For certain statements that a client might execute multiple times during a session, the server converts the statement to an internal structure and caches that structure to be used during execution. Caching enables the server to perform more efficiently because it avoids the overhead of reconverting the statement should it be needed again during the session. Conversion and caching occurs for these statements:
Prepared statements, both those processed at the SQL level
and those processed using the binary client/server protocol
mysql_stmt_prepare() C API
system variable controls the total number of statements the
server caches. (The sum of the number of prepared statements
across all sessions.)
Stored programs (stored procedures and functions, triggers,
and events). In this case, the server converts and caches
the entire program body. The
variable indicates the approximate number of stored programs
the the server caches per session.
The server maintains caches for prepared statements and stored programs on a per-session basis. Statements cached for one session are not accessible to other sessions. When a session ends, the server discards any statements cached for it.
When the server uses a cached internal statement structure, it
must take care that the structure does not go out of date.
Metadata changes can occur for an object used by the statement,
causing a mismatch between the current object definition and the
definition as represented in the internal statement structure.
Metadata changes occur for DDL statements such as those that
create, drop, alter, rename, or truncate tables, or that
analyze, optimize, or repair tables. Table content changes (for
UPDATE) do not change metadata,
Here is an illustration of the problem. Suppose that a client prepares this statement:
PREPARE s1 FROM 'SELECT * FROM t1';
SELECT * expands in the internal
structure to the list of columns in the table. If the set of
columns in the table is modified with
TABLE, the prepared statement goes out of date. If the
server does not detect this change the next time the client
s1, the prepared statement will
return incorrect results.
To avoid problems caused by metadata changes to tables or views
referred to by the prepared statement, the server detects these
changes and automatically reprepares the statement when it is
next executed. That is, the server reparses the statement and
rebuilds the internal structure. Reparsing also occurs after
referenced tables or views are flushed from the table definition
cache, either implicitly to make room for new entries in the
cache, or explicitly due to
Similarly, if changes occur to objects used by a stored program, the server reparses affected statements within the program. (Before MySQL 5.6.6, the server does not detect metadata changes affecting stored programs, so such changes can cause incorrect results or errors.)
The server also detects metadata changes for objects in
expressions. These might be used in statements specific to
stored programs, such as
DECLARE CURSOR or
flow-control statements such as
To avoid reparsing entire stored programs, the server reparses affected statements or expressions within a program only as needed. Examples:
Suppose that metadata for a table or view is changed.
Reparsing occurs for a
SELECT * within
the program that accesses the table or view, but not for a
SELECT * that does not access the table
When a statement is affected, the server reparses it only
partially if possible. Consider this
when_expr3... ... END CASE
If a metadata change affects only
expression is reparsed.
and the other
WHEN expressions are not
Reparsing uses the default database and SQL mode that were in effect for the original conversion to internal form.
The server attempts reparsing up to three times. An error occurs if all attempts fail.
Reparsing is automatic, but to the extent that it occurs, diminishes prepared statement and stored program performance.
For prepared statements, the
status variable tracks the number of repreparations.