Message-ID: <6d0aba397625df79bdc3ca85adc11b9e48ecb0da.camel@j-davis.com>
Subject: Re: Default setting for enable_hashagg_disk
From: Jeff Davis <pgsql@j-davis.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>, David Rowley
 <dgrowleyml@gmail.com>, Stephen Frost <sfrost@snowman.net>, Tom Lane
 <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>, Bruce Momjian
 <bruce@momjian.us>, 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>
Date: Sat, 18 Jul 2020 11:16:26 -0700
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On Fri, 2020-07-17 at 18:38 -0700, Peter Geoghegan wrote:
> There is also the separate question of what to do about the
> hashagg_avoid_disk_plan GUC (this is a separate open item that
> requires a separate resolution). Tom leans slightly towards removing
> it now. Is your position about the same as before?

Yes, I think we should have that GUC (hashagg_avoid_disk_plan) for at
least one release.

Clealy, a lot of plans will change. For any GROUP BY where there are a
lot of groups, there was only one choice in v12 and now there are two
choices in v13. Obviously I think most of those changes will be for the
better, but some regressions are bound to happen. Giving users some
time to adjust, and for us to tune the cost model based on user
feedback, seems prudent.

Are there other examples of widespread changes in plans where we
*didn't* have a GUC? There are many GUCs for controlling parallism,
JIT, etc.

Regards,
	Jeff Davis