Re: [PATCH] Incremental sort (was: PoC: Partial sort)

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: James Coleman <jtc331(at)gmail(dot)com>
Cc: Peter Geoghegan <pg(at)bowt(dot)ie>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Rafia Sabih <rafia(dot)pghackers(at)gmail(dot)com>, Shaun Thomas <shaun(dot)thomas(at)2ndquadrant(dot)com>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: [PATCH] Incremental sort (was: PoC: Partial sort)
Date: 2019-07-07 12:34:49
Message-ID: 20190707123449.w3b2vetwv4y42evj@development
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On Thu, Jul 04, 2019 at 09:29:49AM -0400, James Coleman wrote:
>On Tue, Jun 25, 2019 at 7:22 PM Tomas Vondra
><tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>> On Tue, Jun 25, 2019 at 04:53:40PM -0400, James Coleman wrote:
>> >
>> >Unrelated: if you or someone else you know that's more familiar with
>> >the parallel code, I'd be interested in their looking at the patch at
>> >some point, because I have a suspicion it might not be operating in
>> So I've looked into that, and the reason seems fairly simple - when
>> generating the Gather Merge paths, we only look at paths that are in
>> partial_pathlist. See generate_gather_paths().
>> And we only have sequential + index paths in partial_pathlist, not
>> incremental sort paths.
>> IMHO we can do two things:
>> 1) modify generate_gather_paths to also consider incremental sort for
>> each sorted path, similarly to what create_ordered_paths does
>> 2) modify build_index_paths to also generate an incremental sort path
>> for each index path
>> IMHO (1) is the right choice here, because it automatically does the
>> trick for all other types of ordered paths, not just index scans. So,
>> something like the attached patch, which gives me plans like this:
>> But I'm not going to claim those are total fixes, it's the minimum I
>> needed to do to make this particular type of plan work.
>Thanks for looking into this!
>I intended to apply this to my most recent version of the patch (just
>sent a few minutes ago), but when I apply it I noticed that the
>partition_aggregate regression tests have several of these failures:
>ERROR: could not find pathkey item to sort
>I haven't had time to look into the cause yet, so I decided to wait
>until the next patch revision.

I wanted to investigate this today, but I can't reprodure it. How are
you building and running the regression tests?

Attached is a patch adding the incremental sort below gather merge, and
also tweaking the costing. But that's mostly for and better planning
decisions, I don't get any pathkey errors even with the first patch.


Tomas Vondra
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
parallel-incremental-sort-v2.patch text/plain 2.0 KB

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