DISTINCT_SCAN in agg is only used when certain format of _id is specified

// works

db.newTest.explain(1).aggregate([

   {$sort:{valueDate:1}},

   {$group:{_id: "$valueDate", first: {$first: "$farmId"}}}

]).stages[0]["$cursor"]["executionStats"]

{

        "executionSuccess" : true,

        "nReturned" : 2,

        "executionTimeMillis" : 0,

        "totalKeysExamined" : 2,

        "totalDocsExamined" : 2,

... }

// doesn't work

db.newTest.explain(1).aggregate([

  {$sort:{valueDate:1}},

  {$group:{_id: {v:"$valueDate"}, first: {$first: "$farmId"}}}

]).stages[0]["$cursor"]["executionStats"]

{

        "executionSuccess" : true,

        "nReturned" : 5,

        "executionTimeMillis" : 0,

        "totalKeysExamined" : 10,

        "totalDocsExamined" : 5,

... }