These examples should have proven to you that looking at the counts of blocks hit and read is valuable for determining whether a query plan is really being executed correctly or not. PostgreSQL 9.0 adds a feature to make this easier than before. Instead of looking at the pg_stat* data as done in the previous example, you can request a count of buffers accessed directly when running EXPLAIN:
EXPLAIN (ANALYZE ON, BUFFERS ON) SELECT count(*) FROM t WHERE v=5;
QUERY PLAN
----------
Aggregate (cost=332.92..332.93 rows=1 width=0) (actual time=39.132..39.134 rows=1 loops=1)
Buffers: shared hit=46 read=23
-> Index Scan using i on t (cost=0.00..308.21 rows=9883 width=0) (actual time=0.069..21.843 rows=9993 loops=1)
