转发一下某网友(Howard)的来信，先讨论一下，稍后再给结论
你好：
我现在有个关于mysql的问题一点思路都没有，想问下你，麻烦你有时间了看下。
另外你一般有没有一个活跃的论坛或者小组之类的？
mysql的版本是Server
version: 5.1.51-community-log MySQL Community Server (GPL)
将mysql的tx_isolation设置为repeatable-read
mysql> show global variables
like 'tx_isolation';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tx_isolation | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.00 sec)
mysql> show create table test;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table
|
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------+
| test | CREATE TABLE `test`
(
`id` int(11) DEFAULT
NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
COLLATE=utf8_bin |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
然后同时开启了两个事务，其中事务1先执行select * from test;然后事务2执行alter table test
add column name char(5) default 'a';，事务再执行select * from test;的时候就返回为空了。
第一个transaction：
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test;
+------+
| id |
+------+
| 1 |
| 2 |
| 3 |
| 1 |
| 1 |
| 2 |
+------+
6 rows in set (0.01 sec)
mysql> select * from test;
Empty set (0.00 sec)
第二个transaction
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test;
+------+
| id |
+------+
| 1 |
| 2 |
| 3 |
| 1 |
| 1 |
| 2 |
+------+
6 rows in set (0.00 sec)
mysql> alter table test add column name char(5) default 'a';
Query OK, 6 rows affected (0.00 sec)
Records: 6 Duplicates: 0 Warnings: 0
mysql> select * from test;
+------+------+
| id | name |
+------+------+
| 1 | a |
| 2 | a |
| 3 | a |
| 1 | a |
| 1 | a |
| 2 | a |
+------+------+
6 rows in set (0.00 sec)