SQL请求执行需要很多时间

我有一个SQL查询，有时需要15秒，有时需要1分钟。 请告诉我怎么做

SELECT TOP 100
u.firstName, 
u.id as userID,
ueh.targetID, 
ueh.opened, 
ueh.emailID, 
u.phone,
u.gender
FROM dbo.Students u 
INNER JOIN dbo.tblEmailHistory ueh
ON ueh.studentID = u.ID
WHERE (CONVERT(date,DATEADD(day,6,ueh.sDate))=CONVERT(date,getdate()))
AND IsNull(u.firstName, '') != ''
AND IsNull(u.email, '')     != ''
AND IsNull(u.phone, '')     != ''
AND ueh.status = 'sent'
AND ueh.reject_reason = 'null'
AND ueh.targetID = 28
AND ueh.opened = 0
AND u.deleted = 0
AND NOT EXISTS (SELECT ush.smsSendFullDate, ush.studentID FROM dbo.UsersSmsHistory ush WHERE u.id = ush.studentID AND DATEDIFF(DAY,ush.smsSendFullDate,GETDATE()) = 0)

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这是您的查询，大大简化了：

SELECT TOP 100 u.firstName, u.id as userID,
       ueh.targetID, ueh.opened, ueh.emailID, 
       u.phone, u.gender
FROM dbo.Students u INNER JOIN
     dbo.tblEmailHistory ueh
     ON ueh.studentID = u.ID
WHERE ueh.sDate >= cast(getdate() + 6 as date) AND
      ueh.sDate < csat(getdate() + 7 as date) AND
      u.firstName <> '' AND
      u.email <> '' AND
      u.phone <> '' AND
      ueh.status = 'sent' AND
      ueh.reject_reason = 'null' AND  -- sure you mean a string here?
      ueh.targetID = 28 AND
      ueh.opened = 0 AND
      u.deleted = 0 AND
      NOT EXISTS (SELECT ush.smsSendFullDate, ush.studentID
                  FROM dbo.UsersSmsHistory ush
                  WHERE u.id = ush.studentID AND 
                        convert(date, ush.smsSendFullDate) = convert(date, GETDATE())
                 );

注意：对于几乎所有的比较，与NULL的比较都不是真的，因此ISNULL/COALESCE是不必要的

然后开始添加索引。我建议：

tblEmailHistorytargetid，状态，已打开，已删除，拒绝原因，sdate UsersSmsHistorystudentID，smsSendFullDate

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我猜大多数学生都有姓名和电话号码，所以这些列上的索引不会有帮助。

您的查询看起来没有多余的部分。花费大量时间的原因是，您要连接三次表，其中可能包含大量数据。因此，与其改进查询，不如尝试通过在dbo.tblEmailHistory.studentID、dbo.Students.ID等列上为表添加索引来改进表的性能

是否已检查执行计划？@EdwardGizbreht。你添加索引了吗？很有效！谢谢出于好奇，将convertdate、ush.smsSendFullDate=convertdate、GETDATE子句替换为以下ush.smsSendFullDate>=convertdate、GETDATE和ush.smsSendFullDate@EvaldasBuinauskas不是更好吗。SQL Server将使用索引将日期时间转换为日期。