SQL：在相关子查询中加入（SEMI JOIN）答案

【问题标题】：SQL: JOIN inside correlated subquery (SEMI JOIN)SQL：在相关子查询中加入（SEMI JOIN）
【发布时间】：2021-04-25 05:19:57
【问题描述】：

我需要在相关子查询中加入一个表。但是，postgres 选择的查询计划非常慢。如何优化以下查询：

SELECT c.id
FROM customer c
WHERE EXISTS (
  SELECT 1
  FROM customer_communication cc
  JOIN communication co on co.id = cc.communication_id and co.channel <> 'mobile'
  WHERE cc.user_id = c.id
)

这是EXPLAIN (ANALYZE) 结果：

Nested Loop  (cost=3451561.57..3539012.42 rows=24509 width=8) (actual time=60913.294..64056.970 rows=1036309 loops=1)
  ->  HashAggregate  (cost=3451561.14..3451806.23 rows=24509 width=8) (actual time=60913.264..61187.702 rows=1036310 loops=1)
        Group Key: cc.customer_id
        ->  Hash Join  (cost=2070834.75..3358538.60 rows=37209016 width=8) (actual time=32758.325..52752.383 rows=37209019 loops=1)
              Hash Cond: (cc.communication_id = co.id)
              ->  Seq Scan on customer_communication cc  (cost=0.00..755689.16 rows=37209016 width=16) (actual time=0.011..4949.315 rows=37209019 loops=1)
              ->  Hash  (cost=1772758.38..1772758.38 rows=18168430 width=8) (actual time=32756.662..32756.663 rows=18108924 loops=1)
                    Buckets: 262144  Batches: 128  Memory Usage: 7557kB
                    ->  Seq Scan on communication co  (cost=0.00..1772758.38 rows=18168430 width=8) (actual time=0.007..30024.494 rows=18108924 loops=1)
                          Filter: (channel <> 'mobile')
  ->  Index Only Scan using customerxpk on customer c  (cost=0.43..3.60 rows=1 width=8) (actual time=0.003..0.003 rows=1 loops=1036310)
        Index Cond: (id = cc.customer_id)
        Heap Fetches: 525050
Planning Time: 0.391 ms
Execution Time: 64094.584 ms

【问题讨论】：

标签： sql postgresql performance join subquery

【解决方案1】：

我认为您错误地指定了查询，因为您有冲突的别名。这可能会更好：

SELECT c.id
FROM customer c
WHERE EXISTS (SELECT 1
              FROM customer_communication cc JOIN
                   communication co
                   ON co.id = cc.communication_id AND
                      co.channel <> 'mobile'
              WHERE cc.user_id = c.id
             );

请注意，在子查询中c 指的是外部查询 的customer 和co 指的是communication。

【讨论】：

是的，我更新了查询。但是执行计划还是一样的