From: | Richard Huxton <dev(at)archonet(dot)com> |
---|---|
To: | PostgreSQL general <pgsql-general(at)postgresql(dot)org> |
Subject: | Re: Tuple storage overhead |
Date: | 2010-04-16 11:39:35 |
Message-ID: | 4BC84C77.4080504@archonet.com |
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Lists: | pgsql-general |
On 16/04/10 10:41, Peter Bex wrote:
> Hi all,
>
> I have a table with three columns: one integer and two doubles.
> There are two indexes defined (one on the integer and one on one
> of the doubles). This table stores 700000 records, which take up
> 30 Mb according to pg_relation_size(), and the total relation size
> is 66 Mb.
[snip]
> Is there a way to reduce the per-tuple storage overhead?
Short answer - no.
The database has to track visibility of every row - when it was
inserted, deleted etc to support the MVCC concurrency system.
http://www.postgresql.org/docs/8.4/static/storage-page-layout.html
That means 24 bytes of overhead (on most systems) for each row. That's
higher than some other RDBMS but they'll all have some overhead.
> The reason I'm asking is that I have tons of tables like this,
> and some data sets are much bigger than this. In a relatively
> simple testcase I'm importing data from text files which are
> 5.7 Gb in total, and this causes the db size to grow to 34Gb.
Anything from double to ten times the size isn't unexpected, depending
on row-sizes and how many indexes you are talking about.
--
Richard Huxton
Archonet Ltd
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