只有一個表的SQL查詢 - 花費太多時間

Question

我有下面的sql查詢。

declare @RoleID int 
SELECT @RoleID = RoleID FROM RoleMaster WHERE RoleEnglishDesc = 'Surveyor' 

SELECT * FROM (
SELECT 
    S.ShiftID, 2 AS Flag, ISNULL(U.Latitude, 0) Latitude, ISNULL(U.Longitude, 0) Longitude, 
    U.EmployeeEnglishName AS ToolTipEnglish, U.EmployeeArabicName AS ToolTipArabic, 
    0 AS CaseStageID, COUNT(C.SurveyorAssigned) AS Assigned 
    -- L.EntryTime AS LastEntryTime 
    -- S1.cnt AS OnBreak 
FROM 
    Users U 
    INNER JOIN dbo.SurveyorShift S ON S.UserId = U.UserID AND S.ShiftStatus = 1 
    LEFT JOIN CaseInfoCommandQueue C ON C.SurveyorAssigned = U.UserID 
WHERE U.IsActive = 1 
    AND NULLIF(NULLIF(U.Latitude, '0'), '') IS NOT NULL 
    AND NULLIF(NULLIF(U.Longitude, '0'), '') IS NOT NULL 
    AND ((U.RoleID = @RoleID) OR (U.RoleID2 = @RoleID) OR (U.RoleID3 = @RoleID)) 
GROUP BY U.UserID, RoleID, Latitude, Longitude, EmployeeEnglishName, EmployeeArabicName, ShiftID 
) D 
OUTER APPLY 
(
SELECT TOP 1 EntryTime FROM DeviceUserLocation DLoc 
WHERE DLoc.ShiftID = D.ShiftID ORDER BY DLoc.EntryTime DESC 
) AS L 

我唯一的問題在最後一個外面申請獲取進入時間。因爲外部應用只需要10秒。該表的DeviceUserLocation也有條目。任何克服或步驟使用其他方式的頂部1？

Answer 1

創建覆蓋索引爲您的數據，請務必在每張桌子上有一個聚集索引和上述再按鍵，如果你不知道的聚集和覆蓋索引然後在這裏：http://en.wikipedia.org/wiki/Database_index#Covering_index，尤其是在這裏：https://www.simple-talk.com/sql/learn-sql-server/using-covering-indexes-to-improve-query-performance/您的查詢可能做表掃描（搜索或檢查每一行）而不是索引查找：在索引中查找值並直接轉到該行。 （它不能做索引查找，如果沒有和該列索引，並且它不能直接，如果沒有聚集索引）