From: Dave Dutcher <dave@tridecap.com>
To: 'Scott Feldstein' <scott.feldstein@hyperic.com>
To: pgsql-performance@postgresql.org
Subject: Re: update, truncate and vacuum
Date: Thu, 26 Jul 2007 17:59:58 -0500
Message-ID: <073e01c7cfd8$b0d9d560$2e00a8c0@tridecap.com> (raw)
In-Reply-To: <B195EEA2-7B5D-444F-8F48-46BB88FCFFA8@hyperic.com>
References: <B195EEA2-7B5D-444F-8F48-46BB88FCFFA8@hyperic.com>
> From: Scott Feldstein
> Subject: [PERFORM] update, truncate and vacuum
>
> Hi,
> I have a couple questions about how update, truncate and
> vacuum would work together.
>
> 1) If I update a table foo (id int, value numeric (20, 6))
> with update foo set value = 100 where id = 1
>
> Would a vacuum be necessary after this type of operation
> since the updated value is a numeric? (as opposed to a sql
> type where its size could potentially change i.e varchar)
Yes a vacuum is still necessary. The type doesn't really matter. Postgres
effectively does a delete and insert on all updates.
> 2) After several updates/deletes to a table, if I truncate
> it, would it be necessary to run vacuum in order to reclaim the space?
No a vacuum is not necessary after a truncate because the whole data file is
deleted once a truncate commits. There aren't any dead rows because there
aren't any rows.
Dave
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To: pgsql-performance@postgresql.org
Cc: dave@tridecap.com, scott.feldstein@hyperic.com
Subject: Re: update, truncate and vacuum
In-Reply-To: <073e01c7cfd8$b0d9d560$2e00a8c0@tridecap.com>
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