Re: Default setting for enable_hashagg_disk

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, 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>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Default setting for enable_hashagg_disk
Date: 2020-07-12 12:30:43
Message-ID: 20200712123043.hnjsluya5mbwhnq4@development
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On Sat, Jul 11, 2020 at 08:47:54PM -0400, Tom Lane wrote:
>Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
>> I don't know, but one of the main arguments against simply suggesting
>> people to bump up work_mem (if they're hit by the hashagg spill in v13)
>> was that it'd increase overall memory usage for them. It seems strange
>> to then propose a new GUC set to a default that would result in higher
>> memory usage *for everyone*.
>
>It seems like a lot of the disagreement here is focused on Peter's
>proposal to make hash_mem_multiplier default to 2.0. But it doesn't
>seem to me that that's a critical element of the proposal. Why not just
>make it default to 1.0, thus keeping the default behavior identical
>to what it is now?
>
>If we find that's a poor default, we can always change it later;
>but it seems to me that the evidence for a higher default is
>a bit thin at this point.
>

You're right, I was specifically pushing against that aspect of the
proposal. Sorry for not making that clearer, I assumed it's clear from
the context of this (sub)thread.

I agree making it 1.0 (or equal to work_mem, if it's not a multiplier)
by default, but allowing it to be increased if needed would address most
of the spilling issues.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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