From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
cc: Vik Fearing <vik@postgresfriends.org>,
        Ankit Kumar Pandey <itsankitkp@gmail.com>,
        pghackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Todo: Teach planner to evaluate multiple windows in the optimal order
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Comments: In-reply-to David Rowley <dgrowleyml@gmail.com>
	message dated "Thu, 05 Jan 2023 15:30:59 +1300"
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Date: Wed, 04 Jan 2023 22:12:04 -0500
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David Rowley <dgrowleyml@gmail.com> writes:
> Additionally, it's also not that clear to me that sorting by more
> columns in the sort below the WindowAgg would always be a win over
> doing the final sort for the ORDER BY.  What if the WHERE clause (that
> could not be pushed down before a join) filtered out the vast majority
> of the rows before the ORDER BY. It might be cheaper to do the sort
> then than to sort by the additional columns earlier.

That's certainly a legitimate question to ask, but I don't quite see
where you figure we'd be sorting more rows?  WHERE filtering happens
before window functions, which never eliminate any rows.  So it seems
like a sort just before the window functions must sort the same number
of rows as one just after them.

			regards, tom lane