Re: PoC: Partial sort

From: Andreas Karlsson <andreas(at)proxel(dot)se>
To: Jeremy Harris <jgh(at)wizmail(dot)org>
Cc: pgsql-hackers(at)postgresql(dot)org, Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Subject: Re: PoC: Partial sort
Date: 2014-01-19 01:57:03
Message-ID: 52DB30EF.4060506@proxel.se
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On 01/18/2014 08:13 PM, Jeremy Harris wrote:
> On 31/12/13 01:41, Andreas Karlsson 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.
>
> How does this work in combination with randomAccess ?

As far as I can tell randomAccess was broken by the partial sort patch
even before my change since it would not iterate over multiple
tuplesorts anyway.

Alexander: Is this true or am I missing something?

--
Andreas Karlsson

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