From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: pgsql-hackers@postgresql.org, David Rowley <dgrowleyml@gmail.com>
Subject: Re: Unfortunate pushing down of expressions below sort
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
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Date: Thu, 05 Feb 2026 19:13:09 -0500
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Andres Freund <andres@anarazel.de> writes:
> We push down expressions below a sort node, even though they could be
> evaluated above.

Yeah.  That's a hangover from an ancient decision that sort/limit
would always be applied at the top of the plan tree.  I'm too
lazy to check the details right now, but I think we already relaxed
that in some cases (or maybe it was about evaluating stuff
before/after GROUP BY?).

> That can very substantially increase the space needed for the
> sort.

Could decrease it too, eg if what you are outputting is
md5(some-wide-column).  Not sure we are smart enough to tell
which way is better.

			regards, tom lane