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Re: Possible documentation error

From: Martijn van Oosterhout <kleptog(at)svana(dot)org>
To: "D'Arcy J(dot)M(dot) Cain" <darcy(at)druid(dot)net>
Cc: pgsql-hackers(at)PostgreSQL(dot)org
Subject: Re: Possible documentation error
Date: 2006-12-26 17:12:45
Message-ID: 20061226171245.GD8412@svana.org (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Dec 26, 2006 at 12:04:40PM -0500, D'Arcy J.M. Cain wrote:
> I have been testing this statement and find that it seems not quite
> true. Although ctid changes on update, VACUUM FULL does not change it.
> What it does do is make lower areas available again so an update after a
> VACUUM FULL can reuse lower numbers rather than higher ones before.

A VACUUM FULL will try to compact a table. Thus if there's a lot of
free space at the beginning, it will move tuples near the end to the
beginning.

> Now it certainly seems to me that it should behave as described given
> the definition of VACUUM FULL so I am a little confused by my tests.
> My test table only has two entries in it.  Is that the issue?  In fact,
> I find the same behaviour if I do a simple VACUUM on the table.

On a table with two entries, VACUUM FULL is going to do nothing of
interest. Moving tuples within a page is useless, generally.

Have a nice day,
-- 
Martijn van Oosterhout   <kleptog(at)svana(dot)org>   http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.

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Subject: Re: Possible documentation error
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