12.6.4. Automatic Prepared Statement Repreparation

Metadata changes to tables or views referred to by prepared statements are detected and cause automatic repreparation of the statement when it is next executed. This applies to prepared statements processed at the SQL level (using the PREPARE statement) and those processed using the binary client/server protocol (using the mysql_stmt_prepare() C API function).

Metadata changes occur for DDL statements such as those that create, drop, alter, rename, or truncate tables, or that analyze, optimize, or repair tables. Repreparation 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.

Repreparation is automatic, but to the extent that it occurs, performance of prepared statements is diminished.

When a statement is reprepared, the default database and SQL mode that were in effect for the original preparation are used.

Table content changes (for example, with INSERT or UPDATE) do not cause repreparation, nor do SELECT statements.

An incompatibility with previous versions of MySQL is that a prepared statement may return a different set of columns or different column types from one execution to the next. For example, if the prepared statement is SELECT * FROM t1, altering t1 to contain a different number of columns causes the next execution to return a number of columns different from the previous execution.

Older versions of the client library cannot handle this change in behavior. For applications that use prepared statements with a server that performs automatic repreparation, an upgrade to the new client library is strongly recommended.

The Com_stmt_reprepare status variable tracks the number of repreparations.

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