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Re: turn off caching for performance test

From: Willy-Bas Loos <willybas(at)gmail(dot)com>
To: Pierre C <lists(at)peufeu(dot)com>, acmmailing(at)tweakers(dot)net
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: turn off caching for performance test
Date: 2010-08-27 09:51:21
Message-ID: AANLkTimT19FjqVB+_kjevwbW9wG7FuV6O+nSrK0p6hYc@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
@Pierre: i know.. but first i'd have to find such a query from real-life.
And also, i'm convinced that this query would be faster with a seqscan if
the data wenen't cached.


@Arjen: thanks, that helps.
But that's only the OS cache. There's also the shared_buffers, which are a
postgres specific thing.
I've found DISCARD in the
manual<http://www.postgresql.org/docs/8.3/interactive/sql-discard.html>,
but that only influences a single session, not the shared buffers.

I reckon restarting the cluster should help, would it wipe out the cache?
(pg_ctlcluster 8.3 main restart)
Or is there a more graceful way?

Cheers,

WBL


On Thu, Aug 26, 2010 at 7:37 PM, Pierre C <lists(at)peufeu(dot)com> wrote:

>
>  The bitmap heap scan is 3% faster,
>>
>
> 3% isn't really significant. Especially if the new setting makes one query
> 100 times slower... Like a query which will, by bad luck, get turned into a
> nested loop index scan for a lot of rows, on a huge table which isn't in
> cache...
>



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