Index not used on a DISTINCT ON

Hi all,

I have a table with two column: id_user and login;

and the column login is a Primary Key.

If I made the following view:
CREATE VIEW foo
SELECT DISTINCT ON ( login )
ul.id_login AS login,
ud.last_name AS last_name,
FROM user_login ul LEFT JOIN v_user_traffic v USING ( id_user)
user_data ud
WHERE ul.id_user = ud.id_user;

when is used in this way:

SELECT * FROM foo WHERE login = 'XXXXXX';

the index is not used if I remove the DISTINCT ON
the filter is used!!!

Is it a known issue ?