Oracle evaluates the number of CPUs
available on the server and the number of disks on which the table??™s data is stored in order to
determine the default degree of parallelism.
The maximum available parallelism is set at the instance level. The PARALLEL_MAX_SERVERS
initialization parameter sets the maximum number of parallel query server processes that can be
used at any one time by all the processes in the database. For example, if you set PARALLEL_
MAX_SERVERS to 32 for your instance, and you run a query that uses 30 parallel query server
processes for its query and sorting operations, then only two parallel query server processes are
available for all the rest of the users in the database. Therefore, you need to carefully manage the
parallelism you allow for your queries and batch operations. The PARALLEL_ADAPTIVE_MULTI_
USER parameter, when set to TRUE, enables an adaptive algorithm designed to improve
performance in multiuser environments using parallel execution. The algorithm automatically
reduces the requested degree of parallelism according to the system load at query startup time.
The effective degree of parallelism is based on the default degree of parallelism, or the degree
from the table, or hints, divided by a reduction factor.
For each table, you can set a default degree of parallelism via the parallel clause of the create
table and alter table commands. The degree of parallelism tells Oracle how many parallel query
server processes to attempt to use for each part of the operation.
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