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Re: Query Plan - Index Scan & Seq Scan

From: Scott Marlowe <smarlowe(at)g2switchworks(dot)com>
To: dbadmin(at)nqadmin(dot)com
Cc: pgsql-admin(at)postgresql(dot)org
Subject: Re: Query Plan - Index Scan & Seq Scan
Date: 2005-05-12 15:51:26
Message-ID: 1115913086.22291.1.camel@state.g2switchworks.com (view raw or flat)
Thread:
Lists: pgsql-admin
On Thu, 2005-05-12 at 10:51, Prasanth wrote:
> I agree with you.
> 
> But I have the where conditions on the tables I was expecting the planner to
> user index scan but it went for seq scan.
> 
> I did a little testing using what you said.
> 
> Below are the results.
> 
> SELECT a.id FROM a,b WHERE a.id = b.id AND a.code >2 AND b.account_id = 16221;
> 
> Total runtime: 18194.936 ms
> 
> Then I set the seqscan off and ran the same query.
> 
> Total runtime: 27.554 ms

Good!  This tells us two things, 1:  Your database can use the indexes
(sometimes indexes can't be used for various reasons, which are quickly
disappearing by the way.) and 2:  Your database is making the wrong
choice about when to use a seq scan versus an index.

What does the explain analyze output from that query say about row
estimates versus actual rows returned?

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Subject: Re: Query Plan - Index Scan & Seq Scan
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