小编Lof*_*fty的帖子

我是SQL Server 2005中执行计划的新手,但这让我感到困惑.

当我运行此代码时......

((SELECT COUNT(DISTINCT StudentID)
 FROM vwStudentUnitSchedules AS v
 WHERE v.UnitActive = 1
   AND v.UnitOutcomeCode IS NULL
   AND v.UnitCode = su.UnitCode
   AND v.StartDate = su.StartDate
   AND v.StudentCampus = st.StudentCampus) - 1) AS ClassSize

为了获得班级规模,它会超时并且一般地运行它,它需要30秒

但是当我用这个微小的修改来运行它时......

 ((SELECT COUNT(DISTINCT LTRIM(RTRIM(UPPER(StudentID))))
     FROM vwStudentUnitSchedules AS v
     WHERE v.UnitActive = 1
       AND v.UnitOutcomeCode IS NULL
       AND v.UnitCode = su.UnitCode
       AND v.StartDate = su.StartDate
       AND v.StudentCampus = st.StudentCampus) - 1) AS ClassSize

它几乎立即运行.

是因为LTRIM()RTRIM()和UPPER()函数？他们为什么要让事情变得更快？我想这是因为COUNT(DISTINCT是一个按字符从左到右计算的聚合？是StudentID是VARCHAR(10).

谢谢