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
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Comments: In-reply-to "ldh@laurent-hasson.com" <ldh@laurent-hasson.com>
	message dated "Wed, 28 Jul 2021 02:57:48 -0000"
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Date: Tue, 27 Jul 2021 23:15:52 -0400
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"ldh@laurent-hasson.com" <ldh@laurent-hasson.com> writes:
> One question that popped up in my head. hash_mem_multiplier is an upper-=
bound right: it doesn't reserve memory ahead of time correct? So there is =
no reason for me to spend undue amounts of time fine-tuning this parameter=
? If I have work_mem to 521MB, then I can set hash_mem_multiplier to 8 and=
 should be OK. This doesn't mean that every query will consume 4GB of memo=
ry.

Yeah, I wouldn't sweat over the specific value.  The pre-v13 behavior
was effectively equivalent to hash_mem_multiplier =3D infinity, so if
you weren't having any OOM problems before, just crank it up.

			regards, tom lane