Bug? 8.0 does not use partial index

Hi!

Here's an odd thing. I use a partial index on a table:

group_data
CREATE TABLE group_data (
this_group_id integer NOT NULL,
group_id integer
-- ...
);

create index foo on group_data(this_group_id) where group_id is null;

there are approx 1 million tuples where this_group_id=46, but only 4 (four)
where group_id is null. So I would expect this query to use the index:

select * from group_data where this_group_id=46 and group_id is null.

On 7.4.5, it uses the index, but on 8.0rc5, it does not:

7.4.5=# explain analyze select * from group_data where group_id is null and
this_group_id = 46;
QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------
Index Scan using foo on group_data (cost=0.00..40383.21 rows=108786
width=43) (actual time=0.154..0.176 rows=4 loops=1)
Index Cond: (this_group_id = 46)
Filter: (group_id IS NULL)
Total runtime: 0.241 ms
(4 rows)

8.0.0rc5=# explain analyze select * from group_data where group_id is null
and this_group_id = 46;
QUERY PLAN

---------------------------------------------------------------------------------------------------------------------
Seq Scan on group_data (cost=0.00..140180.91 rows=211378 width=45)
(actual time=383.689..32991.424 rows=4 loops=1)
Filter: ((group_id IS NULL) AND (this_group_id = 46))
Total runtime: 32991.469 ms
(3 rows)

Time: 32992.812 ms

This is bad. But it gets worse:

8.0.0rc5=# explain analyze select * from group_data where group_id is null
and this_group_id = 46 and this_group_id = 46;
QUERY
PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using group_data_tgid_gidnull_idx on group_data
(cost=0.00..145622.85 rows=78985 width=45) (actual time=0.033..0.039 rows=4
loops=1)
Index Cond: ((this_group_id = 46) AND (this_group_id = 46))
Filter: (group_id IS NULL)
Total runtime: 0.086 ms
(4 rows)

Time: 1.912 ms

Don't tell me this is not a bug?

this_group_id = 46 and
this_group_id = 46

seems like a pretty odd way to get a query to use an index?

Need more specific info, please mail me!

Regards,
Palle