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Jun 17, 2020
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system-tables: add a description for columns (#2819) #2929

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56 changes: 54 additions & 2 deletions system-tables/system-table-information-schema.md
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Expand Up @@ -192,6 +192,34 @@ CHARACTER_MAXIMUM_LENGTH: NULL
1 row in set (0.01 sec)
```

The description of columns in the `COLUMNS` table is as follows:

* `TABLE_CATALOG`: The name of the catalog to which the table with the column belongs. The value is always `def`.
* `TABLE_SCHEMA`: The name of the schema in which the table with the column is located.
* `TABLE_NAME`: The name of the table with the column.
* `COLUMN_NAME`: The name of the column.
* `ORDINAL_POSITION`: The position of the column in the table.
* `COLUMN_DEFAULT`: The default value of the column. If the explicit default value is `NULL`, or if the column definition does not include the `default` clause, this value is `NULL`.
* `IS_NULLABLE`: Whether the column is nullable. If the column can store null values, this value is `YES`; otherwise, it is `NO`.
* `DATA_TYPE`: The type of data in the column.
* `CHARACTER_MAXIMUM_LENGTH`: For string columns, the maximum length in characters.
* `CHARACTER_OCTET_LENGTH`: For string columns, the maximum length in bytes.
* `NUMERIC_PRECISION`: The numeric precision of a number-type column.
* `NUMERIC_SCALE`: The numeric scale of a number-type column.
* `DATETIME_PRECISION`: For time-type columns, the fractional seconds precision.
* `CHARACTER_SET_NAME`: The name of the character set of a string column.
* `COLLATION_NAME`: The name of the collation of a string column.
* `COLUMN_TYPE`: The column type.
* `COLUMN_KEY`: Whether this column is indexed. This field might have the following values:
* Empty: This column is not indexed, or this column is indexed and is the second column in a multi-column non-unique index.
* `PRI`: This column is the primary key or one of multiple primary keys.
* `UNI`: This column is the first column of the unique index.
* `MUL`: The column is the first column of a non-unique index, in which a given value is allowed to occur for multiple times.
* `EXTRA`: Any additional information of the given column.
* `PRIVILEGES`: The privilege that the current user has on this column. Currently, this value is fixed in TiDB, and is always `select,insert,update,references`.
* `COLUMN_COMMENT`: Comments contained in the column definition.
* `GENERATION_EXPRESSION`: For generated columns, this value displays the expression used to calculate the column value. For non-generated columns, the value is empty.

The corresponding `SHOW` statement is as follows:

{{< copyable "sql" >}}
Expand All @@ -211,7 +239,7 @@ SHOW COLUMNS FROM t1 FROM test;

### ENGINES table

The `ENGINES` table provides information about storage engines. For compatibility, TiDB will always describe InnoDB as the only supported engine:
The `ENGINES` table provides information about storage engines. For compatibility, TiDB will always describe InnoDB as the only supported engine. In addition, other column values in the `ENGINES` table are also fixed values.

{{< copyable "sql" >}}

Expand All @@ -230,6 +258,15 @@ TRANSACTIONS: YES
1 row in set (0.00 sec)
```

The description of columns in the `ENGINES` table is as follows:

* `ENGINES`: The name of the storage engine.
* `SUPPORT`: The level of support that the server has on the storage engine. In TiDB, the value is always `DEFAULT`.
* `COMMENT`: The brief comment on the storage engine.
* `TRANSACTIONS`：Whether the storage engine supports transactions.
* `XA`: Whether the storage engine supports XA transactions.
* `SAVEPOINTS`: Whether the storage engine supports `savepoints`.

### KEY_COLUMN_USAGE table

The `KEY_COLUMN_USAGE` table describes the key constraints of the columns, such as the primary key constraint:
Expand Down Expand Up @@ -270,6 +307,21 @@ POSITION_IN_UNIQUE_CONSTRAINT: NULL
2 rows in set (0.00 sec)
```

The description of columns in the `KEY_COLUMN_USAGE` table is as follows:

* `CONSTRAINT_CATALOG`: The name of the catalog to which the constraint belongs. The value is always `def`.
* `CONSTRAINT_SCHEMA`: The name of the schema to which the constraint belongs.
* `CONSTRAINT_NAME`: The name of the constraint.
* `TABLE_CATALOG`: The name of the catalog to which the table belongs. The value is always `def`.
* `TABLE_SCHEMA`: The name of the schema to which the table belongs.
* `TABLE_NAME`: The name of the table with constraints.
* `COLUMN_NAME`: The name of the column with constraints.
* `ORDINAL_POSITION`: The position of the column in the constraint, rather than in the table. The position number starts from `1`.
* `POSITION_IN_UNIQUE_CONSTRAINT`: The unique constraint and the primary key constraint are empty. For foreign key constraints, this column is the position of the referenced table's key.
* `REFERENCED_TABLE_SCHEMA`: The name of the schema referenced by the constraint. Currently in TiDB, the value of this column in all constraints is `nil`, except for the foreign key constraint.
* `REFERENCED_TABLE_NAME`: The name of the table referenced by the constraint. Currently in TiDB, the value of this column in all constraints is `nil`, except for the foreign key constraint.
* `REFERENCED_COLUMN_NAME`: The name of the column referenced by the constraint. Currently in TiDB, the value of this column in all constraints is `nil`, except for the foreign key constraint.

### PROCESSLIST table

`PROCESSLIST`, just like `show processlist`, is used to view the requests that are being handled.
Expand Down Expand Up @@ -991,4 +1043,4 @@ The following `INFORMATION_SCHEMA` tables are present in TiDB, but will always r
* `SESSION_STATUS`
* `TABLESPACES`
* `TABLE_PRIVILEGES`
* `TRIGGERS`
* `TRIGGERS`