Subject: Multicolum index and primary key
From: Michele Bendazzoli <mickymouse@mickymouse.it>
To: pgsql-sql@postgresql.org
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Organization: Micky Mouse
Message-Id: <1069088432.8338.16.camel@mickymouse.sintel>
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Date: Mon, 17 Nov 2003 18:00:32 +0100
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I wonder if is useless to set some indexes for columns contained in a
multifield primary key.

Suppose by example that one have a table1 with a primary key over three
field (a, b, c): 

ALTER TABLE public.table1
  ADD CONSTRAINT table1_pkey PRIMARY KEY(a, b, c);

are the indexes over (a) and (a, b) redundant (and so useless)?

I.e. with other words the question is, if I have a primary key set for
the a, b, c fields the queries 

SELECT 1 from table1 where a='a value'

and

SELECT 1 from table1 where a='a value' AND b='another value'

use the implicit index created by the primary key or not?

Thank you in advance for any advice.

ciao, Michele 

p.s. I know, I'll have to begin to use the explain command ...
I promise I'll do it ;-)