sql server中时间戳列的实际用途是什么？

Question

我在sql server中遇到了timestamp数据类型，sql server中timestamp列的实际用途是什么？

Answer 1

让我们以销售订单表为例来说明时间戳的作用。

create table saleorder (ordernumber int, amount int, timestamp);
insert into saleorder (ordernumber, amount) values (1, 100), (2, 100), (3, 200);
select * from saleorder

注意时间戳列中的数据。 Documentation of timestamp（SQL Server 2005）说：这（即时间戳）跟踪数据库中的相对时间，而不是可以与时钟关联的实际时间......每次修改或插入带有时间戳列的行，增加的数据库时间戳值被插入到时间戳列中。

让我们看看数据是什么样子的：

ordernumber amount  timestamp
1           100     0x00000000000007D1
2           100     0x00000000000007D2
3           200     0x00000000000007D3

好的。首先添加订单 1，最后输入订单 3。如果我们要更新订单 1 的数量会怎样？

update saleorder set amount = 200 where ordernumber = 1
select * from saleorder

啊，请注意订单 1 的时间戳现在是 0x7D4（2004 年十进制）。关于其他行，我们知道订单 1 是最近更新的。但是，更重要的是，timestamp 的值是在并发写入发生时出现的。

ordernumber amount  timestamp
1           200     0x00000000000007D4
2           100     0x00000000000007D2
3           200     0x00000000000007D3

假设 John 和 Mary 都在使用以 .NET 开发的 Web 应用程序处理订单 3。约翰调出订单并进行更改。 John 尚未保存数据。玛丽拉了同样的订单并改变了它。约翰先救人。 Mary 尝试保存数据。 .NET 应用程序可以首先查看 Mary 提取的时间戳是否仍然与数据库中订单 3 的时间戳相同。

如果 Mary 使用订单 3 提取的时间戳现在不同（因为 John 保存了数据并且时间戳自动更改），.NET 应用程序可以提醒 Mary 并要求她刷新屏幕上的记录以查看最新更改（或可能会在屏幕上突出显示更改）。

将时间戳视为行版本。有趣的是，SQL Server 的最新版本使用 rowversion 数据类型，它与时间戳数据类型同义。 Documentation of rowversion (SQL Server 2012) 有一些有趣的例子。

Answer 2

我使用时间戳列来跟踪数据何时发生变化，特别是需要同步到一个或多个移动应用程序的数据。您可以使用时间戳列仅返回自某个时间点以来已更改的行（通过提供上一个时间戳）。

Answer 3

我使用TIMESTAMP数据类型（ROWVERSION，SQL2005+）来避免lost update problem：

丢失更新问题：第二个事务写入第二个值 在第一个写入的第一个值之上的数据项（数据） 并发事务，第一个值丢失给其他 并发运行的事务，根据它们的优先级，需要 读取第一个值。读取错误值的事务 以不正确的结果结束。

示例：lost update：

t  : User 1 read payment order (PO) #1 (amount 1000)
t+1: User 2 read payment order (PO) #1 (amount 1000)

t+2: User 1 change the amount for PO #1 to 1005
t+3: User 2 change the amount for PO #1 to 1009 (change make by User 1 is lost because is overwritten by change make by User 2)
t+4: The amount is **1009**.

示例：如何防止lost update：

t  : User 1 read payment order (PO) #1 (amount 1000, timestamp 0x00000000000007D1)
t+1: User 2 read payment order (PO) #1 (amount 1000, timestamp 0x00000000000007D1)

t+2: User 1 change the amount for PO #1 to 1005 and it checks if row has the same `timestamp` (column `RW` in this case; 0x00000000000007D1). The check succeeds and the change is `COMMIT`ed. This will change, also, the timestamp (column 'RW'). The new timestamp is 0x00000000000007D4.
t+3: User 2 change the amount for PO #1 to 1009 and it checks if row has the same `timestamp` (column `RW` in this case; 0x00000000000007D4). The checks fails because the initial timestamp (@rw=0x00000000000007D1) is <> than current timestamp (column `RW`=0x00000000000007D4). An error is raised the catch block "intercepts" the error and this transaction is cancelled (`ROLLBACK`).
t+4: The amount {remains|is} **1005**.

