mysql.user table

  • NetWare

  • new procedures, adding

  • new users, adding, new users, adding

  • no matching rows

  • no-auto-rehash, @command{mysql} option

  • no-beep, @command{mysql} option

  • no-log, @command{mysqld_multi} option

  • no-named-commands, @command{mysql} option

  • no-pager, @command{mysql} option

  • no-tee, @command{mysql} option

  • NO_AUTO_VALUE_ON_ZERO SQL mode

  • NO_DIR_IN_CREATE SQL mode

  • NO_FIELD_OPTIONS SQL mode

  • NO_KEY_OPTIONS SQL mode

  • NO_TABLE_OPTIONS SQL mode

  • NO_UNSIGNED_SUBTRACTION SQL mode

  • NO_ZERO_DATE SQL mode

  • NO_ZERO_IN_DATE SQL mode

  • non-delimited strings

  • Non-transactional tables

  • NOT NULL, constraint

  • Novell NetWare

  • NULL value

  • NULL values, and indexes

  • NULL values, vs. empty values

  • NULL, testing for null, NULL, testing for null, NULL, testing for null, NULL, testing for null

  • NULL values, and AUTO_INCREMENT columns

  • NULL values, and TIMESTAMP columns

  • numbers

  • numeric types

    • 

    O
    


  • ODBC

  • ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility

  • odbcadmin program

  • one-database, @command{mysql} option

  • online location of manual

  • ONLY_FULL_GROUP_BY SQL mode

  • Open Source, defined

  • open tables, open tables

  • open_files_limit variable

  • OpenGIS

  • opening, tables

  • opens

  • OpenSSL

  • operating systems, file-size limits

  • operating systems, supported

  • operating systems, Windows versus Unix

  • operations, arithmetic

  • operators

  • operators, cast, operators, cast

  • optimization, tips

  • optimizations, optimizations

  • optimizer, controlling

  • optimizing, DISTINCT

  • optimizing, filesort

  • optimizing, GROUP BY

  • optimizing, LEFT JOIN

  • optimizing, LIMIT

  • optimizing, tables

  • option files, option files

  • options, @command{configure}

  • options, @command{myisamchk}

  • options, command-line

  • options, command-line, @command{mysqladmin}

  • options, command-line, @command{mysqlcc}

  • options, command-line, @command{mysql}

  • options, provided by MySQL

  • options, replication

  • OR Index Merge optimization

  • Oracle compatibility, Oracle compatibility, Oracle compatibility

  • ORACLE SQL mode

  • ORDER BY, aliases in

  • overview

    • 

    P
    


  • packages, list of

  • page-level locking

  • pager, @command{mysql} option

  • parameters, server

  • partnering with MySQL AB

  • password encryption, reversibility of

  • password, @command{mysqlcc} option

  • password, @command{mysqld_multi} option

  • password, @command{mysql} option

  • password, root user

  • passwords, for users

  • passwords, forgotten

  • passwords, lost

  • passwords, resetting

  • passwords, security

  • passwords, setting, passwords, setting, passwords, setting

  • PATH environment variable

  • pattern matching

  • performance, benchmarks

  • performance, disk issues

  • performance, estimating

  • performance, improving, performance, improving

  • Perl API

  • Perl DBI/DBD, installation problems

  • Perl, installing

  • Perl, installing on Windows

  • permission checks, effect on speed

  • perror

  • PHP API

  • PIPES_AS_CONCAT SQL mode

  • plugins_path, @command{mysqlcc} option

  • port, @command{mysqlcc} option

  • port, @command{mysql} option

  • portability

  • portability, types

  • porting, to other systems

  • post-install, multiple servers

  • post-installation, setup and testing

  • PostgreSQL compatibility

  • POSTGRESQL SQL mode

  • prices, support

  • PRIMARY KEY, constraint

  • primary key, deleting

  • privilege information, location

  • privilege system

  • privilege system, described

  • privilege, changes

  • privileges, access

  • privileges, adding

  • privileges, default

  • privileges, deleting, privileges, deleting

  • privileges, display

  • privileges, dropping, privileges, dropping

  • privileges, granting

  • privileges, revoking

  • problems, access denied errors

  • problems, common errors

  • problems, compiling

  • problems, DATE columns

  • problems, date values

  • problems, installing on IBM-AIX

  • problems, installing on Solaris

  • problems, installing Perl

  • problems, linking

  • problems, ODBC

  • problems, reporting

  • problems, starting the server

  • problems, table locking

  • problems, time zone

  • procedures, adding

  • procedures, stored, procedures, stored

  • process support

  • processes, display

  • processing, arguments

