When the parameter PARALLEL_DEGREE_POLICY is set to AUTO, Oracle Database queues SQL statements that require parallel execution if the necessary number of parallel execution server processes are not available. After the necessary resources become available, the SQL statement is dequeued and allowed to execute. The default dequeue order is a simple first in, first out queue based on the time a statement was issued.
If there are sufficient parallel execution servers available and there are no statements ahead in the queue waiting for the resources, the SQL statement is executed.
If there are not sufficient parallel execution servers available, the SQL statement is queued based on specified conditions and dequeued from the front of the queue when specified conditions are met.
This value is not the maximum number of parallel server processes allowed on the system, but the number available to run parallel statements before parallel statement queuing is used. It is set lower than the maximum number of parallel server processes allowed on the system (PARALLEL_MAX_SERVERS) to ensure each parallel statement gets all of the parallel server resources required and to prevent overloading the system with parallel server processes. Note all serial (nonparallel) statements execute immediately even if parallel statement queuing has been activated.
For information about views for monitoring and analyzing parallel statement queuing, refer to "V$RSRC_SESSION_INFO" and "V$RSRCMGRMETRIC".