From: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
---|---|
To: | Tomasz Ostrowski <tometzky(at)batory(dot)org(dot)pl> |
Cc: | pgsql-bugs(at)postgresql(dot)org, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: query very slow when enable_seqscan=on |
Date: | 2006-07-03 23:05:42 |
Message-ID: | 1151967943.2479.330.camel@localhost.localdomain |
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Lists: | pgsql-bugs pgsql-performance |
On Mon, 2006-07-03 at 22:31 +0200, Tomasz Ostrowski wrote:
> I have a very slow query when enable_seqscan=on and very fast when
> enable_seqscan=off. My schema looks like this (relevant columns
> only):
> PS. Actual table and column names are different (they're in Polish)
> but I've translated them for better readability for english-speaking.
Thanks
> PS. I wonder if it makes sense to "enable_seqscan=off" for every client
> if a database is small enough to fit in OS cache.
You can set this for individual statements if you choose to.
> -> Seq Scan on organization (cost=0.00..480.95 rows=1
> width=4) (actual time=0.071..69.702 rows=1892 loops=1)
> Filter: ((organization_location)::text ~*
> 'warszawa'::text)
The issue is caused by the under-estimation of the number of rows in the
table as a result of the regular expression comparison. As a result the
planner thinks it can choose a nested loops scan, though ends up doing
1892 seq scans of persons, when it thought it would do only one.
The under estimation is a known issue. Posting to -perform for the
record.
--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com
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