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

Daniel,

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: http://www.postgrespro.com
The Russian Postgres Company