5分钟了解MySQL5.7的Online DDL雷区

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Part1:写在最前

Online DDL，当新手听到这个名字的时候，非常高兴，以为无论什么情况下，修改表结构都不会锁表，理想很丰满，现实很骨感！读完本文，教你如何避开这些雷区，安全的修改表结构。话不多说，我们分别来看下MySQL5.6和MySQL5.7在修改表结构上的相同和异同。

Part2:5.6.25的表现

①首先我们构造数据并进行测试

mysql> create database helei;

Query OK, 1 row affected (0.01 sec)

mysql> use helei;

Database changed

mysql> create table helei(

    -> id int(10) unsigned NOT NULL AUTO_INCREMENT,

    -> c1 int(10) NOT NULL DEFAULT '0',

    -> c2 int(10) unsigned DEFAULT NULL,

    -> c5 int(10) unsigned NOT NULL DEFAULT '0',

    -> c3 timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,

    -> c4 varchar(200) NOT NULL DEFAULT '',

    -> PRIMARY KEY(id),

    -> KEY idx_c1(c1),

    -> KEY idx_c2(c2)

    -> )ENGINE=InnoDB ;

Query OK, 0 rows affected (0.02 sec)

mysql> delimiter $$

mysql> drop procedure if exists `insert_helei` $$

mysql> create procedure `insert_helei`(in row_num int )

    -> begin

    ->  declare i int  default 0;

    ->  while i < row_num do

    -> insert into helei(c1, c2, c5,c3, c4) values( floor(rand()*row_num),floor(rand()*row_num),floor(rand()*row_num),now(),repeat('su', floor(rand()*20)));

    -> set i = i+1;

    ->  END while;

    -> end$$

Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;

mysql> call insert_helei(1000000);

Query OK, 1 row affected (18 min 52.88 sec)

mysql> desc helei;

+-------+------------------+------+-----+-------------------+-----------------------------+

| Field | Type             | Null | Key | Default           | Extra                       |

+-------+------------------+------+-----+-------------------+-----------------------------+

| id    | int(10) unsigned | NO   | PRI | NULL              | auto_increment              |

| c1    | int(10)          | NO   | MUL | 0                 |                             |

| c2    | int(10) unsigned | YES  | MUL | NULL              |                             |

| c5    | int(10) unsigned | NO   |     | 0                 |                             |

| c3    | timestamp        | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |

| c4    | varchar(200)     | NO   |     |                   |                             |

+-------+------------------+------+-----+-------------------+-----------------------------+

6 rows in set (0.01 sec)

mysql> alter table helei add c6 varchar(60) not null default '';

Query OK, 0 rows affected (9.66 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> select count(*) from helei;

+----------+

| count(*) |

+----------+

|  1000000 |

+----------+

1 row in set (0.14 sec)

mysql> alter table helei ALGORITHM=INPLACE,modify c6 varchar(80) not null default '';

ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.

mysql> alter table helei modify c6 varchar(80) not null default '';

Query OK, 1000000 rows affected (9.44 sec)

Records: 1000000  Duplicates: 0  Warnings: 0

Tips:心得

可以看出5.6.25这里并不支持ALGORITHM=INPLACE，而直接对表进行修改的话用了9.44s。

②接下来我们测试下变更varchar长度是否锁表

mysql> alter table helei modify c6 varchar(40) not null default '';

Query OK, 1000000 rows affected (9.26 sec)

Records: 1000000  Duplicates: 0  Warnings: 0

mysql> update helei set c1=9999 where id=1;

Query OK, 0 rows affected (7.77 sec)

Rows matched: 1  Changed: 0  Warnings: 0

mysql> show processlist;

+----+------+-----------+-------+---------+------+---------------------------------+-------------------------------------------------------------+

| Id | User | Host      | db    | Command | Time | State                           | Info                                                        |

+----+------+-----------+-------+---------+------+---------------------------------+-------------------------------------------------------------+

|  6 | root | localhost | helei | Query   |    2 | copy to tmp table               | alter table helei modify c6 varchar(40) not null default '' |

|  7 | root | localhost | helei | Query   |    1 | Waiting for table metadata lock | update helei set c1=9999 where id=1                         |

|  8 | root | localhost | NULL  | Query   |    0 | init                            | show processlist                                            |

+----+------+-----------+-------+---------+------+---------------------------------+-------------------------------------------------------------+

3 rows in set (0.00 sec)

可以看到在对c6字段修改表结构时，对c1字段无法更新，被锁

mysql> alter table helei add c7 varchar(60) not null default '';

Query OK, 0 rows affected (8.30 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> update helei set c1=9999 where id=1;

Query OK, 0 rows affected (0.00 sec)

Rows matched: 1  Changed: 0  Warnings: 0

Tips:心得

经测试，增加、删除字段或索引不锁全表，变更字段长度，锁表。

Part3:5.7.15的表现

mysql> select count(*) from helei;

+----------+

| count(*) |

+----------+

|  1000000 |

+----------+

1 row in set (0.11 sec)

mysql> alter table helei ALGORITHM=INPLACE,modify c6 varchar(80) not null default '';

Query OK, 0 rows affected (0.01 sec)

Records: 0  Duplicates: 0  Warnings: 0

Tips:心得

可以看到，这里用时0.01sec，和5.6的9.44sec效率差了相当多吧~

Warning:警告1

不过，ALGORITHM用法只对varcahr类型有效哦，比如我们对c1列int型进行变更：

c1原为int(10) 变更为int(11) unsigned则无效

mysql> alter table helei ALGORITHM=INPLACE,modify c1 int(11) unsigned not null;

ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.

Tips:心得

只变更int的位数，是可以的，不过这没什么意义，因为无论你int多少，最多都只能存10位，这也就是为什么我们生产库开发规范要定义所有的int都用int(10)。

生产库开发规范详见：

http://suifu.blog.51cto.com/9167728/1792604

Warning:警告2

如果字段属性大于并等于varchar(256)（这里的256是指字节（UTF8占用3字节）或者把varchar(80)减少到varchar(70)或者更少)，则仍需要拷贝数据且锁全表。

mysql> alter table helei ALGORITHM=INPLACE,modify c6 varchar(84) not null default '';

Query OK, 0 rows affected (0.01 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table helei ALGORITHM=INPLACE,modify c6 varchar(85) not null default '';

Query OK, 0 rows affected (0.01 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table helei ALGORITHM=INPLACE,modify c6 varchar(86) not null default '';

ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.

mysql> alter table helei ALGORITHM=INPLACE,modify c6 varchar(40) not null default '';

ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.

mysql> alter table helei ALGORITHM=INPLACE,modify c6 varchar(70) not null default '';

ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.

Warning:警告3

添加字段alter table时，对该表的增删改查均不会锁表。而在这之前，当该表被访问时，需要等其执行完毕后才可以执行alter table。

——总结——

在varchar变更字段长度方面，5.7的新特性ALGORITHM参数可以快速调整varchar类型的字段长度。5.7同5.6一样，增加，删除字段或索引不锁全表，删除主键锁全表。因此，在上线时，一定要执行show processlist命令并观察，此刻是否有某个慢SQL对该表进行操作，以免alter table表时出现锁表现象。由于笔者的水平有限，编写时间也很仓促，文中难免会出现一些错误或者不准确的地方，不妥之处恳请读者批评指正。

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