MySQL 8.4 Release Notes
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 (using the
PREPAREstatement) and those processed using the binary client/server protocol (using themysql_stmt_prepare()C API function). Themax_prepared_stmt_countsystem 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
stored_program_cachesystem variable indicates the approximate number of stored programs 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must take care that the structure does not go out of date.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
example, with INSERT or
UPDATE) do not change metadata,
nor do SELECT statements.
Here is an illustration of the problem. Suppose that a client prepares this statement:
PREPARE s1 FROM 'SELECT * FROM t1';
The 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 ALTER
TABLE, the prepared statement goes out of date. If the
server does not detect this change the next time the client
executes s1, the prepared statement returns
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 FLUSH
TABLES.
Similarly, if changes occur to objects used by a stored program, the server reparses affected statements within the program.
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
IF,
CASE, and
RETURN.
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 aSELECT *that does not access the table or view.When a statement is affected, the server reparses it only partially if possible. Consider this
CASEstatement:CASE case_expr WHEN when_expr1 ... WHEN when_expr2 ... WHEN when_expr3 ... ... END CASEIf a metadata change affects only
WHEN, that expression is reparsed.when_expr3case_exprand the otherWHENexpressions are not reparsed.
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
Com_stmt_reprepare
status variable tracks the number of repreparations.