Surprising sequence scan when function call used

I have a table, login, which has a field by the same name; there's an index
on that field. I was surprised to discover that a SELECT which compares the
login field to a constant uses an Index scan, but if it is compared to a
function call--for example, lower()--a sequence scan is forced.

Any idea why?

Here are more details:


vdsq=> select version();                              version
---------------------------------------------------------------------PostgreSQL 7.0.2 on i686-pc-linux-gnu, compiled by
gccegcs-2.91.66
 
(1 row)

vdsq=> \d login                               Table "login" Attribute   |    Type     |                    Modifier
--------------+-------------+-----------------------------------------------
-id           | integer     | not null default nextval('login_id_seq'::text)login        | char(8)     | not
nullpassword    | char(8)     | not nulljobid        | integer     | not nulltype         | smallint    | not
nullentryid     | integer     |lastactivity | timestamp   |trackid      | varchar(50) |roundid      | integer     |
 
Indices: idx_login_entryid,        idx_login_jobid,        idx_login_login,        idx_login_password,
idx_login_type,       login_pkey
 

vdsq=> explain select * from login where login.login = 'foo';
NOTICE:  QUERY PLAN:

Index Scan using idx_login_login on login  (cost=0.00..582.61 rows=609
width=62)

EXPLAIN
vdsq=> explain select * from login where login.login = lower('foo');
NOTICE:  QUERY PLAN:

Seq Scan on login  (cost=0.00..1361.86 rows=609 width=62)

EXPLAIN