过去，PolarDB 在处理归档的冷数据时，遇到的主要挑战之一是对数据定义语言（DDL）操作的兼容性和效率问题，尤其是在执行诸如增加列、调整列宽等操作时。由于这些操作通常要求数据重写，通过 COPY 机制来实现，导致了极高的资源消耗和执行成本。这一局限性在持续归档至 OSS 外表的场景中尤为突出，给数据管理和运维工作带来了不便。
为应对这一挑战，我们引入了 OSS META 功能。该功能通过增强对冷数据的元数据管理能力，使得特定的DDL操作能够以即时（INSTANT）模式执行，从根本上优化了冷数据处理的灵活性和效率。这意味着，在 OSS META 功能启用状态下，系统能够智能识别并应用DDL变更，而无需经历传统意义上的全量数据重写流程。下面举一个例子详细介绍：
例如：客户有一张订单分区表，以订单创建时间作为分区键，每年一个分区。假设超过5年后，我们认为该订单被查询的几率不大，可以归档至 OSS 冷数据。其订单表结构如下：
MySQL> show create table Orders;
+--------+----------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+----------------------------------------------------------------------------------------------+
| Orders | CREATE TABLE `Orders` ( |
| | `order_id` int(11) NOT NULL AUTO_INCREMENT, |
| | `customer_id` int(11) DEFAULT NULL, |
| | `product_id` int(11) DEFAULT NULL, |
| | `order_amount` decimal(10,2) DEFAULT NULL, |
| | `create_time` datetime NOT NULL, |
| | PRIMARY KEY (`order_id`,`create_time`) |
| | ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
| | /*!50500 PARTITION BY RANGE COLUMNS(create_time) */ /*!99990 800020200 INTERVAL(YEAR, 1) */ |
| | /*!50500 (PARTITION p2011 VALUES LESS THAN ('2012-01-01') ENGINE = InnoDB, |
| | PARTITION p2012 VALUES LESS THAN ('2013-01-01') ENGINE = InnoDB, |
| | PARTITION p2013 VALUES LESS THAN ('2014-01-01') ENGINE = InnoDB) */ |
+--------+----------------------------------------------------------------------------------------------+
该表上有 INTERVAL 属性，可以随着表数据的增加，自动创建新的分区。（关于 INTERVAL 功能的介绍，请参考：PolarDB MySQL INTERVAL RANGE分区）。随着数据上升，假设客户插入的数据已经到了2016年，则该表的定义会变为：
MySQL> show create table Orders;
+--------+----------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+----------------------------------------------------------------------------------------------+
| Orders | CREATE TABLE `Orders` ( |
| | `order_id` int(11) NOT NULL AUTO_INCREMENT, |
| | `customer_id` int(11) DEFAULT NULL, |
| | `product_id` int(11) DEFAULT NULL, |
| | `order_amount` decimal(10,2) DEFAULT NULL, |
| | `create_time` datetime NOT NULL, |
| | PRIMARY KEY (`order_id`,`create_time`) |
| | ) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
| | /*!50500 PARTITION BY RANGE COLUMNS(create_time) */ /*!99990 800020200 INTERVAL(YEAR, 1) */ |
| | /*!50500 (PARTITION p2011 VALUES LESS THAN ('2012-01-01') ENGINE = InnoDB, |
| | PARTITION p2012 VALUES LESS THAN ('2013-01-01') ENGINE = InnoDB, |
| | PARTITION p2013 VALUES LESS THAN ('2014-01-01') ENGINE = InnoDB, |
| | PARTITION _p20150101000000 VALUES LESS THAN ('2015-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20160101000000 VALUES LESS THAN ('2016-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20170101000000 VALUES LESS THAN ('2017-01-01 00:00:00') ENGINE = InnoDB) */ |
+--------+----------------------------------------------------------------------------------------------+
此时，由于分区 p2011 的数据距离最新的分区已经超过五年，出于降本的需求，我们将该分区的数据转到另一张冷存表上。首先，打开 OSS META 开关，通过
MySQL> set use_oss_meta = ON;
或者通过控制台设置参数 loose_use_oss_meta = ON ,然后重新连接实例。
随后通过以下存储过程将分区归档至表：
> 该功能将会在 8.0.2.2.25 版本上线，具体请参考官网公告。
MySQL> call dbms_dlm.archive_partition2table('test', 'Orders', 'p2011', 'test','Orders_archive', '');
这个存储过程会把当前表 test.Orders 表的分区 p2011 归档到 OSS 外表 test.Orders_archive，如果没有表test.Orders_archive，则会自动创建一个。归档完成后，看下当前数据库中的所有表，如下：
MySQL> show tables;
+-----------------------------+
| Tables_in_test |
+-----------------------------+
| Orders |
| Orders_archive |
+-----------------------------+
