To see the values that a server will use based on its compiled-in defaults and any option files that it reads, use this command (omit --verbose before MySQL 4.1.1):

mysqld --verbose --help

To see the values that a server will use based on its compiled-in defaults, ignoring the settings in any option files, use this command (omit --verbose before MySQL 4.1.1):

mysqld --no-defaults --verbose --help

To see the current values used by a running server, use the SHOW VARIABLES statement.

Section 5.1.4, “Using System Variables”, discusses the syntax for setting and displaying system variable values.

Section 5.1.4.2, “Dynamic System Variables”, lists the variables that can be set at runtime.

Information on tuning system variables can be found in Section 7.8.2, “Tuning Server Parameters”.

Section 13.2.4, “InnoDB Startup Options and System Variables”, lists InnoDB system variables.

Section 15.3.4.3, “MySQL Cluster System Variables”, lists system variables which are specific to MySQL Cluster.

For information on server system variables specific to replication, see Section 14.8, “Replication and Binary Logging Options and Variables”.

ansi_mode

This is ON if mysqld was started with --ansi. See Section 1.9.3, “Running MySQL in ANSI Mode”. This variable was added in MySQL 3.23.6 and removed in 3.23.41. See the description for sql_mode.

autocommit

The autocommit mode. If set to 1, all changes to a table take effect immediately. If set to 0, you must use COMMIT to accept a transaction or ROLLBACK to cancel it. If autocommit is 0 and you change it to 1, MySQL performs an automatic COMMIT of any open transaction. Another way to begin a transaction is to use a START TRANSACTION or BEGIN statement. See Section 12.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.

By default, client connections begin with autocommit set to 1. To cause clients to begin with a default of 0, set the server's init_connect system variable. See the description of that variable for instructions that show how to do this.

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. Relative path names for other variables usually are resolved relative to the base directory.

bdb_cache_size

The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you do not use BDB tables, you should start mysqld with --skip-bdb to not allocate memory for this cache. This variable was added in MySQL 3.23.14.

bdb_home

The base directory for BDB tables. This should be assigned the same value as the datadir variable. This variable was added in MySQL 3.23.14.

bdb_log_buffer_size

The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you do not use BDB tables, you should set this to 0 or start mysqld with --skip-bdb in order not to allocate memory for this cache. This variable was added in MySQL 3.23.31.

bdb_logdir

The directory where the BDB storage engine writes its log files. This variable can be set with the --bdb-logdir option. This variable was added in MySQL 3.23.14.

bdb_max_lock

The maximum number of locks 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 ...

This variable was added in MySQL 3.23.29.

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.) This variable was added in MySQL 3.23.29.

bdb_tmpdir

The BDB temporary file directory. This variable was added in MySQL 3.23.14.

bdb_version

See the description for version_bdb.

big_tables

If set to 1, all temporary tables are stored on disk rather than in memory. This is a little slower, but the error The table tbl_name is full does not occur for SELECT operations that require a large temporary table. The default value for a new connection is 0 (use in-memory temporary tables). As of MySQL 4.0, you should normally never need to set this variable, because MySQL automatically converts in-memory tables to disk-based tables as necessary.

binlog_cache_size

The size of the cache to hold the SQL statements for the binary log during a transaction. A binary log cache is allocated for each client if the server supports any transactional storage engines and, starting from MySQL 4.1.2, if the server has 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. This variable was added in MySQL 3.23.29. See Section 5.3.4, “The Binary Log”.

bulk_insert_buffer_size

MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE when adding data to nonempty 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. Before MySQL 4.0.3. this variable was named myisam_bulk_insert_tree_size.

character_set

The default character set. This variable was added in MySQL 3.23.3, then removed in MySQL 4.1.1 and replaced by the various character_set_xxx variables.

character_set_client

The character set for statements that arrive from the client. This variable was added in MySQL 4.1.1.

The session value of this variable is set using the character set requested by the client when the client connects to the server. (Many clients support a --default-character-set option to enable this character set to be specified explicitly. See also Section 9.1.4, “Connection Character Sets and Collations”.) The global value of the variable is used to set the session value in cases when the client-requested value is unknown or not available, or the server is configured to ignore client requests:

ucs2 cannot be used as a client character set, which means that it also does not work for SET NAMES or SET CHARACTER SET.

character_set_connection

The character set used for literals that do not have a character set introducer and for number-to-string conversion. This variable was added in MySQL 4.1.1.

character_set_database

The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server. This variable was added in MySQL 4.1.1.

character_set_results

The character set used for returning query results such as result sets or error messages to the client. This variable was added in MySQL 4.1.1.

character_set_server

The server default character set. This variable was added in MySQL 4.1.1.

character_set_system

The character set used by the server for storing identifiers. The value is always utf8. This variable was added in MySQL 4.1.1.

character_sets

The supported character sets. This variable was added in MySQL 3.23.15 and removed in MySQL 4.1.1. (Use SHOW CHARACTER SET for a list of character sets.)

character_sets_dir

The directory where character sets are installed. This variable was added in MySQL 4.1.2.

collation_connection

The collation of the connection character set. This variable was added in MySQL 4.1.1.

collation_database

The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server. This variable was added in MySQL 4.1.1.

collation_server

The server default collation. This variable was added in MySQL 4.1.1.

concurrent_insert

If ON (the default), MySQL permits INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle of the data file. If OFF, concurrent inserts are disabled. If you start mysqld with --skip-new, this variable is set to OFF. This variable was added in MySQL 3.23.7.

