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In-Reply-To: <1393410.1724089927@sss.pgh.pa.us>
From: Robert Haas <robertmhaas@gmail.com>
Date: Mon, 19 Aug 2024 14:20:46 -0400
Message-ID: <CA+Tgmoa6DffJSbSiS0TMcqWGENZwGv0x0K61ohqs2dchWejVow@mail.gmail.com>
Subject: Re: generic plans and "initial" pruning
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Amit Langote <amitlangote09@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, 
	Andres Freund <andres@anarazel.de>, Daniel Gustafsson <daniel@yesql.se>, 
	David Rowley <dgrowleyml@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, 
	Thom Brown <thom@linux.com>
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On Mon, Aug 19, 2024 at 1:52=E2=80=AFPM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
> > But that seems somewhat incidental to what this thread is about.
>
> Perhaps.  But if we're running into issues related to that, it might
> be good to set aside the long-term goal for a bit and come up with
> a cleaner answer for intra-session locking.  That could allow the
> pruning problem to be solved more cleanly in turn, and it'd be
> an improvement even if not.

Maybe, but the pieces aren't quite coming together for me. Solving
this would mean that if we execute a stale plan, we'd be more likely
to get a good error and less likely to get a bad, nasty-looking
internal error, or a crash. That's good on its own terms, but we don't
really want user queries to produce errors at all, so I don't think
we'd feel any more free to rearrange the order of operations than we
do today.

> > Do you have a view on what the way forward might be?
>
> I'm fresh out of ideas at the moment, other than having a hope that
> divide-and-conquer (ie, solving subproblems first) might pay off.

Fair enough, but why do you think that the original approach of
creating a data structure from within the plan cache mechanism
(probably via a call into some new executor entrypoint) and then
feeding that through to ExecutorRun() time can't work? Is it possible
you latched onto some non-optimal decisions that the early versions of
the patch made, rather than there being a fundamental problem with the
concept?

I actually thought the do-it-at-executorstart-time approach sounded
pretty good, even though we might have to abandon planstate tree
initialization partway through, right up until Amit started talking
about moving ExecutorStart() from PortalRun() to PortalStart(), which
I have a feeling is going to create a bigger problem than we can
solve. I think if we want to save that approach, we should try to
figure out if we can teach the plancache to replan one query from a
list without replanning the others, which seems like it might allow us
to keep the order of major operations unchanged. Otherwise, it makes
sense to me to have another go at the other approach, at least to make
sure we understand clearly why it can't work.

> Yeah.  We are working in an extremely not-green field here, which
> means it's a lot easier to see pre-existing reasons why X will not
> work than to have confidence that it will work.  But hey, if this
> were easy then we'd have done it already.

Yeah, true.

--=20
Robert Haas
EDB: http://www.enterprisedb.com