SQL优化技巧 - COUNT标量子查询优化

2024年1月15日 · 阅读需 3 分钟

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定义

在日常开发中，有部分开发人员使用关联标量子查询来进行是否存在的判定，譬如下面的SQL查询有订单的用户列表，

select * from customer where (select count(*) from orders where c_custkey=o_custkey) > 0

这类查询有比较严重的性能问题，它需要对外表的每一条记录，进行一次聚集运算。从上面SQL的执行计划可以看到，它的执行时间为4820.015 ms.

Seq Scan on customer  (cost=0.00..3090818.00 rows=3333 width=181) (actual time=3.944..4819.230 rows=1251 loops=1)
  Filter: ((SubPlan 1) > 0)
  Rows Removed by Filter: 8749
  SubPlan 1
    ->  Aggregate  (cost=309.03..309.04 rows=1 width=8) (actual time=0.478..0.478 rows=1 loops=10000)
          ->  Seq Scan on orders  (cost=0.00..309.01 rows=8 width=0) (actual time=0.421..0.474 rows=1 loops=10000)
                Filter: (customer.c_custkey = o_custkey)
                Rows Removed by Filter: 10000
Planning Time: 9.246 ms
Execution Time: 4820.015 ms

更好的写法是使用EXISTS子查询，或是IN子查询，如下所示：

EXISTS子查询

select * from customer where exists(select 1 from orders where c_custkey=o_custkey)

Nested Loop  (cost=309.31..533.68 rows=1251 width=181) (actual time=2.126..10.256 rows=1251 loops=1)
  ->  HashAggregate  (cost=309.01..321.52 rows=1251 width=4) (actual time=1.769..1.976 rows=1251 loops=1)
        Group Key: orders.o_custkey
        Batches: 1  Memory Usage: 129kB
        ->  Seq Scan on orders  (cost=0.00..284.01 rows=10001 width=4) (actual time=0.010..0.502 rows=10001 loops=1)
  ->  Memoize  (cost=0.30..1.27 rows=1 width=181) (actual time=0.006..0.006 rows=1 loops=1251)
        Cache Key: orders.o_custkey
        Cache Mode: logical
        Hits: 0  Misses: 1251  Evictions: 0  Overflows: 0  Memory Usage: 348kB
        ->  Index Scan using customer_pkey on customer  (cost=0.29..1.26 rows=1 width=181) (actual time=0.006..0.006 rows=1 loops=1251)
              Index Cond: (c_custkey = orders.o_custkey)
Planning Time: 3.119 ms
Execution Time: 10.454 ms

IN子查询

select * from customer where c_custkey in (select o_custkey from orders)

Nested Loop  (cost=309.31..533.68 rows=1251 width=181) (actual time=1.588..3.615 rows=1251 loops=1)
  ->  HashAggregate  (cost=309.01..321.52 rows=1251 width=4) (actual time=1.577..1.676 rows=1251 loops=1)
        Group Key: orders.o_custkey
        Batches: 1  Memory Usage: 129kB
        ->  Seq Scan on orders  (cost=0.00..284.01 rows=10001 width=4) (actual time=0.007..0.469 rows=10001 loops=1)
  ->  Memoize  (cost=0.30..1.27 rows=1 width=181) (actual time=0.001..0.001 rows=1 loops=1251)
        Cache Key: orders.o_custkey
        Cache Mode: logical
        Hits: 0  Misses: 1251  Evictions: 0  Overflows: 0  Memory Usage: 348kB
        ->  Index Scan using customer_pkey on customer  (cost=0.29..1.26 rows=1 width=181) (actual time=0.001..0.001 rows=1 loops=1251)
              Index Cond: (c_custkey = orders.o_custkey)
Planning Time: 0.216 ms
Execution Time: 4.152 ms

定义​

定义