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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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