小编god*_*yan的帖子

我使用的是 Postgres 9.3.4，我有 4 个查询，它们的输入非常相似，但响应时间却大不相同：

查询#1

EXPLAIN ANALYZE SELECT posts.* FROM posts
WHERE posts.source_id IN (19082, 19075, 20705, 18328, 19110, 24965, 18329, 27600, 17804, 20717, 27598, 27599)
AND posts.deleted_at IS NULL
ORDER BY external_created_at desc
LIMIT 100 OFFSET 0;
                                                                                 QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.43..585.44 rows=100 width=1041) (actual time=326092.852..507360.199 rows=100 loops=1)
   ->  Index Scan using index_posts_on_external_created_at on posts  (cost=0.43..14871916.35 rows=2542166 width=1041) (actual time=326092.301..507359.524 rows=100 loops=1)
         Filter: (source_id = ANY ('{19082,19075,20705,18328,19110,24965,18329,27600,17804,20717,27598,27599}'::integer[]))
         Rows Removed by Filter: 6913925
 Total runtime: 507361.944 ms

查询#2

EXPLAIN …