Default Parameter Settings

By default, Oracle Database automatically sets parallel execution parameters, as shown in Table 8-3.

Table 8-3 Parameters and Their Defaults

Parameter	Default	Comments
`PARALLEL_ADAPTIVE_ MULTI_USER`	`TRUE`	Causes parallel execution SQL to throttle degree of parallelism (DOP) requests to prevent system overload.
`PARALLEL_DEGREE_LIMIT`	`CPU_COUNT` X `PARALLEL_THREADS_PER_CPU` X number of instances available	Controls the maximum DOP a statement can have when automatic DOP is in use.
`PARALLEL_DEGREE_POLICY`	`MANUAL`	Controls whether auto DOP, parallel statement queuing and in-memory parallel execution are used. By default, all of these features are disabled.
`PARALLEL_EXECUTION_MESSAGE_SIZE`	16 KB	Specifies the size of the buffers used by the parallel execution servers to communicate among themselves and with the query coordinator. These buffers are allocated out of the shared pool.
`PARALLEL_FORCE_LOCAL`	`FALSE`	Restricts parallel execution to the current Oracle RAC instance.
`PARALLEL_MAX_ SERVERS`	See "PARALLEL_MAX_SERVERS".	Specifies the maximum number of parallel execution processes and parallel recovery processes for an instance. As demand increases, Oracle Database increases the number of processes from the number created at instance startup up to this value. If you set this parameter too low, some queries may not have a parallel execution process available to them during query processing. If you set it too high, memory resource shortages may occur during peak periods, which can degrade performance.
`PARALLEL_MIN_SERVERS`	0	Specifies the number of parallel execution processes to be started and reserved for parallel operations, when Oracle Database is started up. Increasing this setting can help balance the startup cost of a parallel statement, but requires greater memory usage as these parallel execution processes are not removed until the database is shut down.
`PARALLEL_MIN_PERCENT`	0	Specifies the minimum percentage of requested parallel execution processes required for parallel execution. With the default value of 0, a parallel statement executes serially if no parallel server processes are available.
`PARALLEL_MIN_TIME_THRESHOLD`	10 seconds	Specifies the execution time, as estimated by the optimizer, above which a statement is considered for automatic parallel query and automatic derivation of DOP.
`PARALLEL_SERVERS_TARGET`	See "PARALLEL_SERVERS_TARGET".	Specifies the number of parallel execution server processes available to run queries before parallel statement queuing is used. Note that parallel statement queuing is only active if `PARALLEL_DEGREE_POLICY` is set to `AUTO`.
`PARALLEL_THREADS_PER_CPU`	2	Describes the number of parallel execution processes or threads that a CPU can handle during parallel execution.

You can set some parameters in such a way that Oracle Database is constrained. For example, if you set PROCESSES to 20, you are not be able to get 25 child processes.