See also Section 7.6.3, “Concurrent Inserts”.

connect_timeout

The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake. The default value is 5 seconds.

Increasing the connect_timeout value might help if clients frequently encounter errors of the form Lost connection to MySQL server at 'XXX', system error: errno.

convert_character_set

The current character set mapping that was set by SET CHARACTER SET. This variable was removed in MySQL 4.1.

datadir

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

date_format

This variable is unused.

datetime_format

This variable is unused.

default_week_format

The default mode value to use for the WEEK() function. See Section 11.7, “Date and Time Functions”. This variable is available as of MySQL 4.0.14.

delay_key_write

This option applies only to MyISAM tables. It can have one of the following values to affect handling of the DELAY_KEY_WRITE table option that can be used in CREATE TABLE statements.

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 Section 5.1.2, “Server Command Options”, and Section 13.1.1, “MyISAM Startup Options”.

This variable was added in MySQL 3.23.8.

delayed_insert_limit

After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it permits 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.

error_count

The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 12.4.5.11, “SHOW ERRORS Syntax”.

This variable was added in MySQL 4.1.0.

expire_logs_days

The number of days for automatic binary log file removal. The default is 0, which means “no automatic removal.” Possible removals happen at startup and when the binary log is flushed. Log flushing occurs as indicated in Section 5.3, “MySQL Server Logs”. This variable was added in MySQL 4.1.0.

To remove binary log files manually, use the PURGE BINARY LOGS statement. See Section 12.5.1.1, “PURGE BINARY LOGS Syntax”.

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 B.5.4.2, “What to Do If MySQL Keeps Crashing”. This variable is set to ON if you start mysqld with the --flush option. This variable was added in MySQL 3.22.9.

flush_time

If this is set to a nonzero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. This option is best used only on Windows 9x or Me, or on systems with minimal resources. This variable was added in MySQL 3.22.18.

foreign_key_checks

If set to 1 (the default), foreign key constraints for InnoDB tables are checked. If set to 0, they are ignored. Disabling foreign key checking can be useful for reloading InnoDB tables in an order different from that required by their parent/child relationships. This variable was added in MySQL 3.23.52. See Section 13.2.5.4, “FOREIGN KEY Constraints”.

Setting foreign_key_checks to 0 also affects data definition statements: DROP DATABASE drops a database even if it contains tables that have foreign keys that are referred to by tables outside the database, and DROP TABLE drops tables that have foreign keys that are referred to by other tables.

ft_boolean_syntax

The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 11.9.2, “Boolean Full-Text Searches”. This variable was added as a read-only variable in MySQL 4.0.1. It can be modified as of MySQL 4.1.2.

The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:

ft_max_word_len

The maximum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.

ft_min_word_len

The minimum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.

ft_query_expansion_limit

The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION. This variable was added in MySQL 4.1.1.

ft_stopword_file

The file from which to read the list of stopwords for full-text searches. The server looks for the file in the data directory unless an absolute path name is given to specify a different directory. 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. See also Section 11.9.4, “Full-Text Stopwords”. This variable was added in MySQL 4.0.10.

group_concat_max_len

The maximum permitted result length in bytes for the GROUP_CONCAT() function. The default is 1024. This variable was added in MySQL 4.1.0.

have_archive

YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.3.

have_bdb

YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used. This variable was added in MySQL 3.23.30.

have_blackhole_engine

YES if mysqld supports BLACKHOLE tables, NO if not. This variable was added in MySQL 4.1.11.

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. This variable was added in MySQL 4.1.1.

have_crypt

YES if the crypt() system call is available to the server, NO if not. If not, the ENCRYPT() function cannot be used. This variable was added in MySQL 4.0.10.

have_csv

YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.4.

have_example_engine

YES if mysqld supports EXAMPLE tables, NO if not. This variable was added in MySQL 4.1.4.

have_geometry

YES if the server supports spatial data types, NO if not. This variable was added in MySQL 4.1.3.

have_innodb

YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used. This variable was added in MySQL 3.23.37.

have_isam

YES if mysqld supports ISAM tables. DISABLED if --skip-isam is used. This variable was added in MySQL 3.23.30.

have_merge_engine

YES if mysqld supports MERGE tables. DISABLED if --skip-merge is used. This variable was added in MySQL 4.1.21.

have_openssl

YES if mysqld supports SSL (encryption) connections, NO if not. This variable was added in MySQL 3.23.43.

have_query_cache

YES if mysqld supports the query cache, NO if not. This variable was added in MySQL 4.0.2.

have_raid

YES if mysqld supports the RAID option, NO if not. This variable was added in MySQL 3.23.30.

have_rtree_keys

YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM tables.) This variable was added in MySQL 4.1.3.

