Re: PoC: Partial sort

From: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
To: Andreas Karlsson <andreas(at)proxel(dot)se>
Cc: David Rowley <dgrowleyml(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Andres Freund <andres(at)2ndquadrant(dot)com>, Martijn van Oosterhout <kleptog(at)svana(dot)org>
Subject: Re: PoC: Partial sort
Date: 2014-01-13 18:01:20
Message-ID: CAPpHfds-Ssqcxjt_PHt30zJpmwGK3ozo00p-O-1n=67kX=bdNA@mail.gmail.com
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On Tue, Dec 31, 2013 at 5:41 AM, Andreas Karlsson <andreas(at)proxel(dot)se> wrote:

> On 12/29/2013 08:24 AM, David Rowley wrote:
>
>> If it was possible to devise some way to reuse any
>> previous tuplesortstate perhaps just inventing a reset method which
>> clears out tuples, then we could see performance exceed the standard
>> seqscan -> sort. The code the way it is seems to lookup the sort
>> functions from the syscache for each group then allocate some sort
>> space, so quite a bit of time is also spent in palloc0() and pfree()
>>
>> If it was not possible to do this then maybe adding a cost to the number
>> of sort groups would be better so that the optimization is skipped if
>> there are too many sort groups.
>>
>
> It should be possible. I have hacked a quick proof of concept for reusing
> the tuplesort state. Can you try it and see if the performance regression
> is fixed by this?
>
> One thing which have to be fixed with my patch is that we probably want to
> close the tuplesort once we have returned the last tuple from ExecSort().
>
> I have attached my patch and the incremental patch on Alexander's patch.

Thanks. It's included into attached version of patch. As wall as estimation
improvements, more comments and regression tests fix.

------
With best regards,
Alexander Korotkov.

Attachment Content-Type Size
partial-sort-5.patch.gz application/x-gzip 14.5 KB

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