Message-ID: <ee1a7be071349b93c01ffb9ef4cf3a6830d9e780.camel@j-davis.com>
Subject: Re: Default setting for enable_hashagg_disk
From: Jeff Davis <pgsql@j-davis.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: 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: Mon, 22 Jun 2020 10:30:43 -0700
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On Mon, 2020-06-22 at 10:52 -0400, Robert Haas wrote:
> So I feel like the really important thing here is to fix the cases
> that don't come out well with default settings.

...with the caveat that perfection is not something to expect from our
planner.

>  If we can't do that,
> then the feature is half-baked and maybe should not have been
> committed in the first place.

HashAgg started out half-baked at the dawn of time, and stayed that way
through version 12. Disk-based HashAgg was designed to fix it.

Other major planner features generally offer a way to turn them off
(e.g. parallelism, JIT), and we don't call those half-baked.

I agree that the single GUC added in v13 (hashagg_avoid_disk_plan) is
weird because it's half of a disable switch. But it's not weird because
of my changes in v13; it's weird because the planner behavior in v12
was weird. I hope not many people need to set it, and I hope we can
remove it soon.

If you think we will never be able to remove the GUC, then we should
think a little harder about whether we really need it. I am open to
that discussion, but I don't think the presence of this GUC implies
that disk-based hashagg is half-baked.

Regards,
	Jeff Davis