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sql: introduce invisible index #3011

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sql: introduce invisible index
Deardrops May 9, 2020
dc3f134
address comment
Deardrops May 9, 2020
e54d9d7
Merge branch 'master' into invisible-index
Deardrops May 9, 2020
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address comment
Deardrops May 9, 2020
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Merge branch 'invisible-index' of github.com:Deardrops/docs-cn into i…
Deardrops May 9, 2020
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Merge branch 'master' into invisible-index
AilinKid May 11, 2020
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refine format
TomShawn May 11, 2020
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2 changes: 0 additions & 2 deletions reference/mysql-compatibility.md
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Expand Up @@ -105,10 +105,8 @@ TiDB 支持常用的 MySQL 内建函数，但是不是所有的函数都已经

+ Add Index
- 不支持同时创建多个索引
- 不支持 `VISIBLE/INVISIBLE` 的索引
- 其他类型的 Index Type (HASH/BTREE/RTREE) 只有语法支持，功能不支持
+ Add Column
- 不支持同时创建多个列
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@Deardrops Deardrops May 9, 2020 •
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This feature has implemented in TiDB, this line should be removed.

- 不支持将新创建的列设为主键或唯一索引，也不支持将此列设成 AUTO_INCREMENT 属性
+ Drop Column: 不支持删除主键列或索引列
+ Change/Modify Column
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1 change: 1 addition & 0 deletions reference/sql/statements/add-index.md
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Expand Up @@ -108,6 +108,7 @@ EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
* [CREATE INDEX](/reference/sql/statements/create-index.md)
* [DROP INDEX](/reference/sql/statements/drop-index.md)
* [RENAME INDEX](/reference/sql/statements/rename-index.md)
* [ALTER INDEX](/reference/sql/statements/alter-index.md)
* [ADD COLUMN](/reference/sql/statements/add-column.md)
* [CREATE TABLE](/reference/sql/statements/create-table.md)
* [EXPLAIN](/reference/sql/statements/explain.md)
190 changes: 190 additions & 0 deletions reference/sql/statements/alter-index.md
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@@ -0,0 +1,190 @@
---
title: ALTER INDEX
summary: TiDB 数据库中 ALTER INDEX 的使用概况。
category: reference
---

# 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

{{< 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](/reference/sql/statements/create-table.md)
* [CREATE INDEX](/reference/sql/statements/create-index.md)
* [ADD INDEX](/reference/sql/statements/add-index.md)
* [DROP INDEX](/reference/sql/statements/drop-index.md)
* [RENAME INDEX](/reference/sql/statements/rename-index.md)
1 change: 1 addition & 0 deletions reference/sql/statements/alter-table.md
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Expand Up @@ -103,6 +103,7 @@ EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
* [ADD INDEX](/reference/sql/statements/add-index.md)
* [DROP INDEX](/reference/sql/statements/drop-index.md)
* [RENAME INDEX](/reference/sql/statements/rename-index.md)
* [ALTER INDEX](/reference/sql/statements/alter-index.md)
* [CREATE TABLE](/reference/sql/statements/create-table.md)
* [DROP TABLE](/reference/sql/statements/drop-table.md)
* [SHOW CREATE TABLE](/reference/sql/statements/show-create-table.md)
12 changes: 12 additions & 0 deletions reference/sql/statements/create-index.md
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Expand Up @@ -160,6 +160,17 @@ CREATE INDEX idx ON t ((lower(name)));

表达式索引的语法和限制与 MySQL 相同，是通过将索引建立在隐藏的虚拟生成列 (generated virtual column) 上来实现的。因此所支持的表达式继承了虚拟生成列的所有[限制](/reference/sql/generated-columns.md#局限性)。目前，建立了索引的表达式只有在 `FIELD` 子句、`WHERE` 子句和 `ORDER BY` 子句中时，优化器才能使用表达式索引。后续将支持 `GROUP BY` 子句。

## 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。

```sql
CREATE TABLE t1 (c1 INT, c2 INT, UNIQUE(c2));
CREATE UNIQUE INDEX c1 ON t1 (c1) INVISIBLE;
```

