Query is not using index when it should

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something wrong with the majordomo, so I'm sending to general too,
where I'm already subscribed.

My problem is this, using PostgreSQL 7.4.6:

I have a table that looks like this:

Table "public.cjm_object"
Column | Type | Modifiers
-----------+-------------------+-----------
timestamp | bigint | not null
jobid | bigint | not null
objectid | bigint | not null
class | integer | not null
field | character varying | not null
data | bytea |
Indexes:
"cjm_object_pkey" primary key, btree ("timestamp", jobid, objectid, "class", field)
"idx_cjm_object1" btree (objectid, "class", field)

The table has 283465 rows, and the column combination
(objectid,class,field) can occur several times.

Doing a search with all columns in the pkey works, it uses the index:

db=# explain analyze select * from cjm_object where timestamp=1102497954815296 and jobid=9 and objectid=4534 and class=12 and field='paroid';
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
Index Scan using cjm_object_pkey on cjm_object (cost=0.00..32.75 rows=1 width=54) (actual time=0.169..0.172 rows=1 loops=1)
Index Cond: ("timestamp" = 1102497954815296::bigint)
Filter: ((jobid = 9) AND (objectid = 4534) AND ("class" = 12) AND ((field)::text = 'paroid'::text))
Total runtime: 0.381 ms
(4 rows)

But when doing a search with objectid, class and field, it doesn't use
the idx_cjm_object1 index.
db=# explain analyze select * from cjm_object where objectid=4534 and class=12 and field='paroid';
QUERY PLAN
-----------------------------------------------------------------------------------------------------------
Seq Scan on cjm_object (cost=0.00..7987.83 rows=2 width=54) (actual time=21.660..475.664 rows=1 loops=1)
Filter: ((objectid = 4534) AND ("class" = 12) AND ((field)::text = 'paroid'::text))
Total runtime: 475.815 ms
(3 rows)

I have tried to set enable_seqscan to false, but it gives the same
result, except that the estimated cost is higher.

I have also done a vacuum full analyze, and I have reindexed the
database, the table and the index. I have dropped the index and
recreated it, but it still gives the same result.

Please, could someone give me a clue to this?

Tomas