Re: Default setting for enable_hashagg_disk

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Stephen Frost <sfrost(at)snowman(dot)net>
Cc: Peter Geoghegan <pg(at)bowt(dot)ie>, Jeff Davis <pgsql(at)j-davis(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Bruce Momjian <bruce(at)momjian(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, David Rowley <dgrowleyml(at)gmail(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Melanie Plageman <melanieplageman(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Default setting for enable_hashagg_disk
Date: 2020-07-10 21:59:59
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Stephen Frost <sfrost(at)snowman(dot)net> writes:
> I don't see hash_mem as being any kind of proper fix- it's just punting
> to the user saying "we can't figure this out, how about you do it" and,
> worse, it's in conflict with how we already ask the user that question.
> Turning it into a multiplier doesn't change that either.

Have you got a better proposal that is reasonably implementable for v13?
(I do not accept the argument that "do nothing" is a better proposal.)

I agree that hash_mem is a stopgap, whether it's a multiplier or no,
but at this point it seems difficult to avoid inventing a stopgap.
Getting rid of the process-global work_mem setting is a research project,
and one I wouldn't even count on having results from for v14. In the
meantime, it seems dead certain that there are applications for which
the current behavior will be problematic. hash_mem seems like a cleaner
and more useful stopgap than the "escape hatch" approach, at least to me.

regards, tom lane

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