Index not used ! Why?

Hi All,
my database server has very high load in this morning.
I've found the problem. One of my index was not used so far!
it's interesting:

addb=> \d banners                                     Table "banners"Attribute  |           Type           |
         Modifier                      
 
------------+--------------------------+----------------------------------------------------b_no       | integer
         | not null default nextval('banners_b_no_seq'::text)usr_no     | integer                  | b_ext      |
charactervarying(10)    | b_link     | character varying(100)   | b_from     | date                     | b_to       |
date                    | b_lastview | timestamp with time zone | default now()b_maxview  | integer                  |
b_curview | integer                  | default 0b_maxclick | integer                  | b_curclick | integer
     | default 0b_weight   | integer                  | default 1b_curwg    | double precision         | default
0b_active  | boolean                  | default 'f'::boollast_upd   | timestamp with time zone | default now()upd_usr
| integer                  | b_name     | character varying(40)    | 
 
Indices: b_usr_no_idx,        banners_b_no_key

addb=> EXPLAIN SELECT b_link FROM banners WHERE b_no = 3;
NOTICE:  QUERY PLAN:

Seq Scan on banners  (cost=0.00..1.57 rows=1 width=12)

EXPLAIN
addb=> DROP INDEX banners_b_no_key;
DROP
addb=> CREATE INDEX banners_b_no_key ON banners (b_no);
CREATE
addb=> EXPLAIN SELECT b_link FROM banners WHERE b_no = 3;
NOTICE:  QUERY PLAN:

Index Scan using banners_b_no_key on banners  (cost=0.00..4.43 rows=1 width=12)

EXPLAIN
addb=> 

Why index wasn't used ?
postgresql-7.1.2, redhat 7.0, kernel:2.2.19

Thanks, Gabor