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

On Tue, 10 Jan 2023 at 18:36, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> David Rowley <dgrowleyml@gmail.com> writes:
> > Ideally, our sort costing would just be better, but I think that
> > raises the bar a little too high to start thinking of making
> > improvements to that for this patch.
>
> It's trickier than it looks, cf f4c7c410e.  But if you just want
> to add a small correction based on number of columns being sorted
> by, that seems within reach.  See the comment for cost_sort though.
> Also, I suppose for incremental sorts we'd want to consider only
> the number of newly-sorted columns, but I'm not sure if that info
> is readily at hand either.

Yeah, I had exactly that in mind when I mentioned about setting the
bar higher. It seems like a worthy enough goal to improve the sort
costs separately from this work. I'm starting to consider if we might
need to revisit cost_sort() anyway. There's been quite a number of
performance improvements made to sort in the past few years and I
don't recall if anything has been done to check if the sort costs are
still realistic. I'm aware that it's a difficult problem as the number
of comparisons is highly dependent on the order of the input rows.

David