Date: Wed, 24 Jun 2020 12:19:00 -0700
From: Andres Freund <andres@anarazel.de>
To: Bruce Momjian <bruce@momjian.us>
Cc: David Rowley <dgrowleyml@gmail.com>, Jeff Davis <pgsql@j-davis.com>,
	Robert Haas <robertmhaas@gmail.com>,
	Justin Pryzby <pryzby@telsasoft.com>,
	Melanie Plageman <melanieplageman@gmail.com>,
	Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Default setting for enable_hashagg_disk
Message-ID: <20200624191900.t4sbfk236gndwoa3@alap3.anarazel.de>
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Hi,

On 2020-06-24 13:12:03 -0400, Bruce Momjian wrote:
> Well, my point is that merge join works that way, and no one has needed
> a knob to avoid mergejoin if it is going to spill to disk.  If they are
> adjusting work_mem to prevent spill of merge join, they can do the same
> for hash agg.  We just need to document this in the release notes.

I don't think this is comparable. For starters, the IO indirectly
triggered by mergejoin actually leads to plenty people just straight out
disabling it. For lots of workloads there's never a valid reason to use
a mergejoin (and often the planner will never choose one). Secondly, the
planner has better information about estimating the memory usage for the
to-be-sorted data than it has about the size of the transition
values. And lastly, there's a difference between a long existing cause
for bad IO behaviour and one that's suddenly kicks in after a major
version upgrade, to which there's no escape hatch (it's rarely realistic
to disable hash aggs, in contrast to merge joins).

Greetings,

Andres Freund