db stats vs table stats

From: "Ed L(dot)" <pgsql(at)bluepolka(dot)net>
To: pgsql-general(at)postgresql(dot)org
Subject: db stats vs table stats
Date: 2007-02-23 21:02:34
Message-ID: 200702231402.34188.pgsql@bluepolka.net
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I've been periodically collecting the stats stored in
pg_statio_all_tables and pg_stat_database for ~30 different
clusters, and have noticed a curiosity.

I would have thought that for a given period, the change in
pg_stat_database.blks_read would be <= the sum of the changes in
pg_statio_user_tables.heap_blks_read +
pg_statio_user_tables.idx_blks_read +
pg_statio_user_tables.toast_blks_read +

In short, the total would be <= heap + idx + toast + idx for user

It does not appear that way. The table-level IO stats appear to
be typically 1-2 orders of magnitude larger than the db-level
stats. Can anyone explain that?



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