For example, if a query that
performs both table scanning and data sorting operations has a degree of parallelism of 5, there
could be ten parallel query server processes used??”five for scanning and five for sorting. You can
also specify a degree of parallelism for an index when it is created, via the parallel clause of the
create index command.
The minimum number of parallel query server processes started is set via the PARALLEL_
MIN_SERVERS initialization parameter. In general, you should set this parameter to a very low
number (less than 5) unless the system is actively used at all hours of the day. Setting this
parameter to a low value will force Oracle to repeatedly start new query server processes, but it
will greatly decrease the amount of memory held by idle parallel query server processes during
low-use periods. If you set a high value for PARALLEL_MIN_SERVERS, you may frequently have
idle parallel query server processes on your server, holding onto the memory they had previously
acquired but not performing any functions.
Parallelizing operations distributes their processing requirements across multiple CPUs;
however, you should use these features carefully. If you use a degree of parallelism of 5 for a large
query, you will have five separate processes accessing the data. If you have that many processes
accessing the data, you may create contention for the disks on which the data is stored, thus
hurting performance.
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