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Re: Inlining comparators as a performance optimisation

From: Peter Geoghegan <peter(at)2ndquadrant(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Inlining comparators as a performance optimisation
Date: 2011-11-29 18:48:37
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On 29 November 2011 15:31, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> These are exciting advanced you are producing and I am hopeful we can
> get this included in Postgres 9.2.

Thanks Bruce.

>I have mentioned already that I
> think parallelism is the next big Postgres challenge, and of course, one
> of the first areas for parallelism is sorting.

I'm not sure that sorting has that much to recommend it as an initial
target of some new backend parallelism other than being easy to
implement. I've observed the qsort_arg specialisations in this patch
out-perform stock qsort_arg by as much as almost 3 times. However, the
largest decrease in a query's time that I've observed was 45%, and
that was for a contrived worst-case for quicksort, but about 25% is
much more typical of queries similar to the ones I've shown, for more
normative data distributions. While that's a respectable gain, it
isn't a paradigm shifting one, and it makes parallelising qsort itself
for further improvements quite a lot less attractive - there's too
many other sources of overhead.

Peter Geoghegan
PostgreSQL Development, 24x7 Support, Training and Services

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