相关疑难解决方法(0)

使用PostgreSQL 8.4.9,我对查询的PostgreSQL性能有一个奇怪的问题.此查询正在选择3D卷中的一组点,使用a LEFT OUTER JOIN添加相关ID列,其中存在相关ID.x范围的微小变化可能导致PostgreSQL选择不同的查询计划,执行时间从0.01秒到50秒.这是有问题的查询:

SELECT treenode.id AS id,
       treenode.parent_id AS parentid,
       (treenode.location).x AS x,
       (treenode.location).y AS y,
       (treenode.location).z AS z,
       treenode.confidence AS confidence,
       treenode.user_id AS user_id,
       treenode.radius AS radius,
       ((treenode.location).z - 50) AS z_diff,
       treenode_class_instance.class_instance_id AS skeleton_id
  FROM treenode LEFT OUTER JOIN
         (treenode_class_instance INNER JOIN
          class_instance ON treenode_class_instance.class_instance_id
                                                  = class_instance.id
                            AND class_instance.class_id = 7828307)
       ON (treenode_class_instance.treenode_id = treenode.id
           AND treenode_class_instance.relation_id = 7828321)
  WHERE treenode.project_id = 4
    AND (treenode.location).x >= 8000
    AND (treenode.location).x <= (8000 + 4736) …

我有一个简单的查询来连接两个非常慢的表.我发现查询计划在大表email_activities(~10m行)上执行seq扫描,而我认为使用嵌套循环的索引实际上会更快.

我使用子查询重写了查询,试图强制使用索引,然后注意到一些有趣的东西.如果您查看下面的两个查询计划,您将看到当我将子查询的结果集限制为43k时,查询计划确实使用了email_activities上的索引,而将子查询中的限制设置为甚至44k将导致查询计划使用seq扫描email_activities.一个显然比另一个更有效,但Postgres似乎并不关心.

什么可能导致这个？如果其中一个集合大于特定大小,它是否在某处强制使用散列连接？

explain analyze SELECT COUNT(DISTINCT "email_activities"."email_recipient_id") FROM "email_activities" where email_recipient_id in (select "email_recipients"."id" from email_recipients WHERE "email_recipients"."email_campaign_id" = 1607 limit 43000);
                                                                                            QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=118261.50..118261.50 rows=1 width=4) (actual time=224.556..224.556 rows=1 loops=1)
   ->  Nested Loop  (cost=3699.03..118147.99 rows=227007 width=4) (actual time=32.586..209.076 rows=40789 loops=1)
         ->  HashAggregate  (cost=3698.94..3827.94 rows=43000 width=4) (actual time=32.572..47.276 rows=43000 loops=1)
               ->  Limit  (cost=0.09..3548.44 rows=43000 width=4) (actual time=0.017..22.547 rows=43000 loops=1)
                     ->  Index Scan using index_email_recipients_on_email_campaign_id on email_recipients  (cost=0.09..5422.47 rows=65710 width=4) (actual time=0.017..19.168 rows=43000 loops=1) …