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

Question

I have gone through several articles and examples, and have yet to find an efficient way to do this SQL query in MongoDB (where there are millions of rows documen

Answer 1

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;