mysql5.6和mysql5.7对online DDL做了大幅度功能增强，但是仍然存在主库执行DDL，从库存在大幅延迟的情况，故目前生产环境还是通过pt-online-schema-change工具来实现online DDL。但是pt-online-schema-change的使用是否就没有限制呢？
先看看官方文档对pt-online-schema-change的工作原理的描述：
pt-online-schema-change works by creating an empty copy of the table to alter, modifying it as desired, and then copying rows from the original table into the new table. When the copy is complete, it moves away the original table and replaces it with the new one. By default, it also drops the original table. The data copy process is performed in small chunks of data, which are varied to attempt to make them execute in a specific amount of time (see --chunk-time). This process is very similar to how other tools, such as pt-tablechecksum, work. Any modifications to data in the original tables during the copy will be reflected in the new table, because the tool creates triggers on the original table to update the corresponding rows in the new table. The use of triggers means that the tool will not work if any triggers are already defined on the table. When the tool finishes copying data into the new table, it uses an atomic RENAME TABLE operation
接下来通过实验的方式看看pt-online-schema-change是如何工作的，记得打开mysql的general log。通过查看general日志验证pt-online-schema-change的工作机理。
shell>pt-online-schema-change -u linzj -h 192.168.110.131 -p linzj --alter=‘add column vid3 int‘ --execute D=sbtest,t=sbtest
1 创建一个和你要执行 alter 操作的表一样的空表结构：
11 Query CREATE TABLE `sbtest`.`_sbtest_new` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `k` int(10) unsigned NOT NULL DEFAULT ‘0‘, `c` char(120) NOT NULL DEFAULT ‘‘, `pad` char(60) NOT NULL DEFAULT ‘‘, `vid` int(11) DEFAULT NULL, `vid2` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `k` (`k`) ) ENGINE=InnoDB AUTO_INCREMENT=4294967295 DEFAULT CHARSET=utf8
2、执行表结构修改
170407 15:45:46 11 Query ALTER TABLE `sbtest`.`_sbtest_new` add column vid3 int
3、在原表上创建触发器，如果表中已经定义了触发器这个工具就不能工作了。
11 Query CREATE TRIGGER `pt_osc_sbtest_sbtest_del` AFTER DELETE ON `sbtest`.`sbtest` FOR EACH ROW DELETE IGNORE FROM `sbtest `.`_sbtest_new` WHERE `sbtest`.`_sbtest_new`.`id` <=> OLD.`id` 11 Query CREATE TRIGGER `pt_osc_sbtest_sbtest_upd` AFTER UPDATE ON `sbtest`.`sbtest` FOR EACH ROW REPLACE INTO `sbtest`.`_sb test_new` (`id`, `k`, `c`, `pad`, `vid`, `vid2`) VALUES (NEW.`id`, NEW.`k`, NEW.`c`, NEW.`pad`, NEW.`vid`, NEW.`vid2`) 11 Query CREATE TRIGGER `pt_osc_sbtest_sbtest_ins` AFTER INSERT ON `sbtest`.`sbtest` FOR EACH ROW REPLACE INTO `sbtest`.`_sb test_new` (`id`, `k`, `c`, `pad`, `vid`, `vid2`) VALUES (NEW.`id`, NEW.`k`, NEW.`c`, NEW.`pad`, NEW.`vid`, NEW.`vid2`)
4、按主键or唯一索引进行排序，分成若干chunk进行数据copy
11 Query EXPLAIN SELECT * FROM `sbtest`.`sbtest` WHERE 1=1 11 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `sbtest`.`sbtest` FORCE INDEX(`PRIMARY`) ORDER BY `id` LIMIT 1 /*first lo wer boundary*/ 11 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `sbtest`.`sbtest` FORCE INDEX (`PRIMARY`) WHERE `id` IS NOT NULL ORDER BY `id` LIMIT 1 /*key_len*/ 11 Query EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ * FROM `sbtest`.`sbtest` FORCE INDEX (`PRIMARY`) WHERE `id` >= ‘1‘ /*key_le n*/ 11 Query EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `sbtest`.`sbtest` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= ‘1‘)) OR DER BY `id` LIMIT 999, 2 /*next chunk boundary*/ 11 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `sbtest`.`sbtest` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= ‘1‘)) ORDER BY ` id` LIMIT 999, 2 /*next chunk boundary*/ 11 Query SHOW WARNINGS 11 Query SHOW GLOBAL STATUS LIKE ‘Threads_running‘ 11 Query EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `sbtest`.`sbtest` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= ‘1001‘)) ORDER BY `id` LIMIT 19329, 2 /*next chunk boundary*/ 11 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `sbtest`.`sbtest` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= ‘1001‘)) ORDER B Y `id` LIMIT 19329, 2 /*next chunk boundary*/ 11 Query EXPLAIN SELECT `id`, `k`, `c`, `pad`, `vid`, `vid2` FROM `sbtest`.`sbtest` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= ‘ 1001‘)) AND ((`id` <= ‘20330‘)) LOCK IN SHARE MODE /*explain pt-online-schema-change 17219 copy nibble*/ 11 Query INSERT LOW_PRIORITY IGNORE INTO `sbtest`.`_sbtest_new` (`id`, `k`, `c`, `pad`, `vid`, `vid2`) SELECT `id`, `k`, `c` , `pad`, `vid`, `vid2` FROM `sbtest`.`sbtest` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= ‘1001‘)) AND ((`id` <= ‘20330‘)) LOCK IN SHARE MODE /*pt-onlin e-schema-change 17219 copy nibble*/
