pt-online-schema-change用于MySQL的在线DDL。
下面结合官方文档和general log来分析其实现原理。
测试表
mysql> show create table t2\G
*************************** 1. row ***************************
Table: t2
Create Table: CREATE TABLE `t2` (
`id` int(11) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1005764 DEFAULT CHARSET=utf8
1 row in set (0.19 sec)
该表中只有1列，id，自增主键。
其中，表中已经存在一部分数据
mysql> select count(*) from t2;
+----------+
| count(*) |
+----------+
| 1005763 |
+----------+
1 row in set (0.31 sec)
利用pt-online-schema-change对该表新增一列
# pt-online-schema-change --execute --alter "ADD COLUMN c1 DATETIME" D=test,t=t2
Found 2 slaves:
test
hbase
Will check slave lag on:
test
hbase
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 `test`.`t2`...
Creating new table...
CREATE TABLE `test`.`_t2_new` (
`id` int(11) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1005764 DEFAULT CHARSET=utf8
Created new table test._t2_new OK.
Waiting forever for new table `test`.`_t2_new` to replicate to test...
Altering new table...
ALTER TABLE `test`.`_t2_new` ADD COLUMN c1 DATETIME
Altered `test`.`_t2_new` OK.
2016-11-21T12:49:18 Creating triggers...
CREATE TRIGGER `pt_osc_test_t2_del` AFTER DELETE ON `test`.`t2` FOR EACH ROW DELETE IGNORE FROM `test`.`_t2_new` WHERE `test`.`_t2_ne
w`.`id` <=> OLD.`id`CREATE TRIGGER `pt_osc_test_t2_upd` AFTER UPDATE ON `test`.`t2` FOR EACH ROW REPLACE INTO `test`.`_t2_new` (`id`) VALUES (NEW.`id`)
CREATE TRIGGER `pt_osc_test_t2_ins` AFTER INSERT ON `test`.`t2` FOR EACH ROW REPLACE INTO `test`.`_t2_new` (`id`) VALUES (NEW.`id`)
2016-11-21T12:49:18 Created triggers OK.
2016-11-21T12:49:18 Copying approximately 1005075 rows...
INSERT LOW_PRIORITY IGNORE INTO `test`.`_t2_new` (`id`) SELECT `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= ?)) AND (
(`id` <= ?)) LOCK IN SHARE MODE /*pt-online-schema-change 2352 copy nibble*/SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= ?)) ORDER BY `id` LIMIT ?, 2 /*next chun
k boundary*/Copying `test`.`t2`: 40% 00:44 remain
Copying `test`.`t2`: 82% 00:12 remain
2016-11-21T12:50:31 Copied rows OK.
2016-11-21T12:50:31 Analyzing new table...
2016-11-21T12:50:32 Swapping tables...
RENAME TABLE `test`.`t2` TO `test`.`_t2_old`, `test`.`_t2_new` TO `test`.`t2`
2016-11-21T12:50:35 Swapped original and new tables OK.
2016-11-21T12:50:35 Dropping old table...
DROP TABLE IF EXISTS `test`.`_t2_old`
2016-11-21T12:50:36 Dropped old table `test`.`_t2_old` OK.
2016-11-21T12:50:36 Dropping triggers...
DROP TRIGGER IF EXISTS `test`.`pt_osc_test_t2_del`;
DROP TRIGGER IF EXISTS `test`.`pt_osc_test_t2_upd`;
DROP TRIGGER IF EXISTS `test`.`pt_osc_test_t2_ins`;
2016-11-21T12:50:36 Dropped triggers OK.
Successfully altered `test`.`t2`.
