Sql查询穿透行优化

Question

下面是我的表结构

MyTable
-----------

ObjectID int (Identity),           -- Primary Key
FileName varchar(10),
CreatedDate datetime
...........
...........
...........

我需要获取在文件中创建记录所用的时间...即..。同一文件中的前一条记录与同一文件的当前记录之间经过的时间

即..。如果记录是

ObjectID    FileName    CreatedDate (just showing the time part here)
--------    --------    -----------
1           ABC         10:23
2           ABC         10:25
3           DEF         10:26
4           ABC         10:30
5           DEF         10:31
6           DEF         10:35

所需的输出为...

ObjectID    FileName    CreatedDate     PrevRowCreatedDate
--------    --------    ----------- ---------------
1           ABC         10:23           NULL
2           ABC         10:25           10:23
3           DEF         10:26           NULL
4           ABC         10:30           10:25
5           DEF         10:31           10:26
6           DEF         10:35           10:31

到目前为止，我收到了这个查询，但它花费的时间比预期的要长……有没有更好的方法...

    Select  A.ObjectID, 
        A.FileName
        A.CreatedDate as CreatedDate, 
        B.PrevRowCreatedDate,
        datediff("SS", '1900-01-01 00:00:00', Coalesce((A.CreatedDate - B.PrevRowCreatedDate),0)) as secondsTaken
    from MyTable as A 
        Cross Apply (       
        (Select PrevRowCreatedDate = Max(CreatedDate) from MyTable as BB 
                        where   BB.FileName = A.FileName and 
                                BB.CreatedDate < A.CreatedDate
        )
        ) as B  

如果你需要更多信息，请让我知道

谢谢

Answer 1

SELECT t1.FileName, t1.CreatedDate, t2.CreatedDate as PrevCreatedDate
FROM 
   (SELECT FileName, CreateDate,
          ROW_NUMBER() OVER(PARTITION BY FileName ORDER BY CreatedDate) AS OrderNo
   FROM MyTable) t1
LEFT JOIN
   (SELECT FileName, CreateDate,
     ROW_NUMBER() OVER(PARTITION BY FileName ORDER BY CreatedDate) AS OrderNo
     FROM MyTable) t2
ON (t1.FileName = t2.FileName AND t1.OrderNo = t2.OrderNo - 1)

或者更好地使用'WITH'，因为查询是相同的：

WITH t(ObjectID, FileName, CreatedDate, OrderNo) AS
   (SELECT ObjectID, FileName, CreatedDate,
          ROW_NUMBER() OVER(PARTITION BY FileName ORDER BY CreatedDate) AS OrderNo
   FROM MyTable) 
SELECT t1.ObjectID, t1.FileName, t1.CreatedDate, t2.CreatedDate AS PrevCreatedDate,
        DATEDIFF("SS", '1900-01-01 00:00:00', 
           COALESCE((t1.CreatedDate - t2.CreatedDate),0)) AS secondsTaken
FROM t t1 LEFT JOIN t t2 
ON (t1.FileName = t2.FileName AND t1.OrderNo = t2.OrderNo + 1)

Answer 2

我认为Michael的答案确实应该被证明更有效率。但是，在评估效率时，我只想引起人们注意Management Studio中显示的查询成本(相对于批处理)的一个问题。

我建立了一个有23,174行的测试表，并运行了问题中的查询和Michael的查询。查看实际执行计划中的“查询成本(相对于批处理)”，原始查询是1%，Michael的99%的成本，因此看起来效率非常低。

​

​

然而，实际的统计数据告诉我们一个完全不同的故事。

交叉应用方法

SQL Server执行时间: CPU时间=0毫秒，运行时间=0毫秒。

表'MyTable‘。扫描计数23175、逻辑读取49335、物理读取0、预读0、lob逻辑读取0、lob物理读取0、lob预读读取0。

ROW_NUMBER方法

CPU执行时间：

时间=391ms，运行时间=417ms。

表'Worktable‘。扫描计数为0、逻辑读取为0、物理读取为0、预读为0、lob逻辑读取为0、lob物理读取为0、lob预读为0。

表'MyTable‘。扫描计数2、逻辑读取148、物理读取0、预读读取0、lob逻辑读取0、lob物理读取0、lob预读读取0。

在ROW_NUMBER计划中，rownumber=rownumber+1上的合并连接两边都有23,174行。这个值是唯一的，实际输出的行数也是23,174。但是，SQL Server估计该连接产生的行数将为34,812,000，因此它对计划中稍后插入操作的估计成本是非常不准确的。

测试脚本

BEGIN TRAN

CREATE TABLE MyTable
  (
     [ObjectID]    [INT] IDENTITY(1, 1) NOT NULL PRIMARY KEY CLUSTERED,
     [FileName]    [VARCHAR](50) NULL,
     [CreatedDate] [DATETIME] NULL
  )

GO

INSERT INTO MyTable
SELECT ISNULL(type, NEWID()),
       DATEADD(DAY, CAST(RAND(CAST(NEWID() AS VARBINARY)) * 10000 AS INT), GETDATE())
FROM   master.dbo.spt_values,
       (SELECT TOP 10 1 AS X FROM  master.dbo.spt_values) V


DELETE FROM MyTable
WHERE  EXISTS(SELECT *
              FROM   MyTable m2
              WHERE  MyTable.CreatedDate = m2.CreatedDate
                     AND MyTable.FileName = m2.FileName
                     AND MyTable.ObjectID < m2.ObjectID)

CREATE UNIQUE NONCLUSTERED INDEX [IX_MyTable]
  ON MyTable ([FileName] ASC, [CreatedDate] ASC)

SET STATISTICS IO ON
SET STATISTICS TIME ON

SELECT A.ObjectID,
       A.FileName,
       A.CreatedDate                                                                                AS CreatedDate,
       B.PrevRowCreatedDate,
       DATEDIFF("SS", '1900-01-01 00:00:00', COALESCE(( A.CreatedDate - B.PrevRowCreatedDate ), 0)) AS secondsTaken
INTO   #A
FROM   MyTable AS A
       CROSS APPLY ((SELECT PrevRowCreatedDate = MAX(CreatedDate)
                     FROM   MyTable AS BB
                     WHERE  BB.FileName = A.FileName
                            AND BB.CreatedDate < A.CreatedDate)) AS B;

WITH t(ObjectID, FileName, CreatedDate, OrderNo)
     AS (SELECT ObjectID,
                FileName,
                CreatedDate,
                RANK() OVER(PARTITION BY FileName ORDER BY CreatedDate) AS OrderNo
         FROM   MyTable)
SELECT t1.ObjectID,
       t1.FileName,
       t1.CreatedDate,
       t2.CreatedDate                                                                          AS PrevCreatedDate,
       DATEDIFF("SS", '1900-01-01 00:00:00', COALESCE(( t1.CreatedDate - t2.CreatedDate ), 0)) AS secondsTaken
INTO   #B
FROM   t t1
       LEFT JOIN t t2
         ON ( t1.FileName = t2.FileName
              AND t1.OrderNo = t2.OrderNo + 1 )

/*Test the 2 queries give the same result*/
SELECT *
FROM   #A
EXCEPT
SELECT *
FROM   #B

SELECT *
FROM   #B
EXCEPT
SELECT *
FROM   #A

ROLLBACK