  • products, selling

  • program variables, setting

  • programs, client

  • programs, crash-me

  • programs, list of

  • prompt, @command{mysql} option

  • prompts, meanings

  • pronunciation, MySQL

  • Protocol mismatch

  • protocol, @command{mysql} option

  • Python API

    • 

    Q
    


  • queries, entering

  • queries, estimating performance

  • queries, examples

  • queries, speed of

  • queries, Twin Studies project

  • Query Cache

  • query log

  • query, @command{mysqlcc} option

  • questions

  • questions, answering

  • quick, @command{mysql} option

  • quotes, in strings

  • quoting

  • quoting binary data

    • 

    R
    


  • RAID, compile errors

  • RAID, table type

  • raw, @command{mysql} option

  • re-creating, grant tables

  • read_buffer_size @command{myisamchk} variable

  • REAL_AS_FLOAT SQL mode

  • reconfiguring, reconfiguring

  • reconnect, @command{mysql} option

  • recovery, from crash

  • reducing, data size

  • references

  • regex

  • register, @command{mysqlcc} option

  • regular expression syntax, described

  • relational databases, defined

  • release numbers

  • releases, naming scheme

  • releases, testing

  • releases, updating

  • reordering, columns

  • repair options, myisamchk

  • repairing, tables

  • replace utility

  • replication

  • replication limitations

  • replication masters, commands

  • replication options

  • replication slaves, commands

  • reporting, bugs

  • reporting, Connector/ODBC problems

  • reporting, errors, reporting, errors

  • reporting, MyODBC problems

  • reserved words, exceptions

  • restarting, the server

  • retrieving, data from tables

  • return values, UDFs

  • revoking, privileges

  • root password

  • root user, password resetting

  • rounding errors, rounding errors

  • row-level locking

  • rows, counting

  • rows, deleting

  • rows, locking

  • rows, matching problems

  • rows, selecting

  • rows, sorting

  • RPM file

  • RPM Package Manager

  • RTS-threads

  • running @command{configure} after prior invocation

  • running a Web server

  • running, ANSI mode

  • running, batch mode

  • running, multiple servers

  • running, queries

    • 

    S
    


  • safe-updates option

  • safe-updates, @command{mysql} option

  • Sakila

  • schema, altering

  • schema, creating

  • schema, deleting

  • script files

  • scripts, scripts

  • scripts, @command{mysql_install_db}

  • scripts, @command{mysqlbug}

  • scripts, SQL

  • searching, and case sensitivity

  • searching, full-text

  • searching, MySQL Web pages

  • searching, two keys

  • security system

  • security, against attackers

  • SELECT, Query Cache

  • select_limit variable, select_limit variable

  • selecting, databases

  • selling products

  • SEQUENCE

  • sequence emulation

  • sequences

  • server administration

  • server variables, server variables, server variables

  • server, @command{mysqlcc} option

  • server, connecting, server, connecting

  • server, debugging

  • server, disconnecting

  • server, restart

  • server, shutdown

  • server, starting

  • server, starting and stopping

  • server, starting problems

  • servers, multiple

  • services, ISP

  • services, Web

  • SET, size

  • setting passwords

  • setting program variables

  • setting, passwords

  • setup, post-installation

  • shell syntax

  • showing, database information

  • shutdown_timeout variable

  • shutting down, the server

  • silent column changes

  • silent, @command{mysql} option

  • size of tables

  • sizes, display

  • skip-column-names, @command{mysql} option

  • skip-line-numbers, @command{mysql} option

  • slow queries

  • slow query log

  • socket location, changing

  • socket, @command{mysqlcc} option

  • socket, @command{mysql} option

  • Solaris installation problems

  • Solaris troubleshooting

  • sort_buffer_size @command{myisamchk} variable

  • sort_key_blocks @command{myisamchk} variable

  • sorting, character sets

  • sorting, data

  • sorting, grant tables, sorting, grant tables

  • sorting, table rows

  • source distribution, installing

  • source distributions, on Linux

  • Spatial Extensions in MySQL

  • speed, compiling

  • speed, increasing with replication

  • speed, inserting

  • speed, linking

  • speed, of queries, speed, of queries

  • SQL commands, replication masters

  • SQL commands, replication slaves

  • SQL scripts

  • SQL, defined

  • SQL-92, extensions to

  • sql_yacc.cc problems