查看general log中的输出
161017 11:22:56 1052 Connect root@localhost on test
1052 Query set autocommit=1
1052 Query SHOW VARIABLES LIKE 'innodb\_lock_wait_timeout'
1052 Query SET SESSION innodb_lock_wait_timeout=1
1052 Query SHOW VARIABLES LIKE 'lock\_wait_timeout'
1052 Query SET SESSION lock_wait_timeout=60
1052 Query SHOW VARIABLES LIKE 'wait\_timeout'
1052 Query SET SESSION wait_timeout=10000
1052 Query SELECT @@SQL_MODE
1052 Query SET @@SQL_QUOTE_SHOW_CREATE = 1/*!40101, @@SQL_MODE='NO_AUTO_VALUE_ON_ZERO,STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION'*/
1052 Query SELECT @@server_id /*!50038 , @@hostname*/
1053 Connect root@localhost on test
1053 Query set autocommit=1
1053 Query SHOW VARIABLES LIKE 'innodb\_lock_wait_timeout'
1053 Query SET SESSION innodb_lock_wait_timeout=1
1053 Query SHOW VARIABLES LIKE 'lock\_wait_timeout'
1053 Query SET SESSION lock_wait_timeout=60
1053 Query SHOW VARIABLES LIKE 'wait\_timeout'
1053 Query SET SESSION wait_timeout=10000
1053 Query SELECT @@SQL_MODE
1053 Query SET @@SQL_QUOTE_SHOW_CREATE = 1/*!40101, @@SQL_MODE='NO_AUTO_VALUE_ON_ZERO,STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION'*/
1053 Query SELECT @@server_id /*!50038 , @@hostname*/
上述主要是设置会话的变量信息，包括innodb_lock_wait_timeout，wait_timeout和SQL_QUOTE_SHOW_CREATE。
1052 Query SHOW VARIABLES LIKE 'wsrep_on'
1052 Query SHOW VARIABLES LIKE 'version%'
1052 Query SHOW ENGINES
1052 Query SHOW VARIABLES LIKE 'innodb_version'
1052 Query SHOW VARIABLES LIKE 'innodb_stats_persistent'
1052 Query SELECT @@SERVER_ID
1052 Query SHOW GRANTS FOR CURRENT_USER()
1052 Query SHOW FULL PROCESSLIST
1052 Query SHOW SLAVE HOSTS
1052 Query SHOW GLOBAL STATUS LIKE 'Threads_running'
1052 Query SHOW GLOBAL STATUS LIKE 'Threads_running'
1052 Query SELECT CONCAT(@@hostname, @@port)
1052 Query SHOW TABLES FROM `test` LIKE 't2'
1052 Query SHOW TRIGGERS FROM `test` LIKE 't2'
1052 Query /*!40101 SET @OLD_SQL_MODE := @@SQL_MODE, @@SQL_MODE := '', @OLD_QUOTE := @@SQL_QUOTE_SHOW_CREATE, @@SQL_QUOTE_SHOW_CREATE := 1 */
1052 Query USE `test`
1052 Query SHOW CREATE TABLE `test`.`t2`
1052 Query /*!40101 SET @@SQL_MODE := @OLD_SQL_MODE, @@SQL_QUOTE_SHOW_CREATE := @OLD_QUOTE */
1052 Query EXPLAIN SELECT * FROM `test`.`t2` WHERE 1=1
1052 Query SELECT table_schema, table_name FROM information_schema.key_column_usage WHERE referenced_table_schema='test' AND referenced_table_name='t2'
1052 Query SHOW VARIABLES LIKE 'wsrep_on'
1052 Query /*!40101 SET @OLD_SQL_MODE := @@SQL_MODE, @@SQL_MODE := '', @OLD_QUOTE := @@SQL_QUOTE_SHOW_CREATE, @@SQL_QUOTE_SHOW_CREATE := 1 */
解释：
1. 查看参数变量，当前用户的权限，slave的信息，会话变量
2. 确认t2是否存在，t2上是否有触发器
3. 查看执行计划
4. 查看是否t2表是否被其它表外键关联。
39 Query USE `test`
39 Query SHOW CREATE TABLE `test`.`t2`
39 Query /*!40101 SET @@SQL_MODE := @OLD_SQL_MODE, @@SQL_QUOTE_SHOW_CREATE := @OLD_QUOTE */
39 Query CREATE TABLE `test`.`_t2_new` (
`id` int(11) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1005764 DEFAULT CHARSET=utf8
161121 12:49:18 39 Query ALTER TABLE `test`.`_t2_new` ADD COLUMN c1 DATETIME
39 Query /*!40101 SET @OLD_SQL_MODE := @@SQL_MODE, @@SQL_MODE := '', @OLD_QUOTE := @@SQL_QUOTE_SHOW_CREATE, @@SQL_QUOTE_SHOW_CREATE := 1 */
39 Query USE `test`
39 Query SHOW CREATE TABLE `test`.`_t2_new`
39 Query /*!40101 SET @@SQL_MODE := @OLD_SQL_MODE, @@SQL_QUOTE_SHOW_CREATE := @OLD_QUOTE */
39 Query CREATE TRIGGER `pt_osc_test_t2_del` AFTER DELETE ON `test`.`t2` FOR EACH ROW DELETE IGNORE FROM `test`.`_t2_new` WHERE `test`.`_t2_new`.`id` <=> OLD.`id`
39 Query CREATE TRIGGER `pt_osc_test_t2_upd` AFTER UPDATE ON `test`.`t2` FOR EACH ROW REPLACE INTO `test`.`_t2_new` (`id`) VALUES (NEW.`id`)
