| From: | Marti Raudsepp <marti(at)juffo(dot)org> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Alexander Korotkov <aekorotkov(at)gmail(dot)com>, Andreas Karlsson <andreas(at)proxel(dot)se>, David Rowley <dgrowleyml(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Andres Freund <andres(at)2ndquadrant(dot)com>, Martijn van Oosterhout <kleptog(at)svana(dot)org> |
| Subject: | Re: PoC: Partial sort |
| Date: | 2014-02-06 08:39:41 |
| Message-ID: | CABRT9RBxkTGv2QD8rAvynqVq4zhWQSLhE07iuDCeyVLUzmGJSA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Feb 6, 2014 at 5:31 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Hmm, sounds a little steep. Why is it so expensive? I'm probably
> missing something here, because I would have thought that planner
> support for partial sorts would consist mostly of considering the same
> sorts we consider today, but with the costs reduced by the batching.
I guess it's because the patch undoes some optimizations in the
mergejoin planner wrt caching merge clauses and adds a whole lot of
code to find_mergeclauses_for_pathkeys. In other code paths the
overhead does seem to be negligible.
Notice the removal of:
/* Select the right mergeclauses, if we didn't already */
/*
* Avoid rebuilding clause list if we already made one;
* saves memory in big join trees...
*/
Regards,
Marti
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