Message-ID: <45FD5A2D.2090307@boreham.org>
Date: Sun, 18 Mar 2007 09:26:37 -0600
From: David Boreham <david_list@boreham.org>
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To: Barry Moore <barry.moore@genetics.utah.edu>
CC: pgsql-performance@postgresql.org
Subject: Re: Performance Tuning and Disk Cache
References: <1862C4AD-EB83-4D58-B1F2-45BDC47E5608@genetics.utah.edu>
In-Reply-To: <1862C4AD-EB83-4D58-B1F2-45BDC47E5608@genetics.utah.edu>
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Barry Moore wrote:

> I have a very slow query that I'm trying to tune.  I think my  
> performance tuning is being complicated by the system's page cache.
>
> If a run the query after the system has been busy with other tasks  
> for quite a long time then the query can take up to 8-10 minutes to  
> complete.  If I then rerun the same query it will complete in a  
> couple of seconds.
>
> Does anyone know how I can repeatedly run the same query in the  
> "worst case scenario" of no postgres data in the disk cache (e.g.,  
> clear the page cache or force it to be ignored)?

In my experience the only 100% reliable way to do this is to reboot the 
machine.