diskspace usage recovered on db rebuild

From: David Link <dlink(at)soundscan(dot)com>
To: pgsql-general(at)postgresql(dot)org
Subject: diskspace usage recovered on db rebuild
Date: 2005-04-01 17:21:23
Message-ID: 424D8313.9080801@soundscan.com
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Greetings worthymen.

I have a question regarding filesystem disk space usage.

We have a production database containing 5 years of sales data.
Linux 2.6.5; Postgresq 7.4.7. VACUUM ANALZYE the entire database
everynight (about 40min).
It's size, @SUM(pg_class.relpages) * 8192K, is ...

About 66 Gigabytes on disk.

When I rebuild the database (using pg_dump and pgsql ), the new
resultant database is ..

About 48 Gigabytes on disk.

A 27% space savings.

Can someone tell me why that is?

Thank you,
David Link
White Plains, NY
Long live Postgres.

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