  • square brackets

  • SSH

  • SSL and X509 Basics

  • SSL command-line options

  • SSL related options, SSL related options

  • stability

  • Standard SQL, differences from

  • standard SQL, extensions to

  • standards compatibility

  • Starting many servers

  • starting, @command{mysqld}

  • starting, comments

  • starting, the server

  • starting, the server automatically

  • startup options, default

  • startup parameters

  • startup parameters, @command{mysqladmin}

  • startup parameters, @command{mysqlcc}

  • startup parameters, @command{mysql}

  • startup parameters, tuning

  • statically, compiling

  • status command, results

  • status variables, status variables

  • status, tables

  • stopping, the server

  • storage engines, choosing

  • storage of data

  • storage requirements, column type

  • storage space, minimising

  • stored procedures

  • stored procedures and triggers, defined

  • STRICT SQL mode

  • STRICT_ALL_TABLES SQL mode

  • STRICT_TRANS_TABLES SQL mode, STRICT_TRANS_TABLES SQL mode

  • string collating

  • string comparisons, case sensitivity

  • string literal introducer, string literal introducer

  • string replacement, replace utility

  • string types

  • strings, defined

  • strings, escaping characters

  • strings, non-delimited

  • striping, defined

  • subqueries

  • subquery

  • subselects

  • superuser

  • support costs

  • support terms

  • support, for operating systems

  • support, licensing

  • support, mailing address

  • support, types

  • suppression, default values

  • Sybase compatibility

  • symbolic links, symbolic links

  • syntax, @command{mysqlcc} option

  • syntax, regular expression

  • syntax_file, @command{mysqlcc} option

  • system optimization

  • system table

  • system variables, system variables, system variables

  • system, privilege

  • system, security

    • 

    T
    


  • table aliases

  • table cache

  • table is full, table is full

  • table names, case sensitivity

  • table names, case-sensitivity

  • Table scans, avoiding

  • table types, choosing

  • table, @command{mysql} option

  • table, changing, table, changing, table, changing

  • table, deleting

  • table-level locking

  • tables, BDB

  • tables, Berkeley DB

  • tables, changing column order

  • tables, checking

  • tables, closing

  • tables, compressed

  • tables, compressed format

  • tables, constant, tables, constant

  • tables, copying

  • tables, counting rows

  • tables, creating

  • tables, defragment, tables, defragment

  • tables, defragmenting

  • tables, deleting rows

  • tables, displaying

  • tables, displaying status

  • tables, dumping, tables, dumping

  • tables, dynamic

  • tables, error checking

  • tables, flush

  • tables, fragmentation

  • tables, grant

  • tables, HEAP

  • tables, host

  • tables, improving performance

  • tables, information

  • tables, information about

  • tables, InnoDB

  • tables, ISAM

  • tables, loading data

  • tables, maintenance regimen

  • tables, maximum size

  • tables, MEMORY

  • tables, MERGE

  • tables, merging

  • tables, multiple

  • tables, MyISAM

  • tables, names

  • tables, open

  • tables, opening

  • tables, optimizing

  • tables, partitioning

  • tables, RAID

  • tables, repairing

  • tables, retrieving data

  • tables, selecting columns

  • tables, selecting rows

  • tables, sorting rows

  • tables, symbolic links

  • tables, system

  • tables, too many

  • tables, unique ID for last row

  • tables, updating

  • tar, problems on Solaris

  • Tcl API

  • tcp-ip, @command{mysqld_multi} option

  • TCP/IP, TCP/IP

  • technical support, licensing

  • technical support, mailing address

  • tee, @command{mysql} option

  • temporary file, write access

  • temporary tables, problems

  • terminal monitor, defined

  • testing mysqld, mysqltest

  • testing, connection to the server

  • testing, installation

  • testing, of MySQL releases

  • testing, post-installation

  • Texinfo

  • TEXT columns, default values

  • TEXT columns, indexing, TEXT columns, indexing

  • text files, importing

  • TEXT, size

  • thread packages, differences between

  • thread support

  • thread support, non-native

  • threaded clients

  • threads, threads

  • threads, display

  • threads, RTS

  • time types

  • time zone problems

  • timeout, timeout, timeout

  • timeout, connect_timeout variable, timeout, connect_timeout variable, timeout, connect_timeout variable

