CTE Recursion Performance

CTE全名是Common Table Expression，语法基础请参考MSDN文档：https://msdn.microsoft.com/zh-cn/library/ms175972.aspx。

CTE Recursion诞生之时，着实让人惊艳了一把。被很多吃瓜群众以讹传讹之后，“慢”似乎成了CTE Recursion最大的原罪。

很多时候，用到CTE Recursion的场景，无非是千八百条的数据量，最大也不过万八千条，所以“慢”算不上个问题。直到前几天，一个群友问：500W的数据做CTE递归时，怎么做性能优化……

结论：

1. 合理的索引会极大的提升CTE Recursion的性能；
2. 根据实验结果猜测：Sql Server2016对CTE Recursion做了优化，缺失合理索引的前提下，性能有极大的提升（受测试样本影响，结果可能不准确。无论如何，索引可以帮到你）。

Talk is cheap，Show me the code！所以，原因如下图（样本数据：1W零1条^^）：

拒绝耍流氓，测试代码如下：

 IF OBJECT_ID('dbo.TestCte', 'U') IS NOT NULL
 DROP TABLE dbo.TestCte;
 GO
 CREATE TABLE dbo.TestCte
     (
       Id VARCHAR(10) NOT NULL ,
       ParentId VARCHAR(10) NULL
     );
 WITH    cte_001
           AS (    SELECT   1 AS a UNION ALL
                 SELECT   2 AS a UNION ALL
                 SELECT   3 AS a UNION ALL
                 SELECT   4 AS a UNION ALL
                 SELECT   5 AS a UNION ALL
                 SELECT   6 AS a UNION ALL
                 SELECT   7 AS a UNION ALL
                 SELECT   8 AS a UNION ALL
                 SELECT   9 AS a UNION ALL
                 SELECT   10 AS a )
     INSERT  dbo.TestCte
             ( Id, ParentId )
             SELECT    RIGHT( '' + CAST ( T01.Id AS VARCHAR(10) ), 10 ) AS Id ,
                     RIGHT( '' + CAST ( CEILING( T01.Id / 10 ) AS VARCHAR(10) ), 10 ) AS ParentId
             FROM (
                     SELECT  ROW_NUMBER() OVER ( ORDER BY cte_001.a ) AS Id
                     FROM    cte_001
                             CROSS JOIN cte_001 AS A
                             CROSS JOIN cte_001 AS B
                             CROSS JOIN cte_001 AS C
                             CROSS JOIN cte_001 AS D
                             CROSS JOIN cte_001 AS E
                             CROSS JOIN (SELECT TOP 5 * FROM cte_001) AS F ) AS T01;
 GO

 INSERT DBO.TestCte ( Id, ParentId )
 VALUES  ( '',  NULL );
 GO

 --无索引版本
 SET STATISTICS TIME ON;
 SET STATISTICS IO ON;

 IF OBJECT_ID('dbo.T', 'U') IS NOT NULL
 DROP TABLE dbo.T;
 GO
 CREATE TABLE dbo.T
     (
       RN UNIQUEIDENTIFIER PRIMARY KEY,
       Id VARCHAR(10) ,
       ParentId VARCHAR(10) ,
       Memo1 NVARCHAR(128) DEFAULT ( N'我是占位置的！我是占位置的！我是占位置的！我是占位置的！我是占位置的！' ) ,
       Memo2 NVARCHAR(128) DEFAULT ( N'我是很骄傲的！我是很骄傲的！我是很骄傲的！我是很骄傲的！我是很骄傲的！' )
     );
 INSERT  dbo.T
         ( RN ,
           Id ,
           ParentId
         )
         SELECT  N.RN ,
                 N.Id ,
                 N.ParentId
         FROM    ( SELECT    NEWID() AS RN ,
                             Id ,
                             ParentId
                   FROM      dbo.TestCte
                   WHERE     Id < 10001--测试数据量，改这里
                 ) AS N
         ORDER BY RN ASC;
 GO
 WITH    cte_001
           AS ( SELECT   Id ,
                         ParentId
                FROM     dbo.T
                WHERE    ParentId IS NULL
                UNION ALL
                SELECT   T01.Id ,
                         T01.ParentId
                FROM     T AS T01
                         INNER JOIN cte_001 AS T02 ON T02.Id = T01.ParentId
              )
     SELECT  COUNT(*)
     FROM    cte_001;

 --有索引版本
 SET STATISTICS TIME ON;
 SET STATISTICS IO ON;
 IF OBJECT_ID('dbo.T', 'U') IS NOT NULL
 DROP TABLE dbo.T;
 GO
 CREATE TABLE dbo.T
     (
       RN UNIQUEIDENTIFIER PRIMARY KEY,
       Id VARCHAR(10) ,
       ParentId VARCHAR(10) ,
       Memo1 NVARCHAR(128) DEFAULT ( N'我是占位置的！我是占位置的！我是占位置的！我是占位置的！我是占位置的！' ) ,
       Memo2 NVARCHAR(128) DEFAULT ( N'我是很骄傲的！我是很骄傲的！我是很骄傲的！我是很骄傲的！我是很骄傲的！' )
     );
 INSERT  dbo.T ( RN , Id , ParentId )
         SELECT  N.RN , N.Id , N.ParentId
         FROM    ( SELECT    NEWID() AS RN , Id , ParentId
                   FROM      dbo.TestCte
                   WHERE     Id < 10001--测试数据量，改这里
                 ) AS N
         ORDER BY RN ASC;
 GO

 --创建索引
 CREATE NONCLUSTERED INDEX IDX_DBO_T_PARENTID_ID
 ON [dbo].[T] ([ParentId], Id)
 GO

 WITH    cte_001
           AS ( SELECT   Id , ParentId
                FROM     dbo.T
                WHERE    ParentId IS NULL
                UNION ALL
                SELECT   T01.Id , T01.ParentId
                FROM     T AS T01
                         INNER JOIN cte_001 AS T02 ON T02.Id = T01.ParentId
              )
     SELECT  COUNT(*)
     FROM    cte_001;