Date: Tue, 23 Jun 2020 22:14:43 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: David Rowley <dgrowleyml@gmail.com>
Cc: 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: <20200624031443.GV4107@telsasoft.com>
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On Wed, Jun 24, 2020 at 02:11:57PM +1200, David Rowley wrote:
> On Tue, 23 Jun 2020 at 08:24, Jeff Davis <pgsql@j-davis.com> wrote:
> > Another way of looking at it is that the weird behavior is already
> > there in v12, so there are already users relying on this weird behavior
> > as a crutch for some other planner mistake. The question is whether we
> > want to:

Yea - "behavior change" is a scenario for which it's hard to anticipate well
all the range of consequences.

> How about:
> 
> enable_hashagg = "on" -- enables hashagg allowing it to freely spill
> to disk as it pleases.
> enable_hashagg = "trynospill" -- Planner will only choose hash_agg if
> it thinks it won't spill (pre PG13 planner behaviour)
> enable_hashagg = "neverspill" -- executor will *never* spill to disk
> and can still OOM (NOT RECOMMENDED, but does give pre PG13 planner and
> executor behaviour)
> enable_hashagg = "off" -- planner does not consider hash agg, ever.
> Same as what PG12 did for this setting.

+1

I like that this allows the new behavior as an *option* one *can* use rather
than a "behavior change" which is imposed on users and which users then *have*
to accomodate in postgres.

-- 
Justin