巢狀sql的查詢速度比較分析
文章中使用oracle自帶的hr資料庫，故**可以直接進行測試。
**一：
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'a%'
or t.first_name like 'b%'
or t.first_name like 'h%'
or t.first_name like 'k%'
or t.first_name like 'm%'
or t.first_name like 'j%'
or t.first_name like 'n%'
執行計畫：
**二：
select *
from (
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'a%'
or t.first_name like 'b%'
or t.first_name like 'h%'
or t.first_name like 'k%'
or t.first_name like 'm%'
or t.first_name like 'j%'
or t.first_name like 'n%'
)執行計畫：
對比：**1與**2的執行計畫相同
**三：
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'a%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'b%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'h%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'k%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'm%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'j%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'n%'
執行計畫：
**四：
select * from
(select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'a%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'b%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'h%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'k%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'm%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'j%'
union
select t.employee_id, t.first_name, t.phone_number from hr.employees t where t.first_name like 'n%'
)執行計畫：
其他：在乙個sql中，使用「or」語句比使用多個union會花費更短的時間。
**五：
**5-1：
select *
from
(select * from hr.employees tx where tx.department_id = 50) t1,
(select * from hr.departments ty where ty.department_id < 150) t2
where t1.department_id = t2.department_id
**5-2：
select *
from
hr.employees t1,
hr.departments t2
where t1.department_id = t2.department_id and t1.department_id = 50 and t2.department_id < 150
**5-3：
select *
from
hr.employees t1,
hr.departments t2
where t1.department_id = t2.department_id(+) and t1.department_id = 50 and t2.department_id <150
**5-4：
select *
from
hr.employees t1,
hr.departments t2
where t1.department_id(+) = t2.department_id and t1.department_id = 50 and t2.department_id <150
**5-1到**5-4的oracle執行計畫分析結果相同：
對比：**5-1到**5-4的執行計畫相同。oracle是先對t1和t2中資料進行過濾後，再對結果集進行關聯查詢。且oracle對錶過濾內容進行了優化，對錶departments的查詢優化為 ty.department_id=50 而不是 ty.department_id<150
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