OceanBase V4.2 特性解析：MySQL兼容之rename column 语法支持

1. 背景描述

MySQL 自8.0版本起，已支持rename column语法，这一语法允许用户在不更改列定义的情况下，为列重新命名。下面为使用rename column语法的一个具体示例：

alter table t1 rename column col_a to col_b;

在OceanBase过去的版本中，Oracle租户已经支持了rename column语法。而在最新的4.2.1版本及以后，我们进一步扩展了对MySQL租户的支持，引入了rename column语法，从而实现对MySQL 8.0的更多兼容。

2. 功能描述

rename column语法:

ALTER TABLE tbl_name [rename_action [, rename_action] ...];rename_action: RENAME old_col_name To new_col_name;

rename column不改变列定义，仅修改列名。如果目标名称在表中已经存在，那么rename column执行会报错：

OceanBase(root@test)>create table t1 (a int, b int);
Query OK, 0 rows affected (0.100 sec)OceanBase(root@test)>alter table t1 rename column a to b;
ERROR 1060 (42S21): Duplicate column name 'b'

不过也有例外，如果重命名的列出现了循环，可以正常执行。比如下面的是一个合法的alter示例：

OceanBase(root@test)>create table t1 (a int, b int);
Query OK, 0 rows affected (0.046 sec)OceanBase(root@test)>alter table t1 rename column a to b, rename column b to a;
Query OK, 0 rows affected (0.076 sec)OceanBase(root@test)>desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| b     | int(11) | YES  |     | NULL    |       |
| a     | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.003 sec)

如果重命名的列上建有索引，或者有外键约束，rename column可以正常执行，并且索引定义和外键约束会自动级联联修改，比如：

OceanBase(root@test)>create table t1 (a int, b int, index idx_a(a));
Query OK, 0 rows affected (0.057 sec)OceanBase(root@test)>show index from t1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment   | Index_comment | Visible | Expression |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+------------+
| t1    |          1 | idx_a    |            1 | a           | A         |        NULL | NULL     | NULL   | YES  | BTREE      | available |               | YES     | NULL       |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+------------+
1 row in set (0.014 sec)OceanBase(root@test)>alter table t1 rename column a to c;
Query OK, 0 rows affected (0.058 sec)OceanBase(root@test)>show index from t1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment   | Index_comment | Visible | Expression |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+------------+
| t1    |          1 | idx_a    |            1 | c           | A         |        NULL | NULL     | NULL   | YES  | BTREE      | available |               | YES     | NULL       |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+-----------+---------------+---------+------------+

OceanBase(root@test)>create table t1 (a int primary key);
Query OK, 0 rows affected (0.043 sec)OceanBase(root@test)>create table t2(b int, foreign key (b) references t1(a));
Query OK, 0 rows affected (0.066 sec)OceanBase(root@test)>show create table t2;
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                                             |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t2    | CREATE TABLE `t2` (`b` int(11) DEFAULT NULL,CONSTRAINT `t2_OBFK_1691379783330957` FOREIGN KEY (`b`) REFERENCES `test`.`t1`(`a`) ON UPDATE RESTRICT ON DELETE RESTRICT
) DEFAULT CHARSET = utf8mb4 ROW_FORMAT = DYNAMIC COMPRESSION = 'zstd_1.3.8' REPLICA_NUM = 1 BLOCK_SIZE = 16384 USE_BLOOM_FILTER = FALSE TABLET_SIZE = 134217728 PCTFREE = 0 |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.019 sec)OceanBase(root@test)>alter table t1 rename column a to b;
Query OK, 0 rows affected (0.053 sec)OceanBase(root@test)>show create table t2;
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                                             |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t2    | CREATE TABLE `t2` (`b` int(11) DEFAULT NULL,CONSTRAINT `t2_OBFK_1691379783330957` FOREIGN KEY (`b`) REFERENCES `test`.`t1`(`b`) ON UPDATE RESTRICT ON DELETE RESTRICT
) DEFAULT CHARSET = utf8mb4 ROW_FORMAT = DYNAMIC COMPRESSION = 'zstd_1.3.8' REPLICA_NUM = 1 BLOCK_SIZE = 16384 USE_BLOOM_FILTER = FALSE TABLET_SIZE = 134217728 PCTFREE = 0 |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.009 sec)