示例：How to prevent the lost update 的 T-SQL 脚本（警告：您必须使用两个 SSMS 窗口/两个会话）

CREATE DATABASE TestRowVersion;
GO
USE TestRowVersion;
GO

CREATE TABLE dbo.PaymentOrder(
    PaymentOrderID INT IDENTITY(1,1) PRIMARY KEY,
    PaymentOrderDate DATE NOT NULL,
    Amount NUMERIC(18,2) NOT NULL,
    CreateDate DATETIME NOT NULL DEFAULT (GETDATE()),
    UpdateDate DATETIME NULL,
    RW ROWVERSION NOT NULL -- R[ow] V[ersion]
);
GO

INSERT  dbo.PaymentOrder (PaymentOrderDate,Amount)
VALUES  ('2013-07-21',1000);
INSERT  dbo.PaymentOrder (PaymentOrderDate,Amount)
VALUES  ('2013-07-22',2000);
INSERT  dbo.PaymentOrder (PaymentOrderDate,Amount)
VALUES  ('2013-07-23',3000);
GO

SELECT * FROM dbo.PaymentOrder;
/*
PaymentOrderID PaymentOrderDate Amount  CreateDate              UpdateDate RW
-------------- ---------------- ------- ----------------------- ---------- ------------------
1              2013-07-21       1000.00 2013-07-21 09:35:38.750 NULL       0x00000000000007D1
2              2013-07-22       2000.00 2013-07-21 09:35:38.750 NULL       0x00000000000007D2
3              2013-07-23       3000.00 2013-07-21 09:35:38.750 NULL       0x00000000000007D3
*/
GO

-- User 1 (SQL Server Management Studio/SSMS window #1)
    -- [t] Client app, user 1: it loads first PO
    SET NOCOUNT ON;
    GO
    DECLARE @PaymentOrderID INT=1;  -- parameter

    SELECT  po.PaymentOrderID,
            po.PaymentOrderDate,
            po.Amount,
            po.RW 
    FROM    dbo.PaymentOrder po 
    WHERE   po.PaymentOrderID=@PaymentOrderID;

    -- Client app, user 1: during 15 seconds it edit the amount from 1000.00 to 1005.00
    WAITFOR DELAY '00:00:15';
    GO

    -- [t+2] Client app, user 1: it sends this change (new amount) from client app to database server 
    -- with the old row version value
    DECLARE @PaymentOrderID INT=1;              -- parameter
    DECLARE @rw BINARY(8)=0x00000000000007D1;   -- parameter
    DECLARE @NewAmount NUMERIC(18,2)=1005.00;   -- parameter

    BEGIN TRY
        BEGIN TRANSACTION
            UPDATE  dbo.PaymentOrder
            SET     Amount=@NewAmount
            WHERE   PaymentOrderID=@PaymentOrderID
            AND     RW=@rw; -- it checks the timestamp (current timestamp versus original timestamp)
            DECLARE @rowcount INT=@@ROWCOUNT; -- How many rows were affected by the last statement (UPDATE in this case) ?
            SELECT @rowcount AS [@@ROWCOUNT];
            IF @rowcount<>1
                RAISERROR('Lost update or row deleted.', 16, 1);
        COMMIT TRANSACTION
        PRINT 'UPDATE succeded';
    END TRY
    BEGIN CATCH
        IF @@TRANCOUNT>0
            ROLLBACK;

        DECLARE @ErrMsg NVARCHAR(2002);
        SET @ErrMsg=ERROR_MESSAGE();
        RAISERROR(@ErrMsg,16,1);
    END CATCH;
    GO

