EXPLAIN (ANALYZE, BUFFERS)
SELECT COUNT(*)
FROM users
WHERE twitter != '';
…
Aggregate  (cost=845110.21..845110.22 rows=1 width=8) (actual time=1271.157..1271.158 rows=1 loops=1)
  Buffers: shared hit=202662
  ->  Seq Scan on users  (cost=0.00..844969.99 rows=56087 width=0) (actual time=0.019..1265.883 rows=51833 loops=1)
        Filter: ((twitter)::text <> ''::text)
        Rows Removed by Filter: 2487813
        Buffers: shared hit=202662
Planning time: 0.390 ms
Execution time: 1271.180 ms
…
CREATE INDEX CONCURRENTLY twitter_test ON users (twitter);
…
Aggregate  (cost=61002.82..61002.83 rows=1 width=8) (actual time=297.311..297.312 rows=1 loops=1)
  Buffers: shared hit=51854 dirtied=19
  ->  Index Only Scan using twitter_test on users  (cost=0.43..60873.13 rows=51877 width=0) (actual time=279.184..293.532 rows=51833 loops=1)
        Filter: ((twitter)::text <> ''::text)
        Rows Removed by Filter: 2487830
        Heap Fetches: 26037
        Buffers: shared hit=51854 dirtied=19
Planning time: 0.191 ms
Execution time: 297.334 ms

Échauffement