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From: Ankit Kumar Pandey <[email protected]>
To: David Rowley <[email protected]>
Cc: pghackers <[email protected]>
Subject: Re: Todo: Teach planner to evaluate multiple windows in the optimal order
Date: Wed, 4 Jan 2023 17:37:46 +0530
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAApHDvq=g2=ny59f1bvwRVvupsgPHK-KjLPBsSL25fVuGZ4idQ@mail.gmail.com>
References: <[email protected]>
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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

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