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Re: Vacuum-full very slow

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Listmail <lists(at)peufeu(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Steve Crawford <scrawford(at)pinpointresearch(dot)com>, pgsql-general(at)postgresql(dot)org
Subject: Re: Vacuum-full very slow
Date: 2007-04-27 13:18:08
Message-ID: 20070427131808.GE4645@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-generalpgsql-hackers
Martijn van Oosterhout wrote:
> On Thu, Apr 26, 2007 at 12:13:13AM +0200, Listmail wrote:
> > 	VACUUM FULL is slow because it plays with the indexes...
> > 	CLUSTER is slow because it has to order the rows...
> 
> And:
> VACUUM FULL has to seek/read/write all over the disk to get it's job
> done.
> CLUSTER can scan through the table linearly a few times and write out
> the result.
> Now it's true that sorting large files involves overflowing to disk,
> but that path has been pretty well optimised.

Hmm, no, CLUSTER doesn't scan the table linearly; it uses an indexscan,
so it also needs seek/read/seek/read/write.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

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