Subject: Re: Tip ?
From: Scott Marlowe <smarlowe@g2switchworks.com>
To: Alain <alainm@pobox.com>
Cc: Sql-Postgre <pgsql-sql@postgresql.org>
In-Reply-To: <429371EA.30502@pobox.com>
References: <05d801c41bd8$daf4f8f0$0403a8c0@fejleszt4>
	<42930447.2030801@gmail.com>  <429371EA.30502@pobox.com>
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Date: Tue, 24 May 2005 13:34:07 -0500

On Tue, 2005-05-24 at 13:26, Alain wrote:
> This tip was at the end of a message (from Sz=C5=B1cs G=C3=A1bor).
>=20
> > TIP 9: the planner will ignore your desire to choose an index scan if y=
our
> >      joining column's datatypes do not match
>=20
> it looks very important, but I cannot understand it. Sound as a small=20
> and easy mistake that can make things go sour...
>=20
> Can someone explain it please?

Note that this is mostly fixed in V 8.0

What it means is that if you have a table with a field of type numeric,
and you join another table against it with a field of type integer, the
query planner won't know it can use the indexes on those two fields
(assuming you DO have indexes on them) when joining and use a sequential
scan all the time.

Casting one field to the other's type, or changing the type of one to
the other's type will allow the planner to use the index.