aggregation pipeline not correctly on $stdDevPop and $stdDevSample not returning values correctly

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    • Type: Bug
    • Resolution: Done
    • Priority: Major - P3
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    • Affects Version/s: 3.2.0-rc2
    • Component/s: Aggregation Framework
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    • ALL
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      • import ages.json file
      • run the following instructions on shell 
        var pipeline = [
  {$group: {_id:"$city", ages: {$push: "$age"}}},
  {$group: {_id:"$_id", average:{$avg: "$ages"}, stdSamp: {$stdDevSamp: "$ages"},  stdPop: {$stdDevPop: "$ages"}}},
];
var res = db.ages.aggregate(pipeline);
printjson(res);
      Show
      import ages.json file run the following instructions on shell var pipeline = [ {$group: {_id: "$city" , ages: {$push: "$age" }}}, {$group: {_id: "$_id" , average:{$avg: "$ages" }, stdSamp: {$stdDevSamp: "$ages" }, stdPop: {$stdDevPop: "$ages" }}}, ]; var res = db.ages.aggregate(pipeline); printjson(res);
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      Running an aggregation pipeline composed by the following $stdDevPop and $stdDevSamp does not produce the expected output.
      If we have a collection where documents contain individual "age" field and we want to calculate the standard deviation ($stdDevPop) for a given selector (city) the pipeline should be the following:

      db.ages.aggregate([ {$group: { ages: {$push:"$age"}, _id:"$city"  }}, {$group: { _id: null,  std: {$avg: "$ages"}  }}   ] )

      but the output I'm currently getting is the following:

      {
  "waitedMS": NumberLong("0"),
  "result": [
    {
      "_id": null,
      "std": null
    }
  ],
  "ok": 1
}

      Running the same operation but using an intermidiate collection does perform the expected result: 

      db.ages.aggregate([ {$group: { ages: {$push:"$age"}, _id:"$city",  }}, {$out:'aaa'}] )
db.aaa.aggregate( {$project: {_id: "$_id", s: { $stdDevPop: "$ages"}}})
{
  "waitedMS": NumberLong("0"),
  "result": [
    {
      "_id": "NYC",
      "s": 29.037802755871343
    }
  ],
  "ok": 1
}

      Also tested with $avg with the same issue.

      Seems like there is some issue with pipelined $group operations

        1. ages.json
          21 kB

            Assignee:
            Kelsey Schubert
            Reporter:
            Norberto Fernando Rocha Leite (Inactive)
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              Created:
              Updated:
              Resolved: