On Wed, Feb 15, 2017 at 4:43 AM, Amit Khandekar <amitdkhan...@gmail.com> wrote: >> On 14 February 2017 at 22:35, Robert Haas <robertmh...@gmail.com> wrote: >>> For example, suppose that I have a scan of two children, one >>> of which has parallel_workers of 4, and the other of which has >>> parallel_workers of 3. If I pick parallel_workers of 7 for the >>> Parallel Append, that's probably too high. > > In the patch, in such case, 7 workers are indeed selected for Parallel > Append path, so that both the subplans are able to execute in parallel > with their full worker capacity. Are you suggesting that we should not > ?
Absolutely. I think that's going to be way too many workers. Imagine that there are 100 child tables and each one is big enough to qualify for 2 or 3 workers. No matter what value the user has selected for max_parallel_workers_per_gather, they should not get a scan involving 200 workers. What I was thinking about is something like this: 1. First, take the maximum parallel_workers value from among all the children. 2. Second, compute log2(num_children)+1 and round up. So, for 1 child, 1; for 2 children, 2; for 3-4 children, 3; for 5-8 children, 4; for 9-16 children, 5, and so on. 3. Use as the number of parallel workers for the children the maximum of the value computed in step 1 and the value computed in step 2. With this approach, a plan with 100 children qualifies for 8 parallel workers (unless one of the children individually qualifies for some larger number, or unless max_parallel_workers_per_gather is set to a smaller value). That seems fairly reasonable to me. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