2 rows in set
Time: 0.020s
这时已经自动创建好了表 Orders_archive ，表结构和 Orders 基本一致，其建表语句为：
MySQL> show create table Orders_archive;
+----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Orders_archive | CREATE TABLE `Orders_archive` ( |
| | `order_id` int(11) NOT NULL DEFAULT '0', |
| | `customer_id` int(11) DEFAULT NULL, |
| | `product_id` int(11) DEFAULT NULL, |
| | `order_amount` decimal(10,2) DEFAULT NULL, |
| | `create_time` datetime NOT NULL |
| | ) /*!99990 800020213 STORAGE OSS */ ENGINE=CSV DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci /*!99990 800020204 NULL_MARKER='NULL' */ /*!99990 800020223 OSS META=1 */ /*!99990 800020224 OSS_FILE_FILTER='order_id,create_time' */ |
+----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set
Time: 0.006s
可以看到表上带有 OSS META=1 的标记，说明当前表已经开启了 OSS META 功能。查询该表的数据，可以发现 p2011 分区的数据已经在表 Orders_archive 中了，并且 Orders 表中，分区 p2011 也被删除了。
MySQL> show create table Orders;
+--------+----------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+----------------------------------------------------------------------------------------------+
| Orders | CREATE TABLE `Orders` ( |
| | `order_id` int(11) NOT NULL AUTO_INCREMENT, |
| | `customer_id` int(11) DEFAULT NULL, |
| | `product_id` int(11) DEFAULT NULL, |
| | `order_amount` decimal(10,2) DEFAULT NULL, |
| | `create_time` datetime NOT NULL, |
| | PRIMARY KEY (`order_id`,`create_time`) |
| | ) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
| | /*!50500 PARTITION BY RANGE COLUMNS(create_time) */ /*!99990 800020200 INTERVAL(YEAR, 1) */ |
| | /*!50500 (PARTITION p2012 VALUES LESS THAN ('2013-01-01') ENGINE = InnoDB, |
| | PARTITION p2013 VALUES LESS THAN ('2014-01-01') ENGINE = InnoDB, |
| | PARTITION _p20150101000000 VALUES LESS THAN ('2015-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20160101000000 VALUES LESS THAN ('2016-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20170101000000 VALUES LESS THAN ('2017-01-01 00:00:00') ENGINE = InnoDB) */ |
+--------+----------------------------------------------------------------------------------------------+
1 row in set
Time: 0.028s
随后，Orders 表随着数据持续的插入，可能会有更多的分区，例如：
MySQL> show create table Orders;
+--------+----------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+----------------------------------------------------------------------------------------------+
| Orders | CREATE TABLE `Orders` ( |
| | `order_id` int(11) NOT NULL AUTO_INCREMENT, |
| | `customer_id` int(11) DEFAULT NULL, |
| | `product_id` int(11) DEFAULT NULL, |
| | `order_amount` decimal(10,2) DEFAULT NULL, |
| | `create_time` datetime NOT NULL, |
| | PRIMARY KEY (`order_id`,`create_time`) |
| | ) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
| | /*!50500 PARTITION BY RANGE COLUMNS(create_time) */ /*!99990 800020200 INTERVAL(YEAR, 1) */ |
| | /*!50500 (PARTITION p2012 VALUES LESS THAN ('2013-01-01') ENGINE = InnoDB, |
| | PARTITION p2013 VALUES LESS THAN ('2014-01-01') ENGINE = InnoDB, |
| | PARTITION _p20150101000000 VALUES LESS THAN ('2015-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20160101000000 VALUES LESS THAN ('2016-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20170101000000 VALUES LESS THAN ('2017-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20180101000000 VALUES LESS THAN ('2018-01-01 00:00:00') ENGINE = InnoDB) */ |
