相关疑难解决方法(0)

我有一个未使用现有索引的查询，我不明白为什么。

桌子：

mustang=# \d+ bss.amplifier_saturation
                                               Table "bss.amplifier_saturation"
 Column |           Type           |                             Modifiers                             | Storage | Description 
--------+--------------------------+-------------------------------------------------------------------+---------+-------------
 value  | integer                  | not null                                                          | plain   | 
 target | integer                  | not null                                                          | plain   | 
 start  | timestamp with time zone | not null                                                          | plain   | 
 end    | timestamp with time zone | not null                                                          | plain   | 
 id     | integer                  | not null default nextval('amplifier_saturation_id_seq'::regclass) | plain   | 
 lddate | timestamp with time zone | not null default …

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 …

在这个答案中，我解释了 SQL-89 的隐式语法。
但是我在玩的时候注意到不同的查询计划：

EXPLAIN ANALYZE
  SELECT *
  FROM (values(1)) AS t(x), (values(2)) AS g(y);

                                     QUERY PLAN                                     
------------------------------------------------------------------------------------
 Result  (cost=0.00..0.01 rows=1 width=0) (actual time=0.002..0.002 rows=1 loops=1)
 Planning time: 0.052 ms
 Execution time: 0.020 ms
(3 rows)

与此相反：

EXPLAIN ANALYZE
  SELECT *
  FROM (values(1)) AS t(x)                      
  CROSS JOIN (values(2)) AS g(y);
                                           QUERY PLAN                                           
------------------------------------------------------------------------------------------------
 Subquery Scan on g  (cost=0.00..0.02 rows=1 width=4) (actual time=0.004..0.005 rows=1 loops=1)
   ->  Result  (cost=0.00..0.01 rows=1 width=0) (actual time=0.002..0.002 rows=1 loops=1)
 Planning time: 0.075 ms
 Execution time: 0.027 …