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.

This variable was added in MySQL 4.0.0.

identity

This variable is a synonym for the last_insert_id variable. It exists for compatibility with other database systems. As of MySQL 3.23.25, you can read its value with SELECT @@identity. As of MySQL 4.0.3, you can also set its value with SET identity.

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'

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.

This variable was added in MySQL 4.1.2.

init_file

The name of the file specified with the --init-file option when you start the server. This 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. No statement terminator such as ;, \g, or \G should be given at the end of each statement. This variable was added in MySQL 3.23.2.

innodb_xxx

InnoDB system variables are listed in Section 13.2.4, “InnoDB Startup Options and System Variables”.

insert_id

The value to be used by the following INSERT or ALTER TABLE statement when inserting an AUTO_INCREMENT value. This is mainly used with the binary log.

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 minimum size of the buffer that is used for plain index scans, range index scans, and 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. There is no gain from setting the buffer larger than required to hold each matching row, and all joins allocate at least the minimum size, so use caution in setting this variable to a large value globally. It is better to keep the global setting small and change to a larger setting only in sessions that are doing large joins. Memory allocation time can cause substantial performance drops if the global size is larger than needed by most queries that use it.

key_buffer_size

Index blocks for MyISAM and ISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.

The maximum permissible 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. The value of this variable indicates the amount of memory requested. Internally, the server allocates as much memory as possible up to this amount, but the actual allocation might be less.

You can increase the value to get better index handling for all reads and multiple writes; on a system whose primary function is to run MySQL using the MyISAM storage engine, 25% of the machine's total memory is an acceptable value for this variable. However, you should be aware that, if you make the value too large (for example, more than 50% of the machine's total memory), your system might start to page and become extremely slow. This is because MySQL relies on the operating system to perform file system caching for data reads, so you must leave some room for the file system cache. You should also consider the memory requirements of any other storage engines that you may be using in addition to MyISAM.

For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 7.3.2.1, “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 12.4.5, “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. From MySQL 4.1.1 on, the buffer block size is available from the key_cache_block_size server variable. The fraction of the buffer in use is:

1 - ((Key_blocks_unused * key_cache_block_size) / key_buffer_size)

This value is an approximation because some space in the key buffer is allocated internally for administrative structures. Factors that influence the amount of overhead for these structures include block size and pointer size. As block size increases, the percentage of the key buffer lost to overhead tends to decrease. Larger blocks results in a smaller number of read operations (because more keys are obtained per read), but conversely an increase in reads of keys that are not examined (if not all keys in a block are relevant to a query).

Before MySQL 4.1.1, key cache blocks are 1024 bytes, and before MySQL 4.1.2, Key_blocks_unused is unavailable. The Key_blocks_used variable can be used as follows to determine the fraction of the key buffer in use:

(Key_blocks_used × 1024) / key_buffer_size

However, Key_blocks_used indicates the maximum number of blocks that have ever been in use at once, so this formula does not necessarily represent the current fraction of the buffer that is in use.

As of MySQL 4.1, 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 7.5.1, “The MyISAM Key Cache”.

key_cache_age_threshold

This value controls the demotion of buffers from the hot sublist of a key cache to the warm sublist. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. This variable was added in MySQL 4.1.1. See Section 7.5.1, “The MyISAM Key Cache”.

key_cache_block_size

The size in bytes of blocks in the key cache. The default value is 1024. This variable was added in MySQL 4.1.1. See Section 7.5.1, “The MyISAM Key Cache”.

key_cache_division_limit

The division point between the hot and warm sublists of the key cache buffer list. The value is the percentage of the buffer list to use for the warm sublist. Permissible values range from 1 to 100. The default value is 100. This variable was added in MySQL 4.1.1. See Section 7.5.1, “The MyISAM Key Cache”.

language

The directory where error messages are located. See Section 9.3, “Setting the Error Message Language”.

large_files_support

Whether mysqld was compiled with options for large file support. This variable was added in MySQL 3.23.28.

last_insert_id

The value to be returned from LAST_INSERT_ID(). This is stored in the binary log when you use LAST_INSERT_ID() in a statement that updates a table. Setting this variable does not update the value returned by the mysql_insert_id() C API function.

lc_time_names

This variable specifies the locale that controls the language used to display day and month names and abbreviations. This variable affects the output from the DATE_FORMAT(), DAYNAME() and MONTHNAME() functions. Locale names are POSIX-style values such as 'ja_JP' or 'pt_BR'. The default value is 'en_US' regardless of your system's locale setting. For further information, see Section 9.8, “MySQL Server Locale Support”. This variable was added in MySQL 4.1.21.

license

The type of license the server has. This variable was added in MySQL 4.0.19.

