官方文档：

https://dev.mysql.com/doc/refman/5.7/en/innodb-online-ddl-operations.html

虽然官方统计了大部分DDL操作时，是否允许DML，但并不是很全，或者有些支持DML，但仅仅只支持查询，不支持增删改操作。

版本：MySQL 5.7

场景一、

修改 varchar 字段长度时，在 utf8mb4 字符集下，varhcar 长度从【1-63】长度变为 【64及以上】长度时，会锁表，只能查询。反之亦然

State：copy to tmp table

场景二、int 类型个性为 decimal(10,2)，会锁表，只能查询。

State：copy to tmp table

MySQL [(none)]> show processlist;
+----+------+-----------------+-------+---------+------+---------------------------------+-------------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+-------+---------+------+---------------------------------+-------------------------------------------+
| 9 | root | localhost:39614 | ceshi | Sleep | 1604 | | NULL |
| 12 | root | localhost:39620 | NULL | Query | 3 | Waiting for table metadata lock | select * from ceshi.t1 limit 1 for update |
| 13 | root | localhost:39622 | NULL | Query | 0 | starting | show processlist |
| 17 | root | localhost:39630 | ceshi | Query | 5 | copy to tmp table | alter table t1 modify age delimal(10,2) |
+----+------+-----------------+-------+---------+------+---------------------------------+-------------------------------------------+

场景三、

varchar 长度 60 -> 100 ，同时新增加一个 varchar 字段。会锁表，只能查询。

MySQL [(none)]> show processlist;
+----+------+-----------------+-------+---------+------+-------------------+-------------------------------------------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+-------+---------+------+-------------------+-------------------------------------------------------------------------+
| 9 | root | localhost:39614 | ceshi | Sleep | 1903 | | NULL |
| 12 | root | localhost:39620 | NULL | Query | 0 | starting | show processlist |
| 13 | root | localhost:39622 | NULL | Sleep | 299 | | NULL |
| 17 | root | localhost:39630 | ceshi | Query | 6 | copy to tmp table | alter table t1 modify name varchar(100),add column describ varchar(100) |
+----+------+-----------------+-------+---------+------+-------------------+-------------------------------------------------------------------------+
4 rows in set (0.000 sec)

场景四、

varchar 修改为 text。会锁表，只能查询

MySQL [ceshi]> show processlist;
+----+------+-----------------+-------+---------+------+-------------------+------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+-------+---------+------+-------------------+------------------------------------+
| 3 | root | 127.0.0.1:33426 | ceshi | Query | 3 | copy to tmp table | alter table t1 modify describ text |
| 4 | root | 127.0.0.1:33428 | ceshi | Query | 0 | starting | show processlist |
+----+------+-----------------+-------+---------+------+-------------------+------------------------------------+

场景五、

varchar2048 -> 2047 减小会锁表

MySQL [ceshi]> show processlist;
+----+------+-----------------+-------+---------+------+-------------------+------------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+-------+---------+------+-------------------+------------------------------------------+
| 3 | root | 127.0.0.1:33426 | ceshi | Query | 3 | copy to tmp table | alter table t1 modify name varchar(2047) |
| 4 | root | 127.0.0.1:33428 | ceshi | Query | 0 | starting | show processlist |
+----+------+-----------------+-------+---------+------+-------------------+------------------------------------------+
2 rows in set (0.000 sec)