From: Tom Lane <tgl@sss.pgh.pa.us>
To: "ldh@laurent-hasson.com" <ldh@laurent-hasson.com>
cc: Peter Geoghegan <pg@bowt.ie>, David Rowley <dgrowleyml@gmail.com>,
        Justin Pryzby <pryzby@telsasoft.com>,
        "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Big performance slowdown from 11.2 to 13.3
In-reply-to: <543673.1627442152@sss.pgh.pa.us>
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Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Tue, 27 Jul 2021 23:15:52 -0400"
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Date: Tue, 27 Jul 2021 23:31:17 -0400
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Precedence: bulk

I wrote:
> Yeah, I wouldn't sweat over the specific value.  The pre-v13 behavior
> was effectively equivalent to hash_mem_multiplier = infinity, so if
> you weren't having any OOM problems before, just crank it up.

Oh, wait, scratch that: the old executor's behavior is accurately
described by that statement, but the planner's is not.  The planner
will not pick a hashagg plan if it guesses that the hash table
would exceed the configured limit (work_mem before, now work_mem
times hash_mem_multiplier).  So raising hash_mem_multiplier to the
moon might bias the v13 planner to pick hashagg plans in cases
where earlier versions would not have.  This doesn't describe your
immediate problem, but it might be a reason to not just set the
value as high as you can.

BTW, this also suggests that the planner is underestimating the
amount of memory needed for the hashagg, both before and after.
That might be something to investigate at some point.

			regards, tom lane