On Wed, 2020-11-25 at 17:36 +0100, Laurenz Albe wrote: > I have a partitioned table, each partition has "parallel_workers = 10" set. > > SET max_parallel_workers_per_gather = 8; > > SET enable_partitionwise_aggregate = on; > > EXPLAIN (COSTS OFF) > SELECT applicant_name, count(ipc_4) > FROM laurenz.z_flat > GROUP BY applicant_name; > > QUERY PLAN > -------------------------------------------------- > Gather > Workers Planned: 4 > -> Parallel Append > -> HashAggregate > Group Key: z_flat_3.applicant_name > -> Seq Scan on xyz_4 z_flat_3 > -> HashAggregate > Group Key: z_flat.applicant_name > -> Seq Scan on xyz_1 z_flat > [8 more such partition scans] > (33 rows) > > How does the optimizer decide to use 4 parallel workers? > > No matter what I try, I cannot influence that number.
I figured it out. This is automatically calculated from the number of partitions, and the number of parallel workers is ld(#partitions) + 1 where "ld" is the logarithm of base 2 (function "fls" in the source). It might be nice to make this configurable, but since we don't have storage parameters on partitioned tables, I wonder how. Yours, Laurenz Albe -- Cybertec | https://www.cybertec-postgresql.com
