如何优化这个select语句？

Question

以下语句的执行时间几乎为2分钟，而使用相同表和视图且结果相同的select语句的执行时间仅为前者的一半：

SELECT
@NEW_CAREER_NUMBER = MAX(a.[career number])
FROM selecthr20.employee.[career history] a 
    INNER JOIN selecthr20.Employee.[Current Appointments As At Evaluation Date] b
        ON a.[appointment number] = b.[appointment number]  
    INNER JOIN Employee.[BSK Changes in Selected Period] c
        ON a.[career number] = c.[primary key number]
    INNER JOIN selecthr20.employee.[career history extra detail] d
        ON a.[career number] = d.[career number]
            AND c.[primary key number] = d.[career number]
where c.[primary key name] = 'Career Number'
and c.[audit date] < (Select max([audit date]) from Employee.[BSK Changes in Selected Period] where [primary key number] = @LATEST_CAREER_NUMBER)
and b.[person number] = @PERSON_ID

‘'BSK in Selected Period c’是一个视图，其余的是表。

Answer 1

您在d连接上有一个冗余条件。

试一试

SELECT
@NEW_CAREER_NUMBER = MAX(a.[career number])
FROM selecthr20.employee.[career history] a 
JOIN selecthr20.Employee.[Current Appointments As At Evaluation Date] b
      ON b.[appointment number] = a.[appointment number] 
     and b.[person number]      = @PERSON_ID  
JOIN selecthr20.employee.[career history extra detail] d
      ON d.[career number] = a.[career number] 
JOIN Employee.[BSK Changes in Selected Period] c
      ON c.[primary key number] = a.[career number] 
     and c.[primary key name]   = 'Career Number'
     and c.[audit date] < ( Select max([audit date])   
                              from Employee.[BSK Changes in Selected Period]
                             where [primary key number] = @LATEST_CAREER_NUMBER );

自然地将索引放在连接条件上

如果在c连接上有一个循环，那么可以使用一个变量作为审计日期。

我会这么做的--这几乎没有什么坏处

declare @auditdate DateTime = ( Select max([audit date]) 
                                  from Employee.[BSK Changes in Selected Period] 
                                 where [primary key number] = @LATEST_CAREER_NUMBER );

... 

and c.[audit date] < @auditdate