Re: increased max_parallel_workers_per_gather results in fewer workers?

On Wed, Jun 03, 2020 at 06:23:57PM -0400, Philip Semanchuk wrote:
> > On Jun 3, 2020, at 5:15 PM, Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
> > What version postgres ?
>
> This is AWS’ version of Postgres 11.6 (“Aurora”) which of course might make a difference.

> > I guess you should show an explain analyze, specifically "Workers
> > Planned/Launched", maybe by linking to explain.depesz.com
>
> Out of an abundance of caution, our company has a policy of not pasting our plans to public servers. However, I can confirm that when I set max_parallel_workers_per_gather > 4 and the runtime increases, this is what’s in the EXPLAIN ANALYZE output:
>
> Workers Planned: 1
> Workers Launched: 1

Are you referring to a parallel scan/aggregate/hash/??

Are you able to show a plan for a toy query like SELECT count(col) FROM tbl ,
preferably including a CREATE TABLE tbl AS... ; VACUUM ANALYZE tbl;

Are you able to reproduce with an unpatched postgres ?

--
Justin