title: ALTER TABLE summary: TiDB 数据库中 ALTER TABLE 的使用概况。

ALTER TABLE

ALTER TABLE 语句用于对已有表进行修改，以符合新表结构。ALTER TABLE 语句可用于：

语法图

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

示例

创建一张表，并插入初始数据：

  1. CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, c1 INT NOT NULL);
  2. INSERT INTO t1 (c1) VALUES (1),(2),(3),(4),(5);
  1. Query OK, 0 rows affected (0.11 sec)
  2. Query OK, 5 rows affected (0.03 sec)
  3. Records: 5  Duplicates: 0  Warnings: 0

执行以下查询需要扫描全表，因为 c1 列未被索引：

  1. EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
  1. +-------------------------+----------+-----------+---------------+--------------------------------+
  2. | id                      | estRows  | task      | access object | operator info                  |
  3. +-------------------------+----------+-----------+---------------+--------------------------------+
  4. | TableReader_7           | 10.00    | root      |               | data:Selection_6               |
  5. | └─Selection_6           | 10.00    | cop[tikv] |               | eq(test.t1.c1, 3)              |
  6. |   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t1      | keep order:false, stats:pseudo |
  7. +-------------------------+----------+-----------+---------------+--------------------------------+
  8. 3 rows in set (0.00 sec)

你可以使用 ALTER TABLE .. ADD INDEX 语句在 t1 表上添加索引。添加后，EXPLAIN 的分析结果显示 SELECT * FROM t1 WHERE c1 = 3; 查询已使用效率更高的索引范围扫描：

  1. ALTER TABLE t1 ADD INDEX (c1);
  2. EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
  1. Query OK, 0 rows affected (0.30 sec)
  2. +------------------------+---------+-----------+------------------------+---------------------------------------------+
  3. | id                     | estRows | task      | access object          | operator info                               |
  4. +------------------------+---------+-----------+------------------------+---------------------------------------------+
  5. | IndexReader_6          | 10.00   | root      |                        | index:IndexRangeScan_5                      |
  6. | └─IndexRangeScan_5     | 10.00   | cop[tikv] | table:t1, index:c1(c1) | range:[3,3], keep order:false, stats:pseudo |
  7. +------------------------+---------+-----------+------------------------+---------------------------------------------+
  8. 2 rows in set (0.00 sec)

TiDB 允许用户为 DDL 操作指定使用某一种 ALTER 算法。这仅为一种指定，并不改变实际的用于更改表的算法。如果你只想在群集的高峰时段允许即时 DDL 更改，则 ALTER 算法会很有用。示例如下：

  1. ALTER TABLE t1 DROP INDEX c1, ALGORITHM=INSTANT;
  1. Query OK, 0 rows affected (0.24 sec)

如果某一 DDL 操作要求使用 INPLACE 算法，而用户指定 ALGORITHM=INSTANT，会导致报错：

  1. ALTER TABLE t1 ADD INDEX (c1), ALGORITHM=INSTANT;
  1. ERROR 1846 (0A000): ALGORITHM=INSTANT is not supported. Reason: Cannot alter table by INSTANT. Try ALGORITHM=INPLACE.

但如果为 INPLACE 操作指定 ALGORITHM=COPY，会产生警告而非错误，这是因为 TiDB 将该指定解读为该算法或更好的算法。由于 TiDB 使用的算法可能不同于 MySQL，所以这一行为可用于 MySQL 兼容性。

  1. ALTER TABLE t1 ADD INDEX (c1), ALGORITHM=COPY;
  2. SHOW WARNINGS;
  1. Query OK, 0 rows affected, 1 warning (0.25 sec)
  2. +-------+------+---------------------------------------------------------------------------------------------+
  3. | Level | Code | Message                                                                                     |
  4. +-------+------+---------------------------------------------------------------------------------------------+
  5. | Error | 1846 | ALGORITHM=COPY is not supported. Reason: Cannot alter table by COPY. Try ALGORITHM=INPLACE. |
  6. +-------+------+---------------------------------------------------------------------------------------------+
  7. 1 row in set (0.00 sec)

MySQL 兼容性

TiDB 中的 ALTER TABLE 语法主要存在以下限制：

  • 单条 ALTER TABLE 语句不能完成多项操作。
  • 当前不支持有损更改，例如从 BIGINT 类型更改为 INT 类型。
  • 不支持空间数据类型。

其它限制可参考：TiDB 中 DDL 语句与 MySQL 的兼容性情况。

另请参阅