add doc for information_schema.variable_info (#9798)

pingcap · Aug 9, 2022 · e211db5 · e211db5
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diff --git a/TOC.md b/TOC.md
@@ -783,6 +783,7 @@
         - [`TIKV_REGION_STATUS`](/information-schema/information-schema-tikv-region-status.md)
         - [`TIKV_STORE_STATUS`](/information-schema/information-schema-tikv-store-status.md)
         - [`USER_PRIVILEGES`](/information-schema/information-schema-user-privileges.md)
+        - [`VARIABLES_INFO`](/information-schema/information-schema-variables-info.md)
         - [`VIEWS`](/information-schema/information-schema-views.md)
       - [`METRICS_SCHEMA`](/metrics-schema.md)
   - UI

diff --git a/information-schema/information-schema-variables-info.md b/information-schema/information-schema-variables-info.md
@@ -0,0 +1,55 @@
+---
+title: VARIABLES_INFO
+summary: Learn the `VARIABLES_INFO` information_schema table.
+---
+
+# VARIABLES_INFO
+
+The `VARIABLES_INFO` table provides information about the default value, current value, and scope of system variables in the current TiDB instance or TiDB cluster.
+
+```sql
+USE information_schema;
+DESC variables_info;
+```
+
+```sql
++-----------------+---------------------+------+------+---------+-------+
+| Field           | Type                | Null | Key  | Default | Extra |
++-----------------+---------------------+------+------+---------+-------+
+| VARIABLE_NAME   | varchar(64)         | NO   |      | NULL    |       |
+| VARIABLE_SCOPE  | varchar(64)         | NO   |      | NULL    |       |
+| DEFAULT_VALUE   | varchar(64)         | NO   |      | NULL    |       |
+| CURRENT_VALUE   | varchar(64)         | NO   |      | NULL    |       |
+| MIN_VALUE       | bigint(64)          | YES  |      | NULL    |       |
+| MAX_VALUE       | bigint(64) unsigned | YES  |      | NULL    |       |
+| POSSIBLE_VALUES | varchar(256)        | YES  |      | NULL    |       |
+| IS_NOOP         | varchar(64)         | NO   |      | NULL    |       |
++-----------------+---------------------+------+------+---------+-------+
+8 rows in set (0.00 sec)
+```
+
+```sql
+SELECT * FROM variables_info ORDER BY variable_name LIMIT 3;
+```
+
+```sql
++-----------------------------------+----------------+---------------+---------------+-----------+-----------+-----------------+---------+
+| VARIABLE_NAME                     | VARIABLE_SCOPE | DEFAULT_VALUE | CURRENT_VALUE | MIN_VALUE | MAX_VALUE | POSSIBLE_VALUES | IS_NOOP |
++-----------------------------------+----------------+---------------+---------------+-----------+-----------+-----------------+---------+
+| allow_auto_random_explicit_insert | SESSION,GLOBAL | OFF           | OFF           |      NULL |      NULL | NULL            | NO      |
+| auto_increment_increment          | SESSION,GLOBAL | 1             | 1             |         1 |     65535 | NULL            | NO      |
+| auto_increment_offset             | SESSION,GLOBAL | 1             | 1             |         1 |     65535 | NULL            | NO      |
++-----------------------------------+----------------+---------------+---------------+-----------+-----------+-----------------+---------+
+3 rows in set (0.01 sec)
+```
+
+Fields in the `VARIABLES_INFO` table are described as follows:
+
+* `VARIABLE_NAME`: the name of the system variable.
+* `VARIABLE_SCOPE`: the scope of the system variable. `SESSION` means that the system variable is only valid in the current session. `INSTANCE` means that the system variable is valid in the TiDB instance. `GLOBAL` means that the system variable is valid in the TiDB cluster.
+* `DEFAULT_VALUE`: the default value of the system variable.
+* `CURRENT_VALUE`: the current value of the system variable. If the scope includes `SESSION`, `CURRENT_VALUE` is the value in the current session.
+* `MIN_VALUE`: the minimum value allowed for the system variable. If the system variable is not numeric, `MIN_VALUE` is NULL.
+* `MAX_VALUE`: the maximum value allowed for the system variable. If the system variable is not numeric, `MAX_VALUE` is NULL.
+* `POSSIBLE_VALUES`: the possible values of the system variable. If the system variable is not an enum type, `POSSIBLE_VALUES` is NULL.
+* `IS_NOOP`: whether the system variable is a `noop` system variable.
diff --git a/information-schema/information-schema.md b/information-schema/information-schema.md
@@ -45,6 +45,7 @@ Many `INFORMATION_SCHEMA` tables have a corresponding `SHOW` command. The benefi
 | `TABLE_PRIVILEGES`                                                                      | Not implemented by TiDB. Returns zero rows. |
 | `TRIGGERS`                                                                              | Not implemented by TiDB. Returns zero rows. |
 | [`USER_PRIVILEGES`](/information-schema/information-schema-user-privileges.md)          | Summarizes the privileges associated with the current user. |
+| [`VARIABLES_INFO`](/information-schema/information-schema-variables-info.md)            | Provides information about TiDB system variables. |
 | [`VIEWS`](/information-schema/information-schema-views.md)                              | Provides a list of views that the current user has visibility of. Similar to running `SHOW FULL TABLES WHERE table_type = 'VIEW'` |
 
 ## Tables that are TiDB extensions