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。