When you submit a query, the optimizer creates a query plan that determines how the database server executes the query. The optimizer might create one of the two following general kinds of query plans:
If all the required table fragments are on the same coserver, the optimizer may be able to create a serial plan. However, certain conditions (the presence of some operators, for example) for the optimizer to use a parallel plan.
For information about the requirements of serial plans, see Serial Query Execution .
If the table fragments accessed by the query are distributed across disks and coservers, the optimizer creates a parallel query plan, sometimes called an xplan.
For information about how the database server processes parallel plans, see Parallel Query Execution.