num_rows help!

What is the var in the object $db [num_rows], it doesnt seem to be the result rows for a query? and why does it grow continually and not reset with each new query? how to I reset it? [db] => db_mysql Object

        (
            [connection] => Resource id #29
            [phptype] => mysql
            [dbsyntax] => mysql
            [prepare_tokens] => Array
                (
                )

            [prepare_types] => Array
                (
                )

            [num_rows] => Array
                (
                    [33] => 3
                    [35] => 1
                    [36] => 1
                    [41] => 0
                    [42] => 0
                    [43] => 1
                    [44] => 1
                       ..
                    [476162] => 1