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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200512220407.jBM47Dp20260@candle.pha.pa.us>
Subject: Re: Does VACUUM reorder tables on clustered indices
In-Reply-To: <60d5jqja42.fsf@dba2.int.libertyrms.com>
To: Chris Browne <cbbrowne@acm.org>
Date: Wed, 21 Dec 2005 23:07:13 -0500 (EST)
CC: pgsql-sql@postgresql.org
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Chris Browne wrote:
> jnasby@pervasive.com ("Jim C. Nasby") writes:
> > On Wed, Dec 21, 2005 at 12:34:12AM +0100, ipv@tinet.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.

-- 
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