具体可以参考 [不可见索引的介绍](/reference/sql/statements/alter-index.md#不可见索引)。

## 相关 session 变量

和 `CREATE INDEX` 语句相关的全局变量有 `tidb_ddl_reorg_worker_cnt`，`tidb_ddl_reorg_batch_size` 和 `tidb_ddl_reorg_priority`，具体可以参考 [TiDB 特定系统变量](/reference/configuration/tidb-server/tidb-specific-variables.md#tidb_ddl_reorg_worker_cnt)。
Expand All @@ -175,6 +186,7 @@ CREATE INDEX idx ON t ((lower(name)));
* [ADD INDEX](/reference/sql/statements/add-index.md)
* [DROP INDEX](/reference/sql/statements/drop-index.md)
* [RENAME INDEX](/reference/sql/statements/rename-index.md)
* [ALTER INDEX](/reference/sql/statements/alter-index.md)
* [ADD COLUMN](/reference/sql/statements/add-column.md)
* [CREATE TABLE](/reference/sql/statements/create-table.md)
* [EXPLAIN](/reference/sql/statements/explain.md)
1 change: 1 addition & 0 deletions reference/sql/statements/create-table.md
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Expand Up @@ -172,6 +172,7 @@ index_option:
KEY_BLOCK_SIZE [=] value
| index_type
| COMMENT 'string'
| {VISIBLE | INVISIBLE}
```

`index_option` 中 `KEY_BLOCK_SIZE` 目前只是语法上支持。
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1 change: 1 addition & 0 deletions reference/sql/statements/drop-index.md
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Expand Up @@ -112,3 +112,4 @@ Query OK, 0 rows affected (0.30 sec)
* [CREATE INDEX](/reference/sql/statements/create-index.md)
* [ADD INDEX](/reference/sql/statements/add-index.md)
* [RENAME INDEX](/reference/sql/statements/rename-index.md)
* [ALTER INDEX](/reference/sql/statements/alter-index.md)
1 change: 1 addition & 0 deletions reference/sql/statements/rename-index.md
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Expand Up @@ -86,3 +86,4 @@ Create Table: CREATE TABLE `t1` (
* [CREATE INDEX](/reference/sql/statements/create-index.md)
* [DROP INDEX](/reference/sql/statements/drop-index.md)
* [SHOW INDEX](/reference/sql/statements/show-index.md)
* [ALTER INDEX](/reference/sql/statements/alter-index.md)
42 changes: 24 additions & 18 deletions reference/sql/statements/show-indexes.md
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Expand Up @@ -49,12 +49,14 @@ SHOW INDEXES FROM t1;
```

```
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t1 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| t1 | 1 | col1 | 1 | col1 | A | 0 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| t1 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | | YES
| NULL |
| t1 | 1 | col1 | 1 | col1 | A | 0 | NULL | NULL | YES | BTREE | | | YES
| NULL |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
2 rows in set (0.00 sec)
```

Expand All @@ -65,12 +67,14 @@ SHOW INDEX FROM t1;
```

```
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t1 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| t1 | 1 | col1 | 1 | col1 | A | 0 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| t1 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | | YES
| NULL |
| t1 | 1 | col1 | 1 | col1 | A | 0 | NULL | NULL | YES | BTREE | | | YES
| NULL |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
2 rows in set (0.00 sec)
```

Expand All @@ -81,12 +85,14 @@ SHOW KEYS FROM t1;
```

```
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t1 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| t1 | 1 | col1 | 1 | col1 | A | 0 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| t1 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | | YES
| NULL |
| t1 | 1 | col1 | 1 | col1 | A | 0 | NULL | NULL | YES | BTREE | | | YES
| NULL |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
2 rows in set (0.00 sec)
```

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42 changes: 22 additions & 20 deletions reference/system-databases/information-schema.md
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Expand Up @@ -469,26 +469,28 @@ desc statistics;
```

```
+---------------|---------------------|------|------|---------|-------+
| Field | Type | Null | Key | Default | Extra |
+---------------|---------------------|------|------|---------|-------+
| TABLE_CATALOG | varchar(512) | YES | | NULL | |
| TABLE_SCHEMA | varchar(64) | YES | | NULL | |
| TABLE_NAME | varchar(64) | YES | | NULL | |
| NON_UNIQUE | varchar(1) | YES | | NULL | |
| INDEX_SCHEMA | varchar(64) | YES | | NULL | |
| INDEX_NAME | varchar(64) | YES | | NULL | |
| SEQ_IN_INDEX | bigint(2) UNSIGNED | YES | | NULL | |
| COLUMN_NAME | varchar(21) | YES | | NULL | |
| COLLATION | varchar(1) | YES | | NULL | |
| CARDINALITY | bigint(21) UNSIGNED | YES | | NULL | |
| SUB_PART | bigint(3) UNSIGNED | YES | | NULL | |
| PACKED | varchar(10) | YES | | NULL | |
| NULLABLE | varchar(3) | YES | | NULL | |
| INDEX_TYPE | varchar(16) | YES | | NULL | |
| COMMENT | varchar(16) | YES | | NULL | |
| INDEX_COMMENT | varchar(1024) | YES | | NULL | |
+---------------|---------------------|------|------|---------|-------+
+---------------+---------------+------+------+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+---------------+------+------+---------+-------+
| TABLE_CATALOG | varchar(512) | YES | | NULL | |
| TABLE_SCHEMA | varchar(64) | YES | | NULL | |
| TABLE_NAME | varchar(64) | YES | | NULL | |
| NON_UNIQUE | varchar(1) | YES | | NULL | |
| INDEX_SCHEMA | varchar(64) | YES | | NULL | |
| INDEX_NAME | varchar(64) | YES | | NULL | |
| SEQ_IN_INDEX | bigint(2) | YES | | NULL | |
| COLUMN_NAME | varchar(21) | YES | | NULL | |
| COLLATION | varchar(1) | YES | | NULL | |
| CARDINALITY | bigint(21) | YES | | NULL | |
| SUB_PART | bigint(3) | YES | | NULL | |
| PACKED | varchar(10) | YES | | NULL | |
| NULLABLE | varchar(3) | YES | | NULL | |
| INDEX_TYPE | varchar(16) | YES | | NULL | |
| COMMENT | varchar(16) | YES | | NULL | |
| INDEX_COMMENT | varchar(1024) | YES | | NULL | |
| IS_VISIBLE | varchar(3) | YES | | NULL | |
| Expression | varchar(64) | YES | | NULL | |
+---------------+---------------+------+------+---------+-------+
```

下列语句是等价的：
Expand Down