diff --git a/TOC.md b/TOC.md index 39174e885134..1f45188da860 100644 --- a/TOC.md +++ b/TOC.md @@ -235,7 +235,6 @@ - [`ADD INDEX`](/sql-statements/sql-statement-add-index.md) - [`ADMIN`](/sql-statements/sql-statement-admin.md) - [`ALTER DATABASE`](/sql-statements/sql-statement-alter-database.md) - - [`ALTER INDEX`](/sql-statements/sql-statement-alter-index.md) - [`ALTER INSTANCE`](/sql-statements/sql-statement-alter-instance.md) - [`ALTER TABLE`](/sql-statements/sql-statement-alter-table.md) - [`ALTER USER`](/sql-statements/sql-statement-alter-user.md) diff --git a/sql-statements/sql-statement-alter-index.md b/sql-statements/sql-statement-alter-index.md deleted file mode 100644 index 283fee46760b..000000000000 --- a/sql-statements/sql-statement-alter-index.md +++ /dev/null @@ -1,205 +0,0 @@ ---- -title: ALTER INDEX -summary: TiDB 数据库中 ALTER INDEX 的使用概况。 -category: reference -aliases: ['/docs-cn/stable/reference/sql/statements/alter-index/'] ---- - -# 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` 的索引将无法被优化器使用。 - -## 语法图 - -**AlterTableStmt:** - -![AlterTableStmt](/media/sqlgram/AlterTableStmt.png) - -**AlterTableSpec:** - -![AlterTableSpec](/media/sqlgram/AlterTableSpec.png) - -**IndexInvisible:** - -![IndexInvisible](/media/sqlgram/IndexInvisible.png) - -## 语法 - -{{< copyable "sql" >}} - -```sql -ALTER TABLE [table_name] ALTER INDEX [index_name] {VISIBLE | INVISIBLE} -``` - -## 示例 - -可以通过 `ALTER TABLE ... ALTER INDEX ...` 语句，修改索引的可见性： - -{{< copyable "sql" >}} - -```sql -CREATE TABLE t1 (c1 INT, UNIQUE(c1)); -ALTER TABLE t1 ALTER INDEX c1 INVISIBLE; -``` - -```sql -Query OK, 0 rows affected (0.02 sec) -``` - -{{< copyable "sql" >}} - -```sql -SHOW CREATE TABLE t1; -``` - -```sql -+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ -| 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) -``` - -## 不可见索引 - -不可见索引 (Invisible Indexes) 是 MySQL 8.0 引入的新功能，将一个索引设置为不可见，使优化器不会再使用这条索引。这个功能可以方便地验证使用或者不使用一条索引的查询计划，避免了 `Drop index` 或 `Add index` 这种资源消耗较多的操作。 - -{{< copyable "sql" >}} - -```sql -CREATE TABLE t1 (c1 INT, c2 INT, UNIQUE(c2)); -CREATE UNIQUE INDEX c1 ON t1 (c1) INVISIBLE; -``` - -可以通过 `Create Table` 语句查看，不可见的索引会用 `/*!80000 INVISIBLE */` 标识出： - -{{< copyable "sql" >}} - -```sql -SHOW CREATE TABLE t1; -``` - -```sql -+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ -| Table | Create Table - | -+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ -| t1 | CREATE TABLE `t1` ( - `c1` int(11) DEFAULT NULL, - `c2` int(11) DEFAULT NULL, - UNIQUE KEY `c2` (`c2`), - 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**： - -{{< copyable "sql" >}} - -```sql -EXPLAIN SELECT c1 FROM t1 ORDER BY c1; -``` - -```sql -+-------------------------+----------+-----------+---------------+--------------------------------+ -| 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： - -{{< copyable "sql" >}} - -```sql -EXPLAIN SELECT c2 FROM t1 ORDER BY c2; -``` - -```sql -+------------------------+----------+-----------+------------------------+-------------------------------+ -| 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： - -{{< copyable "sql" >}} - -```sql -SELECT * FROM t1 USE INDEX(c1); -``` - -```sql -ERROR 1176 (42000): Key 'c1' doesn't exist in table 't1' -``` - -> **注意：** -> -> “不可见”是仅仅对优化器而言的，不可见索引仍然可以被修改或删除。 - -{{< copyable "sql" >}} - -```sql -ALTER TABLE t1 DROP INDEX c1; -``` - -```sql -Query OK, 0 rows affected (0.02 sec) -``` - -### 限制 - -MySQL 对不可见索引有一条限制：不能将**主键**设置为不可见。TiDB 兼容这条限制，将主键设置为不可见后会抛出错误。 - -{{< copyable "sql" >}} - -```sql -CREATE TABLE t2(c1 INT, PRIMARY KEY(c1) INVISIBLE); -``` - -```sql -ERROR 3522 (HY000): A primary key index cannot be invisible -``` - -这里的**主键**包括既包括显式的主键（通过 `PRIMARY KEY` 关键字指定的主键），也包括隐式的主键。 - -当表中不存在显式的主键时，第一个 `UNIQUE` 索引（要求满足索引上的每一列都是 `NOT NULL`）会自动成为隐式的主键。 - -即不能将隐式的主键设置为不可见的。 - -> **注意：** -> -> TiDB 并不会实际创建一个**隐式的主键**，这个限制仅仅在行为上兼容 MySQL。 - -{{< copyable "sql" >}} - -```sql -CREATE TABLE t2(c1 INT NOT NULL, UNIQUE(c1) INVISIBLE); -``` - -``` -ERROR 3522 (HY000): A primary key index cannot be invisible -``` - -## 另请参阅 - -* [CREATE TABLE](/sql-statements/sql-statement-create-table.md) -* [CREATE INDEX](/sql-statements/sql-statement-create-index.md) -* [ADD INDEX](/sql-statements/sql-statement-add-index.md) -* [DROP INDEX](/sql-statements/sql-statement-drop-index.md) -* [RENAME INDEX](/sql-statements/sql-statement-rename-index.md)