distinct tidb only information function (#14213)

functions-and-operators/information-functions.md

@@ -8,13 +8,12 @@ aliases: ['/docs/dev/functions-and-operators/information-functions/','/docs/dev/
 
 TiDB supports most of the [information functions](https://dev.mysql.com/doc/refman/5.7/en/information-functions.html) available in MySQL 5.7.
 
-## Supported functions
+## TiDB supported MySQL functions
 
 | Name | Description |
 |:-----|:------------|
 | [`BENCHMARK()`](https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_benchmark) | Execute an expression in a loop |
 | [`CONNECTION_ID()`](https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_connection-id) | Return the connection ID (thread ID) for the connection  |
-| `CURRENT_RESOURCE_GROUP()`  | Return the name of the resource group that the current session is bound to |
 | [`CURRENT_USER()`, `CURRENT_USER`](https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_current-user) | Return the authenticated user name and host name |
 | [`DATABASE()`](https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_database) | Return the default (current) database name  |
 | [`FOUND_ROWS()`](https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_found-rows) | For a `SELECT` with a `LIMIT` clause, the number of the rows that are returned if there is no `LIMIT` clause |
@@ -26,6 +25,14 @@ TiDB supports most of the [information functions](https://dev.mysql.com/doc/refm
 | [`USER()`](https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_user) | Return the user name and host name provided by the client    |
 | [`VERSION()`](https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_version) | Return a string that indicates the MySQL server version   |
 
+## TiDB specific functions
+
+The following function is only supported by TiDB, and there is no equivalent function in MySQL.
+
+| Name | Description |
+|:-----|:------------|
+| [`CURRENT_RESOURCE_GROUP()`](/functions-and-operators/tidb-functions.md#current_resource_group)  | Return the name of the resource group that the current session is bound to |
+
 ## Unsupported functions
 
 * `CHARSET()`

functions-and-operators/tidb-functions.md

@@ -21,6 +21,7 @@ The following functions are TiDB extensions, and are not present in MySQL:
 | `VITESS_HASH(str)` | The `VITESS_HASH` function returns the hash of a string that is compatible with Vitess' `HASH` function. This is intended to help the data migration from Vitess. |
 | `TIDB_SHARD()` | The `TIDB_SHARD` function can be used to create a shard index to scatter the index hotspot. A shard index is an expression index with a `TIDB_SHARD` function as the prefix.|
 | `TIDB_ROW_CHECKSUM()` | The `TIDB_ROW_CHECKSUM` function is used to query the checksum value of a row. This function can only be used in `SELECT` statements within the FastPlan process. That is, you can query through statements like `SELECT TIDB_ROW_CHECKSUM() FROM t WHERE id = ?` or `SELECT TIDB_ROW_CHECKSUM() FROM t WHERE id IN (?, ?, ...)`. See also: [Data integrity validation for single-row data](/ticdc/ticdc-integrity-check.md). |
+| `CURRENT_RESOURCE_GROUP()`  | The `CURRENT_RESOURCE_GROUP` function is used to return the resource group name that the current session is bound to. See also: [Use Resource Control to Achieve Resource Isolation](/tidb-resource-control.md). |
 
 </CustomContent>
 
@@ -38,6 +39,7 @@ The following functions are TiDB extensions, and are not present in MySQL:
 | `VITESS_HASH(str)` | The `VITESS_HASH` function returns the hash of a string that is compatible with Vitess' `HASH` function. This is intended to help the data migration from Vitess. |
 | `TIDB_SHARD()` | The `TIDB_SHARD` function can be used to create a shard index to scatter the index hotspot. A shard index is an expression index with a `TIDB_SHARD` function as the prefix.|
 | `TIDB_ROW_CHECKSUM()` | The `TIDB_ROW_CHECKSUM` function is used to query the checksum value of a row. This function can only be used in `SELECT` statements within the FastPlan process. That is, you can query through statements like `SELECT TIDB_ROW_CHECKSUM() FROM t WHERE id = ?` or `SELECT TIDB_ROW_CHECKSUM() FROM t WHERE id IN (?, ?, ...)`. See also: [Data integrity validation for single-row data](https://docs.pingcap.com/tidb/stable/ticdc-integrity-check). |
+| `CURRENT_RESOURCE_GROUP()`  | The `CURRENT_RESOURCE_GROUP` function is used to return the resource group name that the current session is bound to. See also: [Use Resource Control to Achieve Resource Isolation](/tidb-resource-control.md). |
 
 </CustomContent>
 
@@ -337,3 +339,51 @@ The output is as follows:
 +----+------+------+---------------------+
 1 row in set (0.000 sec)
 ```
+
+### CURRENT_RESOURCE_GROUP
+
+The `CURRENT_RESOURCE_GROUP` function is used to show the resource group name that the current session is bound to. When the [Resource control](/tidb-resource-control.md) feature is enabled, the available resources that can be used by SQL statements are restricted by the resource quota of the bound resource group.
+
+When a session is established, TiDB binds the session to the resource group that the login user is bound to by default. If the user is not bound to any resource groups, the session is bound to the `default` resource group. Once the session is established, the bound resource group will not change by default, even if the bound resource group of the user is changed via [modifying the resource group bound to the user](/sql-statements/sql-statement-alter-user.md#modify-basic-user-information). To change the bound resource group of the current session, you can use [`SET RESOURCE GROUP`](/sql-statements/sql-statement-set-resource-group.md).
+
+#### Example
+
+Create a user `user1`, create two resource groups `rg1` and `rg2`, and bind the user `user1` to the resource group `rg1`:
+
+```sql
+CREATE USER 'user1';
+CREATE RESOURCE GROUP 'rg1' RU_PER_SEC = 1000;
+CREATE RESOURCE GROUP 'rg2' RU_PER_SEC = 2000;
+ALTER USER 'user1' RESOURCE GROUP `rg1`;
+```
+
+Use `user1` to log in and view the resource group bound to the current user:
+
+```sql
+SELECT CURRENT_RESOURCE_GROUP();
+```
+
+```
++--------------------------+
+| CURRENT_RESOURCE_GROUP() |
++--------------------------+
+| rg1                      |
++--------------------------+
+1 row in set (0.00 sec)
+```
+
+Execute `SET RESOURCE GROUP` to set the resource group for the current session to `rg2`, and then view the resource group bound to the current user:
+
+```sql
+SET RESOURCE GROUP `rg2`;
+SELECT CURRENT_RESOURCE_GROUP();
+```
+
+```
++--------------------------+
+| CURRENT_RESOURCE_GROUP() |
++--------------------------+
+| rg2                      |
++--------------------------+
+1 row in set (0.00 sec)
+```