\FBMYSQLD\FR(1)

NAME
mysqld - the MySQL server

SYNOPSIS

mysqld [options]

DESCRIPTION

mysqld is the MySQL server. The following discussion covers these MySQL server configuration topics:

•   Startup options that the server supports

•   Server system variables

•   Server status variables

•   How to set the server SQL mode

•   The server shutdown process

\FBMYSQLD\FR COMMAND OPTIONS

When you start the mysqld server, you can specify program options using any of the methods described in Section 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 3.2, “Using Option Files”.

mysqld reads options from the [mysqld] and [server] groups. mysqld_safe reads options from the [mysqld], [server], [mysqld_safe], and [safe_mysqld] groups. 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.

mysqld accepts many command options. For a brief list, execute mysqld --help. To see the full list, use mysqld --verbose --help.

The following list shows some of the most common server options. Additional options are described in other sections:

•   Options that affect security: See Section 4.3, “Security-Related mysqld Options”.

•   SSL-related options: See Section 6.7.5, “SSL Command Options”.

•   Binary log control options: See Section 9.3, “The Binary Log”.

•   Replication-related options: See Section 9, “Replication Startup Options”.

•   Options specific to particular storage engines: See Section 1.1, “MyISAM Startup Options”, Section 5.3, “BDB Startup Options”, Section 2.4, “InnoDB Startup Options and System Variables”, and Section 5.5.1, “MySQL Cluster-Related Command Options for mysqld”.

You can also set the values of server system variables by using variable names as options, as described later in this section.

•   --help, -?

    Display a short help message and exit. Use both the --verbose and --help options to see the full message.

•   --allow-suspicious-udfs

    This option controls whether user-defined functions that have only an xxx symbol for the main function can be loaded. By default, the option is off and only UDFs that have at least one auxiliary symbol can be loaded; this prevents attempts at loading functions from shared object files other than those containing legitimate UDFs. See Section 3.4.6, “User-Defined Function Security Precautions”.

•   --ansi

    Use standard (ANSI) SQL syntax instead of MySQL syntax. For more precise control over the server SQL mode, use the --sql-mode option instead. See Section 9.3, “Running MySQL in ANSI Mode”, and the section called “THE SERVER SQL MODE”.

•   --basedir=path, -b path

    The path to the MySQL installation directory. All paths are usually resolved relative to this directory.

•   --bind-address=IP

    The IP address to bind to.

•   --binlog-format={row|statement}

    Specify whether to use row-based or statement-based replication (statement-based is default). See Section 3, “Row-Based Replication”. This option was added in MySQL 5.1.5.

•   --binlog-row-event-max-size=N

    Specify the maximum size of a row-based binary log event, in bytes. Rows are grouped into events smaller than this size if possible. The value should be a multiple of 256. The default is 1024. See Section 3, “Row-Based Replication”. This option was added in MySQL 5.1.5.

•   --both-log-formats

    Enable both the old and new log formats. (Log files as well as log tables.) This option was added in MySQL 5.1.6.

•   --bootstrap

    This option is used by the mysql_install_db script to create the MySQL privilege tables without having to start a full MySQL server.

•   --character-sets-dir=path

    The directory where character sets are installed. See Section 8.1, “The Character Set Used for Data and Sorting”.

•   --character-set-client-handshake

    Don't ignore character set information sent by the client. To ignore client information and use the default server character set, use --skip-character-set-client-handshake; this makes MySQL behave like MySQL 4.0.

•   --character-set-filesystem=charset_name

    The filesystem character set. This option was added in MySQL 5.1.6.

•   --character-set-server=charset_name, -C charset_name

    Use charset_name as the default server character set. See Section 8.1, “The Character Set Used for Data and Sorting”.

•   --chroot=path

    Put the mysqld server in a closed environment during startup by using the chroot() system call. This is a recommended security measure. Note that use of this option somewhat limits LOAD DATA INFILE and SELECT ... INTO OUTFILE.

•   --collation-server=collation_name

    Use collation_name as the default server collation. See Section 8.1, “The Character Set Used for Data and Sorting”.

•   --console

    (Windows only.) Write error log messages to stderr and stdout even if --log-error is specified. mysqld does not close the console window if this option is used.

•   --core-file

    Write a core file if mysqld dies. For some systems, you must also specify the --core-file-size option to mysqld_safe. See mysqld_safe(1). Note that on some systems, such as Solaris, you do not get a core file if you are also using the --user option.

•   --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 mysqld is doing. The debug_options string often is 'd:t:o,file_name'. The default is 'd:t:i:o,mysqld.trace'. See Section 1.2, “Creating Trace Files”.

•   --default-character-set=charset_name (DEPRECATED)

    Use charset_name as the default character set. This option is deprecated in favor of --character-set-server. See Section 8.1, “The Character Set Used for Data and Sorting”.

•   --default-collation=collation_name

    Use collation_name as the default collation. This option is deprecated in favor of --collation-server. See Section 8.1, “The Character Set Used for Data and Sorting”.

•   --default-storage-engine=type

    Set the default storage engine (table type) for tables. See Chapter 14, Storage Engines and Table Types.

•   --default-table-type=type

    This option is a synonym for --default-storage-engine.

•   --default-time-zone=timezone

    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 is the same as the system time zone (given by the value of the system_time_zone system variable.

•   --delay-key-write[= OFF | ON | ALL]

    Specify how to use delayed key writes. 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 DELAY_KEY_WRITE option. ALL delays key writes for all MyISAM tables. See Section 5.2, “Tuning Server Parameters”, and Section 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 another MySQL server or myisamchk) when the tables are in use. Doing so leads to index corruption.

•   --des-key-file=file_name

    Read the default DES keys from this file. These keys are used by the DES_ENCRYPT() and DES_DECRYPT() functions.

•   --enable-named-pipe

    Enable support for named pipes. This option applies only on Windows NT, 2000, XP, and 2003 systems, and can be used only with the mysqld-nt and mysqld-max-nt servers that support named-pipe connections.

•   --event-scheduler

    Enable the event scheduler. This option was added in MySQL 5.1.6.

•   --exit-info[=flags], -T [flags]

    This is a bit mask of different flags that you can use for debugging the mysqld server. Do not use this option unless you know exactly what it does!

•   --external-locking

    Enable external locking (system locking), which is disabled by default as of MySQL 4.0. Note that if you use this option on a system on which lockd does not fully work (such as Linux), it is easy for 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 must ensure that the following 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 and --query-cache-size=0. (This is not done by default because in many setups it is useful to have a mixture of the preceding options.)

•   --flush

    Flush (synchronize) 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 synchronizing to disk. See Section 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-xxx

    The InnoDB options are listed in Section 2.4, “InnoDB Startup Options and System Variables”.

•   --language=lang_name, -L lang_name

    Return client error messages in the 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 8.2, “Setting the Error Message Language”.

•   --large-pages

    Some hardware/operating system architectures support memory pages greater than the default (usually 4KB). The actual implementation of this support depends on the underlying hardware and OS. Applications that perform a lot of memory accesses may obtain performance improvements by using large pages due to reduced Translation Lookaside Buffer (TLB) misses.

    Currently, MySQL supports only the Linux implementation of large pages support (which is called HugeTLB in Linux). We have plans to extend this support to FreeBSD, Solaris and possibly other platforms.

    Before large pages can be used on Linux, it is necessary to configure the HugeTLB memory pool. For reference, consult the hugetlbpage.txt file in the Linux kernel source.

    This option is disabled by default.

•   --log[=file_name], -l [file_name]

    Log connections and SQL statements received from clients to this file. See Section 9.2, “The General Query Log”. If you omit the filename, MySQL uses host_name.log as the filename.

•   --log-bin=[base_name]

    Enable binary logging. The server logs all statements that change data to the binary log, which is used for backup and replication. See Section 9.3, “The Binary Log”.

    The option value, if given, is the basename for the log sequence. The server creates binary log files in sequence by adding a numeric suffix to the basename. It is recommended that you specify a basename (see Section 8.1, “Open Issues in MySQL”, for the reason). Otherwise, MySQL uses host_name-bin as the basename.

•   --log-bin-index[=file_name]

    The index file for binary log filenames. See Section 9.3, “The Binary Log”. If you omit the filename, and if you didn't specify one with --log-bin, MySQL uses host_name-bin.index as the filename.

•   --log-bin-trust-function-creators[={0|1}]

    With no argument or an argument of 1, this option sets the log_bin_trust_function_creators system variable to 1. With an argument of 0, this option sets the system variable to 0. log_bin_trust_function_creators affects how MySQL enforces restrictions on stored function creation. See Section 4, “Binary Logging of Stored Routines and Triggers”.

•   --log-error[=file_name]

    Log errors and startup messages to this file. See Section 9.1, “The Error Log”. If you omit the filename, MySQL uses host_name.err. If the filename has no extension, the server adds an extension of .err.

•   --log-isam[=file_name]

    Log all MyISAM changes to this file (used only when debugging MyISAM).

•   --log-long-format (DEPRECATED)

    Log extra information to the update log, binary update log, and slow query log, if they have been activated. For example, the username and timestamp are logged for all queries. This option is deprecated, as it now represents the default logging behavior. (See the description for --log-short-format.) 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, queries that do not use indexes are logged to the slow query log. See Section 9.4, “The Slow Query Log”.

•   --log-short-format

    Log less information to the update log, binary update log, and slow query log, if they have been activated. For example, the username and timestamp are not logged for queries.

•   --log-slow-admin-statements

    Log slow administrative statements such as OPTIMIZE TABLE, ANALYZE TABLE, and ALTER TABLE to the slow query log.

•   --log-slow-queries[=file_name]

    Log all queries that have taken more than long_query_time seconds to execute to this file. See Section 9.4, “The Slow Query Log”. See the descriptions of the --log-long-format and --log-short-format options for details.

•   --log-warnings=[level], -W [level]

    Print out warnings such as Aborted connection... to the error log. Enabling this option is recommended, for example, if you use replication (you get more information about what is happening, such as messages about network failures and reconnections). This option is enabled (1) by default, and the default level value if omitted is 1. To disable this option, use --log-warnings=0. Aborted connections are not logged to the error log unless the value is greater than 1. See Section 2.10, “Communication Errors and Aborted Connections”.

•   --low-priority-updates

    Give table-modifying operations (INSERT, REPLACE, DELETE, UPDATE) 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 3.2, “Table Locking Issues”.

•   --memlock

    Lock the 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 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. See Section 4.5, “How to Run MySQL as a Normal User”.

