Re: Autovacuum daemon functionality questions

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Gnanakumar <gnanam(at)zoniac(dot)com>
Cc: pgsql-admin(at)postgresql(dot)org
Subject: Re: Autovacuum daemon functionality questions
Date: 2010-04-08 14:53:28
Message-ID: 20100408145328.GB4126@alvh.no-ip.org
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Gnanakumar wrote:

> 1. Why need to have 2 parameters (base threshold and scale factor) to define
> the threshold value, when either one of the parameter is more than enough to
> define the threshold value. Can you explain the significance of having both
> parameters. What is the real-time advantage of this?

real-time advantage? They are just the two factors in a linear
equation.

> 2. Documentation says ".... If the number of obsolete tuples since the last
> VACUUM exceeds the "vacuum threshold", the table is vacuumed ...".
> I also know about this table "pg_stat_user_tables" which has columns
> n_tup_ins, n_tup_upd, n_tup_del, last_autovaccum and last_autoanalyze.
> Since INSERT, UPDATE and DELETE count gets incremented everytime and do not
> reset after running autovacuum/autoanalyze, how does autovacuum identifies
> obsolete tuples since last VACUUM from this entries.

There are two separate counters for live and dead tuples, IIRC (though
they may not be exposed in the pg_stat views)

> 3. Is there a way to see autovacuum daemon log entries?

Not in 8.2.

--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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