local_infile

Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 5.4.5, “Security Issues with LOAD DATA LOCAL”. This variable was added in MySQL 4.0.3.

locked_in_memory

Whether mysqld was locked in memory with --memlock. This variable was added in MySQL 3.23.25.

log

Whether logging of all statements to the general query log is enabled. See Section 5.3.2, “The General Query Log”.

log_error

The location of the error log. This variable was added in MySQL 4.0.10.

log_slow_queries

Whether slow queries should be logged. “Slow” is determined by the value of the long_query_time variable. This variable was added in MySQL 4.0.2. See Section 5.3.5, “The Slow Query Log”.

log_update

Whether the update log is enabled. This variable was added in MySQL 3.22.18. Note that the binary log is preferable to the update log, which is unavailable as of MySQL 5.0. See Section 5.3.3, “The Update Log”.

log_warnings

Whether to produce additional warning messages. This variable was added in MySQL 4.0.3. It is enabled by default as of MySQL 4.0.19 and 4.1.2. As of MySQL 4.0.21 and 4.1.3, the variable can take values greater than 1 and 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. The default is 10. See Section 5.3.5, “The Slow Query Log”.

low_priority_updates

If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This affects only storage engines that use only table-level locking (such as MyISAM, MEMORY, and MERGE). Before MySQL 3.22.5, this variable was named sql_low_priority_updates.

lower_case_file_system

This variable describes the case sensitivity of file names on the file system where the data directory is located. OFF means file names are case sensitive, ON means they are not case sensitive. This variable is read only because it reflects a file system attribute and setting it would have no effect on the file system. This variable was added in MySQL 4.0.19.

lower_case_table_names

If set to 0, table names are stored as specified and comparisons are case sensitive. If set to 1, table names are stored in lowercase on disk and comparisons are not case sensitive. This variable was added in MySQL 3.23.6. If set to 2 (new in 4.0.18), table names are stored as given but compared in lowercase. From MySQL 4.0.2, this option also applies to database names. From 4.1.1, it also applies to table aliases. For additional information, see Section 8.2.2, “Identifier Case Sensitivity”.

You should not set this variable to 0 if you are running MySQL on a system that does not have case-sensitive file names (such as Windows or Mac OS X). If you set this variable to 0 on such a system and access MyISAM tablenames using different lettercases, index corruption may result. New in 4.0.18: If this variable is not set at startup and the file system on which the data directory is located does not have case-sensitive file names, MySQL automatically sets lower_case_table_names to 2.

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

The setting of this variable has no effect on replication filtering options. See Section 14.9, “How Servers Evaluate Replication Filtering Rules”, for more information.

You should not use different settings for lower_case_table_names on replication masters and slaves. In particular, you should not do this when the slave uses a case-sensitive file system, as this can cause replication to fail. For more information, see Section 14.7.20, “Replication and Variables”.

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 largest BLOB you want to use. The protocol limit for max_allowed_packet is 16MB before MySQL 4.0 and 1GB thereafter. The value should be a multiple of 1024; nonmultiples are rounded down to the nearest multiple.

When you change the message buffer size by changing the value of the max_allowed_packet variable, you should also change the buffer size on the client side if your client program permits it. On the client side, max_allowed_packet has a default of 1GB. Some programs such as mysql and mysqldump enable you to change the client-side value by setting max_allowed_packet on the command line or in an option file.

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. If a connection is established successfully within fewer than max_connect_errors attempts after a previous connection was interrupted, the error count for the host is cleared to zero. However, once a host is blocked, the FLUSH HOSTS statement is the only way to unblock it.

max_connections

The maximum permitted number of simultaneous client connections. By default, this is 100. See Section B.5.2.7, “Too many connections”, for more information.

Increasing this value increases the number of file descriptors that mysqld requires. See Section 7.7.2, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits.

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 was not specified. If you set this to 0, MySQL never creates a thread to handle DELAYED rows; in effect, doing so disables DELAYED entirely. This variable was added in MySQL 3.23.0.

For the SESSION value of this variable, the only valid values are 0 or the GLOBAL value.

max_error_count

The maximum number of error, warning, and note messages to be stored for display by the SHOW ERRORS or SHOW WARNINGS statements. This variable was added in MySQL 4.1.0.

max_heap_table_size

This variable sets the maximum size to which user-created MEMORY (HEAP) tables are permitted 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. A server restart also sets the maximum size of existing MEMORY tables to the global max_heap_table_size value. This variable was added in MySQL 3.23.0.

This variable is also used in conjunction with tmp_table_size to limit the size of internal in-memory tables. See Section 7.7.4, “How MySQL Uses Internal Temporary Tables”.

max_insert_delayed_threads

This variable is a synonym for max_delayed_threads. It was added in MySQL 4.0.19.

max_join_size

Do not permit 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 7.3.1.7, “ORDER BY Optimization”. This variable was added in MySQL 4.1.1

max_prepared_stmt_count

This variable limits the total number of prepared statements in the server. It can be used in environments where there is the potential for denial-of-service attacks based on running the server out of memory by preparing huge numbers of statements. If the value is set lower than the current number of prepared statements, existing statements are not affected and can be used, but no new statements can be prepared until the current number drops below the limit. The default value is 16,382. The permissible range of values is from 0 to 1 million. Setting the value to 0 disables prepared statements. This variable was added in MySQL 4.1.19.