•   --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, each time mysqld opens a MyISAM table, it checks whether the table is marked as crashed or wasn't closed properly. (The last option works only if you are running with external locking disabled.) If this is the case, mysqld runs a check on the table. If the table was corrupted, mysqld attempts to repair it.

    The following options affect how the repair works: OptionDescriptionDEFAULTThe same as not giving any option to --myisam-recover.BACKUPIf the data file was changed during recovery, save a backup of the
                          tbl_name.MYD
                          file as
                          tbl_name-datetime.BAK.FORCERun recovery even if we would lose more than one row from the
                          .MYD file.QUICKDon't check the rows in the table if there aren't any delete blocks.Before the server automatically repairs a table, it writes a note about the repair to 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 forces a repair of a table even if some rows would be deleted, but it keeps the old data file as a backup so that you can later examine what happened.

    See Section 1.1, “MyISAM Startup Options”.

•   --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. See Section 4.4.2, “The MySQL Cluster connectstring”, for syntax.

•   --ndbcluster

    If the binary includes support for the NDB Cluster storage engine, this option enables the engine, which is disabled by default. See Chapter 16, MySQL Cluster.

•   --old-log-format

    Enable only the old log format. This disables log tables and SELECT capability for log contents. This option was added in MySQL 5.1.6.

•   --old-passwords

    Force the server to generate short (pre-4.1) password hashes for new passwords. This is useful for compatibility when the server must support older client programs. See Section 5.9, “Password Hashing as of MySQL 4.1”.

•   --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 1, “Debugging a MySQL Server”.

•   --open-files-limit=count

    Change the number of file descriptors available to mysqld. If this option is not set or is set to 0, mysqld uses the value to reserve file descriptors with setrlimit(). If the value is 0, mysqld reserves max_connections×5 or max_connections + table_open_cache×2 files (whichever is larger). You should try increasing this value if mysqld gives you the error Too many open files.

•   --pid-file=path

    The pathname of the process ID file. This file is used by other programs such as mysqld_safe to determine the server's process ID.

•   --port=port_num, -P port_num

    The port number to use when listening for TCP/IP connections. The port number must be 1024 or higher unless the server is started by the root system user.

•   --port-open-timeout=num

    On some systems, when the server is stopped, the TCP/IP port might not become available immediately. If the server is restarted quickly afterward, its attempt to reopen the port can fail. This option indicates how many seconds the server should wait for the TCP/IP port to become free if it cannot be opened. The default is not to wait. This option was added in MySQL 5.1.5.

•   --safe-mode

    Skip some optimization stages.

•   --safe-show-database (DEPRECATED)

    See Section 5.3, “Privileges Provided by MySQL”.

•   --safe-user-create

    If this option is enabled, a user cannot create new MySQL users by using 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 by clients that attempt to use accounts that have old (pre-4.1) passwords.

•   --shared-memory

    Enable shared-memory connections by local clients. This option is available only on Windows.

•   --shared-memory-base-name=name

    The name of shared memory to use for shared-memory connections. This option is available only on Windows. The default name is MYSQL. The name is case sensitive.

•   --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-external-locking

    Do not use external locking (system locking). With external locking disabled, you must shut down the server to use myisamchk. (See Section 4.3, “MySQL Stability”.) To avoid this requirement, use the CHECK TABLE and REPAIR TABLE statements to check and repair MyISAM tables.

    External locking has been disabled by default since MySQL 4.0.

•   --skip-grant-tables

    This option causes the server not to use the privilege system at all, which gives anyone with access to the server unrestricted access to all databases. You can cause a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload command from a system shell, or by issuing a MySQL FLUSH PRIVILEGES statement after connecting to the server. This option also suppresses loading of plugins and user-defined functions (UDFs).

•   --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 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-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 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; the server allocates memory and other resources for this storage engine only if the --ndbcluster option is given explicitly. See Section 4.3, “Quick Test Setup of MySQL Cluster”, for an example of usage.

•   --skip-networking

    Don't listen for TCP/IP connections at all. All interaction with mysqld must be made via named pipes or shared memory (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are allowed. See Section 5.6, “How MySQL Uses DNS”.

•   --standalone

    Available on Windows NT-based systems only; instructs the MySQL server not to run as a service.

•   --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 db_name.sym file that contains the path to the real directory. See Section 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 6.1.2, “Using Symbolic Links for Tables on Unix”.

•   --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. Note that any global privilege is considered a privilege for the database.

•   --skip-stack-trace

    Don't write stack traces. This option is useful when you are running mysqld under a debugger. On some systems, you also must use this option to get a core file. See Section 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 when listening for local connections. The default value is /tmp/mysql.sock. On Windows, the option specifies the pipe name to use when listening for local connections that use a named pipe. The default value is MySQL (not case sensitive).

•   --sql-mode=value[,value[,value...]]

    Set the SQL mode. See the section called “THE SERVER SQL MODE”.

•   --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 is being allocated to the directory entry cache rather than to the disk cache.

•   --transaction-isolation=level

    Sets the default transaction isolation level. The level value can be READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. See Section 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. 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. For more information about the storage location of temporary files, see Section 4.4, “Where MySQL Stores Temporary Files”. 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 fails.

•   --user={user_name | user_id}, -u {user_name | user_id}

    Run the 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 mysqld as root. The server changes its user ID during its startup sequence, causing it to run as that particular user rather than as root. See Section 4.1, “General Security Guidelines”.

    To avoid a possible security hole where a user adds a --user=root option to a my.cnf file (thus causing the server to run as root), mysqld uses only the first --user option specified and produces a warning if there are multiple --user options. Options in /etc/my.cnf and $MYSQL_HOME/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 is 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.

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 you assign a value to a variable, MySQL might automatically correct the value to stay within a given range, or adjust the value to the closest allowable value if only certain values are allowed.

If you want to restrict the maximum value to which a variable can be set at runtime with SET, you can define this by using the --maximum-var_name command-line option.

It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. This syntax is deprecated.

You can change the values of most system variables for a running server with the SET statement. See Section 5.3, “SET Syntax”.

the section called “SERVER SYSTEM VARIABLES”, provides a full description for all variables, and additional information for setting them at server startup and runtime. Section 5.2, “Tuning Server Parameters”, includes information on optimizing the server by tuning system variables.

SERVER SYSTEM VARIABLES

The mysql server maintains many system variables that indicate how it is configured. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. Most of them can be changed dynamically while the server is running by means of the SET statement, which enables you to modify operation of the server without having to stop and restart it. You can refer to system variable values in expressions.

To see the names and values of system variables, use the SHOW VARIABLES statement. To view the default values for these variables, use mysqld --verbose --help. (For a listing of the default values, see Section 5.2, “Tuning Server Parameters”).

This section provides a description of each system variable. Variables with no version indicated are present in all MySQL 5.1 releases. For historical information concerning their implementation, please see MySQL 5.0 Reference Manual and MySQL 3.23, 4.0, 4.1 Reference Manual.

For additional system variable information, see the section called “USING SYSTEM VARIABLES”, which discusses the syntax for setting and displaying system variable values, and the section called “Dynamic System Variables”, which lists the variables that can be set at runtime. Section 2.4, “InnoDB Startup Options and System Variables”, lists InnoDB system variables. Information on tuning sytem variables can be found in Section 5.2, “Tuning Server Parameters”.

Note: Some of the following variable descriptions refer to “enabling” or “disabling” a variable. These variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. However, to set such a variable on the command line or in an option file, you must set it to 1 or 0; setting it to ON or OFF will not work. For example, on the command line, --delay_key_write=1 works but --delay_key_write=ON does not.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

•   auto_increment_increment

    auto_increment_increment and auto_increment_offset are intended for use with master-to-master replication, and can be used to control the operation of AUTO_INCREMENT columns. Both variables can be set globally or locally, and each can assume an integer value between 1 and 65,535 inclusive. Setting the value of either of these two variables to 0 causes its value to be set to 1 instead. Attempting to set the value of either of these two variables to an integer greater than 65,535 or less than 0 causes its value to be set to 65,535 instead. Attempting to set the value of auto_increment_increment or auto_increment_offset to a non-integer value gives rise to an error, and the actual value of the variable remains unchanged.

    These two variables effect AUTO_INCREMENT column behavior as follows:

    •   auto_increment_increment controls the interval between successive column values. For example:

        mysql> SHOW VARIABLES LIKE 'auto_inc%';
        +--------------------------+-------+
        | Variable_name            | Value |
        +--------------------------+-------+
        | auto_increment_increment | 1     |
        | auto_increment_offset    | 1     |
        +--------------------------+-------+
        2 rows in set (0.00 sec)
        mysql> CREATE TABLE autoinc1
            -> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
          Query OK, 0 rows affected (0.04 sec)
        mysql> SET @@auto_increment_increment=10;
        Query OK, 0 rows affected (0.00 sec)
        mysql> SHOW VARIABLES LIKE 'auto_inc%';
        +--------------------------+-------+
        | Variable_name            | Value |
        +--------------------------+-------+
        | auto_increment_increment | 10    |
        | auto_increment_offset    | 1     |
        +--------------------------+-------+
        2 rows in set (0.01 sec)
        mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
        Query OK, 4 rows affected (0.00 sec)
        Records: 4  Duplicates: 0  Warnings: 0
        mysql> SELECT col FROM autoinc1;
        +-----+
        | col |
        +-----+
        |   1 |
        |  11 |
        |  21 |
        |  31 |
        +-----+
        4 rows in set (0.00 sec)
        (Note how SHOW VARIABLES is used here to obtain the current values for these variables.)

    •   auto_increment_offset determines the starting point for the AUTO_INCREMENT column value. Consider the following, assuming that these statements are executed during the same session as the example given in the description for auto_increment_increment:

        mysql> SET @@auto_increment_offset=5;
        Query OK, 0 rows affected (0.00 sec)
        mysql> SHOW VARIABLES LIKE 'auto_inc%';
        +--------------------------+-------+
        | Variable_name            | Value |
        +--------------------------+-------+
        | auto_increment_increment | 10    |
        | auto_increment_offset    | 5     |
        +--------------------------+-------+
        2 rows in set (0.00 sec)
        mysql> CREATE TABLE autoinc2
            -> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
        Query OK, 0 rows affected (0.06 sec)
        mysql> INSERT INTO autoinc2 VALUES (NULL), (NULL), (NULL), (NULL);
        Query OK, 4 rows affected (0.00 sec)
        Records: 4  Duplicates: 0  Warnings: 0
        mysql> SELECT col FROM autoinc2;
        +-----+
        | col |
        +-----+
        |   5 |
        |  15 |
        |  25 |
        |  35 |
        +-----+
        4 rows in set (0.02 sec)
        If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored.

