From: Josh Berkus <josh@agliodbs.com>
Organization: Aglio Database Solutions
To: Alex Stapleton <alexs@advfn.com>
Subject: Re: autovacuum suggestions for 500,000,000+ row tables?
Date: Mon, 20 Jun 2005 10:46:41 -0700
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Alex,

> Hi, i'm trying to optimise our autovacuum configuration so that it
> vacuums / analyzes some of our larger tables better. It has been set
> to the default settings for quite some time. We never delete
> anything  (well not often, and not much) from the tables, so I am not
> so worried about the VACUUM status, but I am wary of XID wraparound
> nuking us at some point if we don't sort vacuuming out so we VACUUM
> at least once every year ;) 

I personally don't use autovaccuum on very large databases.   For DW, 
vacuuming is far better tied to ETL operations or a clock schedule of 
downtime.

XID wraparound may be further away than you think.   Try checking 
pg_controldata, which will give you the current XID, and you can calculate 
how long you are away from wraparound.  I just tested a 200G data warehouse 
and figured out that we are 800 months away from wraparound, despite hourly 
ETL.

> However not running ANALYZE for such huge 
> periods of time is probably impacting the statistics accuracy
> somewhat, and I have seen some unusually slow queries at times.
> Anyway, does anyone think we might benefit from a more aggressive
> autovacuum configuration?

Hmmm, good point, you could use autovacuum for ANALYZE only.  Just set the 
VACUUM settings preposterously high (like 10x) so it never runs.   Then it'll 
run ANALYZE only.   I generally threshold 200, multiple 0.1x for analyze; 
that is, re-analyze after 200+10% of rows have changed.

-- 
Josh Berkus
Aglio Database Solutions
San Francisco