Add column descriptions for INFORMATION_SCHEMA.PARTITIONS

[mjonss]

Change Request

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.
   https://docs.pingcap.com/tidb/stable/information-schema-partitions only shows the table and its columns, not what the columns mean.

2. Describe your suggestion or addition.
   For each column describe the content, I was recently asked for how to get the partition limits/values for RANGE partitioning :) Which is in PARTITION_DESCRIPTION
   Similar to how https://docs.pingcap.com/tidb/stable/information-schema-columns is documented

3. Provide some reference materials (such as documents and websites) if you could.
   https://dev.mysql.com/doc/refman/8.0/en/information-schema-partitions-table.html

[Oreoxmt]

docs.pingcap.com/tidb/stable/information-schema-columns only shows the table and its columns, not what the columns mean.

@mjonss It seems that the description of columns in the COLUMNS table is already documented in https://docs.pingcap.com/tidb/stable/information-schema-columns.

docs/information-schema/information-schema-columns.md

Lines 81 to 108 in aa1631b

	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.

Does this issue suggest a need to include similar descriptions for columns in https://docs.pingcap.com/tidb/stable/information-schema-partitions?

[mjonss]

@mjonss It seems that the description of columns in the COLUMNS table is already documented in https://docs.pingcap.com/tidb/stable/information-schema-columns.
...
Does this issue suggest a need to include similar descriptions for columns in https://docs.pingcap.com/tidb/stable/information-schema-partitions?

@Oreoxmt yes, sorry for the confusion and wrong link. I have updated the description.

[Oreoxmt]

@lcwangchao Could you please take a look at this issue and create a doc PR to add it?