MySQL DDL:
DDL是一個令所有MySQL dDBA 詬病的一個功能，因為在MySQL中在對錶進行dDDL時，會鎖表，當表比較小比如小於1W行時，對前端影響較小，當時遇到千萬級別的表，就會影響前端應用對錶的寫操作！

InnoDB引擎是透過以下步驟來進行DDL的：
1、按照原始表（original_table）的表結構和DDL語句，新建一個不可見的臨時表（tmp_table）
2、在原表上加write lock，阻塞所有更新操作（insert、delete、update等）
3、執行insert into tmp_table select * from original_table
4、rename original_table和tmp_table，最後drop original_table
5、釋放 write lock。
可以看見在InnoDB執行DDL的時候，原表是隻能讀不能寫的。為此 perconal 推出一個工具 pt-online-schema-change ，其特點是修改過程中不會造成讀寫阻塞。

工作原理：
如果表有外來鍵，除非使用 --alter-foreign-keys-method 指定特定的值，否則工具不予執行。
1 建立一個和你要執行 alter 操作的表一樣的空表結構。
2 執行表結構修改，然後從原表中的資料到copy到表結構修改後的表，
3 在原表上建立觸發器將 copy 資料的過程中，在原表的更新操作更新到新表.
注意:如果表中已經定義了觸發器這個工具就不能工作了。
4 copy 完成以後，用rename table 新表代替原表，預設刪除原表。

用法介紹：
pt-online-schema-change [OPTIONS] DSN
options 可以自行檢視 help，DNS 為你要操作的資料庫和表。這裡有兩個引數需要介紹一下：
--dry-run
這個引數不建立觸發器，不複製資料，也不會替換原表。只是建立和更改新表。
--execute
這個引數的作用和前面工作原理的介紹的一樣，會建立觸發器，來保證最新變更的資料會影響至新表。注意：如果不加這個引數，這個工具會在執行一些檢查後退出。

依賴條件:
操作的表必須有主鍵否則報如下錯誤:
[root@hank-yoon ~]# pt-online-schema-change -u root -pyoon -h127.0.0.1 --alter='add column vname varchar(20)' --execute D=yoon,t=yoon
No slaves found. See --recursion-method if host hank-yoon.com has slaves.
Not checking slave lag because no slaves were found and --check-slave-lag was not specified.
Operation, tries, wait:
copy_rows, 10, 0.25
create_triggers, 10, 1
drop_triggers, 10, 1
swap_tables, 10, 1
update_foreign_keys, 10, 1
Altering `yoon`.`yoon`...
Creating new table...
Created new table yoon._yoon_new OK.
Altering new table...
Altered `yoon`.`_yoon_new` OK.
2016-01-08T17:51:43 Dropping new table...
2016-01-08T17:51:43 Dropped new table OK.
`yoon`.`yoon` was not altered.
The new table `yoon`.`_yoon_new` does not have a PRIMARY KEY or a unique index which is required for the DELETE trigger.

欄位vname沒有新增成功！
mysql> show create table yoon\G
*************************** 1. row ***************************
Table: yoon
Create Table: CREATE TABLE `yoon` (
`actor_id` smallint(8) unsigned NOT NULL DEFAULT '0',
`first_name` varchar(45) NOT NULL,
`last_name` varchar(45) NOT NULL,
`last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8

新增主鍵：
mysql> alter table yoon modify actor_id smallint(8) unsigned primary key;
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0

[root@hank-yoon ~]# pt-online-schema-change -u root -pyoon -h127.0.0.1 --alter='add column vname varchar(20)' --execute D=yoon,t=yoon
No slaves found. See --recursion-method if host hank-yoon.com has slaves.
Not checking slave lag because no slaves were found and --check-slave-lag was not specified.
Operation, tries, wait:
copy_rows, 10, 0.25
create_triggers, 10, 1
drop_triggers, 10, 1
swap_tables, 10, 1
update_foreign_keys, 10, 1
Altering `yoon`.`yoon`...
Creating new table...
Created new table yoon._yoon_new OK.
Altering new table...
Altered `yoon`.`_yoon_new` OK.
2016-01-08T17:57:09 Creating triggers...
2016-01-08T17:57:09 Created triggers OK.
2016-01-08T17:57:09 Copying approximately 200 rows...
2016-01-08T17:57:09 Copied rows OK.
2016-01-08T17:57:09 Swapping tables...
2016-01-08T17:57:09 Swapped original and new tables OK.
2016-01-08T17:57:09 Dropping old table...
2016-01-08T17:57:09 Dropped old table `yoon`.`_yoon_old` OK.
2016-01-08T17:57:09 Dropping triggers...
2016-01-08T17:57:09 Dropped triggers OK.
Successfully altered `yoon`.`yoon`.