5、rename表，默认删除旧表
11 Query RENAME TABLE `sbtest`.`sbtest` TO `sbtest`.`_sbtest_old`, `sbtest`.`_sbtest_new` TO `sbtest`.`sbtest` 11 Query DROP TABLE IF EXISTS `sbtest`.`_sbtest_old`
那这样的话，如果我们在使用pt-online-schema-change工具在线online DDL某个表的时候，同时对该表的主键or唯一索引字段进行DML，是否会存在异常呢？
实验场景如下：
第一个窗口：
shell>pt-online-schema-change -u linzj -h 192.168.110.131 -p linzj --alter=‘add column vid3 int‘ --execute D=sbtest,t=sbtest Found 2 slaves: mysql2 ansible Will check slave lag on: mysql2 ansible 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 `sbtest`.`sbtest`... Creating new table... Created new table sbtest._sbtest_new OK. Waiting forever for new table `sbtest`.`_sbtest_new` to replicate to mysql2... Altering new table... Altered `sbtest`.`_sbtest_new` OK. 2017-04-07T14:52:50 Creating triggers... 2017-04-07T14:52:50 Created triggers OK. 2017-04-07T14:52:50 Copying approximately 986400 rows... Copying `sbtest`.`sbtest`: 86% 00:04 remain 2017-04-07T14:53:27 Copied rows OK. 2017-04-07T14:53:27 Swapping tables... 2017-04-07T14:53:27 Swapped original and new tables OK. 2017-04-07T14:53:27 Dropping old table... 2017-04-07T14:53:27 Dropped old table `sbtest`.`_sbtest_old` OK. 2017-04-07T14:53:27 Dropping triggers... 2017-04-07T14:53:27 Dropped triggers OK. Successfully altered `sbtest`.`sbtest`.
第二个窗口：
root@localhost:mysql3306.sock 15:44: [sbtest]>select count(*) from sbtest; +----------+ | count(*) | +----------+ | 1000000 | +----------+ 1 row in set (0.17 sec)
root@localhost:mysql3306.sock 15:44: [sbtest]>update sbtest set id=9999999 where id =110; Query OK, 1 row affected (1.33 sec) Rows matched: 1 Changed: 1 Warnings: 0
root@localhost:mysql3306.sock 15:45: [sbtest]>update sbtest set id=9999998 where id =111; Query OK, 1 row affected (0.84 sec) Rows matched: 1 Changed: 1 Warnings: 0
root@localhost:mysql3306.sock 15:46: [sbtest]>update sbtest set id=9999997 where id =112; Query OK, 1 row affected (0.75 sec) Rows matched: 1 Changed: 1 Warnings: 0
root@localhost:mysql3306.sock 15:46: [sbtest]>select count(*) from sbtest; +----------+ | count(*) | +----------+ | 1000003 | +----------+ 1 row in set (0.70 sec)
root@localhost:mysql3306.sock 15:46: [sbtest]>select * from sbtest order by id desc limit 5; +---------+---+---+----------------------------------------------------+------+------+------+ | id | k | c | pad | vid | vid2 | vid3 | +---------+---+---+----------------------------------------------------+------+------+------+ | 9999999 | 0 | | qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt | NULL | NULL | NULL | | 9999998 | 0 | | qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt | NULL | NULL | NULL | | 9999997 | 0 | | qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt | NULL | NULL | NULL | | 1000000 | 0 | | qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt | NULL | NULL | NULL | | 999999 | 0 | | qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt | NULL | NULL | NULL | +---------+---+---+----------------------------------------------------+------+------+------+ 5 rows in set (0.00 sec)
root@localhost:mysql3306.sock 15:46: [sbtest]>select * from sbtest where id in (110,111,112); +-----+---+---+----------------------------------------------------+------+------+------+ | id | k | c | pad | vid | vid2 | vid3 | +-----+---+---+----------------------------------------------------+------+------+------+ | 110 | 0 | | qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt | NULL | NULL | NULL | | 111 | 0 | | qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt | NULL | NULL | NULL | | 112 | 0 | | qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt | NULL | NULL | NULL | +-----+---+---+----------------------------------------------------+------+------+------+ 3 rows in set (0.02 sec)
建议大家，在使用pt-online-schema-change的时候，暂停对表主键or唯一索引列的数据更新。
本文出自 “林志坚的博客” 博客，请务必保留此出处http://linzhijian.blog.51cto.com/1047212/1913896
原文地址：http://linzhijian.blog.51cto.com/1047212/1913896
