SELECT ... LOCK IN SHARE MODE和SELECT ... FOR UPDATE locks在RR模式下可以看到最新的记录

14.5.2.4 Locking Reads  锁定读:

如果你查询数据然后插入或者修改相关数据在相同的事务里, 常规的SELECT 语句不能给予足够的保护。

其他事务可以修改或者删除你刚查询相同的记录,InnoDB 支持2个锁定读的类型提供额外的安全:


/*******************

测试MYSQL RR 的重复读:

CREATE TABLE `t3` (
  `sn` int(11) NOT NULL AUTO_INCREMENT COMMENT '自增编号',
  `phoneNo` int(11) DEFAULT NULL,
  PRIMARY KEY (`sn`));

mysql> show create table t3;
+-------

+------------------------------------------------------------------------------------------------------------------------------------------------------------

--------------------+
| Table | Create Table                                                                                                                                        

                           |
+-------

+------------------------------------------------------------------------------------------------------------------------------------------------------------

--------------------+
| t3    | CREATE TABLE `t3` (
  `sn` int(11) NOT NULL AUTO_INCREMENT COMMENT '自增编号',
  `phoneNo` int(11) DEFAULT NULL,
  PRIMARY KEY (`sn`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8     |
+-------

+------------------------------------------------------------------------------------------------------------------------------------------------------------

--------------------+
1 row in set (0.00 sec)


Session 1:
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from t3;
+----+---------+
| sn | phoneNo |
+----+---------+
|  1 |      11 |
|  2 |      22 |
+----+---------+
2 rows in set (0.00 sec)

mysql> select * from t3;
+----+---------+
| sn | phoneNo |
+----+---------+
|  1 |      11 |
|  2 |      22 |
+----+---------+
2 rows in set (0.00 sec)


Session 2:

Database changed
mysql> delete from t3 where sn=2;
Query OK, 1 row affected (0.00 sec)

mysql> commit;
Query OK, 0 rows affected (0.00 sec)


此时session 1看到的还是2条 表明RR是重复读



1.SELECT ... LOCK IN SHARE MODE 设置一个共享锁锁定在任何你要读的记录,其他session 只能读取记录,

因为不能修改它们直到你的事务提交。

如果任何其他记录被其他事务改变 没有被提交,你的查询等待直到那个事务借宿 然后使用最新的数据


mysql> select * from t3;
+----+---------+
| sn | phoneNo |
+----+---------+
|  1 |      11 |
|  2 |      22 |
+----+---------+
2 rows in set (0.00 sec)

mysql> select * from t3 LOCK IN SHARE MODE;








+----+---------+
| sn | phoneNo |
+----+---------+
|  1 |      11 |
|  2 |      99 |
+----+---------+
2 rows in set (4.78 sec)


看到的是最新的数据:


mysql> select * from t3;
+----+---------+
| sn | phoneNo |
+----+---------+
|  1 |      11 |
|  2 |      22 |
+----+---------+
2 rows in set (0.00 sec)

mysql> select * from t3 LOCK IN SHARE MODE;
+----+---------+
| sn | phoneNo |
+----+---------+
|  1 |      11 |
|  2 |      99 |
+----+---------+
2 rows in set (0.00 sec)



2.


mysql> select * from t3 where sn=2 for update   --看到的也是最新的记录
    -> ; 

+----+---------+
| sn | phoneNo |
+----+---------+
|  2 |      99 |
+----+---------+
1 row in set (4.26 sec)

mysql> 
mysql> select * from t3;
+----+---------+
| sn | phoneNo |
+----+---------+
|  1 |      11 |
|  2 |      22 |
+----+---------+
2 rows in set (0.00 sec)



对于所有搜索遇到, SELECT ... FOR UPDATE locks 锁定记录和任何相关的index entries,


相当于你执行一个update语句对于那些记录。


其他事务是被堵塞的来更新那些记录,做 SELECT ... LOCK IN SHARE MODE, 

或者从读取的数据在某些事务隔离级别。


一直读忽略任何锁设置在记录上(老的记录不能被锁定,它们是应用undo log重构的)



 当 LOCK IN SHARE MODE and FOR UPDATE 查询的索引锁是被释放放事务是被提交或者回滚


锁定记录用于更新使用SELECT FOR UPDATE 只使用与当autocommit 被关闭(或者开始事务使用START TRANSACTION or by setting autocommit to 0.

如果自动提交启用,匹配的记录不会被锁定)