    Should one or both of these variables be changed and then new rows inserted into a table containing an AUTO_INCREMENT column, the results may seem counterintuitive because the series of AUTO_INCREMENT values is calculated without regard to any values already present in the column, and the next value inserted is the least value in the series that is greater than the maximum existing value in the AUTO_INCREMENT column. In other words, the series is calculated like so:

    auto_increment_offset + N × auto_increment_increment

    where N is a positive integer value in the series [1, 2, 3, ...]. For example:

    mysql> SHOW VARIABLES LIKE 'auto_inc%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 10    |
    | auto_increment_offset    | 5     |
    +--------------------------+-------+
    2 rows in set (0.00 sec)
    mysql> SELECT col FROM autoinc1;
    +-----+
    | col |
    +-----+
    |   1 |
    |  11 |
    |  21 |
    |  31 |
    +-----+
    4 rows in set (0.00 sec)
    mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
    Query OK, 4 rows affected (0.00 sec)
    Records: 4  Duplicates: 0  Warnings: 0
    mysql> SELECT col FROM autoinc1;
    +-----+
    | col |
    +-----+
    |   1 |
    |  11 |
    |  21 |
    |  31 |
    |  35 |
    |  45 |
    |  55 |
    |  65 |
    +-----+
    8 rows in set (0.00 sec)
    The values shown for auto_increment_increment and auto_increment_offset generate the series 5 + N × 10, that is, [5, 15, 25, 35, 45, ...]. The greatest value present in the col column prior to the INSERT is 31, and the next available value in the AUTO_INCREMENT series is 35, so the inserted values for col begin at that point and the results are as shown for the SELECT query.

    It is important to remember that it is not possible to confine the effects of these two variables to a single table, and thus they do not take the place of the sequences offered by some other database management systems; these variables control the behavior of all AUTO_INCREMENT columns in all tables on the MySQL server. If one of these variables is set globally, its effects persist until the global value is changed or overridden by setting them locally, or until mysqld is restarted. If set locally, the new value affects AUTO_INCREMENT columns for all tables into which new rows are inserted by the current user for the duration of the session, unless the values are changed during that session.

    The default value of auto_increment_increment is 1. See Section 15, “Auto-Increment in Multiple-Master Replication”.

•   auto_increment_offset

    This variable has a default value of 1. For particulars, see the description for auto_increment_increment.

•   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. back_log cannot be set higher than your operating system limit.

•   basedir

    The MySQL installation base directory. This variable can be set with the --basedir option.

•   bdb_cache_parts

    The number of parts to use for the BDB cache. Added in MySQL 5.1.2.

•   bdb_cache_size

    The size of the buffer that is allocated for caching indexes and rows for BDB tables. Some systems support setting this variable larger than 4GB. If you don't use BDB tables, you should start mysqld with --skip-bdb to not allocate memory for this cache.

•   bdb_home

    The base directory for BDB tables. This should be assigned the same value as the datadir variable.

•   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 mysqld with --skip-bdb to not allocate memory for this cache.

•   bdb_logdir

    The directory where the BDB storage engine writes its log files. This variable can be set with the --bdb-logdir option.

•   bdb_max_lock

    The maximum number of locks that can be active for a BDB table (10,000 by default). You should increase this value if errors such as the following occur when you perform long transactions or when mysqld has to examine many rows to calculate a query:

    bdb: Lock table is out of available locks
    Got error 12 from ...

•   bdb_region_size

    The size of the underlying logging area of the BDB environment. This is the size of the memory pool used for storing the filenames used in a transaction. Added in MySQL 5.1.2.

•   bdb_shared_data

    This is ON if you are using --bdb-shared-data to start Berkeley DB in multi-process mode. (Do not use DB_PRIVATE when initializing Berkeley DB.)

•   bdb_tmpdir

    The BDB temporary file directory.

•   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 if the server has the binary log enabled (--log-bin option). If you often use large, multiple-statement transactions, you can increase this cache size 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. See Section 9.3, “The Binary Log”.

•   binlog_format

    The binary logging format, either STATEMENT or ROW. This variable is set by the --binlog-format option. See Section 3, “Row-Based Replication”.

•   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 when adding data to non-empty tables. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. The default value is 8MB.

•   character_set_client

    The character set for statements that arrive from the client.

•   character_set_connection

    The character set used for literals that do not have a character set introducer and for number-to-string conversion.

•   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.

•   character_set_filesystem

    The filesystem character set. The default value is binary. This variable was added in MySQL 5.1.6.

•   character_set_results

    The character set used for returning query results to the client.

•   character_set_server

    The server's default character set.

•   character_set_system

    The character set used by the server for storing identifiers. The value is always utf8.

•   character_sets_dir

    The directory where character sets are installed.

•   collation_connection

    The collation of the connection character set.

•   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.

•   collation_server

    The server's default collation.

•   completion_type

    The transaction completion type:

    •   If the value is 0 (the default), COMMIT and ROLLBACK are unaffected.

    •   If the value is 1, COMMIT and ROLLBACK are equivalent to COMMIT AND CHAIN and ROLLBACK AND CHAIN, respectively. (A new transaction starts immediately with the same isolation level as the just-terminated transaction.)

    •   If the value is 2, COMMIT and ROLLBACK are equivalent to COMMIT RELEASE and ROLLBACK RELEASE, respectively. (The server disconnects after terminating the transaction.)

•   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 mysqld with --safe or --skip-new.

    This variable can take three integer values: ValueDescription0Off1(Default) Enables concurrent insert for MyISAM tables
                          that don't have holes2Enables concurrent inserts for all MyISAM tables. If
                          table has a hole and is in use by another thread
                          the new row will be inserted at end of table. If
                          table is not in use then MySQL does a normal read
                          lock and inserts the new row into the hole.See also Section 3.3, “Concurrent Inserts”.

•   connect_timeout

    The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake.

•   datadir

    The MySQL data directory. This variable can be set with the --datadir option.

•   date_format

    This variable is not implemented.

•   datetime_format

    This variable is not implemented.

•   default_week_format

    The default mode value to use for the WEEK() function. See Section 5, “Date and Time Functions”.

•   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. OptionDescriptionOFFDELAY_KEY_WRITE is ignored.ONMySQL honors any DELAY_KEY_WRITE option specified in
                          CREATE TABLE statements. This
                          is the default value.ALLAll new opened tables are treated as if they were created with the
                          DELAY_KEY_WRITE option enabled.If DELAY_KEY_WRITE is enabled for a table, the key buffer is not flushed for the table on every index update, but only when the table is closed. This speeds 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 the section called “\FBMYSQLD\FR COMMAND OPTIONS”, and Section 1.1, “MyISAM Startup Options”.

    Note that enabling external locking with --external-locking offers no protection against index corruption for tables that use delayed key writes.

•   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 many seconds 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 waits until there is room in the queue again.

•   div_precision_increment

    This variable indicates the number of digits of precision by which to increase the result of division operations performed with the / operator. The default value is 4. The minimum and maximum values are 0 and 30, respectively. The following example illustrates the effect of increasing the default value.

    mysql> SELECT 1/7;
    +--------+
    | 1/7    |
    +--------+
    | 0.1429 |
    +--------+
    mysql> SET div_precision_increment = 12;
    mysql> SELECT 1/7;
    +----------------+
    | 1/7            |
    +----------------+
    | 0.142857142857 |
    +----------------+

•   event_scheduler

    Whether the event scheduler is disabled or enabled. By default, the scheduler is disabled. This variable was added in MySQL 5.1.6.

•   engine_condition_pushdown

    This variable applies to NDB. By default it is 0 (OFF): If you execute a query such as SELECT * FROM t WHERE mycol = 42, where mycol is a non-indexed column, the query is executed as a full table scan on every NDB node. Each node sends every row to the MySQL server, which applies the WHERE condition. If engine_condition_pushdown is set to 1 (ON), the condition is “pushed down” to the storage engine and sent to the NDB nodes. Each node uses the condition to perform the scan, and only sends back to the MySQL server the rows that match the condition.

•   expire_logs_days

    The number of days for automatic binary log removal. The default is 0, which means “no automatic removal.” Possible removals happen at startup and at binary log rotation.

•   flush

    If ON, the server flushes (synchronizes) 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 synchronizing to disk. See Section 4.2, “What to Do If MySQL Keeps Crashing”. This variable is set to ON if you start mysqld with the --flush option.

•   flush_time

    If this is set to a non-zero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. We recommend that this option be used only on Windows 9x or Me, or on systems with minimal resources.

•   ft_boolean_syntax

    The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 7.1, “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.

    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.

    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.

•   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.

    Note: FULLTEXT indexes must be rebuilt after changing this variable or the contents of the stopword file. Use REPAIR TABLE tbl_name QUICK.

•   group_concat_max_len

    The maximum allowed result length for the GROUP_CONCAT() function. The default is 1024.

•   have_archive

    YES if mysqld supports ARCHIVE tables, NO if not.

•   have_bdb

    YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used.

•   have_blackhole_engine

    YES if mysqld supports BLACKHOLE tables, NO if not.

•   have_compress

    YES if the zlib compression library is available to the server, NO if not. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used.

•   have_crypt

    YES if the crypt() system call is available to the server, NO if not. If not, the ENCRYPT() function cannot be used.

•   have_csv

    YES if mysqld supports ARCHIVE tables, NO if not.

•   have_example_engine

    YES if mysqld supports EXAMPLE tables, NO if not.

    have_federated_engine

    YES if mysqld supports FEDERATED tables, NO if not.

•   have_geometry

    YES if the server supports spatial data types, NO if not.

•   have_innodb

    YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used.

•   have_isam

    In MySQL 5.1, this variable appears only for reasons of backward compatibility. It is always NO because ISAM tables are no longer supported.

•   have_ndbcluster

    YES if mysqld supports NDB Cluster tables. DISABLED if --skip-ndbcluster is used.

•   have_partitioning

    YES if mysqld supports partitioning. Added in MySQL 5.1.1 as have_partition_engine and renamed to have_partioning in 5.1.6.

•   have_openssl

    YES if mysqld supports SSL (encryption) of the client/server protocol, NO if not.

•   have_query_cache

    YES if mysqld supports the query cache, NO if not.

•   have_raid

    YES if mysqld supports the RAID option, NO if not.

