MongoDB select count(distinct x) on an indexed column - count unique results for large data sets

1) The easiest way to do this is via the aggregation framework. This takes two "$group" commands: the first one groups by distinct values, the second one counts all of the distinct values

pipeline = [ 
    { $group: { _id: "$myIndexedNonUniqueField"}  },
    { $group: { _id: 1, count: { $sum: 1 } } }
];

//
// Run the aggregation command
//
R = db.runCommand( 
    {
    "aggregate": "myCollection" , 
    "pipeline": pipeline
    }
);
printjson(R);

2) If you want to do this with Map/Reduce you can. This is also a two-phase process: in the first phase we build a new collection with a list of every distinct value for the key. In the second we do a count() on the new collection.

var SOURCE = db.myCollection;
var DEST = db.distinct
DEST.drop();


map = function() {
  emit( this.myIndexedNonUniqueField , {count: 1});
}

reduce = function(key, values) {
  var count = 0;

  values.forEach(function(v) {
    count += v['count'];        // count each distinct value for lagniappe
  });

  return {count: count};
};

//
// run map/reduce
//
res = SOURCE.mapReduce( map, reduce, 
    { out: 'distinct', 
     verbose: true
    }
    );

print( "distinct count= " + res.counts.output );
print( "distinct count=", DEST.count() );

Note that you cannot return the result of the map/reduce inline, because that will potentially overrun the 16MB document size limit. You can save the calculation in a collection and then count() the size of the collection, or you can get the number of results from the return value of mapReduce().

db.myCollection.aggregate( 
   {$group : {_id : "$myIndexedNonUniqueField"} }, 
   {$group: {_id:1, count: {$sum : 1 }}});

straight to result:

db.myCollection.aggregate( 
   {$group : {_id : "$myIndexedNonUniqueField"} }, 
   {$group: {_id:1, count: {$sum : 1 }}})
   .result[0].count;

Following solution worked for me

db.test.distinct('user'); [ "alex", "England", "France", "Australia" ]

db.countries.distinct('country').length 4