事务的隔离级别（Transaction isolation levels）3

在sql server2005里，在 READ COMMITTED隔离级别，SQL server提供了两种不同的方法来阻止一个事务reading dirty data。默认方式，我们已经刚才看过了，用 pessimistic 并发性，锁住正在修改的数据，阻止其它过程读取这个数据，从而使其它的事务处于堵塞状态。

还有一种方法，叫 READ_COMMITTED_SNAPSHOT,用optimistic并发性，允许non-repeatable reads和 phantom reads，阻止 dirty reads的方式不是堵塞另外一个事务，而是相反不堵塞它。

举一个例子。

-- Step 1:
-- First close all other connections to make sure no one is using 
-- the IsolationDB datatabase
-- Step 2:
-- Change the database option to enable "read committed snapshot"
ALTER DATABASE IsolationDB SET READ_COMMITTED_SNAPSHOT ON ;
--<EXECUTE>

-- Step 3:
-- Start a transaction but don't commit it
USE IsolationDB ;
GO
BEGIN TRAN
UPDATE IsolationTest
SET col2 = 'New Value' ;
--<EXECUTE>

开始一个事务，但没有提交。

-- Step 4: 
-- Start a new connection and change your isolation level 
USE IsolationDB ;
GO
SET TRANSACTION ISOLATION LEVEL READ COMMITTED ;
SELECT *
FROM IsolationTest ;
--<EXECUTE>
-- You should notice that the second connection is not blocked, but 
-- it does not return the changed data. The results you get are the
-- original committed data, before the UPDATE in Step 3 was performed
-- no data or messages!

查询之后，发现还是原来的数据，没有堵塞。

-- To finish up, perform the following steps:
-- Step 5:
-- Return to the connection from Step 1 and issue a ROLLBACK
ROLLBACK TRANSACTION ;
--<EXECUTE>
-- Step 6:
-- Now close all other connections to make sure no one is using 
-- the IsolationDB datatabase
-- Step 7:
-- Change the database option to disable "read committed snapshot"
ALTER DATABASE IsolationDB SET READ_COMMITTED_SNAPSHOT OFF ;
--<EXECUTE>

重新把设置改回来吧。