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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

AlterTableStmt
ALTERIgnoreOptionalTABLETableNameAlterTableSpecListOptAlterTablePartitionOptANALYZEPARTITIONPartitionNameListINDEXIndexNameListAnalyzeOptionListOpt
AlterTableSpec
TableOptionListSETTIFLASHREPLICALengthNumLocationLabelListCONVERTTOCharsetKwCharsetNameDEFAULTOptCollateADDColumnKeywordOptIfNotExistsColumnDefColumnPosition(TableElementList)ConstraintPARTITIONIfNotExistsNoWriteToBinLogAliasOptPartitionDefinitionListOptPARTITIONSNUMCHECKTRUNCATEPARTITIONOPTIMIZEREPAIRREBUILDPARTITIONNoWriteToBinLogAliasOptAllOrPartitionNameListCOALESCEPARTITIONNoWriteToBinLogAliasOptNUMDROPColumnKeywordOptIfExistsColumnNameRestrictOrCascadeOptPRIMARYKEYPARTITIONIfExistsPartitionNameListKeyOrIndexIfExistsCHECKIdentifierFOREIGNKEYIfExistsSymbolEXCHANGEPARTITIONIdentifierWITHTABLETableNameWithValidationOptIMPORTDISCARDPARTITIONAllOrPartitionNameListTABLESPACEREORGANIZEPARTITIONNoWriteToBinLogAliasOptReorganizePartitionRuleOptORDERBYAlterOrderItem,DISABLEENABLEKEYSMODIFYColumnKeywordOptIfExistsCHANGEColumnKeywordOptIfExistsColumnNameColumnDefColumnPositionALTERColumnKeywordOptColumnNameSETDEFAULTSignedLiteral(Expression)DROPDEFAULTCHECKIdentifierEnforcedOrNotINDEXIdentifierIndexInvisibleRENAMECOLUMNKeyOrIndexIdentifierTOIdentifierTO=ASTableNameLockClauseAlgorithmClauseFORCEWITHWITHOUTVALIDATIONSECONDARY_LOADSECONDARY_UNLOAD
ColumnKeywordOpt
COLUMN
ColumnDef
ColumnNameTypeSERIALColumnOptionListOpt
ColumnPosition
FIRSTAFTERColumnName

示例

Meta-Only Change

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

Query OK, 0 rows affected (0.11 sec)

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

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

ALTER TABLE t1 MODIFY col1 BIGINT;

Query OK, 0 rows affected (0.09 sec)

SHOW CREATE TABLE t1\G;

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

Reorg-Data Change

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

Query OK, 0 rows affected (0.11 sec)

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

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

ALTER TABLE t1 MODIFY col1 VARCHAR(5);

Query OK, 0 rows affected (2.52 sec)

SHOW CREATE TABLE t1\G;

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

    alter table t1 modify column col1 varchar(4);
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：

    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：

    ALTER TABLE t1 MODIFY col1 BIGINT, MODIFY id BIGINT NOT NULL;
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：

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

    CREATE TABLE t (a int primary key);
ALTER TABLE t MODIFY COLUMN a bigint;
Query OK, 0 rows affected (0.01 sec)

  • 不支持修改生成列的类型。例如：

    CREATE TABLE t (a INT, b INT as (a+1));
ALTER TABLE t MODIFY COLUMN b VARCHAR(10);
ERROR 8200 (HY000): Unsupported modify column: column is generated

  • 不支持修改分区表上的列类型。例如：

    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));
ALTER TABLE t MODIFY COLUMN c1 DATETIME;
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：

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

另请参阅

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