feat: add Show Regions and update REGION_PEERS docs

docs/reference/sql/information-schema/region-peers.md

@@ -19,21 +19,27 @@ DESC REGION_PEERS;
 The output is as follows:
 
 ```sql
-+--------------+--------+------+------+---------+---------------+
-| Column       | Type   | Key  | Null | Default | Semantic Type |
-+--------------+--------+------+------+---------+---------------+
-| region_id    | UInt64 |      | NO   |         | FIELD         |
-| peer_id      | UInt64 |      | YES  |         | FIELD         |
-| peer_addr    | String |      | YES  |         | FIELD         |
-| is_leader    | String |      | YES  |         | FIELD         |
-| status       | String |      | YES  |         | FIELD         |
-| down_seconds | Int64  |      | YES  |         | FIELD         |
-+--------------+--------+------+------+---------+---------------+
++---------------+--------+------+------+---------+---------------+
+| Column        | Type   | Key  | Null | Default | Semantic Type |
++---------------+--------+------+------+---------+---------------+
+| table_catalog | String |      | NO   |         | FIELD         |
+| table_schema  | String |      | NO   |         | FIELD         |
+| table_name    | String |      | NO   |         | FIELD         |
+| region_id     | UInt64 |      | NO   |         | FIELD         |
+| peer_id       | UInt64 |      | YES  |         | FIELD         |
+| peer_addr     | String |      | YES  |         | FIELD         |
+| is_leader     | String |      | YES  |         | FIELD         |
+| status        | String |      | YES  |         | FIELD         |
+| down_seconds  | Int64  |      | YES  |         | FIELD         |
++---------------+--------+------+------+---------+---------------+
 6 rows in set (0.00 sec)
 ```
 
 Fields in the `REGION_PEERS` table are described as follows:
 
+- `table_catalog`: The catalog of the table.
+- `table_schema`: The schema of the table.
+- `table_name`: The name of the table.
 - `region_id`: The ID of the Region.
 - `peer_id`: The ID of the Region peer.
 - `peer_addr`: The address of the peer.

docs/reference/sql/show.md

@@ -380,6 +380,26 @@ SHOW COLUMNS FROM monitor WHERE Field = 'cpu';
 +-------+--------+------+------+---------+-------+---------------+
 ```
 
+To list all regions in a table:
+```sql
+SHOW REGION FROM monitor;
+```
+
+```sql
++----------------+---------------+------+--------+
+| Table          | Region        | Peer | Leader |
++----------------+---------------+------+--------+
+| monitor        | 4398046511104 |    0 | Yes    |
++----------------+---------------+------+--------+
+```
+
+It is the extension of `INFORMATION_SCHEMA.REGION_PEERS` and supports `WHERE` too.
+
+The syntax is
+```sql
+SHOW REGION FROM [table] [IN database] [WHERE where]
+```
+
 Other `SHOW` statements:
 * `SHOW STATUS`  and `SHOW VARIABLES` are not supported, just return empty results.
 * `SHOW TABLE STATUS` is the extension of  `INFORMATION_SCHEMA.TABLES`.

i18n/zh/docusaurus-plugin-content-docs/current/reference/sql/information-schema/region-peers.md

@@ -20,20 +20,26 @@ DESC REGION_PEERS;
 
 ```sql
 +--------------+--------+------+------+---------+---------------+
-| Column       | Type   | Key  | Null | Default | Semantic Type |
-+--------------+--------+------+------+---------+---------------+
-| region_id    | UInt64 |      | NO   |         | FIELD         |
-| peer_id      | UInt64 |      | YES  |         | FIELD         |
-| peer_addr    | String |      | YES  |         | FIELD         |
-| is_leader    | String |      | YES  |         | FIELD         |
-| status       | String |      | YES  |         | FIELD         |
-| down_seconds | Int64  |      | YES  |         | FIELD         |
-+--------------+--------+------+------+---------+---------------+
+| Column        | Type   | Key  | Null | Default | Semantic Type |
++---------------+--------+------+------+---------+---------------+
+| table_catalog | String |      | NO   |         | FIELD         |
+| table_schema  | String |      | NO   |         | FIELD         |
+| table_name    | String |      | NO   |         | FIELD         |
+| region_id     | UInt64 |      | NO   |         | FIELD         |
+| peer_id       | UInt64 |      | YES  |         | FIELD         |
+| peer_addr     | String |      | YES  |         | FIELD         |
+| is_leader     | String |      | YES  |         | FIELD         |
+| status        | String |      | YES  |         | FIELD         |
+| down_seconds  | Int64  |      | YES  |         | FIELD         |
++---------------+--------+------+------+---------+---------------+
 6 rows in set (0.00 sec)
 ```
 
 `REGION_PEERS` 表中的字段描述如下：
 
+- `table_catalog`：表所属的目录。
+- `table_schema`：表所属的数据库。
+- `table_name`：表的名称。
 - `region_id`：Region 的 ID。
 - `peer_id`：Region peer 的 ID。
 - `peer_addr`：peer 的地址。

i18n/zh/docusaurus-plugin-content-docs/current/reference/sql/show.md

@@ -381,6 +381,27 @@ SHOW COLUMNS FROM monitor WHERE Field = 'cpu';
 +-------+--------+------+------+---------+-------+---------------+
 ```
 
+列出表中的所有 Region：
+
+```sql
+SHOW REGION FROM monitor;
+```
+
+```sql
++----------------+---------------+------+--------+
+| Table          | Region        | Peer | Leader |
++----------------+---------------+------+--------+
+| monitor        | 4398046511104 |    0 | Yes    |
++----------------+---------------+------+--------+
+```
+
+这是 `INFORMATION_SCHEMA.REGION_PEERS` 的扩展，并且支持 `WHERE` 子句。
+
+语法是：
+```sql
+SHOW REGION FROM [table] [IN database] [WHERE where]
+```
+
 其他 `SHOW` 扩展语句：
 * `SHOW STATUS` 和 `SHOW VARIABLES` 不支持，仅返回空结果。
 * `SHOW TABLE STATUS` 是 `INFORMATION_SCHEMA.TABLES` 的扩展。