Re: Avoid extra Sort nodes between WindowAggs when sorting can be reused

From: Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>
To: Daniel Gustafsson <daniel(at)yesql(dot)se>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Avoid extra Sort nodes between WindowAggs when sorting can be reused
Date: 2018-07-27 19:12:02
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Thanks for the update.

On 07/25/2018 01:37 AM, Daniel Gustafsson wrote:
>> Hmm, this is missing the eqop fields of SortGroupClause. I haven't
>> tested yet but does the similar degradation happen if two
>> SortGroupCaluses have different eqop and the same other values?
> I wasn’t able to construct a case showing this, but I also think you’re right.
> Do you have an idea of a query that can trigger a regression? The attached
> patch adds a stab at this, but I’m not sure if it’s the right approach.

To trigger that, in your test example you could order by empno::int8 for
one window, and by empno::int2 for another. But don't I think you have
to compare eqop here, because if eqop differs, sortop will differ too. I
removed the comparison from the patch. I also clarified (I hope) the
comments, and did the optimization I mentioned earlier: using array
instead of list for active clauses. Please see the attached v6.
Otherwise I think the patch is good.

Alexander Kuzmenkov
Postgres Professional:
The Russian Postgres Company

Attachment Content-Type Size
0001-Order-windows-on-partition-ordering-prefix-to-reuse-v6.patch text/x-patch 11.1 KB

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