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 pvdprcod
NUMART | integer | not null
NUMDEP | smallint | not null
NUMPRO | smallint | not null
MODELO | varchar(20) | not null
TALLA | varchar(4) | not null
COLOR | varchar(3) | not null

exis=# \d venart
cvetda | smallint | not null
numdep | smallint | not null
numart | integer | not null
mes | smallint | not null
anio | integer | not null
tipotr | 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