Oracle Internals: An Introduction

DML Locks

Library cache pins and the associated row cache enqueue locks protect object definitions for the duration of parse and execute calls. However, for transactions that consist of a series of statements, equivalent locks need to be held for the duration of the transaction.

More than that, the lock mode may need to be raised partway through the transaction. For example, a table may first be queried, and then updated. This, of course, is why lock conversions are necessary. If the existing lock were to be released, even momentarily, it would be possible for the referenced object to be dropped or changed, and the transaction would then be unable to either proceed or roll back.

The possibility of rollback, particularly rollback to a savepoint, adds another dimension of complexity to dictionary locking. Namely, if a transaction is rolled back beyond the point at which a lock was upgraded, then the lock must be downgraded correspondingly, as part of the rollback operation, in order to reduce the risk of artificial deadlocks.

The requirements of dictionary locking for transactions and, in particular, the maintenance of a history of lock conversions, is provided by DML locks in conjunction with TM enqueues. Every transaction holding a DML lock also holds a TM enqueue lock. The basic locking functionality is provided by the enqueue, and the DML lock adds the maintenance of the conversion history.

The fixed array of DML lock structures is sized by the DML_LOCKS parameter. Its free ...