SQL：优化递归CTE

Question

实施例的表结构：

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参照EmployeeId在同一表

目标： 每当行被插入在表中与EmployeeId ，TeamleaderId,CompanyId自动填充TopTeamleaderId,LEVEL和ParentTree与AFTER INSERT触发

代码：

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 

问题： 想象有像百万员工在一个公司，这个查询需要很长的时间来执行。如何优化它，以便只有插入行的父母才会被记录下来？

Answer 1

递归CTE是伟大的，但正如你可以看到，性能可能会受到更大的层次结构的影响。我坚信临时表没有耻辱。

以下将在0.784秒内生成200K点层次结构。

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 

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