Re: Todo: Teach planner to evaluate multiple windows in the optimal order

On Thu, 5 Jan 2023 at 16:12, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> 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.

Yeah, I didn't think the WHERE clause thing out carefully enough. I
think it's only the WindowClause's runCondition that could possibly
filter any rows between the Sort below the WindowAgg and before the
ORDER BY is evaluated.

David