Re: [PATCH] Use optimized single-datum tuplesort in ExecSort

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От Ranier Vilela
Тема Re: [PATCH] Use optimized single-datum tuplesort in ExecSort
Дата
Msg-id CAEudQApnjQ6VqhGY4SxfmYdNoonSdp_LKHCRaQGJxwFfpLHQrQ@mail.gmail.com
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Ответ на Re: [PATCH] Use optimized single-datum tuplesort in ExecSort  (David Rowley <dgrowleyml@gmail.com>)
Ответы Re: [PATCH] Use optimized single-datum tuplesort in ExecSort
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Em qua., 14 de jul. de 2021 às 21:21, David Rowley <dgrowleyml@gmail.com> escreveu:
On Thu, 15 Jul 2021 at 12:10, Ranier Vilela <ranier.vf@gmail.com> wrote:
>
> Em qua., 14 de jul. de 2021 às 20:43, David Rowley <dgrowleyml@gmail.com> escreveu:
>>
>> On Thu, 15 Jul 2021 at 05:55, Ranier Vilela <ranier.vf@gmail.com> wrote:
>> > I repeated (3 times) the benchmark with v8 here,
>> > and the results were not good.
>>
>> Do you have any good theories on why the additional branching that's
>> done in v7b vs v8 might cause it to run faster?
>
>
> Branch Predictions works with *more* probable path,
> otherwise a penalty occurs and the cpu must revert the results.

But, in v8 there is no additional branch, so no branch to mispredict.
I don't really see how your explanation fits.
In v8 the branch occurs at :
+ if (ExecGetResultType(outerPlanState(sortstate))->natts == 1)

datumSort is tested first.

Cpu time is a more expensive resource.
Always is executed two branches, if it is right path, win,
otherwise occurs a penalty time.


It seems much more likely to me that the results were just noisy.  It
would be good to see if you can recreate them consistently.
I do.
Can you please share results with v7b?

regards,
Ranier Vilela

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