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ALTER INDEX
An overview of the usage of ALTER INDEX for the TiDB database.
/docs/dev/sql-statements/sql-statement-alter-index/

ALTER INDEX

The ALTER INDEX statement is used to modify the visibility of the index to Visible or Invisible. Invisible indexes are maintained by DML statements, but will not be used by the query optimizer. This is useful in scenarios where you want to double-check before removing an index permanently. Starting from TiDB v8.0.0, you can make the optimizer select invisible indexes by modifying the system variable tidb_opt_use_invisible_indexes.

Synopsis

AlterTableStmt
         ::= 'ALTER' 'IGNORE'? 'TABLE' TableName AlterIndexSpec ( ',' AlterIndexSpec )*

AlterIndexSpec
         ::= 'ALTER' 'INDEX' Identifier ( 'VISIBLE' | 'INVISIBLE' )

Examples

You can modify the visibility of an index using the ALTER TABLE ... ALTER INDEX ... statement.

{{< copyable "sql" >}}

CREATE TABLE t1 (c1 INT, UNIQUE(c1));
ALTER TABLE t1 ALTER INDEX c1 INVISIBLE;
Query OK, 0 rows affected (0.02 sec)

{{< copyable "sql" >}}

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)

The optimizer cannot use the invisible index of c1.

{{< copyable "sql" >}}

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)

By comparison, c2 is a visible index and can be used by the optimizer.

{{< copyable "sql" >}}

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)

Even if you use the USE INDEX SQL hint to forcibly use indexes, the optimizer still cannot use invisible indexes; otherwise, an error is returned.

{{< copyable "sql" >}}

SELECT * FROM t1 USE INDEX(c1);
ERROR 1176 (42000): Key 'c1' doesn't exist in table 't1'

Note:

"Invisible" here means invisible only to the optimizer. You can still modify or delete invisible indexes.

{{< copyable "sql" >}}

ALTER TABLE t1 DROP INDEX c1;
Query OK, 0 rows affected (0.02 sec)

MySQL compatibility

  • Invisible indexes in TiDB are modeled on the equivalent feature from MySQL 8.0.
  • Similar to MySQL, TiDB does not permit PRIMARY KEY indexes to be made invisible.

See also