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Re: increasing shared buffer slow downs query performance.

From: "Pavan Deolasee" <pavan(dot)deolasee(at)gmail(dot)com>
To: "Andreas Kretschmer" <akretschmer(at)spamfence(dot)net>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: increasing shared buffer slow downs query performance.
Date: 2008-03-25 12:23:58
Message-ID: 2e78013d0803250523w28a16315w3e5ec0846155c8b7@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Mon, Mar 24, 2008 at 3:37 PM, Andreas Kretschmer
<akretschmer(at)spamfence(dot)net> wrote:
> petchimuthu lingam <spmlingam(at)gmail(dot)com> schrieb:
>
>
>  > Hi friends,
>  >
>  > I am using postgresql 8.1, I have shared_buffers = 50000, now i execute the
>  > query, it takes 18 seconds to do sequential scan, when i reduced to 5000, it
>  > takes one 10 seconds, Why.
>
>  Wild guess: the second time the data are in the filesystem cache.
>


Another wild possibility is that the first query sets the hint bits for the
rows involved and hence the second time it runs fast. May be you want
to run the query few times in both the settings and then compare.

Thanks,
Pavan

-- 
Pavan Deolasee
EnterpriseDB http://www.enterprisedb.com

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