From: Tom Lane <tgl@sss.pgh.pa.us>
To: Bob Jolliffe <bobjolliffe@gmail.com>
cc: pgsql-performance@lists.postgresql.org
Subject: Re: does max_connections affect the query planner
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Comments: In-reply-to Bob Jolliffe <bobjolliffe@gmail.com>
	message dated "Tue, 17 Sep 2019 08:40:53 -0000"
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Date: Tue, 17 Sep 2019 10:13:40 -0400
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Bob Jolliffe <bobjolliffe@gmail.com> writes:
> We do know that max_connections is set quite high (600) when we don't
> really expect more than 100.  I wonder does the planner take
> max_connections x work_mem into account when considering the memory it
> has potentially available?

No.  There have been discussions to the effect that it ought to have
a more holistic view about available memory; but nothing's been done
about that, and certainly no existing release does so.

Usually the proximate cause of bad plan choices is bad rowcount
estimates --- you can spot that by comparing estimated and actual
rowcounts in EXPLAIN ANALYZE results.

			regards, tom lane