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. This variable was added in MySQL 4.0.14. See Section 14.3, “Replication Implementation Details”.

max_seeks_for_key

Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer 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 12.4.5.13, “SHOW INDEX Syntax”). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.

This variable was added in MySQL 4.0.14.

max_sort_length

The number of bytes to use when sorting BLOB or TEXT values. Only the first max_sort_length bytes of each value are used; the rest are ignored.

max_tmp_tables

The maximum number of temporary tables a client can keep open at the same time. (This variable does not yet do anything.)

max_user_connections

The maximum number of simultaneous connections permitted to any given MySQL user account. A value of 0 means “no limit.” This variable was added in MySQL 3.23.34.

This variable has only a global form.

max_write_lock_count

After this many write locks, permit some pending read lock requests to be processed in between. This variable was added in MySQL 3.23.7.

myisam_data_pointer_size

The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 7. The default value is 4. This variable was added in MySQL 4.1.2. See Section B.5.2.12, “The table is full”.

myisam_max_extra_sort_file_size

If the temporary file used for fast MyISAM index creation would be larger than using the key cache by the amount specified here, prefer the key cache method. This is mainly used to force long character keys in large tables to use the slower key cache method to create the index. This variable was added in MySQL 3.23.37.

myisam_max_sort_file_size

The maximum size of the temporary file that MySQL is permitted 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. This variable was added in MySQL 3.23.37.

The default value is 2GB. If MyISAM index files exceed this size and disk space is available, increasing the value may help performance. The space must be available in the file system containing the directory where the original index file is located.

myisam_recover_options

The value of the --myisam-recover option. See Section 5.1.2, “Server Command Options”. This variable was added in MySQL 3.23.36.

myisam_repair_threads

If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process. The default value is 1.

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. This variable was added in MySQL 3.23.16.

myisam_stats_method

How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM tables. This variable has three possible values, nulls_equal, nulls_unequal, and nulls_ignored. 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. For nulls_ignored, NULL values are ignored.

The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 7.4.4, “MyISAM Index Statistics Collection”.

Any unique prefix of a valid value may be used to set the value of this variable.

This variable was added in MySQL 4.1.15. For older versions, the statistics collection method is equivalent to nulls_equal.

named_pipe

On Windows, indicates whether the server supports connections over named pipes. This variable was added in MySQL 3.23.50.

net_buffer_length

Each client thread is associated with a connection buffer and result buffer. Both begin with a size given by net_buffer_length but are dynamically enlarged up to max_allowed_packet bytes as needed. The result buffer shrinks to net_buffer_length after each SQL statement.

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 connection buffer is automatically enlarged. The maximum value to which net_buffer_length can be set is 1MB.

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 through 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. This variable was added in MySQL 3.23.20.

net_retry_count

If a read on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads. This variable was added in MySQL 3.23.7.

net_write_timeout

The number of seconds to wait for a block to be written to a connection before aborting the write. This timeout applies only to TCP/IP connections, not to connections made using Unix socket files, named pipes, or shared memory. See also net_read_timeout. This variable was added in MySQL 3.23.20.

new

This variable is used in MySQL 4.0 to turn on some 4.1 behaviors. This variable was added in MySQL 4.0.12.

old_passwords

Whether the server should use pre-4.1-style passwords for MySQL user accounts. This variable was added in MySQL 4.1.1.

one_shot

This is not a variable, but it can be used when setting some variables. It is described in Section 12.4.4, “SET Syntax”.

open_files_limit

The number of files that the operating system permits mysqld to open. This is the real value permitted 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 cannot change the number of open files. This variable was added in MySQL 3.23.20.

pid_file

The path name of the process ID (PID) file. This variable can be set with the --pid-file option. This variable was added in MySQL 3.23.23.

plugin_dir

The path name of the plugin directory. This variable was added in MySQL 4.1.25. If the value is nonempty, user-defined function object files must be located in this directory. If the value is empty, the behavior that is used before 4.1.25 applies: The UDF object files must be located in a directory that is searched by your system's dynamic linker.

If the plugin directory is writable by the server, it may be possible for a user to write executable code to a file in the directory using SELECT ... INTO DUMPFILE. This can be prevented by making plugin_dir read only to the server.

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. This variable was added in MySQL 4.1.1.

prepared_stmt_count

The current number of prepared statements. (The maximum number of statements is given by the max_prepared_stmt_count system variable.) This variable was added in MySQL 4.1.19. In MySQL 4.1.23, it was converted to the global Prepared_stmt_count status variable.

protocol_version

The version of the client/server protocol used by the MySQL server. This variable was added in MySQL 3.23.18.

pseudo_thread_id

This variable is for internal server use.

