本文主要向大家介绍了MySQL数据库之5分钟了解MySQL5.7的Online DDL雷区 ，通过具体的内容向大家展现，希望对大家学习MySQL数据库有所帮助。

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。

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