如果一张比较大的表进行ddl变更，比如说40G，那拷贝的时间让人无法忍受，并且阻塞所有的DML操作,让业务无法继续。
以下是测试过程：
mysql> desc t1;
+--------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| id | int(11) | YES | MUL | NULL | |
| nick | varchar(32) | YES | | NULL | |
| email | varchar(32) | YES | | NULL | |
| gmt_create | datetime | YES | | NULL | |
| gmt_modified | datetime | YES | | NULL | |
+--------------+-------------+------+-----+---------+-------+
mysql> select count(*) from t1;
+----------+
| count(*) |
+----------+
| 2228017 |
+----------+
1 row in set (1.78 sec)
现在对它进行表结构变更，增加一列：
mysql>altertablet1add(telvarchar(20));
QueryOK,2304923rowsaffected(41.03sec)
Records:2304923Duplicates:0Warnings:0
在上述表结构变更过程中，启动另外一个会话，进行select查询操作和一个更新操作：
mysql> select count(*) from t1;
+----------+
| count(*) |
+----------+
| 2304923 |
+----------+
1 row in set (2.10 sec)
mysql> select * from t1 limit 10;
+------+-------+------------------+---------------------+---------------------+
| id | nick | email | gmt_create | gmt_modified |
+------+-------+------------------+---------------------+---------------------+
| 0 | nick0 | nick0@taobao.com | 2008-03-14 00:00:00 | 2008-03-14 00:00:00 |
| 1 | nick1 | nick1@taobao.com | 2008-03-14 00:00:00 | 2008-03-14 00:00:00 |
| 2 | nick2 | nick2@taobao.com | 2008-03-14 00:00:00 | 2008-03-14 00:00:00 |
| 3 | nick3 | nick3@taobao.com | 2008-03-14 00:00:00 | 2008-03-14 00:00:00 |
| 4 | nick4 | nick4@taobao.com | 2008-03-14 00:00:00 | 2008-03-14 00:00:00 |
| 5 | nick5 | nick5@taobao.com | 2008-03-14 00:00:00 | 2008-03-14 00:00:00 |
| 6 | nick6 | nick6@taobao.com | 2008-03-14 00:00:00 | 2008-03-14 00:00:00 |
| 7 | nick7 | nick7@taobao.com | 2008-03-14 00:00:00 | 2008-03-14 00:00:00 |
| 8 | nick8 | nick8@taobao.com | 2008-03-14 00:00:00 | 2008-03-14 00:00:00 |
| 9 | nick9 | nick9@taobao.com | 2008-03-14 00:00:00 | 2008-03-14 00:00:00 |
+------+-------+------------------+---------------------+---------------------+
10 rows in set (0.00 sec)
mysql> update t1 set nick='test_nick' where id=1;
Query OK, 4 rows affected (43. sec) --这里是阻塞的时间
Rows matched: 4 Changed: 4 Warnings: 0
通过以上实验可以看出，对表进行ddl操作时，mysql并不阻塞select查询，但会严重阻塞dml操作。另外，如果你要对表进行ddl操作，由于有一个拷贝操作，你要计算好你的可用空间够不够?如果你的系统经常要进行表结构变更，那么你将不得不要考虑此问题!
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