业务需求：
对一张线上的3000W行的表加字段：
用普通方式，建议小表使用：
ALTER TABLE `idempotent`
ADD COLUMN `orderid` VARCHAR(20) NULL DEFAULT NULL COMMENT ‘业务单据号’ AFTER `isdelete`;
大表会锁表。
对大表加字段，建议用pt-online-schema-change工具，可以不锁表。
对表 idempotent加字段 orderid：
命令：
[root@DB001 ~]#pt-online-schema-change –user=root –password=123456 –host=192.168.1.18 –alter “ADD COLUMN orderid VARCHAR(20) NULL DEFAULT NULL COMMENT ‘业务单据号'” D=ora,t=idempotent –no-check-replication-filters –execute
Found 1 slaves:
localhost.localdomain -> 192.168.1.19:socket
Will check slave lag on:
localhost.localdomain -> 192.168.1.19:socket
Operation, tries, wait:
analyze_table, 10, 1
copy_rows, 10, 0.25
create_triggers, 10, 1
drop_triggers, 10, 1
swap_tables, 10, 1
update_foreign_keys, 10, 1
Altering `ora`.`idempotent`…
Creating new table…
Created new table ora._idempotent_new OK.
Altering new table…
Altered `ora`.`_idempotent_new` OK.
2019-10-23T19:01:00 Creating triggers…
2019-10-23T19:01:00 Created triggers OK.
2019-10-23T19:01:00 Copying approximately 24968401 rows…
Copying `ora`.`idempotent`: 4% 10:53 remain
Copying `ora`.`idempotent`: 8% 10:40 remain
Copying `ora`.`idempotent`: 11% 11:49 remain
Copying `ora`.`idempotent`: 14% 12:08 remain
Copying `ora`.`idempotent`: 17% 12:09 remain
Copying `ora`.`idempotent`: 19% 12:15 remain
Copying `ora`.`idempotent`: 22% 12:15 remain
Copying `ora`.`idempotent`: 24% 12:11 remain
Copying `ora`.`idempotent`: 27% 12:09 remain
Copying `ora`.`idempotent`: 29% 12:03 remain
Copying `ora`.`idempotent`: 31% 11:45 remain
Copying `ora`.`idempotent`: 34% 11:23 remain
Copying `ora`.`idempotent`: 37% 11:01 remain
Copying `ora`.`idempotent`: 39% 10:39 remain
Copying `ora`.`idempotent`: 42% 10:11 remain
Copying `ora`.`idempotent`: 44% 09:50 remain
Copying `ora`.`idempotent`: 47% 09:35 remain
Copying `ora`.`idempotent`: 49% 09:14 remain
Copying `ora`.`idempotent`: 52% 08:44 remain
Copying `ora`.`idempotent`: 55% 08:10 remain
Copying `ora`.`idempotent`: 59% 07:17 remain
Copying `ora`.`idempotent`: 63% 06:21 remain
Copying `ora`.`idempotent`: 67% 05:32 remain
Copying `ora`.`idempotent`: 71% 04:44 remain
Copying `ora`.`idempotent`: 75% 03:59 remain
Copying `ora`.`idempotent`: 80% 03:11 remain
Copying `ora`.`idempotent`: 84% 02:28 remain
Copying `ora`.`idempotent`: 88% 01:49 remain
Copying `ora`.`idempotent`: 92% 01:08 remain
Copying `ora`.`idempotent`: 96% 00:28 remain
2019-10-23T19:18:24 Copied rows OK.
2019-10-23T19:18:24 Analyzing new table…
2019-10-23T19:18:24 Swapping tables…
2019-10-23T19:18:24 Swapped original and new tables OK.
2019-10-23T19:18:24 Dropping old table…
2019-10-23T19:18:26 Dropped old table `ora`.`_idempotent_old` OK.
2019-10-23T19:18:26 Dropping triggers…
2019-10-23T19:18:26 Dropped triggers OK.
Successfully altered `ora`.`idempotent`.
You have mail in /var/spool/mail/root
整个过程持续10几分钟，不锁表。
字段添加完成。
PS：
切记在业务低峰运行。
pt-online-schema-change –user=user –password=xxx –host=ip –port=3306 –alter “add column col1 VARCHAR(64) NULL COMMENT ‘订单号’” D=sy,t=t1 –execute –charset=utf8 –nocheck-replication-filters –max-load=“Threads_running=20”
可以指定字符集，防止加好字段的表的注释出现乱码。
修改注释：
ALTER TABLE idempotent MODIFY COLUMN `sysno` BIGINT(20) NOT NULL AUTO_INCREMENT COMMENT ‘编号’;
