| From: | Peter Geoghegan <pg(at)heroku(dot)com> |
|---|---|
| To: | Greg Stark <stark(at)mit(dot)edu> |
| Cc: | Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Jeremy Harris <jgh(at)wizmail(dot)org> |
| Subject: | Re: Using quicksort and a merge step to significantly improve on tuplesort's single run "external sort" |
| Date: | 2015-07-30 23:18:15 |
| Message-ID: | CAM3SWZT4zVfo864x9oaJudquotGYMccr3F3VBNmSyYQY11oRqg@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Jul 30, 2015 at 4:26 AM, Greg Stark <stark(at)mit(dot)edu> wrote:
> I'm a bit confused where the big win comes from though. Is what's going on
> that the external sort only exceeded memory by a small amount so nearly all
> the tuples are still in memory?
Yes, that's why this can be much faster just as the work_mem threshold
is crossed. You get an "almost internal" sort, which means you can
mostly quicksort, and you can avoid dumping most tuples. It's still a
pretty nice win when less than half of tuples fit in memory, though --
just not as nice. Below that, the optimization isn't used.
--
Peter Geoghegan
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