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 parameter. This variable was added in MySQL 4.0.16.

query_cache_limit

Do not cache results that are larger than this number of bytes. The default value is 1MB. This variable was added in MySQL 4.0.1.

query_cache_min_res_unit

The minimum size for blocks allocated by the query cache. The default value is 4KB. Tuning information for this variable is given in Section 7.5.3.3, “Query Cache Configuration”. This variable is present from MySQL 4.1.

query_cache_size

The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. The permissible values are multiples of 1024; other values are rounded down to the nearest multiple. Note that query_cache_size bytes of memory are allocated even if query_cache_type is set to 0. This variable was added in MySQL 4.0.1.

The query cache needs a minimum size of about 40KB to allocate its structures. (The exact size depends on system architecture.) If you set the value of query_cache_size too small, a warning will occur, as described in Section 7.5.3.3, “Query Cache Configuration”.

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.

This variable defaults to ON.

Any unique prefix of a valid value may be used to set the value of this variable.

This variable was added in MySQL 4.0.3.

query_cache_wlock_invalidate

Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing 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. This variable was added in MySQL 4.0.19.

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.

This variable was added in MySQL 4.0.16.

rand_seed1

The rand_seed1 and rand_seed2 variables exist as session variables only, and can be set but not read. They are not shown in the output of SHOW VARIABLES. These two variables were added in MySQL 4.0.5.

The purpose of these variables is to support replication of the RAND() function. For statements that invoke RAND(), the master passes two values to the slave, where they are used to seed the random number generator. The slave uses these values to set the session variables rand_seed1 and rand_seed2 so that RAND() on the slave generates the same value as on the master.

rand_seed2

See the description for rand_seed1.

range_alloc_block_size

The size of blocks that are allocated when doing range optimization. This variable was added in MySQL 4.0.16.

read_buffer_size

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

read_buffer_size and read_rnd_buffer_size are not specific to any storage engine and apply in a general manner for optimization. See Section 7.8.4, “How MySQL Uses Memory”, for example.

Before MySQL 4.0.3, this variable was named record_buffer.

read_only

This variable is off by default. When it is enabled, the server permits no updates except from users that have the SUPER privilege or (on a slave server) from updates performed by slave threads. On a slave server, this can be useful to ensure that the slave accepts updates only from its master server and not from clients.

read_only exists only as a GLOBAL variable, so changes to its value require the SUPER privilege. Changes to read_only on a master server are not replicated to slave servers. The value can be set on a slave server independent of the setting on the master.

This variable was added in MySQL 4.0.14.

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. See Section 7.3.1.7, “ORDER BY Optimization”. 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.

read_buffer_size and read_rnd_buffer_size are not specific to any storage engine and apply in a general manner for optimization. See Section 7.8.4, “How MySQL Uses Memory”, for example.

Before MySQL 4.0.3, this variable was named record_rnd_buffer.

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).

relay_log_space_limit

The maximum amount of space to use for all relay logs.

safe_show_database

Do not show databases for which the user has no database or table privileges. This can improve security if you are concerned about people being able to see what databases other users have. See also skip_show_database.

This variable was removed in MySQL 4.0.5. Beginning with this version, you should instead use the SHOW DATABASES privilege to control access by MySQL accounts to databases.

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 in the old format (and hence insecure communication over the network). This variable was added in MySQL 4.1.1.

Server startup fails with an error if this option is enabled and the privilege tables are in pre-4.1 format.

server_id

The server ID, used in replication to give each master and slave a unique identity. This variable is set by the --server-id option. For each server participating in replication, you should pick a positive integer in the range from 1 to 2³² – 1 to act as that server's ID.

shared_memory

(Windows only.) Whether the server permits shared-memory connections. This variable was added in MySQL 4.1.1.

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. This variable was added in MySQL 4.1.0.

skip_external_locking

This is OFF if mysqld uses external locking, ON if external locking is disabled. Before MySQL 4.0.3, this variable was named skip_locking.

skip_networking

This is ON if the server permits 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. This variable was added in MySQL 3.22.23.

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 are concerned about people being able to see what databases other users have. See also safe_show_database. This variable was added in MySQL 3.23.4. As of MySQL 4.0.2, its effect also depends on the SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is permitted only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is permitted 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 a privilege for the database.

slow_launch_time

If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable. This variable was added in MySQL 3.23.15.

socket

On Unix 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 session that needs to do a sort allocates a buffer of this size. sort_buffer_size is not specific to any storage engine and applies in a general manner for optimization. See Section 7.3.1.7, “ORDER BY Optimization”, for example.