+--------+----------------------------------------------------------------------------------------------+
1 row in set
Time: 0.006s
此时按照保留5个分区的策略，可以继续把分区 p2012 的数据归档至刚刚的 Orders_archive 表，sql为：
MySQL> call dbms_dlm.archive_partition2table('test', 'Orders', 'p2012', 'test','Orders_archive', '');
Query OK, 0 rows affected
Time: 3.815s
MySQL> show create table Orders;
+--------+----------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+----------------------------------------------------------------------------------------------+
| Orders | CREATE TABLE `Orders` ( |
| | `order_id` int(11) NOT NULL AUTO_INCREMENT, |
| | `customer_id` int(11) DEFAULT NULL, |
| | `product_id` int(11) DEFAULT NULL, |
| | `order_amount` decimal(10,2) DEFAULT NULL, |
| | `create_time` datetime NOT NULL, |
| | PRIMARY KEY (`order_id`,`create_time`) |
| | ) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
| | /*!50500 PARTITION BY RANGE COLUMNS(create_time) */ /*!99990 800020200 INTERVAL(YEAR, 1) */ |
| | /*!50500 (PARTITION p2013 VALUES LESS THAN ('2014-01-01') ENGINE = InnoDB, |
| | PARTITION _p20150101000000 VALUES LESS THAN ('2015-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20160101000000 VALUES LESS THAN ('2016-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20170101000000 VALUES LESS THAN ('2017-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20180101000000 VALUES LESS THAN ('2018-01-01 00:00:00') ENGINE = InnoDB) */ |
+--------+----------------------------------------------------------------------------------------------+
1 row in set
Time: 0.027s
归档后分区表 Orders 的数据进一步减少，可以显著降低存储成本。
但是，如果业务上有修改，例如，对分区表 Orders 需要加一列 price，表示当前订单的价格，加列后表定义为：
MySQL> alter table Orders add column (price decimal (10,2) DEFAULT NULL), algorithm = instant;
Query OK, 0 rows affected
Time: 0.255s
MySQL> show create table Orders;
+--------+----------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+----------------------------------------------------------------------------------------------+
| Orders | CREATE TABLE `Orders` ( |
| | `order_id` int(11) NOT NULL AUTO_INCREMENT, |
| | `customer_id` int(11) DEFAULT NULL, |
| | `product_id` int(11) DEFAULT NULL, |
| | `order_amount` decimal(10,2) DEFAULT NULL, |
| | `create_time` datetime NOT NULL, |
| | `price` decimal(10,2) DEFAULT NULL, |
| | PRIMARY KEY (`order_id`,`create_time`) |
| | ) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
| | /*!50500 PARTITION BY RANGE COLUMNS(create_time) */ /*!99990 800020200 INTERVAL(YEAR, 1) */ |
| | /*!50500 (PARTITION p2013 VALUES LESS THAN ('2014-01-01') ENGINE = InnoDB, |
| | PARTITION _p20150101000000 VALUES LESS THAN ('2015-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20160101000000 VALUES LESS THAN ('2016-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20170101000000 VALUES LESS THAN ('2017-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20180101000000 VALUES LESS THAN ('2018-01-01 00:00:00') ENGINE = InnoDB) */ |
+--------+----------------------------------------------------------------------------------------------+
1 row in set
Time: 0.006s
此时，加完列后，如果我们想继续把分区 p2013 的数据归档至刚刚的 OSS 表 Orders_archive，会报错。这是因为 Orders 表和 Orders_archive 表的定义出现了不一致，报错如下：
MySQL> call dbms_dlm.archive_partition2table('test', 'Orders', 'p2013', 'test','Orders_archive', '');
(8181, '[Data Lifecycle Management] errmsg: The metadata information of the original table is inconsistent with that of the target table.')
