Speed or configuration

The Hermit Hacker wrote:

"use cut-n-paste please, and send us the results of the
EXPLAIN ... stuff
like the cost estimates and whatnot tell us *so* much
..."

This is it:

exis=# \d pvdprcodNUMART    | integer     | not nullNUMDEP    | smallint    | not nullNUMPRO    | smallint    | not
nullMODELO   | varchar(20) | not nullTALLA     | varchar(4)  | not nullCOLOR     | varchar(3)  | not null
 

exis=# \d venartcvetda    | smallint   | not nullnumdep    | smallint   | not nullnumart    | integer    | not nullmes
    | smallint   | not nullanio      | integer    | not nulltipotr    | varchar(2) |importe   | float8     |cantidad  |
float8    |
 

exis=# explain select
cvetda,anio,mes,sum(importe),sum(cantidad)
exis-# from venart where numart in
exis-# (select "NUMART" from pvdprcod where "NUMDEP"=7
and "NUMPRO"=108)
exis-# group by cvetda,numdep,anio,mes;
NOTICE:  QUERY PLAN:

Aggregate  (cost=79015875401357.48..79015875413208.91
rows=79010 width=26) ->  Group  (cost=79015875401357.48..79015875409258.44
rows=790095 width=26)       ->  Sort
(cost=79015875401357.48..79015875401357.48 rows=790095
width=26)             ->  Seq Scan on venart
(cost=100000000.00..79015875283591.09 rows=790095
width=26)                   SubPlan                     ->  Materialize
(cost=100007942.42..100007942.42 rows=34 width=4)                           ->  Seq Scan on pvdprcod
(cost=100000000.00..100007942.42 rows=34 width=4)

EXPLAIN

Why Seq Scan if indexes have been created on the
columns used for access?

Best regards,

Franz J Fortuny