Parallel execution uses multiple processes to run SQL statements on one or more CPUs and is available on both noncluster Oracle databases and Oracle RAC databases.
Oracle RAC takes full advantage of parallel execution by distributing parallel processing across all available instances. The number of processes that can participate in parallel operations depends on the degree of parallelism assigned to each table or index.
See Also:
Oracle Database Performance Tuning Guide for more information about the query optimizer
Oracle Database Concepts for more information about parallel execution