更新时间:2022-04-24 GMT+08:00
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案例:改写SQL消除子查询(案例1)

现象描述

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select 
    1,
    (select count(*) from customer_address_001 a4 where a4.ca_address_sk = a.ca_address_sk) as GZCS 
from customer_address_001 a;

此SQL性能较差,查看发现执行计划中存在SubPlan,具体如下:

优化说明

此优化的核心就是消除子查询。分析业务场景发现a.ca_address_sk不为null,那么从SQL语义出发,可以等价改写SQL为:

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select 
count(*) 
from customer_address_001 a4, customer_address_001 a
where a4.ca_address_sk = a.ca_address_sk
group by  a.ca_address_sk;

为了保证改写的等效性,在customer_address_001. ca_address_sk加了not null约束。

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