Returns the name of the indexed column.
| Parameter | Definition | 
|---|---|
| table-name | A table name. | 
| index-id | The index ID of an index of table-name. | 
| key_# | A key in the index specified by index-id. This parameter specifies the column number in the index. For a single column index, key_# is equal to 0. For a multicolumn index, key_# is equal to 0 for the first column, 1 for the second column, and so on. | 
| user-id | The user ID of the owner of table-name. If user-id is not specified, this value defaults to the caller’s user ID.. |