:: DEVELOPER ZONE
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.
There are several ways to see the names and values of system variables:
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:
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:
mysqld --no-defaults --verbose --help
To see the current values used by a running server, use
the SHOW VARIABLES
statement.
This section provides a description of each system variable. Variables with no version indicated are present in all MySQL 5.0 releases. For historical information concerning their implementation, please see MySQL 3.23, 4.0, 4.1 Reference Manual.
For additional system variable information, see these sections:
Section 5.2.3, “Using System Variables”, discusses the syntax for setting and displaying system variable values.
Section 5.2.3.2, “Dynamic System Variables”, lists the variables that can be set at runtime.
Information on tuning sytem variables can be found in Section 7.5.2, “Tuning Server Parameters”.
Section 14.2.4, “InnoDB
Startup Options and System Variables”, lists
InnoDB
system variables.
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 affect
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 auto_increment_increment
variable
was added in MySQL 5.0.2. Its default value is 1. See
Section 6.13, “Auto-Increment in Multiple-Master Replication”.
auto_increment_offset
This variable was introduced in MySQL 5.0.2. Its default
value is 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_size
The size of the buffer that is allocated for caching
indexes and rows for BDB
tables. If you
don't use BDB
tables, you should start
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_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 5.12.3, “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 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. This variable is used to
interpret string literals that refer to filenames, such as
in the LOAD DATA INFILE
and
SELECT ... INTO OUTFILE
statements and
the LOAD_FILE()
function. Such
filenames are converted from
character_set_client
to
character_set_filesystem
before the
file opening attempt occurs. The default value is
binary
, which means that no conversion
occurs. For systems on which multi-byte filenames are
allowed, a different value may be more appropriate. For
example, if the system represents filenames using UTF-8,
set character_set_filesytem
to
'utf8'
. This variable was added in
MySQL 5.0.19.
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.)
This variable was added in MySQL 5.0.3
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
.
In MySQL 5.0.6, this variable was changed to take three integer values:
Value | Description |
0 | Off |
1 | (Default) Enables concurrent insert for MyISAM tables
that don't have holes |
2 | Enables 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, MySQL does a normal read lock
and inserts the new row into the hole. |
See also Section 7.3.3, “Concurrent Inserts”.
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 12.5, “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.
Option | Description |
OFF |
DELAY_KEY_WRITE is ignored. |
ON |
MySQL honors any DELAY_KEY_WRITE option specified in
CREATE TABLE statements. This
is the default value. |
ALL |
All new opened tables are treated as if they were created with the
DELAY_KEY_WRITE option enabled. |
If DELAY_KEY_WRITE
is enabled 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 5.2.1, “mysqld Command Options”, and
Section 14.1.1, “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 | +----------------+
This variable was added in MySQL 5.0.6.
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.
This variable was added in MySQL 5.0.3. Before that, the
default NDB
behavior is the same as for
a value of OFF
.
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 A.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 12.7.1, “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. This variable was added in
MySQL 5.0.3.
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 5.0, 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_openssl
YES
if mysqld
supports SSL connections, NO
if not.
have_query_cache
YES
if mysqld
supports the query cache, NO
if not.
have_raid
In MySQL 5.0, this variable appears only for
reasons of backward compatibility. It is always
NO
because RAID
tables are no longer supported.
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 14.2.4, “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.
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 7.2.16, “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 13.5.4, “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 7.4.6, “The MyISAM
Key Cache”.
key_cache_age_threshold
This value controls the demotion of buffers from the hot
sub-chain of a key cache to the warm sub-chain. Lower
values cause demotion to happen more quickly. The minimum
value is 100. The default value is 300. See
Section 7.4.6, “The MyISAM
Key Cache”.
key_cache_block_size
The size in bytes of blocks in the key cache. The default
value is 1024. See Section 7.4.6, “The MyISAM
Key Cache”.
key_cache_division_limit
The division point between the hot and warm sub-chains of
the key cache buffer chain. The value is the percentage of
the buffer chain to use for the warm sub-chain. Allowable
values range from 1 to 100. The default value is 100. See
Section 7.4.6, “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. This variable was added in MySQL 5.0.3.
license
The type of license the server has.
local_infile
Whether LOCAL
is supported for
LOAD DATA INFILE
statements. See
Section 5.7.4, “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 5.12.2, “The General Query Log”.
log_bin
Whether the binary log is enabled. See Section 5.12.3, “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
functions 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 17.4, “Binary Logging of Stored Routines and Triggers”.
