MODIFY COLUMN

ALTER TABLE .. MODIFY COLUMN 语句用于修改已有表上的列，包括列的数据类型和属性。若要同时重命名，可改用 CHANGE COLUMN 语句。

从 v5.1.0 版本起，TiDB 开始支持 Reorg 类型变更，包括但不限于：

  • 从 VARCHAR 转换为 BIGINT
  • DECIMAL 精度修改
  • 从 VARCHAR(10) 到 VARCHAR(5) 的长度压缩

语法图

1. AlterTableStmt ::=
2. 'ALTER' IgnoreOptional 'TABLE' TableName ( AlterTableSpecListOpt AlterTablePartitionOpt | 'ANALYZE' 'PARTITION' PartitionNameList ( 'INDEX' IndexNameList )? AnalyzeOptionListOpt )

4. AlterTableSpec ::=
5. TableOptionList
6. |   'SET' 'TIFLASH' 'REPLICA' LengthNum LocationLabelList
7. |   'CONVERT' 'TO' CharsetKw ( CharsetName | 'DEFAULT' ) OptCollate
8. |   'ADD' ( ColumnKeywordOpt IfNotExists ( ColumnDef ColumnPosition | '(' TableElementList ')' ) | Constraint | 'PARTITION' IfNotExists NoWriteToBinLogAliasOpt ( PartitionDefinitionListOpt | 'PARTITIONS' NUM ) )
9. |   ( ( 'CHECK' | 'TRUNCATE' ) 'PARTITION' | ( 'OPTIMIZE' | 'REPAIR' | 'REBUILD' ) 'PARTITION' NoWriteToBinLogAliasOpt ) AllOrPartitionNameList
10. |   'COALESCE' 'PARTITION' NoWriteToBinLogAliasOpt NUM
11. |   'DROP' ( ColumnKeywordOpt IfExists ColumnName RestrictOrCascadeOpt | 'PRIMARY' 'KEY' | 'PARTITION' IfExists PartitionNameList | ( KeyOrIndex IfExists | 'CHECK' ) Identifier | 'FOREIGN' 'KEY' IfExists Symbol )
12. |   'EXCHANGE' 'PARTITION' Identifier 'WITH' 'TABLE' TableName WithValidationOpt
13. |   ( 'IMPORT' | 'DISCARD' ) ( 'PARTITION' AllOrPartitionNameList )? 'TABLESPACE'
14. |   'REORGANIZE' 'PARTITION' NoWriteToBinLogAliasOpt ReorganizePartitionRuleOpt
15. |   'ORDER' 'BY' AlterOrderItem ( ',' AlterOrderItem )*
16. |   ( 'DISABLE' | 'ENABLE' ) 'KEYS'
17. |   ( 'MODIFY' ColumnKeywordOpt IfExists | 'CHANGE' ColumnKeywordOpt IfExists ColumnName ) ColumnDef ColumnPosition
18. |   'ALTER' ( ColumnKeywordOpt ColumnName ( 'SET' 'DEFAULT' ( SignedLiteral | '(' Expression ')' ) | 'DROP' 'DEFAULT' ) | 'CHECK' Identifier EnforcedOrNot | 'INDEX' Identifier IndexInvisible )
19. |   'RENAME' ( ( 'COLUMN' | KeyOrIndex ) Identifier 'TO' Identifier | ( 'TO' | '='? | 'AS' ) TableName )
20. |   LockClause
21. |   AlgorithmClause
22. |   'FORCE'
23. |   ( 'WITH' | 'WITHOUT' ) 'VALIDATION'
24. |   'SECONDARY_LOAD'
25. |   'SECONDARY_UNLOAD'

