| From: | Claudio Freire <klaussfreire(at)gmail(dot)com> |
|---|---|
| To: | "Graeme B(dot) Bell" <grb(at)skogoglandskap(dot)no> |
| Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Merlin Moncure <mmoncure(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, postgres performance list <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Yet another abort-early plan disaster on 9.3 |
| Date: | 2014-09-30 15:59:34 |
| Message-ID: | CAGTBQpbZkgZzrEdc4jYqsaHmi29SOGppBUtcSqbbamCoBh+50Q@mail.gmail.com |
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| Lists: | pgsql-hackers pgsql-performance |
On Tue, Sep 30, 2014 at 8:34 AM, Graeme B. Bell <grb(at)skogoglandskap(dot)no> wrote:
>
>>> The existing cost estimation
>>> code effectively assumes that they're perfectly uniformly distributed;
>>> which is a good average-case assumption but can be horribly wrong in
>>> the worst case.
>
>
> Sorry, just an outsider jumping in with a quick comment.
>
> Every year or two the core count goes up. Can/should/does postgres ever attempt two strategies in parallel, in cases where strategy A is generally good but strategy B prevents bad worst case behaviour? Kind of like a Schrödinger's Cat approach to scheduling.
> What problems would it raise?
Interleaved I/O, that would kill performance for both plans if it
happens on rotating media.
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