SQL：优化递归 CTE

Question

示例table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 在同一个 table

中对 EmployeeId 的引用

目标： 每当使用 EmployeeId、TeamleaderId、CompanyId 在 table 中插入一行时，自动填充 TopTeamleaderId、LEVEL 和 ParentTree 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： 想象一下，一家公司有 1000000 名员工，执行此查询需要很长时间。如何优化它以便只考虑插入行的父级？

Answer 1

递归 CTE 很棒，但如您所见，性能可能会因层次结构较大而受到影响。我坚信 临时表没有什么可耻的。

以下将在 0.784 秒内生成 200K 点层次结构。

例子

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

Returns

SQL：优化递归 CTE

SQL: Optimizing Recursive CTE

sql

sql-server

recursive-query

common-table-expression