Subject: Re: Big differences in plans between 8.0 and 8.1
From: Gabriele Turchi <gabriele.turchi@l39a.com>
To: Joe Conway <mail@joeconway.com>
Cc: Michael Fuhr <mike@fuhr.org>, pgsql-performance@postgresql.org,
	Marco Colombo <marco@esiway.net>
In-Reply-To: <44B94A51.9050108@joeconway.com>
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	<44B94A51.9050108@joeconway.com>
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Organization: L39A S.r.l.
Date: Sat, 15 Jul 2006 22:22:50 +0200
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Il giorno sab, 15/07/2006 alle 13.04 -0700, Joe Conway ha scritto:
> Gabriele Turchi wrote:
> > Running an ANALYZE really change the plan, now it is fast as before
> > (8.0).
> > 
> > On the production system a VACUUM FULL ANALYZE is run every morning
> > after a clean-up, when the "registrazioni" table is empty. During the
> > day this table fills up (about 500 record any day), and apparently the
> > performances are free-falling very quickly. This behaviour has not
> > changed between the old and the new installation.	
> > 
> > Can you suggest an easy way to collect and keep up-to-date these
> > statistics in a very low-impact way?
> > 
> 
> Why not just periodically (once an hour?) run "ANALYZE registrazioni;" 
> during the day. This will only update the statistics, and should be very 
> low impact.
> 

This is my "solution" too... but: is enough? Or else: there is a better
way to do this? If the performance in the better case is 50 times faster
than the worse case, during an hour (50/100 record inserted in
"registrazioni") how much the performance can fall before the new
"ANALYZE" is run? Otherwise, running ANALYZE more frequently can badly
affect the overall performance?

A so big difference in postgres performance, can be considered a bug or
a over-optimization in the plan making? Why (at least apparently) the
8.0 version is not affected?

> HTH,
> 
> Joe
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 6: explain analyze is your friend

Thank you all very much,
Gabriele