27. ColumnKeywordOpt ::= 'COLUMN'?

29. ColumnDef ::=
30. ColumnName ( Type | 'SERIAL' ) ColumnOptionListOpt

32. ColumnPosition ::=
33. ( 'FIRST' | 'AFTER' ColumnName )?

示例

Meta-Only Change

1. CREATE TABLE t1 (id int not null primary key AUTO_INCREMENT, col1 INT);

1. Query OK, 0 rows affected (0.11 sec)

1. INSERT INTO t1 (col1) VALUES (1),(2),(3),(4),(5);

1. Query OK, 5 rows affected (0.02 sec)
2. Records: 5  Duplicates: 0  Warnings: 0

1. ALTER TABLE t1 MODIFY col1 BIGINT;

1. Query OK, 0 rows affected (0.09 sec)

1. SHOW CREATE TABLE t1\G;

1. *************************** 1. row ***************************
2. Table: t1
3. Create Table: CREATE TABLE `t1` (
4. `id` int(11) NOT NULL AUTO_INCREMENT,
5. `col1` bigint(20) DEFAULT NULL,
6. PRIMARY KEY (`id`)
7. ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin AUTO_INCREMENT=30001
8. 1 row in set (0.00 sec)

Reorg-Data Change

1. CREATE TABLE t1 (id int not null primary key AUTO_INCREMENT, col1 INT);

1. Query OK, 0 rows affected (0.11 sec)

1. INSERT INTO t1 (col1) VALUES (12345),(67890);

1. Query OK, 2 rows affected (0.00 sec)
2. Records: 2  Duplicates: 0  Warnings: 0

1. ALTER TABLE t1 MODIFY col1 VARCHAR(5);

1. Query OK, 0 rows affected (2.52 sec)

1. SHOW CREATE TABLE t1\G;

1. *************************** 1. row ***************************
2. Table: t1
3. CREATE TABLE `t1` (
4. `id` int(11) NOT NULL AUTO_INCREMENT,
5. `col1` varchar(5) DEFAULT NULL,
6. PRIMARY KEY (`id`) /*T![clustered_index] CLUSTERED */
7. ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin AUTO_INCREMENT=30001
8. 1 row in set (0.00 sec)

注意：

  • 当所变更的类型与已经存在的数据行产生冲突时，TiDB 会进行报错处理。在上述例子中，TiDB 将进行如下报错：

1. alter table t1 modify column col1 varchar(4); 2. ERROR 1406 (22001): Data Too Long, field len 4, data len 5 - 由于和 Async Commit 功能兼容，DDL 在开始进入到 Reorg Data 前会有一定时间（约 2.5s）的等待处理：

1. Query OK, 0 rows affected (2.52 sec)

MySQL 兼容性

  • 不支持使用单个 ALTER TABLE 语句修改多个列，例如：

1. ALTER TABLE t1 MODIFY col1 BIGINT, MODIFY id BIGINT NOT NULL; 2. ERROR 1105 (HY000): Unsupported multi schema change - 不支持修改主键列上需要 Reorg-Data 的类型，但是支持修改 Meta-Only 的类型。例如：

1. CREATE TABLE t (a int primary key); 2. ALTER TABLE t MODIFY COLUMN a VARCHAR(10); 3. ERROR 8200 (HY000): Unsupported modify column: column has primary key flag

1. CREATE TABLE t (a int primary key); 2. ALTER TABLE t MODIFY COLUMN a bigint; 3. Query OK, 0 rows affected (0.01 sec) - 不支持修改生成列的类型。例如：

1. CREATE TABLE t (a INT, b INT as (a+1)); 2. ALTER TABLE t MODIFY COLUMN b VARCHAR(10); 3. ERROR 8200 (HY000): Unsupported modify column: column is generated - 不支持修改分区表上的列类型。例如：

1. CREATE TABLE t (c1 INT, c2 INT, c3 INT) partition by range columns(c1) ( partition p0 values less than (10), partition p1 values less than (maxvalue)); 2. ALTER TABLE t MODIFY COLUMN c1 DATETIME; 3. ERROR 8200 (HY000): Unsupported modify column: table is partition table - 不支持部分数据类型（例如，部分时间类型、Bit、Set、Enum、JSON 等）的变更，因为 TiDB cast 函数与 MySQL 的行为有一些兼容性问题。例如：

1. CREATE TABLE t (a DECIMAL(13, 7)); 2. ALTER TABLE t MODIFY COLUMN a DATETIME; 3. ERROR 8200 (HY000): Unsupported modify column: change from original type decimal(13,7) to datetime is currently unsupported yet

另请参阅