但是在以下场景，并不会自动级联修改：

重命名的列被生成列表达式引用，执行会报错，不支持修改。
重命名的列被分区表达式引用，执行报错，不支持修改。
重命名的列被check约束引用，执行报错，不支持修改。
重命名的列被视图引用，rename column执行成功，查询视图报错，需要用户手动修改视图定义。
重命名的列被存储过程引用，rename column执行成功，call procedure报错，需要用户手动修改。

OceanBase(root@test)>create table t1(a int, b int as (a + 1), c int, d int, constraint d_check check(d > 0)) partition by hash(c + 1) partitions 2;
Query OK, 0 rows affected (0.057 sec)OceanBase(root@test)>alter table t1 rename column a to e;
ERROR 3108 (HY000): Column 'a' has a generated column dependency
OceanBase(root@test)>alter table t1 rename column c to e;
ERROR 3855 (HY000): Column 'c' has a partitioning function dependency and cannot be dropped or renamed.
OceanBase(root@test)>alter table t1 rename column d to e;
ERROR 3959 (HY000): Check constraint 'd_check' uses column 'd', hence column cannot be dropped or renamed.

OceanBase(root@test)>create table t1 (a int);
Query OK, 0 rows affected (0.043 sec)OceanBase(root@test)>create view v1 as select a + 1 from t1;
Query OK, 0 rows affected (0.040 sec)OceanBase(root@test)>create procedure p() select a + 1 from t1;
Query OK, 0 rows affected (0.036 sec)OceanBase(root@test)>select * from v1;
Empty set (0.010 sec)OceanBase(root@test)>call p();
Empty set (0.014 sec)Query OK, 0 rows affected (0.014 sec)OceanBase(root@test)>alter table t1 rename column a to b;
Query OK, 0 rows affected (0.053 sec)OceanBase(root@test)>select * from v1;
ERROR 1356 (42S22): View 'test.v1' references invalid table(s) or column(s) or function(s) or definer/invoker of view lack rights to use them
OceanBase(root@test)>call p();
ERROR 1054 (42S22): Unknown column 'a' in 'field list'

3. 限制以及注意事项

第一：MySQL 8.0规定ADD PARTITION, DROP PARTITION, DISCARD PARTITION, IMPORT PARTITION, COALESCE PARTITION, REORGANIZE PARTITION, EXCHANGE PARTITION, ANALYZE PARTITION, CHECK PARTITION和REPAIR PARTITION 操作不能和rename column语句一起使用，比如下面一个add partition的例子，执行会报语法错误：

mysql> create table t1 (a int, b int) partition by range(a) (partition p0 values less than (10));
Query OK, 0 rows affected (0.01 sec)mysql> alter table t1 rename column b to c, add partition (partition p1 values less than (20));
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'partition (partition p1 values less than (20))' at line 1

对于同一条alter stmt中同时包含alter_partition_action和alter_column_action的场景，OB当前的实现选择了执行报错（NOT SUPPORTED错误），比如上述SQL：

OceanBase(root@test)>create table t1 (a int, b int) partition by range(a) (partition p0 values less than (10));
Query OK, 0 rows affected (0.052 sec)OceanBase(root@test)>alter table t1 rename column b to c, add partition (partition p1 values less than (20));
ERROR 1235 (0A000): specify alter_column_action and alter_partition_action in a single alter table stmt not supported

第二：MySQL 8.0支持在同一条alter stmt中指定drop column和rename column，如果rename的目标列名和drop的列名相同，rename column可以执行成功，比如：

mysql> create table t1(a int, b int);
Query OK, 0 rows affected (0.01 sec)mysql> alter table t1 drop column a, rename column b to a;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

OB目前不支持上述两个alter_column_action在一次DDL中执行：

OceanBase(root@test)>create table t1 (a int, b int);
Query OK, 0 rows affected (0.047 sec)OceanBase(root@test)>alter table t1 drop column a, rename column b to a;
ERROR 1060 (42S21): Duplicate column name 'a'

建议再drop column和rename column拆成两个DDL语句执行分别执行。