Query Plan - Index Scan & Seq Scan

When joining two tables the query plan is doing a seq scan rather than index
scan. I do have indexes on the columns used for joining the tables.

Example:
SELECT a.id FROM a, b WHERE a.id = b.id;

QUERY PLAN
-----------------------------------------------------------------------------------------
Hash Join (cost=13865.30..326413.23 rows=6451 width=18)
Hash Cond: ("outer".id = "inner".id)
-> Seq Scan on a (cost=0.00..125076.37 rows=6450937 width=18)
-> Hash (cost=10168.64..10168.64 rows=500664 width=4)
-> Seq Scan on b (cost=0.00..10168.64 rows=500664 width=4)
(5 rows)

The planner used to perform a index scan. I have added a lot of data in those
two tables. Right now both tables have millions of records. After adding the new
records the planner is going for a seq scan while doing the join.

Is there any tunning I can do so that the query planner would do a index scan?

I did a vacuum analyze but no change.

Thanks,
-Prasanth.