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From: Bruce Momjian <bruce@momjian.us>
To: Justin Pryzby <pryzby@telsasoft.com>
Cc: David Rowley <dgrowleyml@gmail.com>
Cc: Jeff Davis <pgsql@j-davis.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Default setting for enable_hashagg_disk
Date: Wed, 24 Jun 2020 13:08:34 -0400
Message-ID: <20200624170834.GA17842@momjian.us> (raw)
In-Reply-To: <20200624123843.GX4107@telsasoft.com>
References: <CAAKRu_aisiENMsPM2gC4oUY1hHG3yrCwY-fXUg22C6_MJUwQdA@mail.gmail.com>
	<20200610021544.GA14879@telsasoft.com>
	<8bff2e4e8020c3caa16b61a46918d21b573eaf78.camel@j-davis.com>
	<CA+TgmoagMLfruY4y_40oKLmFFdBRVSGraZsTa5ss9wH546EYhQ@mail.gmail.com>
	<ee1a7be071349b93c01ffb9ef4cf3a6830d9e780.camel@j-davis.com>
	<CA+TgmoY4-zSMFf8XbpO6uRsMX4vNPPLXvtajAZpG3f9eoEyjdA@mail.gmail.com>
	<129c8f1739957589584d9525e1d8782eae3f412f.camel@j-davis.com>
	<CAApHDvqFZikXhAGW=UKZKq1_FzHy+XzmUzAJiNj6RWyTHH4UfA@mail.gmail.com>
	<20200624090628.GA19640@momjian.us>
	<20200624123843.GX4107@telsasoft.com>

On Wed, Jun 24, 2020 at 07:38:43AM -0500, Justin Pryzby wrote:
> On Wed, Jun 24, 2020 at 05:06:28AM -0400, Bruce Momjian wrote:
> > It would seem merge join has almost the same complexities as the new
> > hash join code, since it can spill to disk doing sorts for merge joins,
> > and adjusting work_mem is the only way to control that spill to disk.  I
> > don't remember anyone complaining about spills to disk during merge
> > join, so I am unclear why we would need a such control for hash join.
> 
> It loooks like merge join was new in 8.3.  I don't think that's a good analogy,
> since the old behavior was still available with enable_mergejoin=off.

Uh, we don't gurantee backward compatibility in the optimizer.  You can
turn off hashagg if you want.  That doesn't get you to PG 13 behavior,
but we don't gurantee that.

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

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






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