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  1. Core Server
  2. SERVER-21654

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

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Details

    • Bug
    • Status: Closed
    • Major - P3
    • Resolution: Works as Designed
    • 3.2.0-rc2
    • None
    • Aggregation Framework
    • None
    • ALL
    • Hide
      • 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);

    Description

      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

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            kelsey.schubert@mongodb.com Kelsey Schubert
            norberto.leite Norberto Fernando Rocha Leite (Inactive)
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              Created:
              Updated:
              Resolved: