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Re: Unfortunate pushing down of expressions below sort
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* Re: Unfortunate pushing down of expressions below sort
@ 2026-02-06 00:13 Tom Lane <[email protected]>
  2026-02-06 01:02 ` Re: Unfortunate pushing down of expressions below sort Tom Lane <[email protected]>
  0 siblings, 1 reply; 2+ messages in thread

From: Tom Lane @ 2026-02-06 00:13 UTC (permalink / raw)
  To: Andres Freund <[email protected]>; +Cc: pgsql-hackers; David Rowley <[email protected]>

Andres Freund <[email protected]> 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






^ permalink  raw  reply  [nested|flat] 2+ messages in thread

* Re: Unfortunate pushing down of expressions below sort
  2026-02-06 00:13 Re: Unfortunate pushing down of expressions below sort Tom Lane <[email protected]>
@ 2026-02-06 01:02 ` Tom Lane <[email protected]>
  0 siblings, 0 replies; 2+ messages in thread

From: Tom Lane @ 2026-02-06 01:02 UTC (permalink / raw)
  To: Andres Freund <[email protected]>; +Cc: pgsql-hackers; David Rowley <[email protected]>

I wrote:
> 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?).

Ah, right: see make_sort_input_target() and its very extensive
comment in planner.c.  I wonder why that didn't trigger in
your example.

			regards, tom lane






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