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Don Burleson Blog 







parallel_degree_level Tips

Oracle Database Tips by Donald BurlesonJuly 15, 2015

Question:  How does the parallel_degree_level parameter work in 12c?  Does it accurately help Oracle get the proper degree of parallelism?

Answer:  The parallel_degree_level parameter is new in 12c and it is expressed as a percentage.  The parallel_degree_level only works when you turn-on "automatic" parallelism (via parallel_degree_policy=auto). 

Also see parallel_degree_policy.

The default value is parallel_degree_policy=manual, so both of these parameters must be set together to get "automatic" parallelism.

As a general rule, a "good" degree of parallelism is (cpu_count-1), but there are exceptions and parallel_degree_level allows you to control the degree numbers used for parallel full-table scans and full index scans.

The default for parallel_degree_level is 100 and reducing parallel_degree_level to 25 decreases the estimates for the degree of parallelism to 25%.  Conversely, setting parallel_degree_level to a number above 100 will multiply the degree of parallelism.

This is another stab at automating the degree of parallelism such as the failed parallel_automatic_tuning parameter.

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