This section describes how the query cache works when it is operational. Section 126.96.36.199, “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_nameSelect * from
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.
Because comparison of a query against those in the cache occurs before parsing, the cache is not used for queries of the following types:
Queries that are a subquery of an outer query
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
status variable, not
Section 188.8.131.52, “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
In MySQL 4.0, the query cache is disabled within transactions
(it does not return results). Beginning with MySQL 4.1.1, the
query cache also works within transactions when using
A query that begins with a leading comment may be cached, but cannot be fetched from the cache.
The query cache works for
... queries and stores a value that is returned by a
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
SELECT FOUND_ROWS() query itself
cannot be cached.
A query cannot be cached if it contains any of the functions shown in the following table.
A query also is not cached under these conditions:
It refers to user-defined functions (UDFs).
It refers to user variables.
It refers to tables in the
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 MyODBC section of Chapter 17, Connectors and APIs.
Statements within transactions that use
level also cannot be cached because they use
IN SHARE MODE locking.
It was issued as a prepared statement, even if no placeholders were employed. For example, the query used here is not cached:
char *my_sql_stmt = "SELECT a, b FROM table_c"; /* ... */ mysql_stmt_prepare(stmt, my_sql_stmt, strlen(my_sql_stmt));
It does not use any tables.
It generates warnings.
The user has a column-level privilege for any of the involved tables.