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.
