Transaction Restrictions for Parallel DML

To execute a DML operation in parallel, the parallel execution coordinator acquires parallel execution servers, and each parallel execution server executes a portion of the work under its own parallel process transaction.

Note the following conditions:

  • Each parallel execution server creates a different parallel process transaction.

  • If you use rollback segments instead of Automatic Undo Management, you may want to reduce contention on the rollback segments by limiting the number of parallel process transactions residing in the same rollback segment. See Oracle Database SQL Language Reference for more information.

The coordinator also has its own coordinator transaction, which can have its own rollback segment. To ensure user-level transactional atomicity, the coordinator uses a two-phase commit protocol to commit the changes performed by the parallel process transactions.

A session that is enabled for parallel DML may put transactions in the session in a special mode: If any DML statement in a transaction modifies a table in parallel, no subsequent serial or parallel query or DML statement can access the same table again in that transaction. The results of parallel modifications cannot be seen during the transaction.

Serial or parallel statements that attempt to access a table that has been modified in parallel within the same transaction are rejected with an error message.

If a PL/SQL procedure or block is executed in a parallel DML-enabled session, then this rule applies to statements in the procedure or block.