Help Center> Data Warehouse Service (DWS)> Developer Guide (Kuala Lumpur Region)> Query Performance Optimization> Optimization Cases> Case: Rewriting SQL and Deleting Subqueries (Case 1)
Updated on 2022-08-16 GMT+08:00
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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.

Parent topic: Optimization Cases