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ORACLE锁机制中有两种锁分为:排他锁、共享锁 排他锁:又称X锁,当用户操纵一条数据时,oracle会自动隐式的未该用户操纵的这条记录加上排他锁。加上排他锁后其他事务是不能对该条数据操纵的,只能查看,其他事务也不能再继加X锁。当本事务结束后,其他事务才

ORACLE锁机制中有两种锁分为:排他锁、共享锁

排他锁:又称X锁,当用户操纵一条数据时,oracle会自动隐式的未该用户操纵的这条记录加上排他锁。加上排他锁后其他事务是不能对该条数据操纵的,只能查看,其他事务也不能再继加X锁。当本事务结束后,其他事务才可加X锁操纵这条数据。

共享锁:又称S锁,加锁的用户只要查看权限,而不能增删改。其他用户也可对该数据加S锁,也只能有查看权限不能增删改。


ORACLE中加锁对象的不同,也可分为三种类型的锁。

1、DML锁

DML锁是最常用的锁,为保护表对象数据的一致完整性。该锁再可细分为

行级锁:简单说锁定表的数据行;也称排他锁又称TX锁。当对某些数据行使用DML语句时,oracle自动对该数据行添加行级锁,其他事务只能等到事务提交或释放后才能执行这些数据的DML操作;行级锁不是单独的锁,系统添加行级锁时,oracle也会自动加上该数据对应的表级锁,是为防止其他用户使用DDL语句修改表结构等从而破坏行级锁的约束

表级锁:简单说锁定整个对象表;也称表锁又称TM锁。表级锁可分为5种:

1).行共享

2).行排他

3).共享

4).共享排他

5).排他

2、DDL锁


3、系统锁:系统内部锁,用户无法调用查看的到,不需要了解。

手动加锁

一般情况下,ORACLE执行DML语句,一般时不需要手动加锁的,它会自动加锁。也可以手动的方式去添加锁,手动添加方式为:

1).SELECT 语句 FOR UPDATE [of columns] [wait n | nowait] [skip locked];    --对行记录加锁

2).LOCK TABLE tableName IN lockemode MODE NOWAIT                  --对表加锁

加锁语句练习:

1).使用SELECT 语句 FOR UPDATE语句加锁:

会话1:增加一条记录,不提交事务

SQL> select * from classes;
                                    CID CNAME
--------------------------------------- ----------
                                      1 0901
                                      2 0902
                                      3 0903
                                      4 1111

SQL> update classes set cname='0000' where cid=4;
1 row updated

--在会话1中插入了一条记录但不提交,这时会话1同时拥有TX(行级锁),TM(表级锁)

会话2:使用SELECT * FOR UPDATE 语句加行级锁

SQL> select * from classes for update nowait;
select * from classes for update nowait
ORA-00054: 资源正忙, 但指定以 NOWAIT 方式获取资源

--会话2对表里面行数据添加行级锁失败,因为会话1中还在占用该锁。 如不指定nowait | wait 关键字,会一直处于等待阶段,直到锁被释放才可获取到锁。

SQL> select * from classes for update;
SQL> --等待中....


2).使用LOCK TABLE 语句加锁

会话1:在会话1中修改一条记录,但不提交。

SQL> select * from classes;
                                    CID CNAME
--------------------------------------- ----------
                                      1 0901
                                      2 0902
                                      3 0903
                                      4 0000

SQL> update classes set cname='1111' where cid=4;
1 row updated

--这时会话1中拥有TM,TX锁。


会话2:会话2表中添加行拍他锁

SQL> lock table classes in row exclusive mode nowait;
Table(s) locked

--添加表级锁成功。



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