7.1.3 not using index

Hello,

After fresh install of PostgreSQL 7.1.3 I was having one particular query 
(JOIN), running for several hours. Upon closer investigation, it was returning 
weird EXPLAIN 'optimisations' (in essence, doing 'index' searches on fields 
that were not constrained in the query etc). The same query has reasonable 
EXPLAIN and executes fine under 7.0.2.

I tried to re-create the table by table, starting with the following:

CREATE TABLE r (a integer,b integer,c integer,d integer
);

CREATE INDEX r_d_idx on r(d);

COPY r FROM stdin;
1    4234    4324    4
1    4342    886    8
[...]
\.

(table has ~30k rows)

EXPLAIN SELECT * FROM r where d = 8;

The result is 

NOTICE: QUERY PLAN:

Seq Scan on r (cost=0.00...3041.13 rows=7191 width=4)

Does not matter if I VACUUM ANALYZE the table or the whole database.

Any ideas why this happens?

PostgreSQL is compiled with 

./configure --enable-locale --with-perl --with-python --with-tcl --enable-odbc 
--with-krb4 --with-openssl --enable-syslog --with-includes=/usr/include/kerbero
sIV:/usr/contrib/include

this, under BSD/OS 4.2

Thanks in advance for any ideas,
Daniel