MySQL Manual
6 MySQL Language Reference
- 6.9 MySQL Query Cache
- 6.9.1 How The Query Cache Operates
- 6.9.2 Query Cache Configuration
- 6.9.3 Query Cache Options in
SELECT - 6.9.4 Query Cache Status and Maintenance
6.9.1 How The Query Cache Operates
Queries are compared before parsing, thus
SELECT * FROM tbl_name
and
Select * from tbl_name
are regarded as different queries for query cache, so queries need to be exactly the same (byte for byte) to be seen as identical. In addition, a query may be seen as different if for instance one client is using a new communication protocol format or another character set than another client.
Queries that uses different databases, uses different protocol versions or the uses different default character sets are considered different queries and cached separately.
The cache does work for SELECT SQL_CALC_FOUND_ROWS ... and
SELECT FOUND_ROWS() ... type queries because the number of
found rows is also stored in the cache.
If query result was returned from query cache then status variable
Com_select will not be increased, but Qcache_hits will be.
See section 6.9.4 Query Cache Status and Maintenance.
If a table changes (INSERT, UPDATE, DELETE,
TRUNCATE, ALTER or DROP TABLE|DATABASE),
then all cached queries that used this table (possibly through a
MRG_MyISAM table!) become invalid and are removed from the cache.
Transactional InnoDB tables that have been changed will be invalidated
when a COMMIT is performed.
In MySQL 4.0, the query cache is disabled inside of transactions (it does
not return results). Beginning with MySQL 4.1.1, the query cache will also
work inside of transactions when using InnoDB tables (it will use the
table version number to detect if the data is still current or not).
Before MySQL 5.0, a query that begins with a leading comment might be cached, but could not be fetched from the cache. This problem is fixed in MySQL 5.0.
A query cannot be cached if it contains one of the functions:
| Function | Function | Function |
User-Defined Functions
| CONNECTION_ID
| FOUND_ROWS
|
GET_LOCK
| RELEASE_LOCK
| LOAD_FILE
|
MASTER_POS_WAIT
| NOW
| SYSDATE
|
CURRENT_TIMESTAMP
| CURDATE
| CURRENT_DATE
|
CURTIME
| CURRENT_TIME
| DATABASE
|
ENCRYPT (with one parameter)
| LAST_INSERT_ID
| RAND
|
UNIX_TIMESTAMP (without parameters)
| USER
| BENCHMARK
|
Nor can a query be cached if it contains user variables,
references the mysql system database,
is of the form SELECT ... IN SHARE MODE,
SELECT ... INTO OUTFILE ...,
SELECT ... INTO DUMPFILE ... or
of the form SELECT * FROM AUTOINCREMENT_FIELD IS NULL
(to retrieve last insert id - ODBC work around).
However, FOUND_ROWS() will return the correct value,
even if the preceding query was fetched from the cache.
In case a query does not use any tables, or uses temporary tables, or if the user has a column privilege for any of the involved tables, that query will not be cached.
Before a query is fetched from the query cache, MySQL will check that the user has SELECT privilege to all the involved databases and tables. If this is not the case, the cached result will not be used.
