一则SQL优化案例

原始sql:

select CASE 
WHEN c.deadline >= 24 THEN 6 
WHEN c.deadline >= 12 THEN 5 
WHEN c.deadline >= 6 THEN 4 
WHEN c.deadline >= 3 THEN 3 
WHEN c.deadline >= 1 THEN 2 
else 1 END as sort, count(0) counts
from t_optimization_loan p inner join (select id,CASE WHEN isDayThe = 0 THEN round(deadline/30,2) else deadline end as deadline from t_product_credit) c 
on p.productId = c.id where p.optimizaId = 528596 and p.state = 1 group by sort order by sort asc;

执行计划如下：

在查询过程中存在全表查询，并发量上来肯定db的负载会非常高

优化后：

select CASE 
WHEN temp.deadline >= 24 THEN 6 
WHEN temp.deadline >= 12 THEN 5
WHEN temp.deadline >= 6 THEN 4
WHEN temp.deadline >= 3 THEN 3
WHEN temp.deadline >= 1 THEN 2
else 1 END as sort,count(0) counts 
from (select CASE WHEN isDayThe = 0 THEN round(deadline/30,2) else deadline end as deadline
      from t_optimization_loan p inner join t_product_credit c on p.productId = c.id 
      where p.optimizaId = 528596  and p.state = 1) 
temp group by sort order by sort asc