ALTER INDEX
ALTER INDEX 语句用于修改索引的可见性，可以将索引设置为 Visible 或者 Invisible。设置为 Invisible 的索引即不可见索引 (Invisible Index) 由 DML 语句维护，不会被查询优化器使用。
语法图
- AlterTableStmt
- AlterTableSpec
- IndexInvisible
AlterTableStmt ::=
'ALTER' IgnoreOptional 'TABLE' TableName ( AlterTableSpecListOpt AlterTablePartitionOpt | 'ANALYZE' 'PARTITION' PartitionNameList ( 'INDEX' IndexNameList )? AnalyzeOptionListOpt )
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' IfExists 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 IndexInvisible )
| 'RENAME' ( ( 'COLUMN' | KeyOrIndex ) Identifier 'TO' Identifier | ( 'TO' | '='? | 'AS' ) TableName )
| LockClause
| AlgorithmClause
| 'FORCE'
| ( 'WITH' | 'WITHOUT' ) 'VALIDATION'
| 'SECONDARY_LOAD'
| 'SECONDARY_UNLOAD'
IndexInvisible ::=
'VISIBLE'
| 'INVISIBLE'
示例
可以通过 ALTER TABLE ... ALTER INDEX ... 语句，修改索引的可见性：
CREATE TABLE t1 (c1 INT, UNIQUE(c1));
ALTER TABLE t1 ALTER INDEX c1 INVISIBLE;
Query OK, 0 rows affected (0.02 sec)
SHOW CREATE TABLE t1;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table
|
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1 | CREATE TABLE `t1` (
`c1` int(11) DEFAULT NULL,
UNIQUE KEY `c1` (`c1`) /*!80000 INVISIBLE */
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
优化器将无法使用 c1 这个不可见的索引：
EXPLAIN SELECT c1 FROM t1 ORDER BY c1;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+--------------------------------+
| Sort_4 | 10000.00 | root | | test.t1.c1:asc |
| └─TableReader_8 | 10000.00 | root | | data:TableFullScan_7 |
| └─TableFullScan_7 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.00 sec)
作为对比，c2 是可见的索引，优化器将可以使用索引：
EXPLAIN SELECT c2 FROM t1 ORDER BY c2;
+------------------------+----------+-----------+------------------------+-------------------------------+
| id | estRows | task | access object | operator info |
+------------------------+----------+-----------+------------------------+-------------------------------+
| IndexReader_13 | 10000.00 | root | | index:IndexFullScan_12 |
| └─IndexFullScan_12 | 10000.00 | cop[tikv] | table:t1, index:c2(c2) | keep order:true, stats:pseudo |
+------------------------+----------+-----------+------------------------+-------------------------------+
2 rows in set (0.00 sec)
即使用 SQL Hint USE INDEX 强制使用索引，优化器也无法使用不可见索引，否则 SQL 语句会报错：
SELECT * FROM t1 USE INDEX(c1);
ERROR 1176 (42000): Key 'c1' doesn't exist in table 't1'
ALTER TABLE t1 DROP INDEX c1;
Query OK, 0 rows affected (0.02 sec)
MySQL 兼容性
- TiDB 中的不可见索引是基于 MySQL 8.0 中的同等特性构建的。
- 与 MySQL 类似，TiDB 不允许将主键索引设为不可见。
- MySQL 中提供的优化器开关
use_invisible_indexes=on可将所有的不可见索引重新设为可见。该功能在 TiDB 中不可用。