mysql> show create table yoon\G
*************************** 1. row ***************************
Table: yoon
Create Table: CREATE TABLE `yoon` (
`actor_id` smallint(8) unsigned NOT NULL,
`first_name` varchar(45) NOT NULL,
`last_name` varchar(45) NOT NULL,
`last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`vname` varchar(20) DEFAULT NULL,
PRIMARY KEY (`actor_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

新增多個欄位：
[root@hank-yoon ~]# pt-online-schema-change -u root -pyoon -h127.0.0.1 --alter='add column aname varchar(20),add column bname varchar(30)' --execute D=yoon,t=yoon
No slaves found. See --recursion-method if host hank-yoon.com has slaves.
Not checking slave lag because no slaves were found and --check-slave-lag was not specified.
Operation, tries, wait:
copy_rows, 10, 0.25
create_triggers, 10, 1
drop_triggers, 10, 1
swap_tables, 10, 1
update_foreign_keys, 10, 1
Altering `yoon`.`yoon`...
Creating new table...
Created new table yoon._yoon_new OK.
Altering new table...
Altered `yoon`.`_yoon_new` OK.
2016-01-08T18:04:25 Creating triggers...
2016-01-08T18:04:25 Created triggers OK.
2016-01-08T18:04:25 Copying approximately 200 rows...
2016-01-08T18:04:25 Copied rows OK.
2016-01-08T18:04:25 Swapping tables...
2016-01-08T18:04:26 Swapped original and new tables OK.
2016-01-08T18:04:26 Dropping old table...
2016-01-08T18:04:26 Dropped old table `yoon`.`_yoon_old` OK.
2016-01-08T18:04:26 Dropping triggers...
2016-01-08T18:04:26 Dropped triggers OK.
Successfully altered `yoon`.`yoon`.

mysql> show create table yoon\G
*************************** 1. row ***************************
Table: yoon
Create Table: CREATE TABLE `yoon` (
`actor_id` smallint(8) unsigned NOT NULL,
`first_name` varchar(45) NOT NULL,
`last_name` varchar(45) NOT NULL,
`last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`vname` varchar(20) DEFAULT NULL,
`aname` varchar(20) DEFAULT NULL,
`bname` varchar(30) DEFAULT NULL,
PRIMARY KEY (`actor_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

刪除欄位：
[root@hank-yoon ~]# pt-online-schema-change -u root -pyoon -h127.0.0.1 --alter='drop column aname,drop column bname' --execute D=yoon,t=yoon
No slaves found. See --recursion-method if host hank-yoon.com has slaves.
Not checking slave lag because no slaves were found and --check-slave-lag was not specified.
Operation, tries, wait:
copy_rows, 10, 0.25
create_triggers, 10, 1
drop_triggers, 10, 1
swap_tables, 10, 1
update_foreign_keys, 10, 1
Altering `yoon`.`yoon`...
Creating new table...
Created new table yoon._yoon_new OK.
Altering new table...
Altered `yoon`.`_yoon_new` OK.
2016-01-08T18:05:45 Creating triggers...
2016-01-08T18:05:45 Created triggers OK.
2016-01-08T18:05:45 Copying approximately 200 rows...
2016-01-08T18:05:45 Copied rows OK.
2016-01-08T18:05:45 Swapping tables...
2016-01-08T18:05:45 Swapped original and new tables OK.
2016-01-08T18:05:45 Dropping old table...
2016-01-08T18:05:45 Dropped old table `yoon`.`_yoon_old` OK.
2016-01-08T18:05:45 Dropping triggers...
2016-01-08T18:05:46 Dropped triggers OK.
Successfully altered `yoon`.`yoon`.

新增索引：
[root@hank-yoon ~]# pt-online-schema-change -u root -pyoon -h127.0.0.1 --alter='add key index_first(first_name)' --execute D=yoon,t=yoon No slaves found. See --recursion-method if host hank-yoon.com has slaves.
Not checking slave lag because no slaves were found and --check-slave-lag was not specified.
Operation, tries, wait:
copy_rows, 10, 0.25
create_triggers, 10, 1
drop_triggers, 10, 1
swap_tables, 10, 1
update_foreign_keys, 10, 1
Altering `yoon`.`yoon`...
Creating new table...
Created new table yoon._yoon_new OK.
Altering new table...
Altered `yoon`.`_yoon_new` OK.
2016-01-08T18:06:38 Creating triggers...
2016-01-08T18:06:38 Created triggers OK.
2016-01-08T18:06:38 Copying approximately 200 rows...
2016-01-08T18:06:38 Copied rows OK.
2016-01-08T18:06:38 Swapping tables...
2016-01-08T18:06:38 Swapped original and new tables OK.
2016-01-08T18:06:38 Dropping old table...
2016-01-08T18:06:38 Dropped old table `yoon`.`_yoon_old` OK.
2016-01-08T18:06:38 Dropping triggers...
2016-01-08T18:06:38 Dropped triggers OK.
Successfully altered `yoon`.`yoon`.

MySQL online ddl 工具之pt-online-schema-change

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