The following rule determines whether the INSERT operation should be parallelized in an INSERT SELECT statement:
The INSERT operation is executed in parallel if and only if at least one of the following is true:
The PARALLEL hint is specified after the INSERT in the DML statement.
The table being inserted into (the reference object) has a PARALLEL declaration specification.
An ALTER SESSION FORCE PARALLEL DML statement has been issued previously during the session.
The decision to parallelize the INSERT operation is independent of the SELECT operation, and vice versa.