[SQL] Surprising sequence scan when function call used

[Will Fitzgerald]

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 gcc egcs-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 null
 password     | char(8)     | not null
 jobid        | integer     | not null
 type         | smallint    | not null
 entryid      | 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