使用pt-online-schema-change修改主键时注意

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使用 pt-online-schema-change 做在线 ddl 最添加普通索引、列，修改列类型、添加默认值等使用比较常规，但涉及到要修改的是主键时就有点棘手。在我修改线上实例过程中，有这样的需求，不妨先思考一下怎么做才好：

`1`	`原表上有个复合主键，现在要添加一个自增 id 作为主键，如何进行`

会涉及到以下修改动作：

删除复合主键定义
添加新的自增主键
原复合主键字段，修改成唯一索引

如果你够聪明，应该会把这三个操作放在同一个 alter table 命令执行。percona 手册里有两个地方对修改主键进行了特殊注解：

–alter
A notable exception is when a PRIMARY KEY or UNIQUE INDEX is being created from existing columns as part of the ALTER clause; in that case it will use these column(s) for the DELETE trigger.

–[no]check-alter

DROP PRIMARY KEY
If –alter contain DROP PRIMARY KEY (case- and space-insensitive), a warning is printed and the tool exits unless –dry-run is specified. Altering the primary key can be dangerous, but the tool can handle it. The tool’s triggers, particularly the DELETE trigger, are most affected by altering the primary key because the tool prefers to use the primary key for its triggers. You should first run the tool with –dry-run and –print and verify that the triggers are correct.

由上一篇文章 pt-online-schema-change 使用说明、限制与比较可知，pt-osc 会在原表 t1 上创建 AFTER DELETE/UPDATE/INSERT 三个触发器，修改主键影响最大的就是 DELETE 触发器：新表 t2 上的主键字段在旧表 t1 上不存在，无法根据主键条件触发删除新表 t2 数据。but the tool can handle it，原因是 pt-osc 把触发器改成了下面的形式：

CREATE TRIGGER `pt_osc_confluence_sbtest3_del` AFTER DELETE ON `confluence`.`sbtest3` FOR EACH ROW DELETE IGNORE FROM `confluence`.`_sbtest3_new` 
WHERE `confluence`.`_sbtest3_new`.`id` <=> OLD.`id` AND `confluence`.`_sbtest3_new`.`k` <=> OLD.`k`

注：sbtest3 表上以 (id,k) 作为复合主键

但是如果 id 或 k 列上没有索引，这个删除的代价非常高，所以一定要同时添加复合（唯一）索引 (id,k) .

而对于 INSERT,UPDATE 的触发器，依然是 REPLACE INTO语法，因为它采用的是先插入，如果违反主键或唯一约束，则根据主键或意义约束删除这条数据，再执行插入。（但是注意你不能依赖于新表的主键递增，因为如果原表有 update，新表就会先插入这一条，导致 id 与原表记录所在顺序不一样）

所以如果使用 pt-osc 去修改删除主键，务必同时添加原主键为 UNIQUE KEY，否则很有可能导致性能问题：

$ pt-online-schema-change --user=ecuser --password=ecuser --host=10.0.201.34  \
--alter "DROP PRIMARY KEY,add column pk int auto_increment primary key,add unique key uk_id_k(id,k)" \
D=confluence,t=sbtest3 --print --dry-run

--alter contains 'DROP PRIMARY KEY'.  Dropping and altering the primary key can be dangerous, 
especially if the original table does not have other unique indexes.  ==> 注意 dry-run 的输出

ALTER TABLE `confluence`.`_sbtest3_new` DROP PRIMARY KEY,add column pk int auto_increment primary key,add unique key uk_id_k(id,k)
Altered `confluence`.`_sbtest3_new` OK.
Using original table index PRIMARY for the DELETE trigger instead of new table index PRIMARY because ==> 使用原表主键值判断
the new table index uses column pk which does not exist in the original table.

CREATE TRIGGER `pt_osc_confluence_sbtest3_del` AFTER DELETE ON `confluence`.`sbtest3` FOR EACH ROW DELETE IGNORE FROM `confluence`.`_sbtest3_new` 
WHERE `confluence`.`_sbtest3_new`.`id` <=> OLD.`id` AND `confluence`.`_sbtest3_new`.`k` <=> OLD.`k`