单独查询下，事务控制完美讲解

但是并没有得出明确的结论。先让我们看看事务的定义：

引用

Transactions are described in terms of ACID properties, which are as follows:
n Atomic: all changes to the database made in a transaction are rolled back if any
change fails.
n Consistent: the effects of a transaction take the database from one consistent
state to another consistent state.
n Isolated: the intermediate steps in a transaction are not visible to other users of
the database.
n Durable: when a transaction is completed (committed or rolled back), its effects
persist in the database.


即ACID的定义，从上面看来，似乎除了isolated之外，和只读查询都没有关系。那么是否只读查询不需要事务呢？

再看看Oracle对于只读事务的定义：

引用

Read-Only Transactions
By default, Oracle guarantees statement-level read consistency. The set of data returned by a single query is consistent with respect to a single point in time. However, in some situations, you might also require transaction-level read consistency. This is the ability to run multiple queries within a single transaction, all of which are read-consistent with respect to the same point in time, so that queries in this transaction do not see the effects of intervening committed transactions.

If you want to run a number of queries against multiple tables and if you are not doing any updating, you prefer a read-only transaction. After indicating that your transaction is read-only, you can run as many queries as you like against any table, knowing that the results of each query are consistent with respect to the same point in time.

Oracle默认情况下保证了SQL语句级别的读一致性，即在该条SQL语句执行期间，它只会看到执行前点的数据状态，而不会看到执行期间数据被其他SQL改变的状态。

而Oracle的只读查询(read-only transaction)则保证了事务级别的读一致性，即在该事务范围内执行的多条SQL都只会看到执行前点的数据状态，而不会看到事务期间的任何被其他SQL改变的状态。

因此我们可以得出结论：
如果你一次执行单条查询语句，则没有必要启用事务支持，数据库默认支持SQL执行期间的读一致性；
如果你一次执行多条查询语句，例如统计查询，报表查询，在这种场景下，多条查询SQL必须保证整体的读一致性，否则，在前条SQL查询之后，后条SQL查询之前，

数据被其他用户改变，则该次整体的统计查询将会出现读数据不一致的状态，此时，应该启用事务支持。

只读事务与读写事务区别
对于只读查询，可以指定事务类型为readonly，即只读事务。由于只读事务不存在数据的修改，因此数据库将会为只读事务提供一些优化手段，例如Oracle对于只读事务

，不启动回滚段，不记录回滚log。

在JDBC中，指定只读事务的办法为：
connection.setReadOnly(true);

在Hibernate中，指定只读事务的办法为：
session.setFlushMode(FlushMode.NEVER);
此时，Hibernate也会为只读事务提供Session方面的一些优化手段

在Spring的Hibernate封装中，指定只读事务的办法为：
bean配置文件中，prop属性增加“readOnly”