Queries that require worktable sorts have multistep query plans. The determination of the number of worker processes for a worktable sort is made after the scan of the base table completes. During the phase of the query where data is selected into the worktable, each worker process selects data into a separate partition of the worktable.
Once the worktable is populated, additional worker processes are allocated to perform the sort step. showplan does not report this value; the sort manager reports only whether the sort is performed in serial or parallel. The worker processes used in the previous step do not participate in the sort, but remain allocated to the parallel task until the task completes.
