A DML statement can be parallelized only if you have explicitly enabled parallel DML in the session or in the SQL statement. To enable this mode in a session, run the following SQL statement:
ALTER SESSION ENABLE PARALLEL DML;
To enable parallel DML mode in a specific SQL statement, include the ENABLE_PARALLEL_DML SQL hint. For example:
INSERT /*+ ENABLE_PARALLEL_DML */ …
This mode is required because parallel DML and serial DML have different locking, transaction, and disk space requirements and parallel DML is disabled for a session by default.
When parallel DML is disabled, no DML is executed in parallel even if the PARALLEL hint is used.
When parallel DML is enabled in a session, all DML statements in this session are considered for parallel execution. When parallel DML is enabled in a SQL statement with the ENABLE_PARALLEL_DML hint, only that specific statement is considered for parallel execution. However, even if parallel DML is enabled, the DML operation may still execute serially if there are no parallel hints or no tables with a parallel attribute or if restrictions on parallel operations are violated.
The session's PARALLEL DML mode does not influence the parallelism of SELECT statements, DDL statements, and the query portions of DML statements. If this mode is not set, the DML operation is not parallelized, but scans or join operations within the DML statement may still be parallelized.
When the parallel DML mode has been enabled for a session, you can disable the mode for a specific SQL statement with the DISABLE_PARALLEL_DML SQL hint.
For more information, refer to "Space Considerations for Parallel DML" and "Restrictions on Parallel DML".