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Re: So, is COUNT(*) fast now?

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: So, is COUNT(*) fast now?
Date: 2011-10-23 23:01:01
Message-ID: CAMkU=1wp3FskYnCjks4Sj-WZ6wBqHaSTr-0EF6-EDF1LyTDQ-w@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Fri, Oct 21, 2011 at 12:52 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> Also, this line is kind of expensive:
>
>        if (!visibilitymap_test(scandesc->heapRelation,
>                                ItemPointerGetBlockNumber(tid),
>                                &node->ioss_VMBuffer))
>
> Around 2%.  But I don't see any way to avoid that, or even make it cheaper.

Could we cache by ItemPointerGetBlockNumber(tid) the results of those
tests, for groups of tids on the same index page?

How useful this would be would depend on how well-clustered the table
and index are.


Cheers,

Jeff

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