  • timeout, shutdown_timeout variable

  • TIMESTAMP, and NULL values

  • tips, optimization

  • ToDo list for MySQL

  • TODO, embedded server

  • TODO, symlinks

  • tools, command-line

  • tools, graphical

  • tools, GUI

  • tools, list of

  • tools, mysqld_multi

  • tools, mysqld_safe

  • tools, safe_mysqld

  • trademarks

  • TRADITIONAL SQL mode

  • TRADITONAL SQL mode

  • training

  • transaction-safe tables, transaction-safe tables

  • transactions, support, transactions, support

  • translations_path, @command{mysqlcc} option

  • Translators, list of

  • triggers

  • troubleshooting, FreeBSD

  • troubleshooting, Solaris

  • tutorial

  • Twin Studies, queries

  • type conversions

  • types of support

  • types, columns, types, columns

  • types, data

  • types, date

  • types, Date and Time

  • types, numeric

  • types, of tables

  • types, portability

  • types, strings

  • types, time

  • typographical conventions

    • 

    U
    


  • UCS-2

  • UDFs, compiling

  • UDFs, defined

  • UDFs, return values

  • unbuffered, @command{mysql} option

  • Unicode

  • unique ID

  • UNIQUE, constraint

  • unloading, tables

  • unnamed views

  • update log

  • updating, releases of MySQL

  • updating, tables

  • upgrading

  • upgrading, 3.20 to 3.21

  • upgrading, 3.21 to 3.22

  • upgrading, 3.22 to 3.23

  • upgrading, 3.23 to 4.0

  • upgrading, 4.0 to 4.1

  • upgrading, different architecture

  • upgrading, grant tables

  • upgrading, to 5.0

  • uptime

  • URLs for downloading MySQL

  • user privileges, adding

  • user privileges, deleting, user privileges, deleting

  • user privileges, dropping, user privileges, dropping

  • user table, sorting

  • user variables

  • user, @command{mysqlcc} option

  • user, @command{mysqld_multi} option

  • user, @command{mysql} option

  • user-defined functions, adding, user-defined functions, adding

  • usernames, and passwords

  • users, adding, users, adding

  • users, deleting, users, deleting

  • users, root

  • uses, of MySQL

  • using multiple disks to start data

  • UTF-8

  • UTF8

    • 

    V
    


  • valid numbers, examples

  • VARCHAR, size

  • variables, @command{mysqld}

  • variables, server, variables, server

  • variables, status, variables, status

  • variables, System

  • variables, system, variables, system

  • variables, user

  • variables, values

  • verbose, @command{mysql} option

  • version, @command{mysqlcc} option

  • version, @command{mysqld_multi} option

  • version, @command{mysql} option

  • version, choosing

  • version, latest

  • vertical, @command{mysql} option

  • views, views

  • views, updatable, views, updatable

  • virtual memory, problems while compiling

  • Visual Basic

    • 

    W
    


  • wait, @command{mysql} option

  • Web server, running

  • Well-Known Binary format

  • Well-Known Text format

  • What is an X509/Certificate?

  • What is encryption

  • widths, display

  • wildcards, and LIKE

  • wildcards, in mysql.columns_priv table

  • wildcards, in mysql.db table

  • wildcards, in mysql.host table

  • wildcards, in mysql.tables_priv table

  • wildcards, in mysql.user table

  • Windows

  • Windows, compiling on

  • Windows, open issues

  • Windows, upgrading

  • Windows, versus Unix

  • WKB format

  • WKT format

  • Word program

  • wrappers, Eiffel

  • write access, tmp

  • write_buffer_size @command{myisamchk} variable

    • 

    X
    


  • xml, @command{mysql} option

    • 

    Y
    


  • Year 2000 compliance

  • Year 2000 issues

    • 


    


    
This document was generated on 23 October 2004 using the
texi2html
translator version 1.52 (extended by [email protected]).