动态查询中的 TSQL 隔离级别

Question

存储过程中指定的隔离级别是否会传递到该存储过程中的动态查询？

CREATE PROCEDURE MySP AS 
BEGIN
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
DECLARE @dSQL VARCHAR(max) = 'SELECT col FROM table';
EXEC(@dSQL);
END

那么，该动态查询中的隔离级别是多少？

Answer 1

它确实可以进行动态查询，您可以使用以下内容进行检查：

DBCC useroptions;

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
DECLARE @dSQL VARCHAR(max) = 'DBCC useroptions';
EXEC(@dSQL);

Answer 2

通常不仅仅是TRANSACTION ISOLATION LEVEL，SET 语句是会话级别的，因此可以传递到子流程。但是，虽然它们可以在子流程中更改，但这些更改不会带回调用/父上下文。

有关更多详细信息，请参阅 SET Statements 的 MSDN 页面。

您可以通过以下操作轻松测试：

SSMS 查询选项卡 1：

-- DROP TABLE ##tmp;
CREATE TABLE ##tmp (Col1 INT);
INSERT INTO ##tmp (Col1) VALUES (1);

BEGIN TRAN
SELECT * FROM ##tmp WITH (HOLDLOCK, TABLOCKX);
WAITFOR DELAY '00:02:00.000'; -- 2 minute timer
ROLLBACK TRAN;

SSMS 查询选项卡 2：

-- First, highlight the following and run. It will hang. Cancel the query.
-- SELECT * FROM ##tmp;

-- Second, hit F5. It will return the row twice.
-- If the SET command in the EXEC affected the parent process, the 2nd SELECT
-- would hang.
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;

EXEC (N' SELECT * FROM ##tmp; SET TRANSACTION ISOLATION LEVEL READ COMMITTED;');

SELECT * FROM ##tmp;

Answer 3

我相信您确实具有隔离级别。我做了这个测试，我在动态查询中获得了隔离级别 1

-- Create the SP (my test is on AdventureWorks2012)
CREATE PROCEDURE MySP AS 
BEGIN
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
DECLARE @dSQL VARCHAR(max) = 'SELECT * FROM Person.Person; WAITFOR DELAY ''02:00'';';
EXEC(@dSQL);
END

-- Exec SP
EXEC MySP

-- Check the isolation level while your dynamic code is running
SELECT transaction_isolation_level, * FROM sys.sysprocesses t1
JOIN sys.dm_exec_requests t2 on t1.spid = t2.session_id
WHERE t1.spid > 50