39 Query CREATE TRIGGER `pt_osc_test_t2_ins` AFTER INSERT ON `test`.`t2` FOR EACH ROW REPLACE INTO `test`.`_t2_new` (`id`) VALUES (NEW.`id`)
解释：
1. 根据目标表结构创建一张新表。
2. 对新表添加字段，可以看出pt-online-shema-change对表结构进行变更依赖的还是MySQL自身的Online DDL。
3. 针对目标表创建三个触发器，DELETE，UPDATE和INSERT，因为REPLACE操作只有在主键或唯一索引存在的情况下才有意义，这也就解释了为什么目标表上要有主键或唯一索引。
39 Query EXPLAIN SELECT * FROM `test`.`t2` WHERE 1=1
39 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) ORDER BY `id` LIMIT 1 /*first lower boundary*/
39 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`t2` FORCE INDEX (`PRIMARY`) WHERE `id` IS NOT NULL ORDER BY `id` LIMIT 1 /*key_len*/
39 Query EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ * FROM `test`.`t2` FORCE INDEX (`PRIMARY`) WHERE `id` >= '1' /*key_len*/
39 Query EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1')) ORDER BY `id` LIMIT 999, 2 /*next chunk boundary*/
39 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1')) ORDER BY `id` LIMIT 999, 2 /*next chunk boundary*/
39 Query EXPLAIN SELECT `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1')) AND ((`id` <= '1000')) LOCK IN SHARE MODE /*explain pt-online-schema-change 2352 copy nibble*/
39 Query INSERT LOW_PRIORITY IGNORE INTO `test`.`_t2_new` (`id`) SELECT `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1')) AND ((`id` <= '1000')) LOCK IN SHARE MODE /*pt-online-schema-change 2352 copy nibble*/
39 Query SHOW WARNINGS
39 Query SELECT @@SERVER_ID
39 Query SHOW GRANTS FOR CURRENT_USER()
39 Query SHOW FULL PROCESSLIST
39 Query SELECT @@SERVER_ID
39 Query SHOW GRANTS FOR CURRENT_USER()
39 Query SHOW FULL PROCESSLIST
161121 12:49:20 39 Query SELECT 'pt-online-schema-change keepalive'
161121 12:49:21 39 Query SELECT @@SERVER_ID
39 Query SHOW GRANTS FOR CURRENT_USER()
39 Query SHOW FULL PROCESSLIST
39 Query SHOW GLOBAL STATUS LIKE 'Threads_running'
39 Query EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1001')) ORDER BY `id` LIMIT 28516, 2 /*next chunk boundary*/
39 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1001')) ORDER BY `id` LIMIT 28516, 2 /*next chunk boundary*/
39 Query EXPLAIN SELECT `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1001')) AND ((`id` <= '29517')) LOCK IN SHARE MODE /*explain pt-online-schema-change 2352 copy nibble*/
39 Query INSERT LOW_PRIORITY IGNORE INTO `test`.`_t2_new` (`id`) SELECT `id` FROM `test`.`t2` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1001')) AND ((`id` <= '29517')) LOCK IN SHARE MODE /*pt-online-schema-change 2352 copy nibble*/
解释：
上述输出只包含两个chunk的选择。其它chunk的选择基本相同。
1. SHOW GLOBAL STATUS LIKE 'Threads_running'用于监控当前的系统负载。
2. 可以看出pt-online-schema-change是以chunk为单位进行目标表数据的拷贝。
3. 在拷贝的过程中，对目标表的相关记录加了共享锁，此时，会堵塞客户端对这些记录的DML操作。
39 Query ANALYZE TABLE `test`.`_t2_new` /* pt-online-schema-change */
161121 12:50:32 39 Query RENAME TABLE `test`.`t2` TO `test`.`_t2_old`, `test`.`_t2_new` TO `test`.`t2`
161121 12:50:35 39 Query DROP TABLE IF EXISTS `test`.`_t2_old`
161121 12:50:36 39 Query DROP TRIGGER IF EXISTS `test`.`pt_osc_test_t2_del`
39 Query DROP TRIGGER IF EXISTS `test`.`pt_osc_test_t2_upd`
39 Query DROP TRIGGER IF EXISTS `test`.`pt_osc_test_t2_ins`
39 Query SHOW TABLES FROM `test` LIKE '\_t2\_new'
161121 12:50:37 40 Quit
39 Quit
解释：
1. 在完成数据的拷贝后，会对新表执行ANALYZE操作，这样，可及时更新新表的统计信息。
官档的解释如下：
This circumvents a potentially serious issue related to InnoDB optimizer statistics. If the table being alerted is
busy and the tool completes quickly, the new table will not have optimizer statistics after being swapped. This
can cause fast, index-using queries to do full table scans until optimizer statistics are updated (usually after 10
seconds). If the table is large and the server very busy, this can cause an outage.
2. 对目标表和新表进行RENAME操作。
3. 删除原来的目标表
4. 删除触发器。