When the parameter PARALLEL_DEGREE_POLICY is set to ADAPTIVE or AUTO, Oracle will queue SQL statements that require parallel execution, if the necessary parallel server processes are not available. In a single instance database, statement queuing will begin once the number of parallel server processes active on the system is equal to or greater than PARALLEL_SERVERS_TARGET. In an Oracle RAC database, a statement running on one instance can allocate parallel server processes on another instance when necessary. For example, if a statement running on one instance requires parallel execution, but the number of parallel server processes active on that instance is equal to or greater than the value of PARALLEL_SERVERS_TARGET for that instance, then the statement can allocate parallel server processes on a different instance. Therefore, statement queuing in an Oracle RAC database will begin only when every instance has reached its PARALLEL_SERVER_TARGET threshold.

By default, PARALLEL_SERVERS_TARGET is set lower than the maximum number of parallel server processes allowed on the system (PARALLEL_MAX_SERVERS) to ensure each parallel statement will get all of the parallel server resources required and to prevent overloading the system with parallel server processes.

The number of concurrent parallel users running at default degree of parallelism on an instance depends on the memory initialization parameter settings for the instance. For example, if the MEMORY_TARGET or SGA_TARGET initialization parameter is set, then the number of concurrent_parallel_users = 4. If neither MEMORY_TARGET or SGA_TARGET is set, then PGA_AGGREGATE_TARGET is examined. If a value is set for PGA_AGGREGATE_TARGET, then concurrent_parallel_users = 2. If a value is not set for PGA_AGGREGATE_TARGET, then concurrent_parallel_users = 1.

Note that all serial (non-parallel) statements will execute immediately even if statement queuing has been activated.

By default, all PDB queries are subjected to queuing at the PDB level first by the PDB's PARALLEL_SERVERS_TARGET value, and then at the CDB level by the CDB's PARALLEL_SERVERS_TARGET value. This default behavior prevents any SQL statement inside a PDB from getting downgraded if parallel servers are exhausted in the CDB.

Parallel statement queuing is enabled by default at the CDB level because the CDB has a default value for PARALLEL_SERVERS_TARGET. You can disable parallel statement queuing at the CDB level by using ALTER SYSTEM to set PARALLEL_SERVERS_TARGET to 0 for the CDB.

The default value for PARALLEL_SERVERS_TARGET for a PDB is determined using the calculation in the table above with the PDB’s CPU_COUNT value.