Why does it not use the index?

This one is irritating. Here's some psql output:

================================================================================
dumps=# \d fal_profdel
Table "fal_profdel"
Attribute | Type | Modifier
-----------+--------------------------+----------
sid | character(4) | not null
card_num | character(19) | not null
date_del | timestamp with time zone |
filename | character varying(30) |
Indices: fal_prfdel_cn,
fal_prfdel_date,
fal_prfdel_pk

dumps=# \d fal_prfdel_cn
Index "fal_prfdel_cn"
Attribute | Type
-----------+---------------
card_num | character(19)
unique btree

dumps=# explain select card_num from fal_profdel where card_num = 'removed_for_privacy';
NOTICE: QUERY PLAN:

Seq Scan on fal_profdel (cost=0.00..120546.39 rows=46649 width=12)

EXPLAIN
================================================================================

Now, why the heck is the select query not using the index? I've tried it by having an exact 19 character card_num as well - still explains as a 'Seq Scan' (tablespace scan) - and each query takes up to 37 seconds (thus confirming that it is indeed doing scans and not using the index).

I've tried dropping and re-creating the indexes, still it explains as tablespace scans.

I am running postgresql 7.1.3 - a bit old, I know, but I have had no reason to upgrade just yet.

By the way, the fal_profdel table has 4,664,867 rows in it currently - thus I really don't want full table scans!

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