mysql 幻象读

[sql] view plain copy

CREATE TABLE `t100` (
`id` bigint(20) NOT NULL default '0',
`value` varchar(32) default NULL,
PRIMARY KEY (`id`)
) ;

mysql>select @@global.tx_isolation, @@tx_isolation;
+-----------------------+-----------------+
| @@global.tx_isolation | @@tx_isolation  |
+-----------------------+-----------------+
| REPEATABLE-READ       | REPEATABLE-READ |
+-----------------------+-----------------+

试验4-1：

SESSION A:
set autocommit=0;



SESSION B:




tSessionA                                                     Session B
|
set autocommit=0;                                         set autocommit=0;
| START TRANSACTION;                                  START TRANSACTION;
|
| SELECT * FROM t100;
| empty set
|                                                   INSERT INTO t100 VALUES (1, 'a');
|
|
| SELECT * FROM t100;
| empty set
|                                                                      COMMIT;
|
| SELECT * FROM t100;
| empty set
|
| INSERT INTO t100 VALUES (1, 'a');
| ERROR 1062 (23000):
| Duplicate entry '1' for key 1
v (shit,刚刚明明告诉我没有这条记录的)

如此就出现了幻读，以为表里没有数据，其实数据已经存在了，傻乎乎的提交后，才发现数据冲突了。