If you see many Sort_merge_passes per second in SHOW GLOBAL STATUS output, you can consider increasing the sort_buffer_size value to speed up ORDER BY or GROUP BY operations that cannot be improved with query optimization or improved indexing. The entire buffer is allocated even if it is not all needed, so setting it larger than required globally will slow down most queries that sort. It is best to increase it as a session setting, and only for the sessions that need a larger size. On Linux, there are thresholds of 256KB and 2MB where larger values may significantly slow down memory allocation, so you should consider staying below one of those values. Experiment to find the best value for your workload. See Section B.5.4.4, “Where MySQL Stores Temporary Files”.

sql_auto_is_null

If this variable is set to 1 (the default), then after a statement that successfully inserts an automatically generated AUTO_INCREMENT value, you can find that value by issuing a statement of the following form:

SELECT * FROM tbl_name WHERE auto_col IS NULL

If the statement returns a row, the value returned is the same as if you invoked the LAST_INSERT_ID() function. For details, including the return value after a multiple-row insert, see Section 11.13, “Information Functions”. If no AUTO_INCREMENT value was successfully inserted, the SELECT statement returns no row.

The behavior of retrieving an AUTO_INCREMENT value by using an IS NULL comparison is used by some ODBC programs, such as Access. See Section 17.1.7.1.1, “Obtaining Auto-Increment Values”. This behavior can be disabled by setting sql_auto_is_null to 0.

sql_big_selects

If set to 0, MySQL aborts SELECT statements that are likely to take a very long time to execute (that is, statements for which the optimizer estimates that the number of examined rows exceeds the value of max_join_size). This is useful when an inadvisable WHERE statement has been issued. The default value for a new connection is 1, which permits all SELECT statements.

If you set the max_join_size system variable to a value other than DEFAULT, sql_big_selects is set to 0.

sql_buffer_result

If set to 1, sql_buffer_result forces results from SELECT statements to be put into temporary tables. This helps MySQL free the table locks early and can be beneficial in cases where it takes a long time to send results to the client. The default value is 0. This variable was added in MySQL 3.23.13.

sql_log_bin

If set to 0, no logging is done to the binary log for the client. The client must have the SUPER privilege to set this option. The default value is 1. This variable was added in MySQL 3.23.16.

sql_log_off

If set to 1, no logging is done to the general query log for this client. The client must have the SUPER privilege to set this option. The default value is 0.

sql_log_update

If set to 0, no logging is done to the update log for the client. The client must have the SUPER privilege to set this option. The default value is 1. This variable was added in MySQL 3.22.5.

sql_mode

The current server SQL mode. This variable was added in MySQL 3.23.41. It can be set dynamically as of MySQL 4.1.1. See Section 5.1.6, “Server SQL Modes”.

sql_notes

If set to 1 (the default), warnings of Note level are recorded. If set to 0, Note warnings are suppressed. mysqldump includes output to set this variable to 0 so that reloading the dump file does not produce warnings for events that do not affect the integrity of the reload operation. sql_notes was added in MySQL 4.1.11.

sql_quote_show_create

If set to 1 (the default), the server quotes identifiers for SHOW CREATE TABLE and SHOW CREATE DATABASE statements. If set to 0, quoting is disabled. This option is enabled by default so that replication works for identifiers that require quoting. See Section 12.4.5.7, “SHOW CREATE TABLE Syntax”, and Section 12.4.5.6, “SHOW CREATE DATABASE Syntax”. This variable was added in MySQL 3.23.26.

sql_safe_updates

If set to 1, MySQL aborts UPDATE or DELETE statements that do not use a key in the WHERE clause or a LIMIT clause. This makes it possible to catch UPDATE or DELETE statements where keys are not used properly and that would probably change or delete a large number of rows. The default value is 0. This variable was added in MySQL 3.22.32.

sql_select_limit

The maximum number of rows to return from SELECT statements. The default value for a new connection is the maximum number of rows that the server permits per table, which depends on the server configuration and may be affected if the server build was configured with --with-big-tables. Typical default values are (2³²)–1 or (2⁶⁴)–1. If you have changed the limit, the default value can be restored by assigning a value of DEFAULT.

If a SELECT has a LIMIT clause, the LIMIT takes precedence over the value of sql_select_limit.

sql_select_limit does not apply to SELECT statements executed within stored routines. It also does not apply to SELECT statements that do not produce a result set to be returned to the client. These include SELECT statements in subqueries, CREATE TABLE ... SELECT, and INSERT INTO ... SELECT.

sql_warnings

This variable controls whether single-row INSERT statements produce an information string if warnings occur. The default is 0. Set the value to 1 to produce an information string. This variable was added in MySQL 3.22.11.

storage_engine

This variable is a synonym for table_type. It was added in MySQL 4.1.2.

sync_frm

If this variable is set to 1, when any nontemporary 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. This was added as a command-line option in MySQL 4.0.18. It is also a settable global variable as of MySQL 4.1.3.

