system_tables: add column description for several tables

information-schema/information-schema-character-sets.md

@@ -43,4 +43,11 @@ SELECT * FROM `character_sets`;
 | binary             | binary               | binary        |      1 |
 +--------------------+----------------------+---------------+--------+
 5 rows in set (0.00 sec)
-```
+```
+
+The description of columns in the `CHARACTER_SETS` table is as follows:
+
+* `CHARACTER_SET_NAME`: The name of the character set.
+* `DEFAULT_COLLATE_NAME` The default collation name of the character set.
+* `DESCRIPTION` The description of the character set.
+* `MAXLEN` The maximum length required to store a character in this character set.

information-schema/information-schema-collation-character-set-applicability.md

@@ -38,3 +38,13 @@ SELECT * FROM collation_character_set_applicability WHERE character_set_name='ut
 +----------------+--------------------+
 1 row in set (0.00 sec)
 ```
+
++----------------+--------------------+
+| COLLATION_NAME | CHARACTER_SET_NAME |
+| utf8mb4_bin | utf8mb4 |
++----------------+--------------------+
+
+The description of columns in the `COLLATION_CHARACTER_SET_APPLICABILITY` table is as follows:
+
+* `COLLATION_NAME`: The name of the collation.
+* `CHARACTER_SET_NAME`: The name of the character set which the collation belongs to.

information-schema/information-schema-collations.md

@@ -42,3 +42,17 @@ SELECT * FROM collations WHERE character_set_name='utf8mb4';
 +----------------+--------------------+------+------------+-------------+---------+
 1 row in set (0.00 sec)
 ```
+
+The description of columns in the `COLLATION` table is
+as follows:
+
+* `COLLATION_NAME`: The name of the collation.
+* `CHARACTER_SET_NAME`: The name of the character set
+which the collation belongs to.
+* `ID`: The ID of the collation.
+* `IS_DEFAULT`: Whether this collation is the default
+collation of the character set it belongs to.
+* `IS_COMPILED`: Whether the character set is compiled
+into the server.
+* `SORTLEN`: The minimum length of memory allocated when
+the collation sorts characters.

information-schema/information-schema-session-variables.md

@@ -46,4 +46,10 @@ SELECT * FROM session_variables ORDER BY variable_name LIMIT 10;
 | bind_address                      | *                |
 +-----------------------------------+------------------+
 10 rows in set (0.00 sec)
-```
+```
+
+The description of columns in the `SESSION_VARIABLES` table is as follows:
+
+* `VARIABLE_NAME`: The name of the session-level variable in the database.
+* `VARIABLE_VALUE`: The value of the session-level
+variable in the database.

information-schema/information-schema-tables.md

@@ -91,6 +91,30 @@ SHOW TABLES
   [LIKE 'wild']
 ```
 
+The description of columns in the `TABLES` table is as follows:
+
+* `TABLE_CATALOG`: The name of the catalog which the table belongs to. The value is always `def`.
+* `TABLE_SCHEMA`: The name of the schema which the table belongs to.
+* `TABLE_NAME`: The name of the table.
+* `TABLE_TYPE`: The type of the table.
+* `ENGINE`: The type of the storage engine. The value is currently `InnoDB`.
+* `VERSION`: Version. The value is `10` by default.
+* `ROW_FORMAT`: The row format. The value is currently `Compact`.
+* `TABLE_ROWS`: The number of rows in the table in statistics.
+* `AVG_ROW_LENGTH`: The average row length of the table. `AVG_ROW_LENGTH` = `DATA_LENGTH` / `TABLE_ROWS`.
+* `DATA_LENGTH`: Data length. `DATA_LENGTH` = `TABLE_ROWS` \* the sum of storage lengths of the columns in the tuple. The replicas of TiKV are not taken into account.
+* `MAX_DATA_LENGTH`: The maximum data length. The value is currently `0`, which means the data length has no upper limit.
+* `INDEX_LENGTH`: The index length. `INDEX_LENGTH` = `TABLE_ROWS` \* the sum of lengths of the columns in the index tuple. The replicas of TiKV are not taken into account.
+* `DATA_FREE`: Data fragment. The value is currently `0`.
+* `AUTO_INCREMENT`: The current step of the auto- increment primary key.
+* `CREATE_TIME`: The time at which the table is created.
+* `UPDATE_TIME`: The time at which the table is updated.
+* `CHECK_TIME`: The time at which the table is checked.
+* `TABLE_COLLATION`: The collation of strings in the table.
+* `CHECKSUM`: Checksum.
+* `CREATE_OPTIONS`: Creates options.
+* `TABLE_COMMENT`: The comments and notes of the table.
+
 Most of the information in the table is the same as MySQL. Only two columns are newly defined by TiDB:
 
 * `TIDB_TABLE_ID`: to indicate the internal ID of a table. This ID is unique in a TiDB cluster.

information-schema/information-schema-tidb-hot-regions.md

@@ -32,5 +32,15 @@ DESC tidb_hot_regions;
 10 rows in set (0.00 sec)
 ```
 
-- `TABLE_ID` and `INDEX_ID` are IDs generated for the corresponding table and index in TiDB.
-- `TYPE` is the type for a hot Region. Its value can be `READ` or `WRITE`.
+The description of columns in the `TIDB_HOT_REGIONS` table is as follows:
+
+* `TABLE_ID`: ID of located.
+* `INDEX_ID`: ID of the table in which the hot Region is the index in which the hot Region is located.
+* `DB_NAME`: The database name of the object in which the hot Region is located.
+* `TABLE_NAME`: The name of the table in which the hot Region is located.
+* `INDEX_NAME`: The name of the index in which the hot Region is located.
+* `REGION_ID`: ID of the hot Region.
+* `TYPE`: The type of the hot Region.
+* `MAX_HOT_DEGREE`: The maximum hot degree of the Region.
+* `REGION_COUNT`: The number of Regions in the instance. 
+* `FLOW_BYTES`: The number of bytes written and read in the Region.