SQL：優化遞歸CTE

Question

實施例的表結構：

EmployeeId TeamleaderId TopTeamleaderId LEVEL ParentTree CompanyId 
1   0    0     0  NULL   1 
2   1    1     1  2>1   1 
3   2    1     2  3>2>1   1 

TeamleaderId是foreignKey參照EmployeeId在同一表

目標： 每當行被插入在表中與EmployeeId ，TeamleaderId,CompanyId自動填充TopTeamleaderId,LEVEL和ParentTree與AFTER INSERT觸發

代碼：

WITH CTE AS (
    SELECT EmployeeId, TeamleaderId,0 AS [Level], CAST(EmployeeId AS varchar(100)) AS Heirarchy, TopTeamleaderId 
    FROM dbo.Employee 
    WHERE EmployeeId IN (SELECT EmployeeId FROM Employee WHERE TeamleaderId IS NULL 
    AND CompanyId IN(SELECT DISTINCT CompanyId FROM INSERTED)) 

    UNION ALL 

    SELECT mgr.EmployeeId, mgr.TeamleaderId, CTE.[Level] +1 AS [Level], 
     CAST((CAST(mgr.EmployeeId AS VARCHAR(100)) + '>' + CTE.Heirarchy) AS varchar(100)) AS Heirarchy, CTE.TopTeamleaderId 
    FROM CTE 
    INNER JOIN dbo.Employee AS mgr 
     ON TaskCTE.EmployeeId = mgr.ParentTeamleaderId 
) 
UPDATE Employee SET [LEVEL] = TC.[LEVEL], ParentTree = TC.Heirarchy, TopTeamleaderId = TC.TopTeamleaderId 
FROM dbo.Employee AS Employee 
JOIN (SELECT * FROM CTE WHERE EmployeeId IN(SELECT DISTINCT EmployeeId FROM INSERTED) AND ParentTeamleaderId IS NOT NULL) TC 
ON 
Employee.EmployeeId = TC.EmployeeId 

問題： 想象有像百萬員工在一個公司，這個查詢需要很長的時間來執行。如何優化它，以便只有插入行的父母纔會被記錄下來？

Answer 1

遞歸CTE是偉大的，但正如你可以看到，性能可能會受到更大的層次結構的影響。我堅信臨時表沒有恥辱。

以下將在0.784秒內生成200K點層次結構。

例

Select EmployeeId 
     ,TeamleaderId 
     ,Lvl=1 
     ,TopTeamleaderId = 0 
     ,ParentTree=cast(EmployeeId as varchar(500)) 
     ,CompanyID 
Into #TempBld 
From Employee 
Where TeamleaderId is null 

Declare @Cnt int=1 
While @Cnt<=30  --<< Set Your Max Level 
    Begin 
     Insert Into #TempBld 
     Select A.EmployeeId 
       ,A.TeamleaderId 
       ,B.Lvl+1 
       ,IIF(B.Lvl=1,B.EmployeeId,B.TopTeamleaderId) 
       ,concat(A.EmployeeId,'>',B.ParentTree) 
       ,A.CompanyID 
     From Employee A 
     Join #TempBld B on (B.Lvl=@Cnt and A.TeamleaderId=B.EmployeeId) 
     Set @Cnt=@Cnt+1 
    End 

--Select * from #TempBld Order by ParentTree 

返回