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From: "Josh Berkus" <josh@agliodbs.com>
Subject: Re: Help on indexes
To: Stephane DEWITTE <stephane@smeso.fr>, <pgsql-sql@postgresql.org>
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Date: Wed, 15 May 2002 08:19:46 -0700
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Stephane,

> I don't understanf why it makes a seq scan on table mldecs on cases 2
> and 3,
> according that mldecs(ldec_dec_cod) has an index (named
> ldec_dec_cod_mldecs_key). The base is vacuumed and analyzed every
> night. How
> can I use the ldec_dec_cod_mldecs_key index on mldecs ?

First question: Have you actually measured the execution time of the
various queries?  There are many cases where a Seq Scan is faster than
an Index Scan.  For example, if in Case2, 50% of the entries were
values 'X' of 'Y', then a Seq Scan is the better approach.

Please determine that you actually have a problem with slow execution.
 Otherwise, the query parser is just doing its job.

-Josh Berkus