Date: Wed, 24 Jun 2020 13:08:34 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Justin Pryzby <pryzby@telsasoft.com>
Cc: David Rowley <dgrowleyml@gmail.com>, Jeff Davis <pgsql@j-davis.com>,
	Robert Haas <robertmhaas@gmail.com>,
	Melanie Plageman <melanieplageman@gmail.com>,
	Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	pgsql-hackers@postgresql.org
Subject: Re: Default setting for enable_hashagg_disk
Message-ID: <20200624170834.GA17842@momjian.us>
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>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200624123843.GX4107@telsasoft.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

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