•   have_row_based_replication

    YES if the server can perform replication using row-based binary logging. If the value is NO, the server can use only statement-based logging. See Section 3, “Row-Based Replication”. This variable was added in MySQL 5.1.5.

•   have_rtree_keys

    YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM tables.)

•   have_symlink

    YES if symbolic link support is enabled, NO if not. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options, and on Windows for support of data directory symlinks.

•   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 system 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 that have the SUPER privilege. This is done so that an erroneous value for init_connect does not prevent all clients from connecting. For example, the value might contain a statement that has a syntax error, thus causing client connections to fail. Not executing init_connect for users that have the SUPER privilege enables them to open a connection and fix the init_connect value.

•   init_file

    The name of the file specified with the --init-file option when you start the server. This should be 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.

•   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.

•   innodb_xxx

    InnoDB system variables are listed in Section 2.4, “InnoDB Startup Options and System Variables”.

•   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 joins that do not use indexes and thus perform full table scans. 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 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. Consider also the memory requirements of other storage engines.

    For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 2.16, “Speed of INSERT Statements”.

    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 5.4, “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_unused status variable and the buffer block size, which is available from the key_cache_block_size system variable:

    1 - ((Key_blocks_unused × key_cache_block_size) / key_buffer_size)
    This value is an approximation because some space in the key buffer may be allocated internally for administrative structures.

    It is possible to create multiple MyISAM key caches. The size limit of 4GB applies to each cache individually, not as a group. See Section 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. See Section 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. See Section 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. See Section 4.6, “The MyISAM Key Cache”.

•   language

    The language used for error messages.

•   large_file_support

    Whether mysqld was compiled with options for large file support.

•   large_pages

    Whether large page support is enabled.

•   license

    The type of license the server has.

•   local_infile

    Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 4.4, “Security Issues with LOAD DATA LOCAL”.

•   locked_in_memory

    Whether mysqld was locked in memory with --memlock.

•   log

    Whether logging of all statements to the general query log is enabled. See Section 9.2, “The General Query Log”.

•   log_bin

    Whether the binary log is enabled. See Section 9.3, “The Binary Log”.

•   log_bin_trust_function_creators

    This variable applies when binary logging is enabled. It controls whether stored function creators can be trusted not to create stored functions that will cause unsafe events to be written to the binary log. If set to 0 (the default), users are not allowed to create or alter stored routines unless they have the SUPER privilege in addition to the CREATE ROUTINE or ALTER ROUTINE privilege. A setting of 0 also enforces the restriction that a function must be declared with the DETERMINISTIC characteristic, or with the READS SQL DATA or NO SQL characteristic. If the variable is set to 1, MySQL does not enforce these restrictions on stored function creation. See Section 4, “Binary Logging of Stored Routines and Triggers”.

•   log_error

    The location of the error log.

•   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. See Section 9, “Replication Startup Options”.

•   log_slow_queries

    Whether slow queries should be logged. “Slow” is determined by the value of the long_query_time variable. See Section 9.4, “The Slow Query Log”.

•   log_warnings

    Whether to produce additional warning messages. It is enabled (1) by default. 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 server increments the Slow_queries status variable. 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. The minimum value is 1. See Section 9.4, “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.

•   lower_case_file_system

    This variable describes the case sensitivity of filenames on the filesystem where the data directory is located. OFF means filenames are case sensitive, ON means they are not case sensitive.

•   lower_case_table_names

    If set to 1, table names are stored in lowercase on disk and table name comparisons are not case sensitive. If set to 2 table names are stored as given but compared in lowercase. This option also applies to database names and table aliases. See Section 2.2, “Identifier Case Sensitivity”.

    If you are using InnoDB tables, you should set this variable to 1 on all platforms to force names to be converted to lowercase.

    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). 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 large (possibly incorrect) packets.

    You must increase this value if you are using large 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 1GB.

•   max_binlog_cache_size

    If a multiple-statement transaction requires more than this amount of memory, the server generates a Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage error.

•   max_binlog_size

    If a write to the binary log causes the current log file size to exceed the value of this variable, the server rotates the binary logs (closes the current file and opens the next one). You cannot set this variable to more than 1GB or to less than 4096 bytes. The default value is 1GB.

    A transaction is written in one chunk to the binary log, so it is never split between several binary logs. Therefore, if you have big transactions, you might see binary logs larger than max_binlog_size.

    If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well.

•   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 mysqld requires. See Section 4.8, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits. See also Section 2.6, “Too many connections”.

•   max_delayed_threads

    Do not 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 is 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.

•   max_error_count

    The maximum number of error, warning, and note messages to be stored for display by the SHOW ERRORS and SHOW WARNINGS statements.

•   max_heap_table_size

    This variable sets the maximum size to which MEMORY 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 altered with ALTER TABLE or TRUNCATE TABLE.

•   max_insert_delayed_threads

    This variable is a synonym for max_delayed_threads.

•   max_join_size

    Do not allow SELECT statements that probably need to examine more than max_join_size rows (for single-table statements) or row combinations (for multiple-table statements) or that 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 value of SQL_BIG_SELECTS to 0. If you set the SQL_BIG_SELECTS value again, the max_join_size variable is ignored.

    If a query result is in the query cache, no result size check is performed, because the result has previously 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 2.12, “ORDER BY Optimization”.

•   max_relay_log_size

    If a write by a replication slave to its relay log causes the current log file size to exceed the value of this variable, the slave rotates the relay logs (closes the current file and opens the next one). If max_relay_log_size is 0, the server uses max_binlog_size for both the binary log and the relay log. If max_relay_log_size is greater than 0, it constrains the size of the relay log, which enables you to have different sizes for the two logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. See Section 4, “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 assumes that no more than this number of key seeks are required when searching for matching rows in a table by scanning an index, regardless of the actual cardinality of the index (see Section 5.4.12, “SHOW INDEX Syntax”). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.

•   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 does not 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 has both a global scope and a (read-only) session scope. The session variable has the same value as the global variable unless the current account has a non-zero MAX_USER_CONNECTIONS resource limit. In that case, the session value reflects the account limit.

•   max_write_lock_count

    After this many write locks, allow some pending read lock requests to be processed in between.

•   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 7. The default value is 6. See Section 2.11, “The table is full”.

•   myisam_max_extra_sort_file_size (DEPRECATED)

    Note: This variable is not supported in MySQL 5.1. See MySQL 5.0 Reference Manual for more information.

•   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 larger than this value, the index is created using the key cache instead, which is slower. The value is given in bytes.

•   myisam_recover_options

    The value of the --myisam-recover option. See the section called “\FBMYSQLD\FR COMMAND OPTIONS”.

•   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 beta-quality code.

•   myisam_sort_buffer_size

    The size of the buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE.

•   myisam_stats_method

    How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM tables. This variable has two possible values, nulls_equal and nulls_unequal. For nulls_equal, all NULL index values are considered equal and form a single value group that has a size equal to the number of NULL values. For nulls_unequal, NULL values are considered unequal, and each NULL forms a distinct value group of size 1.

    The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 4.7, “MyISAM Index Statistics Collection”.

•   myisam_use_mmap

    Use memory mapping for reading and writing MyISAM tables. This variable was added in MySQL 5.1.4.

•   multi_read_range

    Specifies the maximum number of ranges to send to a storage engine during range selects. The default value is 256. Sending multiple ranges to an engine is a feature that can improve the performance of certain selects dramatically, particularly for NDBCLUSTER. This engine needs to send the range requests to all nodes, and sending many of those requests at once reduces the communication costs significantly.

•   named_pipe

    (Windows only.) Indicates whether the server supports connections over named pipes.

•   ndb_extra_logging

    This variable can be set to a non-zero value to enable extra NDB logging. This variable was added in MySQL 5.1.6.

•   net_buffer_length

    The communication buffer is reset to this size between SQL statements. This variable should not normally be changed, but if you have very little memory, you can set it to the expected length of 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. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. 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.

•   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.

•   net_write_timeout

    The number of seconds to wait for a block to be written to a connection before aborting the write. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. See also net_read_timeout.

•   new

    This variable was used in MySQL 4.0 to turn on some 4.1 behaviors, and is retained for backward compatibility. In MySQL 5.1, its value is always OFF.

•   old_passwords

    Whether the server should use pre-4.1-style passwords for MySQL user accounts. See Section 2.3, “Client does not support authentication protocol”.

•   one_shot

    This is not a variable, but it can be used when setting some variables. It is described in Section 5.3, “SET Syntax”.

•   open_files_limit

    The number of files that the operating system allows mysqld to open. This is the real value allowed by the system and might be different from the value you gave using the --open-files-limit option to mysqld or mysqld_safe. The value is 0 on systems where MySQL can't change the number of open files.

•   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.

•   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 algorithm used in MySQL 5.0.0 (and previous versions) for performing searches.

•   pid_file

    The pathname of the process ID (PID) file. This variable can be set with the --pid-file option.

•   plugin_dir

    The pathname of the plugins directory. This variable was added in MySQL 5.1.2.

•   port

    The number of 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.

•   protocol_version

    The version of the client/server protocol used by the MySQL server.

•   query_alloc_block_size

    The allocation size of memory blocks that are allocated for objects created during statement parsing and execution. If you have problems with memory fragmentation, it might help to increase this a bit.

•   query_cache_limit

    Don't cache results that are larger than this number of bytes. The default value is 1MB.

•   query_cache_min_res_unit

    The minimum size (in bytes) for blocks allocated by the query cache. The default value is 4096 (4KB). Tuning information for this variable is given in Section 11.3, “Query Cache Configuration”.

•   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 is allocated even if query_cache_type is set to 0. See Section 11.3, “Query Cache Configuration”, for more information.

•   query_cache_type

    Set the 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. Possible values are shown in the following table: OptionDescription0 or OFFDon't cache or retrieve results. Note that this does not deallocate the
                          query cache buffer. To do that, you should set
                          query_cache_size to 0.1 or ONCache all query results except for those that begin with SELECT
                          SQL_NO_CACHE.2 or DEMANDCache results only for queries that begin with SELECT
                          SQL_CACHE.This variable defaults to ON.

•   query_cache_wlock_invalidate

    Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing statements that read from 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.

•   query_prealloc_size

    The size of the persistent buffer used for statement parsing and execution. This buffer is not freed between statements. 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.

•   range_alloc_block_size

    The size of blocks that are allocated when doing range optimization.

•   read_buffer_size

    Each thread that does a sequential scan allocates a buffer of this size (in bytes) for each table it scans. If you do many sequential scans, you might want to increase this value, which defaults to 131072.

