SQL：优化递归 CTE

Question

示例表结构：

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 对同一张表中EmployeeId 的引用

目标： 每当使用EmployeeId、TeamleaderId、CompanyId 在表中插入一行时，使用AFTER INSERT 触发器自动填充TopTeamleaderId、LEVEL 和ParentTree

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： 假设一家公司有 1000000 名员工，这个查询需要很长时间才能执行。如何优化它以便只考虑插入行的父母？

Answer 1

递归 CTE 很棒，但正如您所见，性能可能会受到更大层次结构的影响。我坚信临时表没有羞耻感。

以下将在 0.784 秒内生成 200K 点层次结构。

示例

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

退货