更新时间：2022-06-13 GMT+08:00
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。
父主题： 实际调优案例