This variable was added in MySQL 5.0.16.
log_bin_trust_routine_creators
This is the old name for
log_bin_trust_function_creators
. Before
MySQL 5.0.16, it also applies to stored procedures, not
just stored functions. As of 5.0.16, this variable is
deprecated. It is recognized for backward compatibility
but its use results in a warning.
This variable was added in MySQL 5.0.6.
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 variable to have any effect. See Section 6.8, “Replication Startup Options”.
log_slow_queries
Whether slow queries should be logged. “Slow”
is determined by the value of the
long_query_time
variable. See
Section 5.12.4, “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.
The default is 10. See Section 5.12.4, “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 9.2.2, “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 largest 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 7.4.8, “How MySQL Opens and Closes Tables”, for comments on file
descriptor limits. See also
Section A.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 7.2.12, “ORDER BY
Optimization”.
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. The default value is 16,382. The allowable range of values is from 0 to 1 milliion. 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. This variable was added in MySQL 5.0.21.
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 6.3, “Replication Implementation Details”.
max_seeks_for_key
Limit the assumed maximum number of seeks when looking up
rows based on a key. The MySQL optimizer 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 13.5.4.13, “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.”
Before MySQL 5.0.3, this variable has only global scope.
Beginning with MySQL 5.0.3, it also has 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 (4 before MySQL 5.0.6). This variable
was added in MySQL 4.1.2. See
Section A.2.11, “The table is full
”.
myisam_max_extra_sort_file_size
(DEPRECATED)
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. The value is given in
bytes.
Note: This variable was removed in MySQL 5.0.6.
myisam_max_sort_file_size
The maximum size of the temporary file that 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 Section 5.2.1, “mysqld 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 7.4.7, “MyISAM
Index Statistics Collection”.
This variable was added in MySQL 5.0.14. For older
versions, the statistics collection method is equivalent
to nulls_equal
.
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. This variable was added in MySQL 5.0.3.
named_pipe
(Windows only.) Indicates whether the server supports connections over named pipes.
ndb_autoincrement_prefetch_sz
Determines the probability of gaps in an autoincremented
column. Set to 1
to minimize this. Set
to a high value for optimization — makes inserts
faster, but decreases the likelihood that consecutive
autoincrement numbers will be used in a batch of inserts.
Default value: 32
. Mimimum value:
1
.
ndb_cache_check_time
The number of seconds to wait before checking the
NDB
query cache. Setting this to
0
(the default and minimum value) means
that the NDB
query cache will be
checked for validation upon each query. A large value
means the NDB
query cache will seldom
be checked and invalidated because of updates on another
mysqld. It is generally not desirable
to set this to a value greater than 2
.
ndb_force_send
Forces sending of buffers to NDB
immediately, without waiting for other threads. Defaults
to ON
.
ndb_index_stat_cache_entries
Sets the granularity of the statistics by determining the
number of starting and ending keys to store in the
statistics memory cache. Zero means no caching takes
place; in this case, the data nodes are always queries
directly. Default value: 32
.
ndb_index_stat_enable
Use NDB
index statistics in query
optimization. Defaults to ON
.
ndb_index_stat_update_freq
How often to query data nodes instead of the statistics
cache. For example, a value of 20
(the
default) means to direct every
20th query to the data nodes.
ndb_report_thresh_binlog_epoch_slip
This is a threshold on the number of epochs to be behind
before reporting binlog status. For example, a value of
3
(the default) means that if the
difference between which epoch has been received from the
storage nodes and which epoch has been applied to the
binlog is 3 or more, a status message will be sent to the
cluster log.
