Make $collStats with count never return result field

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    • Query 2020-11-30, Query 2020-12-14
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      New Description:
      This ticket tracks the removal of the "result" field from the `$collStats` count results.

      Old Description:
      On a sharded cluster, running $collStats for a namespace that doesn't exist returns an empty 'result' array:

      >>> pprint.pprint(c.bar.command('aggregate', 'bar', pipeline=[{"$collStats": {"count": {}}}], cursor={}, check=False)){'$clusterTime': {'clusterTime': Timestamp(1528553991, 1),
                  'signature': {'hash': b'\x00\x00\x00\x00\x00\x00\x00\x00'
                                        b'\x00\x00\x00\x00\x00\x00\x00\x00'
                                        b'\x00\x00\x00\x00',
                                'keyId': 0}},
 'cursor': {'firstBatch': [], 'id': 0, 'ns': 'bar.bar'},
 'ok': 1.0,
 'operationTime': Timestamp(1528553991, 1),
 'result': []}

      When the namespace does exist, it returns a document for each shard, but no 'result' array:

      >>> pprint.pprint(c.foo.command('aggregate', 'bar', pipeline=[{"$collStats": {"count": {}}}], cursor={}, check=False)){'$clusterTime': {'clusterTime': Timestamp(1528554032, 1),
                  'signature': {'hash': b'\x00\x00\x00\x00\x00\x00\x00\x00'
                                        b'\x00\x00\x00\x00\x00\x00\x00\x00'
                                        b'\x00\x00\x00\x00',
                                'keyId': 0}},
 'cursor': {'firstBatch': [{'count': 174899,
                            'host': 'devbox:29018',
                            'localTime': datetime.datetime(2018, 6, 9, 14, 20, 35, 202000),
                            'ns': 'foo.bar',
                            'shard': 'shard0001'},
                           {'count': 125101,
                            'host': 'devbox:29017',
                            'localTime': datetime.datetime(2018, 6, 9, 14, 20, 35, 202000),
                            'ns': 'foo.bar',
                            'shard': 'shard0000'}],
            'id': 0,
            'ns': 'foo.bar'},
 'ok': 1.0,
 'operationTime': Timestamp(1528554032, 1)}

      Standalones and replica sets never return a 'result' field:

      >>> pprint.pprint(c.foo.command('aggregate', 'bar', pipeline=[{"$collStats": {"count": {}}}], cursor={}, check=False))
{'cursor': {'firstBatch': [{'count': 1,
                            'host': 'devbox:57017',
                            'localTime': datetime.datetime(2018, 6, 9, 14, 21, 0, 164000),
                            'ns': 'foo.bar'}],
            'id': 0,
            'ns': 'foo.bar'},
 'ok': 1.0}
>>> pprint.pprint(c.bar.command('aggregate', 'bar', pipeline=[{"$collStats": {"count": {}}}], cursor={}, check=False))
{'code': 40481,
 'codeName': 'Location40481',
 'errmsg': 'Unable to retrieve count in $collStats stage: Database [bar] not '
           'found.',
 'ok': 0.0}

      I have no idea why mongos returns a result field when the namespace doesn't exist, but a result field would be really useful if it returned the sum of the counts from all shards. That would avoid application or driver developers having to $group and $sum the results from each shard. If we were to do that, mongod should also return a result field for consistency between topology types.

            Assignee:
            Sam Mercier
            Reporter:
            Bernie Hackett
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              Created:
              Updated:
              Resolved: