Join faster than single table query

Hi:

I must have missed something, but how is it possible that a join on
tables A and B is faster (a lot faster) than a query to one of the
tables with the same conditions?

The problem seems to be with the query plan, in the case os a query to
table_a only, the planner executes a "Seq Scan", in the case of a join,
an "Index Scan". table_a has about 4M records, so the difference is
quite noticeable.

explain
select * from table_a where field_1=1 and field_2='20030808' and
field_3='963782342';
NOTICE: QUERY PLAN:

Seq Scan on table_a (cost=0.00..373661.73 rows=12 width=227)

EXPLAIN

explain
select * FROM table_b, table_a
WHERE
table_b.field_1 = table_a.field_1
AND table_b.field_3 = table_a.field_3
AND table_b.field_3 in ('963782342')

AND table_a.field_2 = '20030808'
;
NOTICE: QUERY PLAN:

Nested Loop (cost=0.00..317.07 rows=3 width=351)
-> Seq Scan on table_b (cost=0.00..308.80 rows=1 width=124)
-> Index Scan using table_a_i01 on table_a (cost=0.00..8.24 rows=2
width=227)

EXPLAIN

Index on table_a is defined on field_1, field_2 and field_3.

Thanks a lot for any help.
Ruben.