The optimizer considers parallel query plans only when Adaptive Server and the current session are properly configured for parallelism, as described in “Controlling the degree of parallelism”.
If both the Adaptive Server and the current session are configured for parallel queries, then all queries within the session are eligible for parallel query optimization. Individual queries can also attempt to enforce parallel query optimization by using the optimizer hint parallel N for parallel or parallel 1 for serial.
If the Adaptive Server or the current session is not configured for parallel queries, or if a given query uses optimizer hints to enforce serial execution, then the optimizer considers serial access methods; the parallel access methods described in this chapter are not considered.
Adaptive Server does not execute parallel queries against system tables.