Date: Wed, 24 Jun 2020 19:18:10 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Andres Freund <andres@anarazel.de>
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: <20200624231810.GB13627@momjian.us>
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On Wed, Jun 24, 2020 at 12:19:00PM -0700, Andres Freund wrote:
> 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).

Well, this sounds like an issue of degree, rather than kind.  It sure
sounds like "ignore work_mem for this join type, but not the other".

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee