Re: [HACKERS] [PATCH] Incremental sort

From: Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>
To: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
Cc: Antonin Houska <ah(at)cybertec(dot)at>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] [PATCH] Incremental sort
Date: 2018-03-20 16:33:42
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Revised patch is attached. It's rebased to the last master.

On Fri, Mar 16, 2018 at 3:55 PM, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>

> On 03/16/2018 09:47 AM, Alexander Korotkov wrote:
> > On Fri, Mar 16, 2018 at 5:12 AM, Tomas Vondra
> > <tomas(dot)vondra(at)2ndquadrant(dot)com <mailto:tomas(dot)vondra(at)2ndquadrant(dot)com>>
> wrote:
> >
> > I agree those don't seem like an issue in the Incremental Sort patch,
> > but like a more generic costing problems.
> >
> >
> > Yes, I think so too.
> I wonder if we could make the costing a bit more pessimistic, to make
> these loses less likely, while still keeping the main wins (particularly
> for the LIMIT queries). But that seems a bit like a lost case, I guess.

Making costing more pessimistic makes sense. Revised patch does
it in quite rough way: volumes of groups in incremental sort are multiplied
by 1.5. That makes one query in regression tests to fallback to fullsort
from incremental sort. Could you test it? If this will shorten number of
cases where incremental sort causes regression, then it might be an
acceptable way to do more pessimistic costing.

> Do you think we can mark this patch RFC assuming that it have
> > already got pretty much of review previously.
> >
> Actually, I was going to propose to switch it to RFC, so I've just done
> that. I think the patch is clearly ready for a committer to take a
> closer look. I really like this improvement.
> I'm going to rerun the tests, but that's mostly because I'm interested
> if the change from i++ to i-- in cmpSortPresortedCols makes a measurable
> difference. I don't expect to find any issues, so why wait with the RFC?

Good, thanks.

Alexander Korotkov
Postgres Professional:
The Russian Postgres Company

Attachment Content-Type Size
incremental-sort-18.patch application/octet-stream 106.1 KB

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