On Sun, Oct 13, 2024 at 6:31 AM Marc SCHAEFER <alphanet-postgresql-gene...@alphanet.ch> wrote: > template1=> SELECT COUNT(*) FROM pg_class a, pg_class b, pg_class c; > > I see only one 100% CPU PostgreSQL process.
If you set set min_parallel_table_scan_size = 0 then it uses parallelism, and completes much faster. The planner generally works by comparing the estimated cost of various plans (it is a "cost based" optimiser), but the decision to actually consider parallelism at all is essentially "rule based", and the rules aren't smart enough for this query with default settings. pg_class is considered too small to bother parallelising the scan, and here you have a 3-way cross-join which generates an enormous of work for each tuple so it is actually a good idea to parallelise it. I guess people don't actually do that too often.
