ALTER TABLE
此语句用于修改现有表以符合新的表结构。ALTER TABLE
可以用来:
概述
- AlterTableStmt
- TableName
- AlterTableSpec
- PlacementPolicyOption
AlterTableStmt ::=
'ALTER' IgnoreOptional 'TABLE' TableName (
AlterTableSpecListOpt AlterTablePartitionOpt |
'ANALYZE' 'PARTITION' PartitionNameList ( 'INDEX' IndexNameList )? AnalyzeOptionListOpt |
'COMPACT' ( 'PARTITION' PartitionNameList )? 'TIFLASH' 'REPLICA'
)
TableName ::=
Identifier ('.' Identifier)?
AlterTableSpec ::=
TableOptionList
| 'SET' 'TIFLASH' 'REPLICA' LengthNum LocationLabelList
| 'CONVERT' 'TO' CharsetKw ( CharsetName | 'DEFAULT' ) OptCollate
| 'ADD' ( ColumnKeywordOpt IfNotExists ( ColumnDef ColumnPosition | '(' TableElementList ')' ) | Constraint | 'PARTITION' IfNotExists NoWriteToBinLogAliasOpt ( PartitionDefinitionListOpt | 'PARTITIONS' NUM ) )
| ( ( 'CHECK' | 'TRUNCATE' ) 'PARTITION' | ( 'OPTIMIZE' | 'REPAIR' | 'REBUILD' ) 'PARTITION' NoWriteToBinLogAliasOpt ) AllOrPartitionNameList
| 'COALESCE' 'PARTITION' NoWriteToBinLogAliasOpt NUM
| 'DROP' ( ColumnKeywordOpt IfExists ColumnName RestrictOrCascadeOpt | 'PRIMARY' 'KEY' | 'PARTITION' IfExists PartitionNameList | ( KeyOrIndex IfExists | 'CHECK' ) Identifier | 'FOREIGN' 'KEY' Symbol )
| 'EXCHANGE' 'PARTITION' Identifier 'WITH' 'TABLE' TableName WithValidationOpt
| ( 'IMPORT' | 'DISCARD' ) ( 'PARTITION' AllOrPartitionNameList )? 'TABLESPACE'
| 'REORGANIZE' 'PARTITION' NoWriteToBinLogAliasOpt ReorganizePartitionRuleOpt
| 'ORDER' 'BY' AlterOrderItem ( ',' AlterOrderItem )*
| ( 'DISABLE' | 'ENABLE' ) 'KEYS'
| ( 'MODIFY' ColumnKeywordOpt IfExists | 'CHANGE' ColumnKeywordOpt IfExists ColumnName ) ColumnDef ColumnPosition
| 'ALTER' ( ColumnKeywordOpt ColumnName ( 'SET' 'DEFAULT' ( SignedLiteral | '(' Expression ')' ) | 'DROP' 'DEFAULT' ) | 'CHECK' Identifier EnforcedOrNot | 'INDEX' Identifier ("VISIBLE" | "INVISIBLE") )
| 'RENAME' ( ( 'COLUMN' | KeyOrIndex ) Identifier 'TO' Identifier | ( 'TO' | '='? | 'AS' ) TableName )
| LockClause
| AlgorithmClause
| 'FORCE'
| ( 'WITH' | 'WITHOUT' ) 'VALIDATION'
| 'SECONDARY_LOAD'
| 'SECONDARY_UNLOAD'
| ( 'AUTO_INCREMENT' | 'AUTO_ID_CACHE' | 'AUTO_RANDOM_BASE' | 'SHARD_ROW_ID_BITS' ) EqOpt LengthNum
| ( 'CACHE' | 'NOCACHE' )
| (
'TTL' EqOpt TimeColumnName '+' 'INTERVAL' Expression TimeUnit (TTLEnable EqOpt ( 'ON' | 'OFF' ))?
