查询，有必要用事务吗

编程入门行业动态更新时间:2024-10-15 00:27:47

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查询，有必要用事务吗

如果只提交一个查询，有必要用事务吗？这个问题之前已经讨论过

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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”