Slow query

Hi List,

I need your help, I don't know what happend with my queries when I use this 
table "simon".When I like display the result the query is very slow and take 
five minutes or more.
The number of registers in simon is:2421580
and the other table "llamada" the number of registers is :1779741 but the 
time of my query is faster that when i run the query but using my table 
simon
The struct of my table "simon" is:
                           Table "simon"
Attribute |    Type     |                    Modifier
-----------+-------------+-------------------------------------------------
num       | integer     | not null default nextval('simon_num_seq'::text)
ip        | varchar(16) |
fecha     | date        |
hora      | time        |
tras      | smallint    |
reci      | smallint    |
perd      | smallint    |
min       | float4      |
avg       | float4      |
max       | float4      |
Indices: 
avg_si,fecha_si,hora_si,ip_si,max_si,min_si,perd_si,reci_si,simon_hora,        simon_pkey,tras_si

I run Explain and this is the result:
EXPLAIN SELECT * from simon where fecha='20030122' order by hora desc;
NOTICE:  QUERY PLAN:
Sort  (cost=57700.73..57700.73 rows=23682 width=46) ->  Seq Scan on simon  (cost=0.00..55591.05 rows=23682 width=46)

and when I run explain for another table this is te result:
EXPLAIN SELECT * from llamada where fecha='20030122' order by hora desc;
NOTICE:  QUERY PLAN:
Sort  (cost=41102.70..41102.70 rows=12413 width=82) ->  Index Scan using fecha_lla on llamada  (cost=0.00..39944.95
rows=12413
 
width=82)

This is the sintax when i created my index:
CREATE INDEX fecha_si ON simon USING btree(fecha);


If somebody can help me or tell me why my queries using the table simon are 
slow, I will thankfull

Regards,
Ricardo

PD:Sorry  for my grammar Enghish

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