优化具有多个连接的大表查询的索引

Question

我们有一个images包含大约 2500 万条记录的表，当我根据多个连接的值查询该表时，规划器的估计与行计数的实际结果有很大不同。我们还有其他查询，这些查询大致相同，但没有所有连接，而且速度要快得多。我想知道我可以采取哪些步骤来调试和优化查询。另外，是使用一个索引覆盖连接和子句中包含的所有列where，还是使用多个索引，一个索引用于每个连接列，另一个索引包含子句中的所有字段where？

查询：

EXPLAIN ANALYZE
SELECT "images".* FROM "images" 
INNER JOIN "locations" ON "locations"."id" = "images"."location_id" 
INNER JOIN "users" ON "images"."creator_id" = "users"."id" 
INNER JOIN "user_groups" ON "users"."id" = "user_groups"."user_id" 
WHERE "images"."deleted_at" IS NULL 
AND "user_groups"."group_id" = 7 
AND "images"."creator_type" = 'User' 
AND "images"."status" = 2 
AND "locations"."active" = TRUE 
ORDER BY date_uploaded DESC 
LIMIT 50 
OFFSET 0;

解释一下：

Limit  (cost=25670.61..25670.74 rows=50 width=585) (actual time=1556.250..1556.278 rows=50 loops=1)
  ->  Sort  (cost=25670.61..25674.90 rows=1714 width=585) (actual time=1556.250..1556.264 rows=50 loops=1)
        Sort Key: images.date_uploaded
        Sort Method: top-N heapsort  Memory: 75kB
        ->  Nested Loop  (cost=1.28..25613.68 rows=1714 width=585) (actual time=0.097..1445.777 rows=160886 loops=1)
              ->  Nested Loop  (cost=0.85..13724.04 rows=1753 width=585) (actual time=0.069..976.326 rows=161036 loops=1)
                    ->  Nested Loop  (cost=0.29..214.87 rows=22 width=8) (actual time=0.023..0.786 rows=22 loops=1)
                          ->  Seq Scan on user_groups  (cost=0.00..95.83 rows=22 width=4) (actual time=0.008..0.570 rows=22 loops=1)
                                Filter: (group_id = 7)
                                Rows Removed by Filter: 5319
                          ->  Index Only Scan using users_pkey on users  (cost=0.29..5.40 rows=1 width=4) (actual time=0.006..0.008 rows=1 loops=22)
                                Index Cond: (id = user_groups.user_id)
                                Heap Fetches: 18
                    ->  Index Scan using creator_date_uploaded_Where_pub_not_del on images  (cost=0.56..612.08 rows=197 width=585) (actual time=0.062..40.992 rows=7320 loops=22)
                          Index Cond: ((creator_id = users.id) AND ((creator_type)::text = 'User'::text) AND (status = 2))
              ->  Index Scan using locations_pkey on locations  (cost=0.43..6.77 rows=1 width=4) (actual time=0.002..0.002 rows=1 loops=161036)
                    Index Cond: (id = images.location_id)
                    Filter: active
                    Rows Removed by Filter: 0
Planning time: 1.694 ms
Execution time: 1556.352 ms

我们在 RDS db.m4.large 实例上运行 Postgres 9.4。

Answer 1

至于查询本身，您唯一能做的就是跳过users表。从EXPLAIN你可以看到它只是做了一个Index Only Scan而不实际接触桌子。因此，从技术上讲，您的查询可能如下所示：

SELECT images.* FROM images
INNER JOIN locations ON locations.id = images.location_id
INNER JOIN user_groups ON images.creator_id = user_groups.user_id
WHERE images.deleted_at IS NULL 
AND user_groups.group_id = 7 
AND images.creator_type = 'User' 
AND images.status = 2 
AND locations.active = TRUE 
ORDER BY date_uploaded DESC 
OFFSET 0 LIMIT 50

剩下的就是关于索引的了。locations似乎数据很少，所以这里的优化不会给你带来任何好处。user_groups另一方面可以从索引ON (user_id) WHERE group_id = 7或ON (group_id, user_id). 这应该消除对表内容的一些额外过滤。

-- Option 1
CREATE INDEX ix_usergroups_userid_groupid7
ON user_groups (user_id)
WHERE group_id = 7;

-- Option 2
CREATE INDEX ix_usergroups_groupid_userid
ON user_groups (group_id, user_id);

当然，这里最大的事情就是images。目前，刨床会进行索引扫描creator_date_uploaded_Where_pub_not_del，我怀疑该扫描不完全符合要求。在这里，根据您的使用模式，您会想到多个选项 - 来自搜索参数相当常见的选项：

-- Option 1
CREATE INDEX ix_images_creatorid_typeuser_status2_notdel
ON images (creator_id)
WHERE creator_type = 'User' AND status = 2 AND deleted_at IS NULL;

到具有完全动态参数的一个：

-- Option 2
CREATE INDEX ix_images_status_creatortype_creatorid_notdel
ON images (status, creator_type, creator_id)
WHERE deleted_at IS NULL;

第一个索引更可取，因为它较小（值被过滤掉而不是索引）。

总而言之，除非您受到内存（或其他因素）的限制，否则我会在user_groups和上添加索引images。指标的正确选择必须通过经验来确认，因为通常有多种选择，具体情况取决于数据的统计分布。