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.