index is not used

Hello,
i need help for my strange problem.

I have a table of ~ 9 000 000 rows,

Table "mytable"
Column | Type | Modifiers
----------+---------+-----------
field1 | integer | not null
field2 | integer | not null
field3 | real |
Indexes: mytable_field1_key,
mytable_field3_key
Primary key: mytable_field1_field2_pkey

But the indices are not used, even when I added a separate index on
field1 !

trc=# explain select * from mytable where field1 = 6;
NOTICE: QUERY PLAN:

Seq Scan on mytable (cost=0.00..182382.74 rows=49579 width=4)

EXPLAIN

OR:

trc=# explain select * from mytable where field3 = 0.3;
NOTICE: QUERY PLAN:

Seq Scan on mytable (cost=0.00..182382.74 rows=49579 width=4)

EXPLAIN

I am using postgres 7.2.1 on RedHat.

On the same installation in another database similar table of 100 rows
uses the indices when needed.

thanks in advance,
mila.