ndb_report_thresh_binlog_mem_usage
This is a threshold on the percentage of free memory
remaining before reporting binlog status. For example, a
value of 10
(the default) means that if
the amount of available memory for receiving binlog data
from the data nodes falls below 10%, a status message will
be sent to the cluster log.
ndb_use_exact_count
Forces NDB
to use an count of records
during SELECT COUNT(*)
query planning
to speed up this type of query. The default value is
ON
. For faster queries overall, disable
this feature by setting the value of
ndb_use_exact_count
to
OFF
.
ndb_use_transactions
You can disable NDB
transaction support
by setting this variable's values to
OFF
(not recommended). The default is
ON
.
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 5.0, its value is always
OFF
.
old_passwords
Whether the server should use pre-4.1-style passwords for
MySQL user accounts. See Section A.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 13.5.3, “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. This variable was added in MySQL 5.0.1.
optimizer_search_depth
The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value. If set to the maximum number of tables used in a query plus 2, the optimizer switches to the algorithm used in MySQL 5.0.0 (and previous versions) for performing searches. This variable was added in MySQL 5.0.1.
pid_file
The pathname of the process ID (PID) file. This variable
can be set with the --pid-file
option.
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.
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 5.0.21.
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 5.14.3, “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 query_cache_size
bytes of
memory are allocated even if
query_cache_type
is set to 0. See
Section 5.14.3, “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:
Option | Description |
0 or OFF
|
Don't cache results in or retrieve results from the query cache. Note
that this does not deallocate the query cache
buffer. To do that, you should set
query_cache_size to 0. |
1 or ON
|
Cache all query results except for those that begin with SELECT
SQL_NO_CACHE . |
2 or DEMAND
|
Cache results only for queries that begin with SELECT
SQL_CACHE . |
This variable 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. As of MySQL
5.0.16, 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 A.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.
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 priot to MySQL 4.0.3
is 0. You must explicitly set the value greater than 0 to
enable the “retry” behavior, which is
probably a good idea. In MySQL 5.0.3 or newer, the default
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 A.4.4, “Where MySQL Stores Temporary Files”.
sql_mode
The current server SQL mode, which can be set dynamically. See Section 5.2.5, “The Server SQL Mode”.
sql_slave_skip_counter
The number of events from the master that a slave server
should skip. See
Section 13.6.2.6, “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
Section 5.2.1, “mysqld 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 time zone for each client that connects.
See Section 5.11.8, “MySQL Server Time Zone Support”.
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 5.2.4, “Server Status Variables”. If the value of
Opened_tables
is large and you don't 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 7.4.8, “How MySQL Opens and Closes Tables”.
table_lock_wait_timeout
Specifies a wait timeout for table-level locks, in
seconds. The default timeout is 50 seconds. The timeout is
active only if the connection has open cursors. This
variable can also be set globally at runtime (you need the
SUPER
privilege to do this). It's
available as of MySQL 5.0.10.
table_type
This variable is a synonym for
storage_engine
. In MySQL
5.0, 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 Section 5.2.4, “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 7.1.4, “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 5.11.8, “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 13.4.6, “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.
This variable was added in MySQL 5.0.2.
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
.
User Comments
if you set
[mysqld]
ft_min_word_len=3
you should also set
[myisamchk]
ft_min_word_len=3
if you use myisamchk
If your queries are mysteriously failing after running for sometime, even though the SQL syntax is correct, check how long the query was running compared to the interactive_timeout and wait_timeout variables. Prior to mysql 4.1.16 on BSD systems (including Mac OS X), the timeout period was not enforced. If you've recently upgraded to >4.1.16 and are getting these failures... this is most likely the problem.
Note that you can increase these variables, but show variables will not show the new setting until you login again because these variables are set at thread creation - so you'll need to start a new thread to see the changes.
http://bugs.mysql.com/bug.php?id=8731 for the curious.
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