sql-server – T-SQL是更新限制Atomic的子查询吗？

我在MS Sql Server 2008 R2中有一个简单的队列实现.这是队列的本质：

CREATE TABLE ToBeProcessed 
(
    Id BIGINT IDENTITY(1,1) PRIMARY KEY NOT NULL,[Priority] INT DEFAULT(100) NOT NULL,IsBeingProcessed BIT default (0) NOT NULL,SomeData nvarchar(MAX) NOT null
)

我想原子地选择按优先级排序的前n行和IsBeingProcessed为false的id,并更新这些行以表示它们正在被处理.我以为我会使用Update,Top,Output和Order By的组合,但不幸的是你不能在Update语句中使用top和order by.

所以我创建了一个in子句来限制更新,并且子查询按顺序执行(见下文).我的问题是,这整个语句是原子的,还是我需要将它包装在一个事务中？

DECLARE @numberToProcess INT = 2

CREATE TABLE #IdsToProcess
(
    Id BIGINT NOT null
)

UPDATE 
    ToBeProcessed
SET
    ToBeProcessed.IsBeingProcessed = 1
OUTPUT 
    INSERTED.Id 
INTO
    #IdsToProcess   
WHERE
    ToBeProcessed.Id IN 
    (
        SELECT TOP(@numberToProcess) 
            ToBeProcessed.Id 
        FROM 
            ToBeProcessed 
        WHERE
            ToBeProcessed.IsBeingProcessed = 0
        ORDER BY 
            ToBeProcessed.Id,ToBeProcessed.Priority DESC)

SELECT 
    *
FROM 
    #IdsToProcess

DROP TABLE #IdsToProcess

这是一些插入一些虚拟行的SQL：

INSERT INTO ToBeProcessed (SomeData) VALUES (N'');
INSERT INTO ToBeProcessed (SomeData) VALUES (N'');
INSERT INTO ToBeProcessed (SomeData) VALUES (N'');
INSERT INTO ToBeProcessed (SomeData) VALUES (N'');
INSERT INTO ToBeProcessed (SomeData) VALUES (N'');

解决方法

在那种情况下,我会使用这种方法.

;WITH cte As
(
SELECT TOP(@numberToProcess) 
            *
        FROM 
            ToBeProcessed WITH(UPDLOCK,ROWLOCK,READPAST) 
        WHERE
            ToBeProcessed.IsBeingProcessed = 0
        ORDER BY 
            ToBeProcessed.Id,ToBeProcessed.Priority DESC
)            
UPDATE 
    cte
SET
    IsBeingProcessed = 1
OUTPUT 
    INSERTED.Id 
INTO
    #IdsToProcess

我之前有点不确定SQL Server是否会在使用子查询处理您的版本时使用U锁,从而阻止两个并发事务读取相同的TOP N行.这似乎不是这种情况.

测试表

CREATE TABLE JobsToProcess
(
priority INT IDENTITY(1,1),isprocessed BIT,number INT
)

INSERT INTO JobsToProcess
SELECT TOP (1000000) 0,0
FROM master..spt_values v1,master..spt_values v2

测试脚本(在2个并发SSMS会话中运行)

BEGIN TRY
DECLARE @FinishedMessage VARBINARY (128) = CAST('TestFinished' AS  VARBINARY (128))
DECLARE @SynchMessage VARBINARY (128) = CAST('TestSynchronising' AS  VARBINARY (128))
SET CONTEXT_INFO @SynchMessage

DECLARE @OtherSpid int

WHILE(@OtherSpid IS NULL)
SELECT @OtherSpid=spid 
FROM sys.sysprocesses 
WHERE context_info=@SynchMessage and spid<>@@SPID

SELECT @OtherSpid


DECLARE @increment INT = @@spid
DECLARE @number INT = @increment

WHILE (@number = @increment AND NOT EXISTS(SELECT * FROM sys.sysprocesses WHERE context_info=@FinishedMessage))
UPDATE JobsToProcess 
SET @number=number +=@increment,isprocessed=1
WHERE priority = (SELECT TOP 1 priority 
                   FROM JobsToProcess 
                   WHERE isprocessed=0 
                   ORDER BY priority DESC)

SELECT * 
FROM JobsToProcess 
WHERE number not in (0,@OtherSpid,@@spid)
SET CONTEXT_INFO @FinishedMessage
END TRY
BEGIN CATCH
SET CONTEXT_INFO @FinishedMessage
SELECT ERROR_MESSAGE(),ERROR_NUMBER()
END CATCH

几乎立即执行停止,因为两个并发事务都更新同一行,因此在识别TOP 1优先级时采用的S锁必须在获取U锁之前释放,然后2个事务继续按顺序获得行U和X锁.

如果添加了CI ALTER TABLE JobsToProcess ADD PRIMARY KEY CLUSTERED(优先级),那么死锁几乎立即发生,因为在这种情况下,行S锁没有被释放,一个事务获取行上的U锁并等待将其转换为一个X锁,另一个事务仍在等待将其S锁转换为U锁.

如果上面的查询更改为使用MIN而不是TOP

WHERE priority = (SELECT MIN(priority)
                   FROM JobsToProcess 
                   WHERE isprocessed=0 
                   )

然后SQL Server设法完全消除计划中的子查询并一直采用U锁.

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