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Re: Why those queries do not utilize indexes?

From: Artimenko Igor <igorart7(at)yahoo(dot)com>
To: Dennis Bjorklund <db(at)zigo(dot)dhs(dot)org>
Subject: Re: Why those queries do not utilize indexes?
Date: 2004-08-27 21:08:58
Message-ID: 20040827210858.16196.qmail@web11904.mail.yahoo.com (view raw or flat)
Thread:
Lists: pgsql-performance
I could force Postgres to use the best index by removing condition "msgstatus = CAST( 0 AS
smallint );" from WHERE clause & set enable_seqscan to off;
Total runtime in this case dropped from 1883 ms ( sequential reads ) to 1.598 ms ( best index ).

But unfortunatelly It does not resolve my problem. I can not remove above condition. I need to
find a way to use whole condition "WHERE user_id = CAST( 20000 AS BIGINT ) and msgstatus = CAST( 0
AS smallint );" and still utilyze index.  

Yes you are right. Using "messagesStatus" index is even worse for my data set then sequential
scan.

Igor Artimenko

--- Dennis Bjorklund <db(at)zigo(dot)dhs(dot)org> wrote:

> On Fri, 27 Aug 2004, Artimenko Igor wrote:
> 
> > 1. Sequential search and very high cost if set enable_seqscan to on;
> > Seq scan on messageinfo ( cost=0.00..24371.30, rows =36802 )
> > 
> > 2. Index scan but even bigger cost if set enable_seqscan to off;
> > Index messagesStatus on messageinfo ( Cost=0.00..27220.72, rows=36802 )
> 
> So pg thinks that a sequential scan will be a little bit faster (The cost 
> is a little bit smaller). If you compare the actual runtimes maybe you 
> will see that pg was right. In this case the cost is almost the same so 
> the runtime is probably almost the same.
> 
> When you have more data pg will start to use the index since then it will 
> be faster to use an index compared to a seq. scan.
> 
> -- 
> /Dennis Bjrklund
> 
> 



		
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