| 'REMOVE' 'TTL'
| TTLEnable EqOpt ( 'ON' | 'OFF' )
| TTLJobInterval EqOpt stringLit
)
| PlacementPolicyOption
PlacementPolicyOption ::=
"PLACEMENT" "POLICY" EqOpt PolicyName
| "PLACEMENT" "POLICY" (EqOpt | "SET") "DEFAULT"
示例
创建一个带有初始数据的表:
CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, c1 INT NOT NULL);
INSERT INTO t1 (c1) VALUES (1),(2),(3),(4),(5);
Query OK, 0 rows affected (0.11 sec)
Query OK, 5 rows affected (0.03 sec)
Records: 5 Duplicates: 0 Warnings: 0
以下查询需要进行全表扫描,因为列 c1 没有建立索引:
EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+--------------------------------+
| TableReader_7 | 10.00 | root | | data:Selection_6 |
| └─Selection_6 | 10.00 | cop[tikv] | | eq(test.t1.c1, 3) |
| └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.00 sec)
可以使用 ALTER TABLE .. ADD INDEX
来为 t1 表添加索引。EXPLAIN
确认,原始查询现在使用索引范围扫描,效率更高:
ALTER TABLE t1 ADD INDEX (c1);
EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
Query OK, 0 rows affected (0.30 sec)
+------------------------+---------+-----------+------------------------+---------------------------------------------+
| id | estRows | task | access object | operator info |
+------------------------+---------+-----------+------------------------+---------------------------------------------+
| IndexReader_6 | 10.00 | root | | index:IndexRangeScan_5 |
| └─IndexRangeScan_5 | 10.00 | cop[tikv] | table:t1, index:c1(c1) | range:[3,3], keep order:false, stats:pseudo |
+------------------------+---------+-----------+------------------------+---------------------------------------------+
2 rows in set (0.00 sec)
TiDB 支持断言 DDL 更改使用特定的 ALTER
算法。注意,这只是一个断言,并不会改变实际用于修改表的算法:
ALTER TABLE t1 DROP INDEX c1, ALGORITHM=INSTANT;
Query OK, 0 rows affected (0.24 sec)
在需要 INPLACE
算法的操作上使用 ALGORITHM=INSTANT
断言会导致语句错误:
ALTER TABLE t1 ADD INDEX (c1), ALGORITHM=INSTANT;
ERROR 1846 (0A000): ALGORITHM=INSTANT is not supported. Reason: Cannot alter table by INSTANT. Try ALGORITHM=INPLACE.
然而,在 INPLACE
操作中使用 ALGORITHM=COPY
断言会产生警告而非错误。这是因为 TiDB 将断言理解为 this algorithm or better。这种行为差异对于 MySQL 兼容性很有用,因为 TiDB 使用的算法可能与 MySQL 不同:
ALTER TABLE t1 ADD INDEX (c1), ALGORITHM=COPY;
SHOW WARNINGS;
Query OK, 0 rows affected, 1 warning (0.25 sec)
+-------+------+---------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+---------------------------------------------------------------------------------------------+
| Error | 1846 | ALGORITHM=COPY is not supported. Reason: Cannot alter table by COPY. Try ALGORITHM=INPLACE. |
+-------+------+---------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
MySQL 兼容性
在 TiDB 中,ALTER TABLE
主要限制如下:
当在单个
ALTER TABLE
语句中修改多个 schema 对象时:- 不支持对同一对象进行多次修改。
- TiDB 在执行前会验证表的 schema 在执行之前。例如,执行
ALTER TABLE t ADD COLUMN c1 INT, ADD COLUMN c2 INT AFTER c1;
时会报错,因为表中不存在列c1
。 - 对于
ALTER TABLE
语句,TiDB 的执行顺序是从左到右逐个执行每个变更,这在某些情况下与 MySQL 不兼容。
不支持对主键列进行 Reorg-Data 类型的变更。
不支持对分区表的列类型变更。
不支持对生成列的列类型变更。
由于 TiDB 和 MySQL 在
CAST
函数行为上的兼容性问题,不支持某些数据类型(例如部分 TIME、Bit、Set、Enum 和 JSON 类型)的变更。不支持空间数据类型。
ALTER TABLE t CACHE | NOCACHE
是 TiDB 对 MySQL 语法的扩展。详情请参见 Cached Tables。
关于更多限制,请参见 MySQL Compatibility。