select returns no line

Hi, 

Our database has a SELECT problem using varchar columns in WHERE clause
(but not in all rows!!!). 
We can fix the whole table (or just the row) as shown below but later it
seems wrong again (and after the fix the row isn't UPDATEd).

Any idea?

Thanks       Attila

********************************************************************************
Environment:

[PostgreSQL 6.5.3 on i686-pc-linux-gnu, compiled by gcc 2.95.2]

goodwill=>\d users
Table    = users
+----------------------------------+----------------------------------+-------+
|              Field               |              Type                | Length|
+----------------------------------+----------------------------------+-------+
| user_id                          | int4 not null default nextval (  |     4 |
| user_login                       | varchar() not null               |    15 |
| user_passwd                      | varchar() not null               |    15 |
| user_exp                         | timestamp                        |     4 |
+----------------------------------+----------------------------------+-------+
Indices:  users_pkey        
users_user_login_key                                                                                        

********************************************************************************
The problem:

goodwill=>select * from users where user_login='test';
user_id|user_login|user_passwd|user_exp
-------+----------+-----------+--------
(0 rows)

goodwill=> select * from users where user_id=4;
user_id|user_login|user_passwd  |user_exp
-------+----------+-------------+--------     4|test      |0PDv7a2EESjZo|
(1 row)

goodwill=> update users set user_login=user_login where user_id=4;
UPDATE
1                                                                                                              

goodwill=>select * from users where user_login='test';
user_id|user_login|user_passwd  |user_exp
-------+----------+-------------+--------     4|test      |0PDv7a2EESjZo|
(1 row)


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