MySQL 多会话之间更新数据的小实例

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)
	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)
mysql> SELECT * FROM t; +----+------+ | id | num | +----+------+ | 1 | 100 | | 2 | 200 | +----+------+ 2 rows in set (0.00 sec)
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！
mysql> SELECT * FROM t; +----+------+ | id | num | +----+------+ | 1 | 100 | | 2 | 200 | | 3 | 1000 | +----+------+ 3 rows in set (0.00 sec)

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)
	mysql> INSERT INTO t VALUES(4,400); Query OK, 1 row affected (0.01 sec)
mysql> SELECT * FROM t; +----+------+ | id | num | +----+------+ | 1 | 100 | | 2 | 200 | | 3 | 1000 | +----+------+ 3 rows in set (0.00 sec)
mysql> COMMIT; Query OK, 0 rows affected (0.00 sec)
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