Date: Wed, 3 Jun 2020 17:36:41 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Philip Semanchuk <philip@americanefficient.com>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: increased max_parallel_workers_per_gather results in fewer
 workers?
Message-ID: <20200603223641.GV30144@telsasoft.com>
References: <B2C0B20E-4562-4F0B-A876-922EA4AC22A8@americanefficient.com>
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On Wed, Jun 03, 2020 at 06:23:57PM -0400, Philip Semanchuk wrote:
> > On Jun 3, 2020, at 5:15 PM, Justin Pryzby <pryzby@telsasoft.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