此时，我们需要给表 Orders_archive 也加上 price 列，就可以正常归档了，sql 为：
MySQL> alter table Orders_archive add column (price decimal (10,2) DEFAULT NULL), algorithm = instant;
Query OK, 0 rows affected
Time: 0.348s
注意，
OSS META = 1 标记时，才能执行 instant DDL 操作，否则会变成 copy DDL。DDL sql 中强制指定算法为 INSTANT，避免出现预期外的 COPY 导致代价过大。此时看下表 Order_archive 的表结构：
MySQL> show create table Orders_archive;
+----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Orders_archive | CREATE TABLE `Orders_archive` ( |
| | `order_id` int(11) NOT NULL DEFAULT '0', |
| | `customer_id` int(11) DEFAULT NULL, |
| | `product_id` int(11) DEFAULT NULL, |
| | `order_amount` decimal(10,2) DEFAULT NULL, |
| | `create_time` datetime NOT NULL, |
| | `price` decimal(10,2) DEFAULT NULL |
| | ) /*!99990 800020213 STORAGE OSS */ ENGINE=CSV DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci /*!99990 800020204 NULL_MARKER='NULL' */ /*!99990 800020223 OSS META=1 */ /*!99990 800020224 OSS_FILE_FILTER='order_id,create_time' */ |
+----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set
Time: 0.250s
已经和表 Orders 一致了，此时再归档分区 p2013，就不会报错了。
MySQL> call dbms_dlm.archive_partition2table('test', 'Orders', 'p2013', 'test','Orders_archive', '');
Query OK, 0 rows affected
Time: 4.969s
MySQL> show create table Orders;
+--------+------------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+------------------------------------------------------------------------------------------------+
| Orders | CREATE TABLE `Orders` ( |
| | `order_id` int(11) NOT NULL AUTO_INCREMENT, |
| | `customer_id` int(11) DEFAULT NULL, |
| | `product_id` int(11) DEFAULT NULL, |
| | `order_amount` decimal(10,2) DEFAULT NULL, |
| | `create_time` datetime NOT NULL, |
| | `price` decimal(10,2) DEFAULT NULL, |
| | PRIMARY KEY (`order_id`,`create_time`) |
| | ) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
| | /*!50500 PARTITION BY RANGE COLUMNS(create_time) */ /*!99990 800020200 INTERVAL(YEAR, 1) */ |
| | /*!50500 (PARTITION _p20150101000000 VALUES LESS THAN ('2015-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20160101000000 VALUES LESS THAN ('2016-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20170101000000 VALUES LESS THAN ('2017-01-01 00:00:00') ENGINE = InnoDB, |
| | PARTITION _p20180101000000 VALUES LESS THAN ('2018-01-01 00:00:00') ENGINE = InnoDB) */ |
+--------+------------------------------------------------------------------------------------------------+
1 row in set
Time: 0.024s
可以看到分区数据已经归档至 Orders_archive 表里面了。
通过以上可以看出，OSS META 开启后，对当前冷存表的 DDL 能力有极大的增强。更多的信息可以参考官网文档OSS DDL 能力介绍。
当然，如果您当前表已经归档成 OSS 表了，希望开启 OSS META 功能以支持更好的 DDL 操作，可以通过 REPAIR TABLE 语句实现，例如我们有一张归档表 t，
MySQL> show create table t;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------+
| t | CREATE TABLE `t` ( |
| | `id` int(11) DEFAULT NULL |
| | ) /*!99990 800020213 STORAGE OSS */ ENGINE=CSV DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci /*!99990 800020204 NULL_MARKER='NULL' */ |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set
Time: 0.008s
此时表 t 上没有 oss meta 标记，可以通过 repair table 增加：
MySQL> repair table t;
+--------+--------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+--------+--------+----------+----------+
| test.t | repair | status | OK |
+--------+--------+----------+----------+
1 row in set
Time: 0.863s
MySQL> show create table t;
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t | CREATE TABLE `t` ( |
| | `id` int(11) DEFAULT NULL |
| | ) /*!99990 800020213 STORAGE OSS */ ENGINE=CSV DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci /*!99990 800020204 NULL_MARKER='NULL' */ /*!99990 800020223 OSS META=1 */ |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set
Time: 0.014s
注意，repair table 是一个 inplace 操作，对于 OSS 表来说，其整体时间与表的大小相关。
当然，也可以关闭当前表的 OSS META 功能，通过 disable OSS META 实现：
MySQL> ALTER TABLE t DISABLE OSS META;
Query OK, 0 rows affected
Time: 0.049s
MySQL> show create table t;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------+
| t | CREATE TABLE `t` ( |
| | `id` int(11) DEFAULT NULL |
| | ) /*!99990 800020213 STORAGE OSS */ ENGINE=CSV DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci /*!99990 800020204 NULL_MARKER='NULL' */ |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set
关闭 OSS META 是一个 instant 操作，只修改表的元数据。
PolarDB MySQL 的 8.0.2.2.24 版本引入了冷数据 OSS META 功能，显著优化了冷数据的 DDL 操作。在过去，由于 OSS 冷数据的 DDL 能力有限，大部分 DDL 操作都需要使用 COPY 算法，导致修改表定义的成本很高。现在，通过增强对冷数据元数据的管理，某些DDL操作可以直接应用，无需全量数据重写，提高了效率和灵活性。当启用 OSS META功能后，例如在处理分区归档到OSS的场景中，可以更方便地进行表结构的修改。总的来说，OSS META 功能提升了PolarDB MySQL在处理冷数据DDL时的性能，降低了维护成本，使得冷数据管理更加便捷高效。