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

On 04/01/23 09:32, David Rowley wrote:
>
> It looks like that works by accident. I see no mention of this either
> in the comments or in [1].

This kind of troubles me because function name
/select_active_windows///doesn't tell me if its only job is

to reorder window clauses for optimizing sort. From code, I don't see it
doing anything else either.

> If we don't have one already, then we should likely add a regression
> test that ensures that this remains true. Since it does not seem to
> be documented in the code anywhere, it seems like something that could
> easily be overlooked if we were to ever refactor that code.
>
I don't see any tests in windows specific to sorting operation (and in
what order). I will add those.

Also, one thing, consider the following query:

explain analyze select row_number() over (order by a,b),count(*) over
(order by a) from abcd order by a,b,c;

In this case, sorting is done on (a,b) followed by incremental sort on c
at final stage.

If we do just one sort: a,b,c at first stage then there won't be need to
do another sort (incremental one).

Now, I am not sure if which one would be faster: sorting (a,b,c) vs
sort(a,b) + incremental sort(c)

because even though datum sort is fast, there can be n number of combos
where we won't be doing that.

I might be looking at extreme corner cases though but still wanted to share.

--
Regards,
Ankit Kumar Pandey