Case: Rewriting SQL and Deleting Subqueries (Case 1)
Symptom
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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;
|
This SQL performance is poor. SubPlan exists in the execution plan as follows:
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:
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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;
|
To ensure that the modified statements have the same functions, not null is added to customer_address_001. ca_address_sk.
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