oracle事务隔离级别,用jdbc体验

item

item_value

action_time

id

aaa

LOOCKY

06-12-2006 15:23:54

1

tsindex

users

06-12-2006 15:23:54

2

tstemp

temp

06-12-2006 15:23:54

3

Read committed

This is the default transaction isolation level. Each query executed by a transaction sees only data that was committed before the query (not the transaction) began. An Oracle query never reads dirty (uncommitted) data.

Because Oracle does not prevent other transactions from modifying the data read by a query, that data can be changed by other transactions between two executions of the query. Thus, a transaction that runs a given query twice can experience both nonrepeatable read and phantoms.

Serializable

Serializable transactions see only those changes that were committed at the time the transaction began, plus those changes made by the transaction itself through INSERT , UPDATE , and DELETE statements. Serializable transactions do not experience nonrepeatable reads or phantoms.

Dirty write

Not possible

Not possible

Dirty read

Not possible

Not possible

Nonrepeatable read

Possible

Not possible

Phantoms

Possible

Not possible

Read committed

This is the default transaction isolation level. Each query executed by a transaction sees only data that was committed before the query (not the transaction) began. An Oracle query never reads dirty (uncommitted) data.

Because Oracle does not prevent other transactions from modifying the data read by a query, that data can be changed by other transactions between two executions of the query. Thus, a transaction that runs a given query twice can experience both nonrepeatable read and phantoms.

默认的隔离级别设置。事务中的查询只能看到在此查询之前（ 而非事务开始之前 ）提交的数据。

由于 oracle 不会因为查询数据而阻止另外一个事务修改数据，因此数据可以在一个事务中的 2 次查询中，查到不同的结果。因此

可能出现 nonrepeatable read and phantoms 的情况

Serializable

Serializable transactions see only those changes that were committed at the time the transaction began, plus those changes made by the transaction itself through INSERT , UPDATE , and DELETE statements. Serializable transactions do not experience nonrepeatable reads or phantoms.

根绝我的理解解释一下：

serializable transactions 在事务执行： 2 次同一条数据查询的时候（就是两次执行查询，就是说执行完第一个 .executeQuery ，然后执行第二个 .executeQuery ），如果在第一个 .executeQuery 开始执行而另外一个事务已经开始修改数据，并且已经提交，那么两次读取的数据是另外一个事务修改前的数据。

如果在第一个 .executeQuery 之前，另外一个事务修改了数据，那么两次读取的数据是另外一个事务修改后的数据。

这恰恰反映了， repeatable read ，两次结果一致

这与 Read committed 完全不同，

要是 Read committed ，第一个 .executeQuery 未执行完第二事务，而在第二个 .executeQuery 前第二个事务执行完毕，那么第一个 .executeQuery 得到的是初始数据，而第二个 .executeQuery 得到的是修改后的数据

恰恰说明了 nonrepeatable read ，两次结果不一致的情况