Using bigint needs explicit cast to use the index

Hi,

I've recently converted a database to use bigint for the indices. Suddenly
simple queries like

select * from new_test_result where parent_id = 2

are doing full table scans instead of using the index. The table has over 4
million rows, of which only 30 or so would be selected by the query.

The database table in question was fully vacuumed and clustered on the
index.

I tried disabling seqscan, but it still did full table scan. After browsing
around a bit, I had a hunch it might be failing to use the index because it
is perhaps converting the parent_id to an integer, and I don't have a
functional index on that (wouldn't seem correct either).

I tested my hunch by casting the constant to bigint (as can be seen below)
and suddenly the query is using the index again.

We are currently using pg 7.3.4. Is this intended behaviour? Should the
constant be cast to the type of the table column where possible, or should
it be the other way around? If this is considered a bug, is it already
fixed, in 7.3.6 or 7.4.x?

Kind Regards,
Steve Butler

steve=# \d new_test_result;
Table "public.new_test_result"
Column | Type | Modifiers
-----------+---------+------------------------------------------------------
-----------
id | bigint | not null default
nextval('public.new_test_result_id_seq'::text)
parent_id | bigint |
testcode | text |
testtype | text |
testdesc | text |
pass | integer |
latency | integer |
bytessent | integer |
bytesrecv | integer |
defect | text |
Indexes: test_result_parent_id_fk btree (parent_id)
Foreign Key constraints: $1 FOREIGN KEY (parent_id) REFERENCES
new_test_run(id) ON UPDATE NO ACTION ON DELETE CASCADE

steve=# explain select * from new_test_result where parent_id = 2;
QUERY PLAN
-----------------------------------------------------------------------
Seq Scan on new_test_result (cost=0.00..123370.57 rows=23 width=125)
Filter: (parent_id = 2)
(2 rows)

steve=# explain select * from new_test_result where parent_id = 2::bigint;
QUERY PLAN
----------------------------------------------------------------------------
-----------------------
Index Scan using test_result_parent_id_fk on new_test_result
(cost=0.00..3.32 rows=23 width=125)
Index Cond: (parent_id = 2::bigint)
(2 rows)