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turn off caching for performance test

From: Willy-Bas Loos <willybas(at)gmail(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: turn off caching for performance test
Date: 2010-08-26 10:32:57
Message-ID: AANLkTi=dBXkp2qMHZOR_gd9eD+cjwMqOwg-N6POGq+ji@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
Hi,

I have a colleague that is convinced that the website is faster if
enable_seqscan is turned OFF.
I'm convinced of the opposite (better to leave it ON), but i would like to
show it, prove it to him.
Now the first query we tried, would do a bitmap heap scan instead of a
seqscan when the latter were disabled, to exclude about 50% of the records
(18K of 37K records).
The bitmap heap scan is 3% faster, so that didn't really plea my case.
The thing is that by the time we tried it, the data had been cached, so
there is no penalty for the use of the index (HDD retention on random
access). So it's logical that the index lookup is faster, it looks up less
records.

Now i'm looking for a way to turn off the caching, so that we'll have a fair
test.

It makes no sense to me to set shared_buffers really low. Any tips?

Cheers,

WBL


-- 
"Patriotism is the conviction that your country is superior to all others
because you were born in it." -- George Bernard Shaw

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Next:From: Arjen van der MeijdenDate: 2010-08-26 16:50:11
Subject: Re: turn off caching for performance test
Previous:From: A. KretschmerDate: 2010-08-26 05:24:20
Subject: Re: SubQuery Performance

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