private function _mapKeyIndex($arr,$keys)
    {
        $maps = array();
        foreach ($arr as $index=>$row) {
            $key = "n";
            foreach ($keys as $cName) {
                $key.=$row[$cName];
            }
            if (!isset($maps[$key])) {
                $maps[$key] = array();
            }
            array_push($maps[$key], $index);
        }
        return $maps;
    }