| From: | Peter Geoghegan <pg(at)heroku(dot)com> |
|---|---|
| To: | Greg Stark <stark(at)mit(dot)edu> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
| Subject: | Re: Using quicksort for every external sort run |
| Date: | 2016-02-08 00:50:44 |
| Message-ID: | CAM3SWZS8Ppe3pC8HJSKc+6HgOxUseBDm-d7uoaKzT1PdH6wb=g@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Sun, Feb 7, 2016 at 10:51 AM, Greg Stark <stark(at)mit(dot)edu> wrote:
>> > You don't want to change the behavior of the current patch for the
>> > second or subsequent run; that should remain a quicksort, pure and
>> > simple. Do I have that right?
>>
>> Yes.
>
> I'm not even sure this is necessary. The idea of missing out on
> producing a single sorted run sounds bad but in practice since we
> normally do the final merge on the fly there doesn't seem like there's
> really any difference between reading one tape or reading two or three
> tapes when outputing the final results. There will be the same amount
> of I/O happening and a 2-way or 3-way merge for most data types should
> be basically free.
I basically agree with you, but it seems possible to fix the
regression (generally misguided though those regressed cases are).
It's probably easiest to just fix it.
--
Peter Geoghegan
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