As with any PostgreSQL question, the first place you should look for answer is the PostgreSQL docs. I was recently reviewing the EXPLAIN docs as freshen up on some query tuning fu and I came across this little gem:
EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 100; QUERY PLAN ------------------------------------------------------------------------------ Bitmap Heap Scan on tenk1 (cost=2.37..232.35 rows=106 width=244) Recheck Cond: (unique1 < 100) -> Bitmap Index Scan on tenk1_unique1 …