无法在mongodb上获得聚合中的确切错误

When i am trying this aggregation query on a sample mongo database of 25 entries, It works. But when the same method is applied for entire mongodb of 317000 entries, it doesn't respond. Even it's not showing any kind of error. What to do for such problems.

$mongo = new Mongo("localhost");
$collection = $mongo->selectCollection("sampledb","analytics");

$cursor = $collection->aggregate(
    array(
        array(
           '$group' => array(
               '_id' => array(
                   'os'=>'$feat_os',
                   'minprice'=>'$finder_input_1',
                   'max_price'=>'$finder_input_2',
                   'feat_quadcode'=>'$feat_quadcore'
               ),
               'count' => array('$sum' => 1)
            )
        ),
        array('$out'=>"result") 
    ) 
);