•   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 that have the SUPER privilege. This can be useful to ensure that a slave server accepts updates only from its master server and not from clients. This variable does not apply to TEMPORARY tables.

•   relay_log_purge

    Disables or enables automatic purging of relay log files as soon as they are not needed any more. The default value is 1 (ON).

•   read_rnd_buffer_size

    When reading rows in sorted order following a key-sorting operation, 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.

•   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 use of passwords employing the old format (and hence insecure communication over the network).

    Server startup fails with an error if this option is enabled and the privilege tables are in pre-4.1 format. See Section 2.3, “Client does not support authentication protocol”.

•   server_id

    The server ID. This value is set by the --server-id option. It is used for replication to enable master and slave servers to identify themselves uniquely.

•   shared_memory

    (Windows only.) Whether the server allows shared-memory connections.

•   shared_memory_base_name

    (Windows only.) The name of shared memory to use for shared-memory connections. This is useful when running multiple MySQL instances on a single physical machine. The default name is MYSQL. The name is case sensitive.

•   skip_external_locking

    This is OFF if mysqld uses external locking, ON if external locking is disabled.

•   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 or shared memory. On NetWare, only TCP/IP connections are supported, so do not set this variable to ON. This variable can be set to ON with the --skip-networking option.

•   skip_show_database

    This prevents people from using the SHOW DATABASES statement if they do not have the SHOW DATABASES privilege. This can improve security if you have concerns about users being able to see databases belonging to other users. Its effect 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 the names of only those databases for which the user has the SHOW DATABASES or other privilege.

•   slave_compressed_protocol

    Whether to use compression of the slave/master protocol if both the slave and the master support it.

•   slave_load_tmpdir

    The name of the directory where the slave creates temporary files for replicating LOAD DATA INFILE statements.

•   slave_net_timeout

    The number of seconds to wait for more data from a master/slave connection before aborting the read. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory.

•   slave_skip_errors

    The replication errors that the slave should skip (ignore).

•   slave_transaction_retries

    If a replication slave SQL thread fails to execute a transaction because of an InnoDB deadlock or exceeded InnoDB's innodb_lock_wait_timeout or NDBCluster's TransactionDeadlockDetectionTimeout or TransactionInactiveTimeout, it automatically retries slave_transaction_retries times before stopping with an error. The default value is 10.

•   slow_launch_time

    If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable.

•   socket

    On Unix platforms, this variable is the name of the socket file that is used for local client connections. The default is /tmp/mysql.sock. (For some distribution formats, the directory might be different, such as /var/lib/mysql for RPMs.)

    On Windows, this variable is the name of the named pipe that is used for local client connections. The default value is MySQL (not case sensitive).

•   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 4.4, “Where MySQL Stores Temporary Files”.

•   sql_mode

    The current server SQL mode, which can be set dynamically. See the section called “THE SERVER SQL MODE”.

•   sql_slave_skip_counter

    The number of events from the master that a slave server should skip. See Section 6.2.6, “SET GLOBAL SQL_SLAVE_SKIP_COUNTER Syntax”.

•   storage_engine

    The default storage engine (table type). To set the storage engine at server startup, use the --default-storage-engine option. See the section called “\FBMYSQLD\FR COMMAND OPTIONS”.

•   sync_binlog

    If the value of this variable is positive, the MySQL server synchronizes its binary log to disk (using fdatasync()) after every sync_binlog writes to the binary log. Note that there is one write to the binary log per statement if autocommit is enabled, and one write per transaction otherwise. The default value is 0, which does no synchronizing to disk. A value of 1 is the safest choice, because in the event of a crash you lose at most one statement or transaction from the binary log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast).

    If the value of sync_binlog is 0 (the default), no extra flushing is done. The server relies on the operating system to flush the file contents occasionaly as for any other file.

•   sync_frm

    If this variable is set to 1, when any non-temporary table is created its .frm file is synchronized to disk (using fdatasync()). This is slower but safer in case of a crash. The default is 1.

•   system_time_zone

    The server system time zone. When the server begins executing, it inherits a time zone setting from the machine defaults, possibly modified by the environment of the account used for running the server or the startup script. The value is used to set system_time_zone. Typically the time zone is specified by the TZ environment variable. It also can be specified using the --timezone option of the mysqld_safe script.

    The system_time_zone variable differs from time_zone. Although they might have the same value, the latter variable is used to initialize the tome zone for each client that connects. See Section 8.8, “MySQL Server Time Zone Support”.

•   table_cache

    This is the old name of table_open_cache before MySQL 5.1.3. From 5.1.3 on, use table_open_cache instead.

•   table_definition_cache

    The number of table definitions that can be stored in the definition cache. If you use a large number of tables, you can create a large table definition cache to speed up opening of tables. The table definition cache takes less space and does not use file descriptors, unlike the normal table cache. This variable was added in MySQL 5.1.3.

•   table_open_cache

    The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See the section called “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_open_cache variable. For more information about the table cache, see Section 4.8, “How MySQL Opens and Closes Tables”. Before MySQL 5.1.3, this variable is called table_cache.

•   table_type

    This variable is a synonym for storage_engine. In MySQL 5.1, storage_engine is the preferred name.

•   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 are fewer than 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 provide a notable performance improvement if you have a good thread implementation.) By examining the difference between the Connections and Threads_created status variables, you can see how efficient the thread cache is. For details, see the section called “SERVER STATUS VARIABLES”.

•   thread_concurrency

    On Solaris, mysqld calls thr_setconcurrency() with this value. This function enables applications to give the threads system a hint about the desired number of threads that should be run at the same time.

•   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 1.4, “The MySQL Benchmark Suite”. The default is 192KB.

•   time_format

    This variable is not implemented.

•   time_zone

    The current time zone. This variable is used to initialize the tome zone for each client that connects. By default, the initial value of this is 'SYSTEM' (which means, “use the value of system_time_zone”). The value can be specified explicitly at server startup with the --default-time-zone option. See Section 8.8, “MySQL Server Time Zone Support”.

•   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. 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.

    The multiple-directory 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 fails. However, if you are using MySQL 4.0.0 or later, you can set the slave's temporary directory using the slave_load_tmpdir variable. In that case, the slave won't use the general tmpdir value and you can set tmpdir to a non-permanent location.

•   transaction_alloc_block_size

    The amount in bytes by which to increase a per-transaction memory pool which needs memory. See the description of transaction_prealloc_size.

•   transaction_prealloc_size

    There is a per-transaction memory pool from which various transaction-related allocations take memory. The initial size of the pool in bytes is transaction_prealloc_size. For every allocation that cannot be satisfied from the pool because it has insufficient memory available, the pool is increased by transaction_alloc_block_size bytes. When the transaction ends, the pool is truncated to transaction_prealloc_size bytes.

    By making transaction_prealloc_size sufficiently large to contain all statements within a single transaction, you can avoid many malloc() calls.

•   tx_isolation

    The default transaction isolation level. Defaults to REPEATABLE-READ.

    This variable is set by the SET TRANSACTION ISOLATION LEVEL statement. See Section 4.6, “SET TRANSACTION Syntax”. If you set tx_isolation directly to an isolation level name that contains a space, the name should be enclosed within quotes, with the space replaced by a dash. For example:

    SET tx_isolation = 'READ-COMMITTED';

•   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.

•   version

    The version number for the server.

•   version_bdb

    The BDB storage engine version.

•   version_comment

    The 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.

•   version_compile_machine

    The type of machine or architecture on which MySQL was built.

•   version_compile_os

    The type of operating system on which MySQL was built.

•   wait_timeout

    The number of seconds the server waits for activity on a non-interactive connection before closing it. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory.

    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.

USING SYSTEM VARIABLES

The mysql server maintains many system variables that indicate how it is configured. the section called “SERVER SYSTEM VARIABLES”, describes the meaning of these variables. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. Most of them can be changed dynamically while the server is running by means of the SET statement, which enables you to modify operation of the server without having to stop and restart it. You can refer to system variable values in expressions.

The server maintains two kinds of system variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections. A given system variable can have both a global and a session value. Global and session system variables are related as follows:

•   When the server starts, it initializes all global variables to their default values. These defaults can be changed by options specified on the command line or in an option file. (See Section 3, “Specifying Program Options”.)

•   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 example, the client's SQL mode is controlled by the session sql_mode value, which is initialized when the client connects to the value of the global sql_mode value.

System variable values can be set globally at server startup by using options on the command line or in an option file. When you use a startup option to set a variable that takes a numeric value, the value can be given with a suffix of K, M, or G (either uppercase or lowercase) to indicate a multiplier of 1024, 10242 or 10243; that is, units of kilobytes, megabytes, or gigabygtes, respectively. Thus, the following command starts the server with a query cache size of 16 megabytes and a maximum packet size of one gigabyte:

mysqld --query_cache_size=16M --max_allowed_packet=1G

Within an option file, those variables are set like this:

[mysqld]
query_cache_size=16M
max_allowed_packet=1G

The lettercase of suffix letters does not matter; 16M and 16m are equivalent, as are 1G and 1g.

If you want to restrict the maximum value to which a system variable can be set at runtime 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.

Many system variables are dynamic and can be changed while the server runs by using the SET statement. For a list, see the section called “Dynamic System Variables”. To change a system variable with SET, refer to it as var_name, optionally preceded by a modifier:

•   To indicate explicitly that a variable is a global variable, precede its name by GLOBAL or @@global.. The SUPER privilege is required to set global variables.

•   To indicate explicitly that a variable is a session variable, precede its name by SESSION, @@session., or @@. Setting a session variable requires no special privilege, but a client can change only its own session variables, not those of any other client.

•   LOCAL and @@local. are synonyms for SESSION and @@session..

•   If no modifier is present, SET changes the session variable.

A SET statement can contain multiple variable assignments, separated by commas. If you set several system variables, the most recent GLOBAL or SESSION modifier in the statement is used for following variables that have no modifier specified.

Examples:

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;

When you assign a value to a system variable with SET, you cannot use suffix letters in the value (as can be done with startup options). However, the value can take the form of an expression:

SET sort_buffer_size = 10 * 1024 * 1024;

The @@var_name syntax for system variables is supported for compatibility with some other database systems.

If you change a session system variable, the value remains in effect until your session ends or until you change the variable to a different value. The change is not visible to other clients.

