From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
---|---|
To: | Chris Browne <cbbrowne(at)acm(dot)org> |
Cc: | pgsql-sql(at)postgresql(dot)org |
Subject: | Re: Does VACUUM reorder tables on clustered indices |
Date: | 2005-12-22 04:07:13 |
Message-ID: | 200512220407.jBM47Dp20260@candle.pha.pa.us |
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Lists: | pgsql-hackers pgsql-sql |
Chris Browne wrote:
> jnasby(at)pervasive(dot)com ("Jim C. Nasby") writes:
> > On Wed, Dec 21, 2005 at 12:34:12AM +0100, ipv(at)tinet(dot)org wrote:
> >> Hi,
> >>
> >> Utilize <b>CLUSTER;</b> (after vacuum) to reorder the data.
> >
> > Why would you vacuum when cluster is just going to wipe out the dead
> > tuples anyway?
>
> There is one reason to VACUUM before running CLUSTER...
>
> That is that VACUUM will be *guaranteed* to draw all the pages into memory.
>
> Subsequently, you can be certain that the pages are in cache, and that
> the CLUSTER should need to do minimal I/O to read data into memory.
>
> If I'm considering clustering the Slony-I "sl_log_1" table, forcing it
> into memory *is* something I'll consider doing in order to minimize
> the time that would-be writers are blocked from writing...
Why don't you just do SELECT * FROM tab WHERE col != 'lkjasdflkjadsf'.
That should pull things into memory without the VACUUM overhead.
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
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