mysql DDL时出现的锁等待状态

如下表格所示：

session1:	session2:
10:30:27 root@localhost:[testdb] mysql.sock>select * from t2; +------+--------+-------+ | id   | course | score | +------+--------+-------+ |    1 | math   |  NULL | +------+--------+-------+ 1 row in set (0.00 sec)
	10:30:20 root@localhost:[testdb] mysql.sock>select * from t2; +------+--------+-------+ | id   | course | score | +------+--------+-------+ |    1 | math   |  NULL | +------+--------+-------+ 1 row in set (0.00 sec)
	10:30:24 root@localhost:[testdb] mysql.sock>begin; Query OK, 0 rows affected (0.00 sec)
10:30:31 root@localhost:[testdb] mysql.sock>begin; Query OK, 0 rows affected (0.00 sec)
10:30:33 root@localhost:[testdb] mysql.sock>alter table t2 drop column score; Query OK, 0 rows affected (0.04 sec) Records: 0  Duplicates: 0  Warnings: 0   10:30:54 root@localhost:[testdb] mysql.sock>alter table t2 add column score int default 0; Query OK, 0 rows affected (0.03 sec) Records: 0  Duplicates: 0  Warnings: 0
	10:30:38 root@localhost:[testdb] mysql.sock>select * from t2; +------+--------+-------+ | id   | course | score | +------+--------+-------+ |    1 | math   |     0 | +------+--------+-------+ 1 row in set (0.00 sec)
10:31:24 root@localhost:[testdb] mysql.sock>alter table t2 drop column score; waiting……
	10:31:52 root@localhost:[testdb] mysql.sock>commit; Query OK, 0 rows affected (0.00 sec)
10:31:24 root@localhost:[testdb] mysql.sock>alter table t2 drop column score; Query OK, 0 rows affected (50.78 sec) Records: 0  Duplicates: 0  Warnings: 0

在session2中并没有进行select * from t2 for update,但是session1中的alter出现了等待，而session2在没有进行select * from t2时，session1是可以正常进行alter操作的。（把上面的select * from t2换成select 1 from t2的效果也一样。）
即mysql中的DDL操作需要没有其它线程操作其表时（即使是select也算），才会进行，要不会进行锁等待情况。