If you change a global system variable, the value is remembered and used for new connections until the server restarts. (To make a global system variable setting permanent, you should set it in an option file.) The change is visible to any client that accesses that global variable. However, the change affects the corresponding session variable only for clients that connect after the change. The global variable change does not affect the session variable for any client that is currently connected (not even that of the client that issues the SET GLOBAL statement).

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 (or @@global.) when setting a global variable.

To set a SESSION variable to the GLOBAL value or a GLOBAL value to the compiled-in MySQL default value, use the DEFAULT keyword. 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;

Not all system variables can be set to DEFAULT. In such cases, use of DEFAULT results in an error.

You can refer to the values of specific global or sesson system variables in expressions by using one of the @@-modifiers. For example, you can retrieve values in a SELECT statement like this:

SELECT @@global.sql_mode, @@session.sql_mode, @@sql_mode;

When you refer to a system variable in an expression as @@var_name (that is, when you do not specify @@global. or @@session.), MySQL returns the session value if it exists and the global value otherwise. (This differs from SET @@var_name = value, which always refers to the session value.)

Note: Some system variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. However, to set such a variable on the command line or in an option file, you must set it to 1 or 0; setting it to ON or OFF will not work. For example, on the command line, --delay_key_write=1 works but --delay_key_write=ON does not.

To display system variable names and values, use the SHOW VARIABLES statement.

mysql> SHOW VARIABLES;
+---------------------------------+--------------------------------------+
| Variable_name                   | Value                                |
+---------------------------------+--------------------------------------+
| auto_increment_increment        | 1                                    |
| auto_increment_offset           | 1                                    |
| automatic_sp_privileges         | ON                                   |
| back_log                        | 50                                   |
| basedir                         | /home/jon/bin/mysql/                 |
| binlog_cache_size               | 32768                                |
| bulk_insert_buffer_size         | 8388608                              |
| character_set_client            | latin1                               |
| character_set_connection        | latin1                               |
| character_set_database          | latin1                               |
| character_set_results           | latin1                               |
| character_set_server            | latin1                               |
| character_set_system            | utf8                                 |
| character_sets_dir              | /usr/bin/mysql/share/mysql/charsets/ |
| collation_connection            | latin1_swedish_ci                    |
| collation_database              | latin1_swedish_ci                    |
| collation_server                | latin1_swedish_ci                    |
| innodb_additional_mem_pool_size | 1048576                              |
| innodb_autoextend_increment     | 8                                    |
| innodb_buffer_pool_awe_mem_mb   | 0                                    |
| innodb_buffer_pool_size         | 8388608                              |
| innodb_checksums                | ON                                   |
| innodb_commit_concurrency       | 0                                    |
| innodb_concurrency_tickets      | 500                                  |
| innodb_data_file_path           | ibdata1:10M:autoextend               |
| innodb_data_home_dir            |                                      |
| version                         | 5.1.6-alpha-log                      |
| version_comment                 | Source distribution                  |
| version_compile_machine         | i686                                 |
| version_compile_os              | suse-linux                           |
| wait_timeout                    | 28800                                |
+---------------------------------+--------------------------------------+

With a LIKE clause, the statement displays only those variables that match the pattern. To obtain a specific variable name, use a LIKE clause as shown:

SHOW VARIABLES LIKE 'max_join_size';
SHOW SESSION VARIABLES LIKE 'max_join_size';

To get a list of variables whose name match a pattern, use the ‘%’ wildcard character in a LIKE clause:

SHOW VARIABLES LIKE '%size%';
SHOW GLOBAL VARIABLES LIKE '%size%';

Wildcard characters can be used in any position within the pattern to be matched. Strictly speaking, because ‘_’ is a wildcard that matches any single character, you should escape it as ‘\_’ to match it literally. In practice, this is rarely necessary.

For SHOW VARIABLES, if you specify neither GLOBAL nor SESSION, MySQL returns SESSION values.

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 were to remove a SESSION variable that has 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.

  Structured System Variables

A structured variable differs from a regular system variable in two respects:

•   Its value is a structure with components that specify server parameters considered to be closely related.

•   There might be several instances of a given type of structured variable. Each one has a different name and refers to a different resource maintained by the server.

MySQL 5.1 supports one structured variable type, which specifies parameters governing the operation of key caches. A key cache structured variable has these components:

•   key_buffer_size

•   key_cache_block_size

•   key_cache_division_limit

•   key_cache_age_threshold

This section describes 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 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.

Structured variable instances and components follow these naming rules:

•   For a given type of structured variable, each instance must have a name that is unique within variables of that type. However, instance names need not be unique across structured variable types. For example, each structured variable has an instance named default, so default is not unique across variable types.

•   The names of the components of each structured variable type must be unique across all system variable names. If this were not true (that is, if two different types of structured variables could share component member names), it would not be clear which default structured variable to use for references to member names that are not qualified by an instance name.

•   If a structured variable instance name is not legal as an unquoted identifier, refer to it as a quoted identifier using backticks. For example, hot-cache is not legal, but `hot-cache` is.

•   global, session, and local are not legal instance names. This avoids a conflict with notation such as @@global.var_name for referring to non-structured system variables.

Currently, 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, you can 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, use this syntax:

[mysqld]
hot_cache.key_buffer_size=64K

If you start the server with this 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.

  Dynamic System Variables

Many server system variables are dynamic and can be set at runtime using SET GLOBAL or SET SESSION. You can also obtain their values using SELECT. See the section called “USING 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. The table also lists session options that can be set with the SET statement. Section 5.3, “SET Syntax”, discusses these options.

Variables that have a type of “string” take a string value. Variables that have a type of “numeric” take a numeric value. Variables that have a type of “boolean” can be set to 0, 1, ON or OFF. (If you set them on the command line or in an option file, use the numeric values.) 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 enumerated system variables, the first enumeration value corresponds to 0. This differs from ENUM columns, for which the first enumeration value corresponds to 1. Variable NameValue TypeTypeautocommitbooleanSESSIONbig_tablesbooleanSESSIONbinlog_cache_sizenumericGLOBALbulk_insert_buffer_sizenumericGLOBAL | SESSIONcharacter_set_clientstringGLOBAL | SESSIONcharacter_set_connectionstringGLOBAL | SESSION
                    character_set_resultsstringGLOBAL | SESSIONcharacter_set_serverstringGLOBAL | SESSIONcollation_connectionstringGLOBAL | SESSIONcollation_serverstringGLOBAL | SESSIONcompletion_typenumericGLOBAL | SESSIONconcurrent_insertbooleanGLOBALconnect_timeoutnumericGLOBALconvert_character_setstringGLOBAL | SESSIONdefault_week_formatnumericGLOBAL | SESSIONdelay_key_writeOFF | ON | ALLGLOBALdelayed_insert_limitnumericGLOBALdelayed_insert_timeoutnumericGLOBALdelayed_queue_sizenumericGLOBALdiv_precision_incrementnumericGLOBAL | SESSIONengine_condition_pushdownbooleanGLOBAL | SESSIONerror_countnumericSESSIONevent_schedulerbooleanGLOBALexpire_logs_daysnumericGLOBALflushbooleanGLOBALflush_timenumericGLOBALforeign_key_checksbooleanSESSIONft_boolean_syntaxnumericGLOBALgroup_concat_max_lennumericGLOBAL | SESSIONidentitynumericSESSIONinnodb_autoextend_incrementnumericGLOBALinnodb_commit_concurrencynumericGLOBALinnodb_concurrency_ticketsnumericGLOBALinnodb_max_dirty_pages_pctnumericGLOBALinnodb_max_purge_lagnumericGLOBALinnodb_support_xabooleanGLOBAL | SESSIONinnodb_sync_spin_loopsnumericGLOBALinnodb_table_locksbooleanGLOBAL | SESSIONinnodb_thread_concurrencynumericGLOBALinnodb_thread_sleep_delaynumericGLOBALinsert_idbooleanSESSIONinteractive_timeoutnumericGLOBAL | SESSIONjoin_buffer_sizenumericGLOBAL | SESSIONkey_buffer_sizenumericGLOBAL
                    last_insert_idnumericSESSIONlocal_infilebooleanGLOBALlog_warningsnumericGLOBALlong_query_timenumericGLOBAL | SESSIONlow_priority_updatesbooleanGLOBAL | SESSIONmax_allowed_packetnumericGLOBAL | SESSIONmax_binlog_cache_sizenumericGLOBALmax_binlog_sizenumericGLOBALmax_connect_errorsnumericGLOBALmax_connectionsnumericGLOBALmax_delayed_threadsnumericGLOBALmax_error_countnumericGLOBAL | SESSIONmax_heap_table_sizenumericGLOBAL | SESSIONmax_insert_delayed_threadsnumericGLOBALmax_join_sizenumericGLOBAL | SESSIONmax_relay_log_sizenumericGLOBALmax_seeks_for_keynumericGLOBAL | SESSIONmax_sort_lengthnumericGLOBAL | SESSIONmax_tmp_tablesnumericGLOBAL | SESSIONmax_user_connectionsnumericGLOBALmax_write_lock_countnumericGLOBALmyisam_stats_methodenumGLOBAL | SESSIONmulti_read_rangenumericGLOBAL | SESSIONmyisam_data_pointer_sizenumericGLOBALlog_bin_trust_function_creatorsbooleanGLOBALmyisam_max_sort_file_sizenumericGLOBAL | SESSIONmyisam_repair_threadsnumericGLOBAL | SESSIONmyisam_sort_buffer_sizenumericGLOBAL | SESSIONmyisam_use_mmapbooleanGLOBALndb_extra_loggingnumericGLOBALnet_buffer_lengthnumericGLOBAL | SESSIONnet_read_timeoutnumericGLOBAL | SESSIONnet_retry_countnumericGLOBAL | SESSIONnet_write_timeoutnumericGLOBAL | SESSIONold_passwordsnumericGLOBAL | SESSIONoptimizer_prune_levelnumericGLOBAL | SESSIONoptimizer_search_depthnumericGLOBAL | SESSIONpreload_buffer_sizenumericGLOBAL | SESSIONquery_alloc_block_sizenumericGLOBAL | SESSIONquery_cache_limitnumericGLOBALquery_cache_sizenumericGLOBALquery_cache_typeenumerationGLOBAL | SESSIONquery_cache_wlock_invalidatebooleanGLOBAL | SESSIONquery_prealloc_sizenumericGLOBAL | SESSIONrange_alloc_block_sizenumericGLOBAL | SESSIONread_buffer_sizenumericGLOBAL | SESSIONread_onlynumericGLOBALread_rnd_buffer_sizenumericGLOBAL | SESSIONrpl_recovery_ranknumericGLOBALsafe_show_databasebooleanGLOBALsecure_authbooleanGLOBALserver_idnumericGLOBALslave_compressed_protocolbooleanGLOBALslave_net_timeoutnumericGLOBALslave_transaction_retriesnumericGLOBALslow_launch_timenumericGLOBALsort_buffer_sizenumericGLOBAL | SESSIONsql_auto_is_nullbooleanSESSIONsql_big_selectsbooleanSESSIONsql_big_tablesbooleanSESSIONsql_buffer_resultbooleanSESSIONsql_log_binbooleanSESSIONsql_log_offbooleanSESSIONsql_log_updatebooleanSESSIONsql_low_priority_updatesbooleanGLOBAL | SESSIONsql_max_join_sizenumericGLOBAL | SESSIONsql_modeenumerationGLOBAL | SESSIONsql_notesbooleanSESSIONsql_quote_show_createbooleanSESSIONsql_safe_updatesbooleanSESSIONsql_select_limitnumericSESSIONsql_slave_skip_counternumericGLOBALupdatable_views_with_limitenumerationGLOBAL | SESSIONsql_warningsbooleanSESSIONsync_binlognumericGLOBALsync_frmbooleanGLOBALstorage_engineenumerationGLOBAL | SESSIONtable_definition_cachenumericGLOBALtable_open_cachenumericGLOBALtable_typeenumerationGLOBAL | SESSIONthread_cache_sizenumericGLOBALtime_zonestringGLOBAL | SESSIONtimestampbooleanSESSIONtmp_table_sizeenumerationGLOBAL | SESSIONtransaction_alloc_block_sizenumericGLOBAL | SESSIONtransaction_prealloc_sizenumericGLOBAL | SESSIONtx_isolationenumerationGLOBAL | SESSIONunique_checksbooleanSESSIONwait_timeoutnumericGLOBAL | SESSIONwarning_countnumericSESSION.SH "SERVER STATUS VARIABLES"

