为什么左连接会导致优化器忽略索引？

Question

使用 postgres 9.6.11，我有一个类似的架构：

所有者：

id: BIGINT (PK)
dog_id: BIGINT NOT NULL (FK)
cat_id: BIGINT NULL (FK)

index DOG_ID_IDX (dog_id)
index CAT_ID_IDX (cat_id)

动物：

id: BIGINT (PK)
name: VARCHAR(50) NOT NULL

index NAME_IDX (name)

在一些示例数据中：

所有者表：

| id | dog_id | cat_id |
| -- | ------ | ------ |
| 1  | 100    | 200    |
| 2  | 101    | NULL   |

动物桌：

| id  | name     |
| --- | -------- |
| 100 | "fluffy" |
| 101 | "rex"    |
| 200 | "tom"    |

我需要执行的一个常见查询是通过宠物的名字来查找主人，我认为可以通过如下查询来完成：

select *
from owner o
    join animal dog on o.dog_id = dog.id
    left join animal cat on o.cat_id = cat.id
where dog.name = "fluffy" or cat.name = "fluffy";

但我不明白我从中得到的计划：

Hash Join  (cost=30304.51..77508.31 rows=3 width=899)
  Hash Cond: (dog.id = owner.dog_id)
  Join Filter: (((dog.name)::text = 'fluffy'::text) OR ((cat.name)::text = 'fluffy'::text))
  ->  Seq Scan on animal dog  (cost=0.00..17961.23 rows=116623 width=899)
  ->  Hash  (cost=28208.65..28208.65 rows=114149 width=19)
        ->  Hash Left Join  (cost=20103.02..28208.65 rows=114149 width=19)
              Hash Cond: (owner.cat_id = cat.id)
              ->  Seq Scan on owner o  (cost=0.00..5849.49 rows=114149 width=16)
              ->  Hash  (cost=17961.23..17961.23 rows=116623 width=19)
                    ->  Seq Scan on animal cat  (cost=0.00..17961.23 rows=116623 width=19)

我不明白为什么查询计划会进行顺序扫描。 我认为优化器会足够聪明，可以使用name 索引扫描animal 表一次，甚至两次，然后根据这个结果加入到所有者表中，但结果却是一个非常意外的查询计划。

我采用了一个更简单的情况，我们只想查找狗名，并且查询的行为与我预期的一样：

select *
from owner o
    join animal dog on o.dog_id = dog.id
where dog.name = "fluffy";

这个查询产生一个我理解的计划，使用animal.name上的索引：

Nested Loop  (cost=0.83..16.88 rows=1 width=1346)
  ->  Index Scan using DOG_ID_IDX on animal dog  (cost=0.42..8.44 rows=1 width=899)
        Index Cond: ((name)::text = 'fluffy'::text)
  ->  Index Scan using dog_id on owner o  (cost=0.42..8.44 rows=1 width=447)
        Index Cond: (dog_id = b.id)

即使使用两个内部连接进行查询也会产生我期望的查询计划：

select * 
from owner o
  join animal dog on o.dog_id = dog.id
  join animal cat on o.cat_id = cat.id
where dog.name = 'fluffy' or cat.name = 'fluffy';

Merge Join  (cost=35726.09..56215.53 rows=3 width=2245)
  Merge Cond: (owner.cat_id = cat.id)
  Join Filter: (((dog.name)::text = 'fluffy'::text) OR ((cat.name)::text = 'fluffy'::text))
  ->  Nested Loop  (cost=0.83..132348.38 rows=114149 width=1346)
        ->  Index Scan using CAT_ID_IDX on owner o  (cost=0.42..11616.07 rows=114149 width=447)
        ->  Index Scan using animal_pkey on animal dog  (cost=0.42..1.05 rows=1 width=899)
              Index Cond: (id = owner.dog_id)
  ->  Index Scan using animal_pkey on animal cat  (cost=0.42..52636.91 rows=116623 width=899)

所以看起来animal 的左连接导致优化器忽略索引。

为什么对animal 进行额外的左连接似乎会导致优化器忽略索引？

编辑： 解释（分析，缓冲）产量：

Hash Left Join  (cost=32631.95..150357.57 rows=3 width=2245) (actual time=6696.935..6696.936 rows=0 loops=1)
  Hash Cond: (o.cat_id = cat.id)
  Filter: (((dog.name)::text = 'fluffy'::text) OR ((cat.name)::text = 'fluffy'::text))
  Rows Removed by Filter: 114219
  Buffers: shared hit=170464 read=18028 dirtied=28, temp read=13210 written=13148
  ->  Merge Join  (cost=0.94..65696.37 rows=114149 width=1346) (actual time=1.821..860.643 rows=114219 loops=1)
        Merge Cond: (o.dog_id = dog.id)
        Buffers: shared hit=170286 read=1408 dirtied=28
        ->  Index Scan using DOG_ID_IDX on owner o  (cost=0.42..11402.48 rows=114149 width=447) (actual time=1.806..334.431 rows=114219 loops=1)
              Buffers: shared hit=84787 read=783 dirtied=13
        ->  Index Scan using animal_pkey on animal dog  (cost=0.42..52636.91 rows=116623 width=899) (actual time=0.006..300.507 rows=116977 loops=1)
              Buffers: shared hit=85499 read=625 dirtied=15
  ->  Hash  (cost=17961.23..17961.23 rows=116623 width=899) (actual time=5626.780..5626.780 rows=116977 loops=1)
        Buckets: 8192  Batches: 32  Memory Usage: 3442kB
        Buffers: shared hit=175 read=16620, temp written=12701
        ->  Seq Scan on animal cat  (cost=0.00..17961.23 rows=116623 width=899) (actual time=2.519..5242.106 rows=116977 loops=1)
              Buffers: shared hit=175 read=16620
Planning time: 1.245 ms
Execution time: 6697.357 ms

Answer 1

left join 需要保留第一个表中的所有行。因此，它通常会扫描该表，即使 where 条件会根据这些条件过滤其他表。

Postgres 产生的查询计划不足为奇。