Re: Seq scans roadmap

From: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
To: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
Cc: Zeugswetter Andreas ADI SD <ZeugswetterA(at)spardat(dot)at>, CK Tan <cktan(at)greenplum(dot)com>, Luke Lonergan <LLonergan(at)greenplum(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Jeff Davis <pgsql(at)j-davis(dot)com>, Simon Riggs <simon(at)enterprisedb(dot)com>
Subject: Re: Seq scans roadmap
Date: 2007-05-10 17:33:28
Message-ID: 46435768.9020909@enterprisedb.com
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Heikki Linnakangas wrote:
> However, it caught me by total surprise that the performance with 1
> buffer is so horrible. Using 2 buffers is enough to avoid whatever the
> issue is with just 1 buffer. I have no idea what's causing that. There
> must be some interaction that I don't understand.

Ok, I found the reason for that. I was using this query for the selects:
SELECT COUNT(*) FROM (SELECT 1 FROM stock_copytest LIMIT 10000000) AS a;

Stock_copytest is larger than RAM size, that's why I used the LIMIT to
make the result set memory resident. That had the side effect that
apparently the limit-node kept the single buffer pinned which defeated
the buffer ring completely. To avoid issues like that we apparently want
to use 2-4 buffers instead of just 1.

I'll review my test methodology and keep testing...

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

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