Group by bug?

hi,

wd_test=# \d t1
Table "public.t1"
Column | Type | Modifiers
--------+---------+-------------------------------------------------
id | integer | not null default nextval('t1_id_seq'::regclass)
tag | text |

wd_test=# select * from t1;
id | tag
----+-----
1 | a
2 | a
3 | b
4 | c
5 | b
(5 rows)

---- this sql will not group by the case result.
wd_test=# select case t1.tag when 'a' then '1' else '0' end as tag,
count(*) from t1 group by tag;
tag | count
-----+-------
0 | 1
0 | 2
1 | 2
(3 rows)

---- this sql will group by the case result.
wd_test=# select case t1.tag when 'a' then '1' else '0' end as ttag,
count(*) from t1 group by ttag;
ttag | count
------+-------
0 | 3
1 | 2
(2 rows)