如何取消/重置 SQL Server 的事务隔离级别？

Question

也许我误解了有关事务或 SQL Server 正在做什么的事情，但请考虑以下 T-SQL：

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
GO

BEGIN TRANSACTION

-- DO SOME READS AND OTHER THINGS

COMMIT

-- OK, WHAT HAPPENS TO THE ISOLATION LEVEL AFTER THIS?

也许这并不重要，但我喜欢那种温暖模糊的感觉，即我让事情保持原来的样子。完成我正在做的事情。是否可以将隔离级别重置回原始状态，无论之前的状态如何？

Answer 1

如果您的代码在存储过程中执行，则更改仅适用于存储过程的范围内 - 当存储过程返回时，连接的隔离级别将自动恢复到之前的级别：（

create procedure dbo.IsoTest
as
    set transaction isolation level serializable
    begin transaction

    select transaction_isolation_level FROM sys.dm_exec_sessions where session_id = @@SPID

    select object_id from sys.objects

    commit
go
select transaction_isolation_level FROM sys.dm_exec_sessions where session_id = @@SPID
exec dbo.IsoTest
select transaction_isolation_level FROM sys.dm_exec_sessions where session_id = @@SPID

忽略 sys.objects 的结果集，输出 2、4 和 2 作为隔离级别）。

Answer 2

您可以通过 sys.dm_exec_sessions.transaction_isolation_level 了解当前级别

如果需要跨批次，请使用 SET CONTEXT_INFO 保留该值，稍后也可以从 sys.dm_exec_sessions 读取该值。

DECLARE @CurrentIsolationLevel smallint

SELECT @CurrentIsolationLevel = transaction_isolation_level
FROM sys.dm_exec_sessions
WHERE session_id = @@SPID

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION
-- DO SOME READS AND OTHER THINGS
COMMIT

DECLARE @SQL varchar(200)
SELECT @SQL = 'SET TRANSACTION ISOLATION LEVEL ' +
       CASE  @CurrentIsolationLevel
           WHEN 1 THEN 'READ UNCOMMITTED'
           WHEN 2 THEN 'READ COMMITTED'
           WHEN 3 THEN 'REPEATABLE READ'
           WHEN 4 THEN 'SERIALIZABLE'
           WHEN 5 THEN 'SNAPSHOT'
       END
EXEC (@sql)

Answer 3

GBN 解决方案对我不起作用。我怀疑它在其他地方是否有效。

问题是返回到先前的隔离级别仅在 EXEC 上下文中有效。我的脚本如下。请注意，如果当前隔离包括快照，它也不会尝试更改隔离。 （如果你尝试，就会失败）。

DECLARE @initalIsoloationLevel nvarchar(25)

SELECT @initalIsoloationLevel =
      CASE
          WHEN transaction_isolation_level = 1 
             THEN 'READ UNCOMMITTED' 
          WHEN transaction_isolation_level = 2 
               AND is_read_committed_snapshot_on = 1 
             THEN 'READ COMMITTED SNAPSHOT' 
          WHEN transaction_isolation_level = 2 
               AND is_read_committed_snapshot_on = 0 
             THEN 'READ COMMITTED' 
          WHEN transaction_isolation_level = 3 
             THEN 'REPEATABLE READ' 
          WHEN transaction_isolation_level = 4 
             THEN 'SERIALIZABLE' 
          WHEN transaction_isolation_level = 5 
             THEN 'SNAPSHOT' 
          ELSE NULL
       END 
FROM   sys.dm_exec_sessions AS s
WHERE  session_id = @@SPID

  /* Changing isolation level is not supported with snapshots */
IF @initalIsoloationLevel NOT LIKE '%SNAP%' AND @initalIsoloationLevel is NOT NULL
    SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;


/****** DO  YOUR  STUFF HERE   ******/


  /* Changing isolation level is not supported with snapshots */
IF @initalIsoloationLevel = 'READ COMMITTED'  
     SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
ELSE IF @initalIsoloationLevel = 'REPEATABLE READ' 
     SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
ELSE IF @initalIsoloationLevel = 'SERIALIZABLE'
     SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;