Message-ID: <646c3d09475e49a5d1145c91af0ce3fdc3cdee96.camel@j-davis.com>
Subject: Re: allowing extensions to control planner behavior
From: Jeff Davis <pgsql@j-davis.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: Wed, 28 Aug 2024 16:15:51 -0700
In-Reply-To: <CA+TgmoY4yBRawLY-u0GM6aW7f0N4W8RU8WsZRb5+qhpWWfmZcQ@mail.gmail.com>
References: 
	<CA+TgmoZY+baV-T-5ifDn6P=L=aV-VkVBrPmi0TQkcEq-5Finww@mail.gmail.com>
	 <95e423ea042a8d2f85c76ee28feb7a5b10265f4d.camel@j-davis.com>
	 <CA+TgmoY4yBRawLY-u0GM6aW7f0N4W8RU8WsZRb5+qhpWWfmZcQ@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
User-Agent: Evolution 3.44.4-0ubuntu2 
MIME-Version: 1.0
Archived-At: <https://www.postgresql.org/message-id/646c3d09475e49a5d1145c91af0ce3fdc3cdee96.camel%40j-davis.com>
Precedence: bulk

On Wed, 2024-08-28 at 16:35 -0400, Robert Haas wrote:
> On Wed, Aug 28, 2024 at 4:29=E2=80=AFPM Jeff Davis <pgsql@j-davis.com> wr=
ote:
> > Preserving a path for the right amount of time seems like the
> > primary
> > challenge for most of the use cases you raised (removing paths is
> > easier than resurrecting one that was pruned too early). If we try
> > to
> > keep a path around, that implies that we need to keep parent paths
> > around too, which leads to an explosion if we aren't careful.
> >=20
> > But we already solved all of that for pathkeys. We keep the paths
> > around if there's a reason to (a useful pathkey) and there's not
> > some
> > other cheaper path that also satisfies the same reason.
>=20
> But we've already solved it for this case, too. This is exactly what
> incrementing disabled_nodes does.

Hints are often described as something positive: use this index, use a
hash join here, etc. Trying to force a positive thing by adding
negative attributes to everything else is awkward. We've all had the
experience where we disable one plan type hoping for a good plan, and
we end up getting a different crazy plan that we didn't expect, and
need to disable a few more plan types.

Beyond awkwardness, one case where it matters is the interaction
between an extension that provides hints and an extension that offers a
CustomScan. How is the hints extension supposed to disable a path it
doesn't know about?

Regards,
	Jeff Davis