    -- [t+4] Client app, user 1: it reloads first PO
    DECLARE @PaymentOrderID INT=1;  -- parameter

    SELECT  po.PaymentOrderID,
            po.PaymentOrderDate,
            po.Amount,
            po.RW 
    FROM    dbo.PaymentOrder po 
    WHERE   po.PaymentOrderID=@PaymentOrderID;  
    GO

-- User 2 (warning: run this script in another SQL Server Management Studio window: File > New Database Engine Query !; SSMS window #2)
    -- [t+1] Client app, user 1: it loads first PO
    SET NOCOUNT ON;
    GO
    DECLARE @PaymentOrderID INT=1;  -- parameter

    SELECT  po.PaymentOrderID,
            po.PaymentOrderDate,
            po.Amount,
            po.RW 
    FROM    dbo.PaymentOrder po 
    WHERE   po.PaymentOrderID=@PaymentOrderID;

    -- Client app, user 1: during 20 seconds it edit the amount from 1000.00 to 1005.00
    WAITFOR DELAY '00:00:20';
    GO

    -- [t+4] Client app, user 1: it sends this change (new amout) from client app to database server 
    -- with the old row version value
    DECLARE @PaymentOrderID INT=1;              -- parameter
    DECLARE @rw BINARY(8)=0x00000000000007D1;   -- parameter
    DECLARE @NewAmount NUMERIC(18,2)=1009.00;   -- parameter

    BEGIN TRY
        BEGIN TRANSACTION
            UPDATE  dbo.PaymentOrder
            SET     Amount=@NewAmount
            WHERE   PaymentOrderID=@PaymentOrderID
            AND     RW=@rw; -- it checks the timestamp (current timestamp versus original timestamp)
            DECLARE @rowcount INT=@@ROWCOUNT; -- How many rows were affected by the last statement (UPDATE in this case) ?
            SELECT @rowcount AS [@@ROWCOUNT];
            IF @rowcount<>1
                RAISERROR('Lost update or row deleted.', 16, 1);
        COMMIT TRANSACTION
        PRINT 'UPDATE succeded';
    END TRY
    BEGIN CATCH
        IF @@TRANCOUNT>0
            ROLLBACK;

        DECLARE @ErrMsg NVARCHAR(2002);
        SET @ErrMsg=ERROR_MESSAGE();
        RAISERROR(@ErrMsg,16,1);
    END CATCH;
    GO

    -- [t+4] Client app, user 1: it reloads first PO
    DECLARE @PaymentOrderID INT=1;  -- parameter

    SELECT  po.PaymentOrderID,
            po.PaymentOrderDate,
            po.Amount,
            po.RW 
    FROM    dbo.PaymentOrder po 
    WHERE   po.PaymentOrderID=@PaymentOrderID;  
    GO

用户 1 的结果（金额 1000 -> 1005）：

PaymentOrderID PaymentOrderDate Amount                                  RW
-------------- ---------------- --------------------------------------- ------------------
1              2013-07-21       1000.00                                 0x00000000000007D1

@@ROWCOUNT <- Timestamp check succeeds 
-----------
1

UPDATE succeded
PaymentOrderID PaymentOrderDate Amount                                  RW
-------------- ---------------- --------------------------------------- ------------------
1              2013-07-21       1005.00                                 0x00000000000007D4

用户 2 的结果（金额 1000 -> 1009）：

PaymentOrderID PaymentOrderDate Amount                                  RW
-------------- ---------------- --------------------------------------- ------------------
1              2013-07-21       1000.00                                 0x00000000000007D1

@@ROWCOUNT <- Timestamp check fails 
-----------
0

Msg 50000, Level 16, State 1, Line 27
Lost update.
PaymentOrderID PaymentOrderDate Amount                                  RW
-------------- ---------------- --------------------------------------- ------------------
1              2013-07-21       1005.00                                 0x00000000000007D4

注意：将错误信息改为RAISERROR('Lost update or row deleted.', 16, 1);