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
ALTERIGNORETABLETableNameAlterIndexSpec,
AlterIndexSpec
ALTERINDEXIdentifierVISIBLEINVISIBLE

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。

另请参阅

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