5分钟了解MySQL5.7的Online DDL雷区

江南才子

　　
　　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。