To ensure transaction serializability, the server must not allow one session to perform a data definition language (DDL) statement on a table that is used in an uncompleted transaction in another session.

As of MySQL 5.4.4, the server achieves this by acquiring metadata locks on tables used within a transaction and deferring release of those locks until the transaction ends. A metadata lock on a table prevents changes to the table's structure. This locking approach has the implication that a table that is being used by a transaction within one session cannot be used in DDL statements by other sessions until the transaction ends. For example, if a table t1 is in use by a transaction, another session that attempts to execute DROP TABLE t1 will block until the transaction ends.

Metadata locks acquired during a PREPARE statement are released once the statement has been prepared, even if preparation occurs within a multiple-statement transaction.

Before MySQL 5.4.4, when a transaction acquired a metadata lock for a table used within a statement, it released the lock at the end of the statement. This approach had the disadvantage that if a DDL statement occurred for a table that was being used by another session in an active transaction, statements could be written to the binary log in the wrong order.