index problem (uses one index but not the other)

Hello,

I have a table with about 600000 records and two indexes:
  CREATE TABLE transactions (
    type INT2,
    order_id INT4,
    account_id INT4,
    market_id INT4,
    qty INT4,
    price INT2,
    timestamp INT8,
    memo varchar(30));

  CREATE INDEX trans_aid_idx ON transactions USING HASH (account_id);
  CREATE INDEX trans_oid_idx ON transactions USING HASH (order_id);

When I make a query on an account_id it uses the trans_aid_idx index,
but when I make a query on an order_id it does a sequential scan instead
of using the trans_oid_idx index:

  nf=# explain select * from transactions where account_id = 12345;
  NOTICE:  QUERY PLAN:

  Index Scan using trans_aid_idx on transactions  (cost=0.00..496.99
rows=139 width=40)

  EXPLAIN
  nf=# explain select * from transactions where order_id = 12345;
  NOTICE:  QUERY PLAN:

  Seq Scan on transactions  (cost=0.00..11490.45 rows=5061 width=40)

  EXPLAIN

I have vacuum analyzed the entire DB and just the transactions table, I
have
dropped and recreated the table and the indexes. But it does seem to help.

The weird thing is that this used to work (i.e. trans_oid_idx was used)
when there was about 200000 records (about 1/3 of now). Also, there is
about
9000 distinct values of account_id, but about 300000 values of order_id.

Is there a problem when the number of distinct values grows too large?

(I am running Postgresql 7.1.3 on Redhat 7.1)

Thanks for any help/advice.

--Maurice