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

 
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