The server maintains many status variables that provide information about its operation. 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                 | 3          |
| Created_tmp_tables                | 2          |
| Threads_created                   | 217        |
| Threads_running                   | 88         |
| Uptime                            | 1389872    |
+-----------------------------------+------------+

Many status variables are reset to 0 by the FLUSH STATUS statement.

The status variables have the following meanings. Variables with no version indicated were already present prior to MySQL 5.1. For information regarding their implementation history, see MySQL 5.0 Reference Manual.

•   Aborted_clients

    The number of connections that were aborted because the client died without closing the connection properly. See Section 2.10, “Communication Errors and Aborted Connections”.

•   Aborted_connects

    The number of failed attempts to connect to the MySQL server. See Section 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.

•   Binlog_cache_use

    The number of transactions that used the temporary binary log cache.

•   Bytes_received

    The number of bytes received from all clients.

•   Bytes_sent

    The number of bytes sent to all clients.

•   Com_xxx

    The Com_xxx statement counter variables indicate 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, respectively.

    The Com_stmt_xxx status variables are as follows:

    •   Com_stmt_prepare

    •   Com_stmt_execute

    •   Com_stmt_fetch

    •   Com_stmt_send_long_data

    •   Com_stmt_reset

    •   Com_stmt_close

    Those variables stand for prepared statement commands. Their names refer to the COM_xxx command set used in the network layer. In other words, their values increase whenever prepared statement API calls such as mysql_stmt_prepare(), mysql_stmt_execute(), and so forth are executed. However, Com_stmt_prepare, Com_stmt_execute and Com_stmt_close also increase for PREPARE, EXECUTE, or DEALLOCATE PREPARE, respectively. Additionally, the values of the older (available since MySQL 4.1.3) statement counter variables Com_prepare_sql, Com_execute_sql, and Com_dealloc_sql increase for the PREPARE, EXECUTE, and DEALLOCATE PREPARE statements. Com_stmt_fetch stands for the total number of network round-trips issued when fetching from cursors.

    All of the Com_stmt_xxx variables are increased even if a prepared statement argument is unknown or an error occurred during execution. In other words, their values correspond to the number of requests issued, not to the number of requests successfully completed.

•   Compression

    Whether the client connection uses compression in the client/server protocol. Added in MySQL 5.1.2.

•   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.

•   Created_tmp_files

    How many temporary files mysqld has created.

•   Created_tmp_tables

    The number of in-memory temporary tables created automatically by the server while executing statements. If Created_tmp_disk_tables is large, 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.

•   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 times that tables have been discovered via this mechanism.

•   Handler_delete

    The number of times that rows have been deleted from tables.

•   Handler_read_first

    The number of times the first entry was read from an index. If this value 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 value is high, it is a good indication that your tables are properly indexed for your queries.

•   Handler_read_next

    The number of requests to read the next row in key order. This value is 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.

•   Handler_read_rnd

    The number of requests to read a row based on a fixed position. This value is 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 entire 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 value is 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.

•   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.

•   Innodb_buffer_pool_pages_data

    The number of pages containing data (dirty or clean).

•   Innodb_buffer_pool_pages_dirty

    The number of pages currently dirty.

•   Innodb_buffer_pool_pages_flushed

    The number of buffer pool page-flush requests.

•   Innodb_buffer_pool_pages_free

    The number of free pages.

•   Innodb_buffer_pool_pages_latched

    The number of latched pages in InnoDB buffer pool. These are pages currently being read or written or that cannot be flushed or removed for some other reason.

•   Innodb_buffer_pool_pages_misc

    The number of pages that are busy because they have been allocated for administrative overhead such as row locks or the adaptive hash index. This value can also be calculated as Innodb_buffer_pool_pages_total – Innodb_buffer_pool_pages_free – Innodb_buffer_pool_pages_data.

•   Innodb_buffer_pool_pages_total

    The total size of the buffer pool, in pages.

•   Innodb_buffer_pool_read_ahead_rnd

    The number of “random” read-aheads initiated by InnoDB. This happens when a query scans a large portion of a table but in random order.

•   Innodb_buffer_pool_read_ahead_seq

    The number of sequential read-aheads initiated by InnoDB. This happens when InnoDB does a sequential full table scan.

•   Innodb_buffer_pool_read_requests

    The number of logical read requests InnoDB has done.

•   Innodb_buffer_pool_reads

    The number of logical reads that InnoDB could not satisfy from the buffer pool and had to do a single-page read.

•   Innodb_buffer_pool_wait_free

    Normally, writes to the InnoDB buffer pool happen in the background. However, if it is necessary to read or create a page and no clean pages are available, it is also necessary to wait for pages to be flushed first. This counter counts instances of these waits. If the buffer pool size has been set properly, this value should be small.

•   Innodb_buffer_pool_write_requests

    The number writes done to the InnoDB buffer pool.

•   Innodb_data_fsyncs

    The number of fsync() operations so far.

•   Innodb_data_pending_fsyncs

    The current number of pending fsync() operations.

•   Innodb_data_pending_reads

    The current number of pending reads.

•   Innodb_data_pending_writes

    The current number of pending writes.

•   Innodb_data_read

    The amount of data read so far, in bytes.

•   Innodb_data_reads

    The total number of data reads.

•   Innodb_data_writes

    The total number of data writes.

•   Innodb_data_written

    The amount of data written so far, in bytes.

•   Innodb_dblwr_writes, Innodb_dblwr_pages_written

    The number of doublewrite operations that have been performed and the number of pages that have been written for this purpose. See Section 2.14.1, “Disk I/O”.

•   Innodb_log_waits

    The number of times that the log buffer was too small and a wait was required for it to be flushed before continuing.

•   Innodb_log_write_requests

    The number of log write requests.

•   Innodb_log_writes

    The number of physical writes to the log file.

•   Innodb_os_log_fsyncs

    The number of fsync() writes done to the log file.

•   Innodb_os_log_pending_fsyncs

    The number of pending log file fsync() operations.

•   Innodb_os_log_pending_writes

    The number of pending log file writes.

•   Innodb_os_log_written

    The number of bytes written to the log file.

•   Innodb_page_size

    The compiled-in InnoDB page size (default 16KB). Many values are counted in pages; the page size allows them to be easily converted to bytes.

•   Innodb_pages_created

    The number of pages created.

•   Innodb_pages_read

    The number of pages read.

•   Innodb_pages_written

    The number of pages written.

•   Innodb_row_lock_current_waits

    The number of row locks currently being waited for.

•   Innodb_row_lock_time

    The total time spent in acquiring row locks, in milliseconds.

•   Innodb_row_lock_time_avg

    The average time to acquire a row lock, in milliseconds.

•   Innodb_row_lock_time_max

    The maximum time to acquire a row lock, in milliseconds.

•   Innodb_row_lock_waits

    The number of times a row lock had to be waited for.

•   Innodb_rows_deleted

    The number of rows deleted from InnoDB tables.

•   Innodb_rows_inserted

    The number of rows inserted into InnoDB tables.

•   Innodb_rows_read

    The number of rows read from InnoDB tables.

•   Innodb_rows_updated

    The number of rows updated in InnoDB tables.

•   Key_blocks_not_flushed

    The number of key blocks in the key cache that have changed but have not yet been flushed to disk.

•   Key_blocks_unused

    The number of unused 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 the section called “SERVER SYSTEM VARIABLES”.

•   Key_blocks_used

    The number of used blocks in the key cache. This value is a high-water mark that indicates the maximum number of blocks that have ever been in use at one time.

•   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 large, 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. This is useful for comparing the cost of different query plans for the same query. The default value of 0 means that no query has been compiled yet. The default value is 0. Last_query_cost has session scope.

•   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.

•   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_open_cache value is probably too small.

•   Qcache_free_blocks

    The number of free memory blocks in the query cache.

•   Qcache_free_memory

    The amount of free memory for the query cache.

•   Qcache_hits

    The number of query cache hits.

•   Qcache_inserts

    The number of queries added to the query cache.

•   Qcache_lowmem_prunes

    The number of queries that were deleted from the query cache because of low memory.

•   Qcache_not_cached

    The number of non-cached queries (not cacheable, or not cached due to the query_cache_type setting).

•   Qcache_queries_in_cache

    The number of queries registered in the query cache.

•   Qcache_total_blocks

    The total number of blocks in the query cache.

•   Questions

    The number of statements that clients have sent to the server.

•   Rpl_status

    The status of failsafe replication (not yet implemented).

•   Select_full_join

    The number of joins that perform table scans because they do not use indexes. If this value is not 0, you should carefully check the indexes of your tables.

