public function add_fields($tablename = null, $table_structure = null)
    {
        if (empty($tablename) || empty($table_structure)) return false;

        if (!is_string($table_structure))
            $table_structure_ar = self::build_fields_array( $table_structure );
        else
            $table_structure_ar = array($table_structure);

        if (self::table_exists($tablename)) {
            $query = "ALTER TABLE $tablename ADD COLUMN ( ".join(',', $table_structure_ar)." );";
    /*
            $this->queries[] = $query;
            $results = $this->db->queryExec($query, $err);
            if ($err) trigger_error( "Can not alter table '$tablename' [$err]!", E_USER_ERROR );
    */
            $results = $this->query($query, null, 'queryExec');
            $this->addCachedQuery( $query );
        }
        return true;
    }