When an instance starts, Oracle Database creates a pool of parallel execution servers, which are available for any parallel operation. The initialization parameter PARALLEL_MIN_SERVERS specifies the number of parallel execution servers that Oracle Database creates at instance startup.
When executing a parallel operation, the parallel execution coordinator obtains parallel execution servers from the pool and assigns them to the operation. If necessary, Oracle Database can create additional parallel execution servers for the operation. These parallel execution servers remain with the operation throughout execution. After the statement has been processed, the parallel execution servers return to the pool.
If the number of parallel operations increases, Oracle Database creates additional parallel execution servers to handle incoming requests. However, Oracle Database never creates more parallel execution servers for an instance than the value specified by the initialization parameter PARALLEL_MAX_SERVERS.
If the number of parallel operations decreases, Oracle Database terminates any parallel execution servers that have been idle for a threshold interval. Oracle Database does not reduce the size of the pool less than the value of PARALLEL_MIN_SERVERS, no matter how long the parallel execution servers have been idle.