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 time zone for each client that connects. See Section 9.7, “MySQL Server Time Zone Support”.

system_time_zone was added in MySQL 4.1.3.

table_cache

The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See Section 5.1.5, “Server Status Variables”. If the value of Opened_tables is large and you do not do FLUSH TABLES often (which just forces all tables to be closed and reopened), then you should increase the value of the table_cache variable. For more information about the table cache, see Section 7.7.2, “How MySQL Opens and Closes Tables”.

table_type

The default table type (storage engine). To set the table type at server startup, use the --default-table-type option. This variable was added in MySQL 3.23.0. See Section 5.1.2, “Server Command Options”.

thread_cache_size

How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there 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 does not provide a notable performance improvement if you have a good thread implementation. However, if your server sees hundreds of connections per second you should normally set thread_cache_size high enough so that most new connections use cached threads. By examining the difference between the Connections and Threads_created status variables, you can see how efficient the thread cache is. For details, see Section 5.1.5, “Server Status Variables”. This variable was added in MySQL 3.23.16.

thread_concurrency

This variable is specific to Solaris systems, for which mysqld invokes the thr_setconcurrency() with the variable 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. This variable was added in MySQL 3.23.7.

thread_stack

The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. The default is large enough for normal operation. See Section 7.1.3, “The MySQL Benchmark Suite”. The default is 64KB before MySQL 4.0.10 and 192KB thereafter. If the thread stack size is too small, it limits the complexity of the SQL statements that the server can handle, the recursion depth of stored procedures, and other memory-consuming actions.

time_format

This variable is unused.

time_zone

The current time zone. This variable is used to initialize the time 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 9.7, “MySQL Server Time Zone Support”. This variable was added in MySQL 4.1.3.

timestamp = {timestamp_value | DEFAULT}

Set the time for this client. This is used to get the original timestamp if you use the binary log to restore rows. timestamp_value should be a Unix epoch timestamp, not a MySQL timestamp.

timezone

The time zone for the server. This is set from the TZ environment variable when mysqld is started. The time zone also can be set by giving a --timezone argument to mysqld_safe. This variable was added in MySQL 3.23.15. As of MySQL 4.1.3, it is obsolete and has been replaced by the system_time_zone variable. See Section B.5.4.6, “Time Zone Problems”.

tmp_table_size

The maximum size of internal in-memory temporary tables. (The actual limit is determined as the minimum of tmp_table_size and max_heap_table_size.) If an in-memory temporary table exceeds the limit, MySQL automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size (and max_heap_table_size if necessary) if you do many advanced GROUP BY queries and you have lots of memory. This variable does not apply to user-created MEMORY tables.

You can compare the number of internal on-disk temporary tables created to the total number of internal temporary tables created by comparing the values of the Created_tmp_disk_tables and Created_tmp_tables variables.

See also Section 7.7.4, “How MySQL Uses Internal Temporary Tables”.

tmpdir

The directory used for temporary files and temporary tables. Starting from MySQL 4.1, this variable can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (“:”) on Unix and semicolon characters (“;”) on Windows, NetWare, and OS/2.

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 file system 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 will not use the general tmpdir value and you can set tmpdir to a nonpermanent location.

This variable was added in MySQL 3.22.4.

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. This variable was added in MySQL 4.0.16.

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. This variable was added in MySQL 4.0.16.

tx_isolation

The default transaction isolation level. This variable was added in MySQL 4.0.3.

This variable is set by the SET TRANSACTION ISOLATION LEVEL statement. See Section 12.3.6, “SET TRANSACTION Syntax”. If you set tx_isolation directly to an isolation level name that contains a space, the name should be enclosed within quotation marks, with the space replaced by a dash. For example:

SET tx_isolation = 'READ-COMMITTED';

Any unique prefix of a valid value may be used to set the value of this variable.

unique_checks

If set to 1 (the default), uniqueness checks for secondary indexes in InnoDB tables are performed. If set to 0, storage engines are permitted to assume that duplicate keys are not present in input data. If you know for certain that your data does not contain uniqueness violations, you can set this to 0 to speed up large table imports to InnoDB. This variable was added in MySQL 3.23.52.

Note that setting this variable to 0 does not require storage engines to ignore duplicate keys. An engine is still permitted to check for them and issue duplicate-key errors if it detects them.

version

The version number for the server.

version_bdb

The BDB storage engine version. This variable was added in MySQL 3.23.31 with the name bdb_version and renamed to version_bdb in MySQL 4.1.1.

version_comment

The configure script has a --with-comment option that permits a comment to be specified when building MySQL. This variable contains the value of that comment. This variable was added in MySQL 4.0.17.

version_compile_machine

The type of machine or architecture on which MySQL was built. This variable was added in MySQL 4.1.1.

version_compile_os

The type of operating system on which MySQL was built. This variable was added in MySQL 4.0.19.

wait_timeout

The number of seconds the server waits for activity on a noninteractive connection before closing it. This timeout applies only to TCP/IP and Unix socket file connections, not to connections made using 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.

warning_count

The number of errors, warnings, and notes that resulted from the last statement that generated messages. This variable is read only. See Section 12.4.5.26, “SHOW WARNINGS Syntax”.

This variable was added in MySQL 4.1.0.