A System Administrator configures the upper limit to the degree of parallelism using server-wide configuration parameters. Session-wide and query-level options can further limit the degree of parallelism. These limits set both the total number of worker processes that can be used in a parallel query and the total number of worker processes that can be used for hash-based access methods.
The optimizer removes from consideration any parallel access methods that would require more worker processes than the upper limit for the query. (If the upper limit to the degree of parallelism is 1, the optimizer does not consider any parallel access methods.)
See “Configuration parameters for controlling parallelism” for more information about configuration parameters that control the upper limit to the degree of parallelism.
