From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
cc: pgsql-committers@lists.postgresql.org,
        Richard Guo <guofenglinux@gmail.com>
Subject: Re: pgsql: Fix LATERAL join test in test memoize.sql
In-reply-to: 
 <CAApHDvruLvSdzmP2JE=LsfXcD3mH-m1dv81Vc8tNCWmmO82Z5A@mail.gmail.com>
References: <E1pK6H4-0058ux-8l@gemulon.postgresql.org>
 <2144818.1674517061@sss.pgh.pa.us>
 <CAApHDvruLvSdzmP2JE=LsfXcD3mH-m1dv81Vc8tNCWmmO82Z5A@mail.gmail.com>
Comments: In-reply-to David Rowley <dgrowleyml@gmail.com>
	message dated "Tue, 24 Jan 2023 13:08:01 +1300"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <2149152.1674519788.1@sss.pgh.pa.us>
Date: Mon, 23 Jan 2023 19:23:08 -0500
Message-ID: <2149153.1674519788@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/2149153.1674519788%40sss.pgh.pa.us>
Precedence: bulk

David Rowley <dgrowleyml@gmail.com> writes:
> You're right that OFFSET 0 would have been a better choice. I just
> wasn't aware that we were considering changing the code so we pull up
> subqueries with an ORDER BY.

No such plan is in the offing AFAIK, but it doesn't seem entirely
out of the question either.

> In fact, I thought there were several
> good reasons we're not doing that. e.g. allow aggregation in outer
> query by some order specified by the subquery. But perhaps any changes
> we make in this area would be more conditional than just ignoring the
> fact that the subquery has an ORDER BY.

Yeah, we've historically honored subquery ORDER BY and I doubt we'd
want to stop.  But I'm not sure that that that completely precludes
subquery flattening, especially for single-relation subqueries such
as these.  You could imagine, for example, still doing the pull-up
and then only accepting paths for the rel that satisfy the ORDER BY.
This could still lead to better optimization of the outer query
than what happens now, I think.

Anyway, it's not something I expect to happen soon, but maybe
someday.

			regards, tom lane