From: Martin Nickel <martin@portant.com>
To: pgsql-performance@postgresql.org
Subject: Re: Sequential scan on FK join
Date: Mon, 17 Oct 2005 14:56:43 -0500
Message-ID: <pan.2005.10.17.19.56.36.629275@portant.com> (raw)
References: <pan.2005.10.12.20.40.22.703085@portant.com>
<4353648C.8040507@archonet.com>
<pan.2005.10.17.13.07.53.915839@portant.com>
<4353E342.2060708@archonet.com>
When I turn of seqscan it does use the index - and it runs 20 to 30%
longer. Based on that, the planner is correctly choosing a sequential
scan - but that's just hard for me to comprehend. I'm joining on an int4
key, 2048 per index page - I guess that's a lot of reads - then the data
-page reads. Still, the 8-minute query time seems excessive.
On Mon, 17 Oct 2005 18:45:38 +0100, Richard Huxton wrote:
> Martin Nickel wrote:
>> Subject: Re: Sequential scan on FK join From: Martin Nickel
>> <martin@portant.com> Newsgroups: pgsql.performance
>> Date: Wed, 12 Oct 2005 15:53:35 -0500
>>
>> Richard, here's the EXPLAIN ANALYZE. I see your point re: the 2.7M
>> expected vs the 2 actual, but I've run ANALYZE on the lead table and it
>> hasn't changed the plan. Suggestions?
>>
>> Hash Join (cost=62.13..2001702.55 rows=2711552 width=20) (actual
>> time=40.659..244709.315 rows=2 125270 loops=1)
> ^^^
> Hmm - is that not just a formatting gap there? Is it not 2,125,270 rows
> matching which would suggest PG is getting it more right than wrong.
>
> Try issuing "SET enable_seqscan=false" before running the explain analyse
> - that will force the planner to use any indexes it can find and should
> show us whether the index would help. --
> Richard Huxton
> Archonet Ltd
>
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To: pgsql-performance@postgresql.org
Cc: martin@portant.com
Subject: Re: Sequential scan on FK join
In-Reply-To: <pan.2005.10.17.19.56.36.629275@portant.com>
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