The following bug has been logged online:

Bug reference: 1408
Logged by: andrzej
Email address: wppl16 (AT) wp (DOT) pl
PostgreSQL version: 8.0.0-rc5
Operating system: windows XP
Description: don't see index
Details:

Create table t1
(
v varchar(5) not null,
m varchar(20),
d date,
l varchar(10),
primary key(v)
);

explain select * from t1 where v='abc'
result:
Seq Scan on t1 (....
Filter(v)::text='abc'::text)
DON'T SEE INDEX PKEY ???????!!!!

but Postgres 7.4.2 Linux
explain select * from t1 where v='abc'
result:
Index Scan using t1_pkey on t1(...
Index Cond((v)::text='abc'::text)
SEE INDEX PKEY OK !!!!!

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TIP 8: explain analyze is your friend

On Tue, Jan 18, 2005 at 09:01:29 +0000,
andrzej <wppl16 (AT) wp (DOT) pl> wrote:
This probably isn't a bug. In some circumstances a sequential scan is faster
than an index scan.

How many rows are in this table?
Have you run an ANALYZE on the table?

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TIP 4: Don't 'kill -9' the postmaster