•   Select_full_range_join

    The number of joins that used a range search on a reference table.

•   Select_range

    The number of joins that used ranges on the first table. This is normally not a critical issue even if the value is quite large.

•   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.

•   Select_scan

    The number of joins that did a full scan of the first table.

•   Slave_open_temp_tables

    The number of temporary tables that the slave SQL thread currently has open.

•   Slave_running

    This is ON if this server is a slave that is connected to a master.

•   Slave_retried_transactions

    The total number of times since startup that the replication slave SQL thread has retried transactions.

•   Slow_launch_threads

    The number of threads that have taken more than slow_launch_time seconds to create.

•   Slow_queries

    The number of queries that have taken more than long_query_time seconds. See Section 9.4, “The Slow Query Log”.

•   Sort_merge_passes

    The number of merge passes that 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.

•   Sort_range

    The number of sorts that were done using ranges.

•   Sort_rows

    The number of sorted rows.

•   Sort_scan

    The number of sorts that were done by scanning the table.

•   Ssl_xxx

    Variables used for SSL connections.

•   Table_locks_immediate

    The number of times that a table lock was acquired immediately.

•   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.

•   Threads_cached

    The number of threads in the thread cache.

•   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.

•   Threads_running

    The number of threads that are not sleeping.

•   Uptime

    The number of seconds that the server has been up.

THE SERVER SQL MODE

The MySQL server can operate in different SQL modes, and can apply these modes differently for different clients. This capability enables each application to tailor the server's operating mode 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 mysqld with the --sql-mode="modes" option. modes is a list of different modes separated by comma (‘,’) characters. The default value is empty (no modes set). The modes value also can be empty (--sql-mode="") if you want to clear it explicitly.

You can change the SQL mode at runtime by using a SET [GLOBAL|SESSION] sql_mode='modes' statement to set the sql_mode system value. 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 own session sql_mode value at any time.

You can retrieve the current global or session sql_mode value with the following statements:

SELECT @@global.sql_mode;
SELECT @@session.sql_mode;

The most important sql_mode values are probably these:

•   ANSI

    Change syntax and behavior to be more conformant to standard SQL.

•   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.

•   TRADITIONAL

    Make MySQL behave like a “traditional” SQL database system. A simple description of this mode is “give an error instead of a warning” when inserting an incorrect value into a column. Note: The INSERT/UPDATE aborts as soon as the error is noticed. This may not be what you want if you are using a non-transactional storage engine, because data changes made prior to the error are not be rolled back, resulting in a “partially done” update.

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 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.

    The server requires that month and day values be legal, and not merely in the range 1 to 12 and 1 to 31, respectively. With strict mode disabled, invalid dates such as '2004-04-31' are converted to '0000-00-00' and a warning is generated. With strict mode enabled, invalid dates generate an error. To allow such dates, enable ALLOW_INVALID_DATES.

•   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 identifiers with this mode enabled. With ANSI_QUOTES enabled, you cannot use double quotes to quote a literal string, because it is interpreted as an identifier.

•   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 or UPDATE. If this mode is not enabled, MySQL instead returns NULL for divisions by zero. If used in INSERT IGNORE or UPDATE IGNORE, MySQL generates a warning for divisions by zero, but the result of the operation is NULL.

•   HIGH_NOT_PRECEDENCE

    The precedence of the NOT operator is such that expressions such as NOT a BETWEEN b AND c are parsed as NOT (a BETWEEN b AND c). In some older versions of MySQL, the expression was parsed as (NOT a) BETWEEN b AND c. The old higher-precedence behavior can be obtained by enabling the HIGH_NOT_PRECEDENCE SQL mode.

    mysql> SET sql_mode = '';
    mysql> SELECT NOT 1 BETWEEN -5 AND 5;
            -> 0
    mysql> SET sql_mode = 'broken_not';
    mysql> SELECT NOT 1 BETWEEN -5 AND 5;
            -> 1

•   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";
    The IGNORE_SPACE SQL mode applies to built-in functions, not to stored routines. it is always allowable to have spaces after a routine name, regardless of whether IGNORE_SPACE is enabled.

•   NO_AUTO_CREATE_USER

    Prevent GRANT from automatically creating new users if it would otherwise do so, unless a non-empty password also is specified.

•   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.

    This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (Storing 0 is not a recommended practice, by the way.) For example, if you dump the table with mysqldump and then reload it, MySQL normally generates new sequence numbers when it encounters the 0 values, resulting in a table with contents different from the one that was dumped. Enabling NO_AUTO_VALUE_ON_ZERO before reloading the dump file solves this problem. mysqldump now automatically includes in its output a statement that enables NO_AUTO_VALUE_ON_ZERO, to avoid this problem.

•   NO_BACKSLASH_ESCAPES

    Disable the use of the backslash character (‘\’) as an escape character within strings. With this mode enabled, backslash becomes any ordinary character like any other.

•   NO_DIR_IN_CREATE

    When creating a table, ignore all INDEX DIRECTORY and DATA DIRECTORY directives. This option is useful on slave replication servers.

•   NO_ENGINE_SUBSTITUTION

    Prevents automatic substitution of the default storage engine when a statement such as CREATE TABLE specifies a storage engine that is disabled or not compiled in.

•   NO_FIELD_OPTIONS

    Do not print MySQL-specific column options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.

•   NO_KEY_OPTIONS

    Do not print MySQL-specific index options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.

•   NO_TABLE_OPTIONS

    Do not print MySQL-specific table options (such as ENGINE) in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.

•   NO_UNSIGNED_SUBTRACTION

    In subtraction operations, do not mark the result as UNSIGNED if one of the operands is unsigned. Note that this makes BIGINT UNSIGNED not 100% usable in all contexts. See Section 8, “Cast Functions and Operators”.

•   NO_ZERO_DATE

    In strict mode, don't allow '0000-00-00' as a valid date. You can still insert zero dates with the IGNORE option. When not in strict mode, the date is accepted but a warning is generated.

•   NO_ZERO_IN_DATE

    In strict mode, don't accept dates where the month or day part is 0. If used with the IGNORE option, MySQL inserts a '0000-00-00' date for any such date. When not in strict mode, the date is accepted but a warning is generated.

•   ONLY_FULL_GROUP_BY

    Do not allow queries for which the GROUP BY clause refers to a column that is not present in the output column list.

•   PIPES_AS_CONCAT

    Treat || as a string concatenation operator (same as CONCAT()) rather than as a synonym for OR.

•   REAL_AS_FLOAT

    Treat REAL as a synonym for FLOAT. By default, MySQL treats REAL as a synonym for DOUBLE.

•   STRICT_ALL_TABLES

    Enable strict mode for all storage engines. Invalid data values are rejected. Additional detail follows.

•   STRICT_TRANS_TABLES

    Enable strict mode for transactional storage engines, and when possible for non-transactional storage engines. Additional details follow.

Strict mode controls how MySQL handles input values that are invalid or missing. A value can be invalid for several reasons. For example, it might have the wrong data type for the column, or it might be out of range. A value is missing when a new row to be inserted does not contain a value for a column that has no explicit DEFAULT clause in its definition.

For transactional tables, an error occurs for invalid or missing values in a statement when either of the STRICT_ALL_TABLES or STRICT_TRANS_TABLES modes are enabled. The statement is aborted and rolled back.

For non-transactional tables, the behavior is the same for either mode, if the bad value occurs in the first row to be inserted or updated. The statement is aborted and the table remains unchanged. If the statement inserts or modifies multiple rows and the bad value occurs in the second or later row, the result depends on which strict option is enabled:

•   For STRICT_ALL_TABLES, MySQL returns an error and ignores the rest of the rows. However, in this case, the earlier rows still have been inserted or updated. This means that you might get a partial update, which might not be what you want. To avoid this, it's best to use single-row statements because these can be aborted without changing the table.

•   For STRICT_TRANS_TABLES, MySQL converts an invalid value to the closest valid value for the column and insert the adjusted value. If a value is missing, MySQL inserts the implicit default value for the column data type. In either case, MySQL generates a warning rather than an error and continues processing the statement. Implicit defaults are described in Section 1.4, “Data Type Default Values”.

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 as well, enable the NO_ZERO_IN_DATE and NO_ZERO_DATE SQL modes in addition to strict mode.

If you are not using strict mode (that is, neither STRICT_TRANS_TABLES nor STRICT_ALL_TABLES is enabled), MySQL inserts adjusted values for invalid or missing values and produces warnings. In strict mode, you can produce this behavior by using INSERT IGNORE or UPDATE IGNORE. See Section 5.4.25, “SHOW WARNINGS Syntax”.

The following special modes are provided as shorthand for combinations of mode values from the preceding list.

The descriptions include all mode values that are available in the most recent version of MySQL. For older versions, a combination mode does not include individual mode values that are not available except in newer versions.

•   ANSI

    Equivalent to REAL_AS_FLOAT, PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE. See Section 9.3, “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, NO_AUTO_CREATE_USER.

•   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, HIGH_NOT_PRECEDENCE.

•   MYSQL40

    Equivalent to NO_FIELD_OPTIONS, HIGH_NOT_PRECEDENCE.

•   ORACLE

    Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS, NO_AUTO_CREATE_USER.

•   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, NO_AUTO_CREATE_USER.

THE MYSQL SERVER SHUTDOWN PROCESS

The server shutdown process takes place as 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 mysqladmin shutdown command. mysqladmin can be used on any platform supported by MySQL. Other operating system-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 appears 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, and shared memory on Windows.

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 statements check their state periodically and take longer to die. For additional information about thread termination, see Section 5.5.3, “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 transaction 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), and 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, unless innodb_fast_shutdown is 2, writes the current LSN to the tablespace, and terminates its own internal threads.

6.  The server exits.

SEE ALSO
msql2mysql(1), myisamchk(1), myisamlog(1), myisampack(1), mysql(1), mysql.server(1), mysql_config(1), mysql_fix_privilege_tables(1), mysql_zap(1), mysqlaccess(1), mysqladmin(1), mysqlbinlog(1), mysqlcheck(1), mysqld_multi(1), mysqld_safe(1), mysqldump(1), mysqlhotcopy(1), mysqlimport(1), mysqlmanager(1), mysqlshow(1), mysqlslap(1), perror(1), replace(1), safe_mysqld(1)

For more information, please refer to the MySQL Reference Manual, which may already be installed locally and which is also available online at http://dev.mysql.com/doc/.

AUTHOR
MySQL AB (http://www.mysql.com/). This software comes with no warranty.