Date: Wed, 10 Jan 2007 19:27:09 -0600
From: "Jim C. Nasby" <jim@nasby.net>
To: Jeremy Haile <jhaile@fastmail.fm>
Cc: pgsql-performance@postgresql.org
Subject: Re: High inserts, bulk deletes - autovacuum vs scheduled vacuum
Message-ID: <20070111012709.GG12217@nasby.net>
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On Wed, Jan 10, 2007 at 04:48:42PM -0500, Jeremy Haile wrote:
> > BTW, that's the default values for analyze... the defaults for vacuum
> > are 2x that.
> 
> Yeah - I was actually more concerned that tables would need to be
> analyzed more often than I was about vacuuming too often, so I used
> analyze as the example.  Since my app is inserting constantly throughout
> the day and querying for "recent" data - I want to make sure the query
> planner realizes that there are lots of rows with new timestamps on
> them.  In other words, if I run a query "select * from mytable where
> timestamp > '9:00am'" - I want to make sure it hasn't been a day since
> the table was analyzed, so the planner thinks there are zero rows
> greater than 9:00am today.
 
Well, analyze is pretty cheap. At most it'll read only 30,000 pages,
which shouldn't take terribly long on a decent system. So you can be a
lot more aggressive with it.

> > What's more important
> > is to make sure critical tables (such as queue tables) are getting
> > vacuumed frequently so that they stay small. 
> 
> Is the best way to do that usually to lower the scale factors?  Is it
> ever a good approach to lower the scale factor to zero and just set the
> thresholds to a pure number of rows? (when setting it for a specific
> table)

The problem is what happens if autovac goes off and starts vacuuming
some large table? While that's going on your queue table is sitting
there bloating. If you have a separate cronjob to handle the queue
table, it'll stay small, especially in 8.2.
-- 
Jim Nasby                                            jim@nasby.net
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)