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Message-ID: <466EE403.5080200@enterprisedb.com>
Date: Tue, 12 Jun 2007 19:20:51 +0100
From: Heikki Linnakangas <heikki@enterprisedb.com>
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To: Vladimir Stankovic <V.Stankovic@city.ac.uk>
CC: PostgreSQL Performance <pgsql-performance@postgresql.org>
Subject: Re: Variable (degrading) performance
References: <466D848F.2010505@city.ac.uk> <466D8BAF.604@enterprisedb.com>
	<466EBAAD.9030905@city.ac.uk>
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Vladimir Stankovic wrote:
> What I am hoping to see is NOT the same value for all the executions of 
> the same type of transaction (after some transient period). Instead, I'd 
> like to see that if I take appropriately-sized set of transactions  I 
> will see at least steady-growth in transaction average times, if not 
> exactly the same average. Each chunk would possibly include sudden 
> performance drop due to the necessary vacuum and checkpoints. The 
> performance might be influenced by the change in the data set too.
> I am unhappy about the fact that durations of experiments can differ 
> even 30% (having in mind that they are not exactly the same due to the 
> non-determinism on the client side) . I would like to eliminate this 
> variability. Are my expectations reasonable? What could be the cause(s) 
> of this variability?

You should see that if you define your "chunk" to be long enough. Long 
enough is probably hours, not minutes or seconds. As I said earlier, 
checkpoints and vacuum are a major source of variability.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com