Case: Rewriting SQL and Deleting Subqueries (Case 1)

Symptom

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;

This SQL performance is poor. SubPlan exists in the execution plan as follows:

image1

Optimization

The core of this optimization is to eliminate subqueries. Based on the service scenario analysis, a**.**ca_address_sk is not null. In terms of SQL syntax, you can rewrite the SQL statement as follows:

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;

Note

To ensure that the modified statements have the same functions, not null is added to customer_address_001. ca_address_sk.