标签:style blog io ar strong for 数据 sp div
1:创建一个实验表
mysql> use test; mysql> CREATE TABLE t -> (id int(11) NOT NULL DEFAULT 0, -> num int(11) DEFAULT NULL, -> PRIMARY KEY(id)) -> ENGINE=INNODB DEFAULT CHARSET=gbk; Query OK, 0 rows affected (0.02 sec)
mysql> INSERT INTO t VALUES(1,100); mysql> INSERT INTO t VALUES(2,200);
Session A | Session B |
mysql> BEGIN; | |
mysql> SELECT * FROM t; +----+------+ | id | num | +----+------+ | 1 | 100 | | 2 | 200 | +----+------+ 2 rows in set (0.00 sec)
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mysql> USE test Reading table information for completion of table and column names You can turn off this feature to get a quicker startup with -A Database changed mysql> INSERT INTO t VALUES(3,300); Query OK, 1 row affected (0.01 sec)
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mysql> SELECT * FROM t; +----+------+ | id | num | +----+------+ | 1 | 100 | | 2 | 200 | +----+------+ 2 rows in set (0.00 sec)
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mysql> UPDATE t SET num=1000 WHERE id=3; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 上述查看并没有id=3的列,这里居然成功了! |
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mysql> SELECT * FROM t; +----+------+ | id | num | +----+------+ | 1 | 100 | | 2 | 200 | | 3 | 1000 | +----+------+ 3 rows in set (0.00 sec)
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从Session A整个过程看来,它试图更新一个不存在的记录(id=3),结果更新成功,并且之后这个记录可以访问。 为什么SessionA第二次检索仍然是2条记录呢? Innodb内部每个事务开始时,都会有一个事务id,同时事务对象中还有一个read_view变量,用于控制该事务可见的记录范围(MVCC)。 对于每个访问到的记录行,会根据read_view的trx_id(事务id)与行记录的trx_id比较,判断记录是否逻辑上可见。 Session B中插入的记录不可见,原因即为Session A先于session B,因此新插入的数据经过判断,不在可见范围内。对应的源码在row/row0sel.c [4040-4055].
Session A | Session B |
mysql> SELECT * FROM t; +----+------+ | id | num | +----+------+ | 1 | 100 | | 2 | 200 | | 3 | 1000 | +----+------+ 3 rows in set (0.00 sec)
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mysql> INSERT INTO t VALUES(4,400); Query OK, 1 row affected (0.01 sec)
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mysql> SELECT * FROM t; +----+------+ | id | num | +----+------+ | 1 | 100 | | 2 | 200 | | 3 | 1000 | +----+------+ 3 rows in set (0.00 sec)
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mysql> COMMIT; Query OK, 0 rows affected (0.00 sec)
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mysql> SELECT * FROM t; +----+------+ | id | num | +----+------+ | 1 | 100 | | 2 | 200 | | 3 | 1000 | | 4 | 400 | +----+------+ 4 rows in set (0.00 sec) 提交之后正常可见 |
标签:style blog io ar strong for 数据 sp div
原文地址:http://www.cnblogs.com/xiaoit/p/4003290.html