MySQL 5.6 Reference Manual Including MySQL NDB Cluster 7.3-7.4 Reference Guide
The have_query_cache
server
system variable indicates whether the query cache is
available:
mysql> SHOW VARIABLES LIKE 'have_query_cache';
+------------------+-------+
| Variable_name | Value |
+------------------+-------+
| have_query_cache | YES |
+------------------+-------+
When using a standard MySQL binary, this value is always
YES
, even if query caching is disabled.
Several other system variables control query cache operation.
These can be set in an option file or on the command line when
starting mysqld. The query cache system
variables all have names that begin with
query_cache_
. They are described briefly in
Section 5.1.7, “Server System Variables”, with additional
configuration information given here.
To set the size of the query cache, set the
query_cache_size
system
variable. Setting it to 0 disables the query cache, as does
setting query_cache_type=0
.
By default, the query cache is disabled. This is achieved
using a default size of 1M, with a default for
query_cache_type
of 0.
To reduce overhead significantly, start the server with
query_cache_type=0
if you do
not intend to use the query cache.
When using the Windows Configuration Wizard to install or
configure MySQL, the default value for
query_cache_size
is
configured automatically for you based on the different
configuration types available. When using the Windows
Configuration Wizard, the query cache may be enabled (that
is, set to a nonzero value) due to the selected
configuration. The query cache is also controlled by the
setting of the
query_cache_type
variable.
Check the values of these variables as set in your
my.ini
file after configuration has
taken place.
When you set query_cache_size
to a nonzero value, keep in mind that 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 too small, you'll get a warning, as in this example:
mysql>SET GLOBAL query_cache_size = 40000;
Query OK, 0 rows affected, 1 warning (0.00 sec) mysql>SHOW WARNINGS\G
*************************** 1. row *************************** Level: Warning Code: 1282 Message: Query cache failed to set size 39936; new query cache size is 0 mysql>SET GLOBAL query_cache_size = 41984;
Query OK, 0 rows affected (0.00 sec) mysql>SHOW VARIABLES LIKE 'query_cache_size';
+------------------+-------+ | Variable_name | Value | +------------------+-------+ | query_cache_size | 41984 | +------------------+-------+
For the query cache to actually be able to hold any query results, its size must be set larger:
mysql>SET GLOBAL query_cache_size = 1000000;
Query OK, 0 rows affected (0.04 sec) mysql>SHOW VARIABLES LIKE 'query_cache_size';
+------------------+--------+ | Variable_name | Value | +------------------+--------+ | query_cache_size | 999424 | +------------------+--------+ 1 row in set (0.00 sec)
The query_cache_size
value is
aligned to the nearest 1024 byte block. The value reported may
therefore be different from the value that you assign.
If the query cache size is greater than 0, the
query_cache_type
variable
influences how it works. This variable can be set to the
following values:
A value of 0
or OFF
prevents caching or retrieval of cached results.
A value of 1
or ON
enables caching except of those statements that begin with
SELECT SQL_NO_CACHE
.
A value of 2
or
DEMAND
causes caching of only those
statements that begin with SELECT
SQL_CACHE
.
If query_cache_size
is 0, you
should also set
query_cache_type
variable to
0. In this case, the server does not acquire the query cache
mutex at all, which means that the query cache cannot be
enabled at runtime and there is reduced overhead in query
execution.
Setting the GLOBAL
query_cache_type
value
determines query cache behavior for all clients that connect
after the change is made. Individual clients can control cache
behavior for their own connection by setting the
SESSION
query_cache_type
value. For
example, a client can disable use of the query cache for its
own queries like this:
mysql> SET SESSION query_cache_type = OFF;
If you set query_cache_type
at server startup (rather than at runtime with a
SET
statement), only the numeric values are permitted.
To control the maximum size of individual query results that
can be cached, set the
query_cache_limit
system
variable. The default value is 1MB.
Be careful not to set the size of the cache too large. Due to the need for threads to lock the cache during updates, you may see lock contention issues with a very large cache.
You can set the maximum size that can be specified for the
query cache at runtime with the
SET
statement by using the
--maximum-query_cache_size=
option on the command line or in the configuration file.
32M
When a query is to be cached, its result (the data sent to the
client) is stored in the query cache during result retrieval.
Therefore the data usually is not handled in one big chunk.
The query cache allocates blocks for storing this data on
demand, so when one block is filled, a new block is allocated.
Because memory allocation operation is costly (timewise), the
query cache allocates blocks with a minimum size given by the
query_cache_min_res_unit
system variable. When a query is executed, the last result
block is trimmed to the actual data size so that unused memory
is freed. Depending on the types of queries your server
executes, you might find it helpful to tune the value of
query_cache_min_res_unit
:
The default value of
query_cache_min_res_unit
is 4KB. This should be adequate for most cases.
If you have a lot of queries with small results, the
default block size may lead to memory fragmentation, as
indicated by a large number of free blocks. Fragmentation
can force the query cache to prune (delete) queries from
the cache due to lack of memory. In this case, decrease
the value of
query_cache_min_res_unit
.
The number of free blocks and queries removed due to
pruning are given by the values of the
Qcache_free_blocks
and
Qcache_lowmem_prunes
status variables.
If most of your queries have large results (check the
Qcache_total_blocks
and
Qcache_queries_in_cache
status variables), you can increase performance by
increasing
query_cache_min_res_unit
.
However, be careful to not make it too large (see the
previous item).