126.96.36.199 How the Query Cache Operates
The query cache is deprecated as of MySQL 5.7.20, and is removed in MySQL 8.0.
This section describes how the query cache works when it is operational. Section 188.8.131.52, “Query Cache Configuration”, describes how to control whether it is operational.
Incoming queries are compared to those in the query cache before parsing, so the following two queries are regarded as different by the query cache:
SELECT * FROM tbl_name Select * from tbl_name
Queries must be exactly the same (byte for byte) to be seen as identical. In addition, query strings that are identical may be treated as different for other reasons. Queries that use different databases, different protocol versions, or different default character sets are considered different queries and are cached separately.
The cache is not used for queries of the following types:
Queries that are a subquery of an outer query
Queries executed within the body of a stored function, trigger, or event
Before a query result is fetched from the query cache, MySQL checks whether the user has
SELECT privilege for all databases and tables involved. If this is not the case, the cached result is not used.
If a query result is returned from query cache, the server increments the
Qcache_hits status variable, not
Com_select. See Section 184.108.40.206, “Query Cache Status and Maintenance”.
If a table changes, all cached queries that use the table become invalid and are removed from the cache. This includes queries that use
MERGE tables that map to the changed table. A table can be changed by many types of statements, such as
DROP TABLE, or
The query cache also works within transactions when using
The result from a
SELECT query on a view is cached.
The query cache works for
SELECT SQL_CALC_FOUND_ROWS ... queries and stores a value that is returned by a following
SELECT FOUND_ROWS() query.
FOUND_ROWS() returns the correct value even if the preceding query was fetched from the cache because the number of found rows is also stored in the cache. The
SELECT FOUND_ROWS() query itself cannot be cached.
Prepared statements that are issued using the binary protocol using
mysql_stmt_execute() (see Section 27.7.7, “C API Prepared Statements”), are subject to limitations on caching. Comparison with statements in the query cache is based on the text of the statement after expansion of
? parameter markers. The statement is compared only with other cached statements that were executed using the binary protocol. That is, for query cache purposes, prepared statements issued using the binary protocol are distinct from prepared statements issued using the text protocol (see Section 13.5, “Prepared Statements”).
A query cannot be cached if it uses any of the following functions:
ENCRYPT()with one parameter
UNIX_TIMESTAMP()with no parameters
A query also is not cached under these conditions:
It refers to user-defined functions (UDFs) or stored functions.
It refers to user variables or local stored program variables.
It refers to tables in the
It refers to any partitioned tables.
It is of any of the following forms:
SELECT ... LOCK IN SHARE MODE SELECT ... FOR UPDATE SELECT ... INTO OUTFILE ... SELECT ... INTO DUMPFILE ... SELECT * FROM ... WHERE autoincrement_col IS NULL
The last form is not cached because it is used as the ODBC workaround for obtaining the last insert ID value. See the Connector/ODBC section of Chapter 27, Connectors and APIs.
Statements within transactions that use
SERIALIZABLEisolation level also cannot be cached because they use
LOCK IN SHARE MODElocking.
It does not use any tables.
It generates warnings.
The user has a column-level privilege for any of the involved tables.