Details
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Bug
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Resolution: Fixed
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Major - P3
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None
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Fully Compatible
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ALL
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Query 2020-11-30, Query 2020-12-14
Description
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)){'$clusterTime': {'clusterTime': Timestamp(1528553991, 1),
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'signature': {'hash': b'\x00\x00\x00\x00\x00\x00\x00\x00'
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b'\x00\x00\x00\x00\x00\x00\x00\x00'
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b'\x00\x00\x00\x00',
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'keyId': 0}},
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'cursor': {'firstBatch': [], 'id': 0, 'ns': 'bar.bar'},
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'ok': 1.0,
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'operationTime': Timestamp(1528553991, 1),
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'result': []}
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When the namespace does exist, it returns a document for each shard, but no 'result' array:
>>> pprint.pprint(c.foo.command('aggregate', 'bar', pipeline=[{"$collStats": {"count": {}}}], cursor={}, check=False)){'$clusterTime': {'clusterTime': Timestamp(1528554032, 1),
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'signature': {'hash': b'\x00\x00\x00\x00\x00\x00\x00\x00'
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b'\x00\x00\x00\x00\x00\x00\x00\x00'
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b'\x00\x00\x00\x00',
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'keyId': 0}},
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'cursor': {'firstBatch': [{'count': 174899,
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'host': 'devbox:29018',
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'localTime': datetime.datetime(2018, 6, 9, 14, 20, 35, 202000),
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'ns': 'foo.bar',
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'shard': 'shard0001'},
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{'count': 125101,
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'host': 'devbox:29017',
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'localTime': datetime.datetime(2018, 6, 9, 14, 20, 35, 202000),
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'ns': 'foo.bar',
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'shard': 'shard0000'}],
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'id': 0,
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'ns': 'foo.bar'},
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'ok': 1.0,
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'operationTime': Timestamp(1528554032, 1)}
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Standalones and replica sets never return a 'result' field:
>>> pprint.pprint(c.foo.command('aggregate', 'bar', pipeline=[{"$collStats": {"count": {}}}], cursor={}, check=False))
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{'cursor': {'firstBatch': [{'count': 1,
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'host': 'devbox:57017',
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'localTime': datetime.datetime(2018, 6, 9, 14, 21, 0, 164000),
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'ns': 'foo.bar'}],
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'id': 0,
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'ns': 'foo.bar'},
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'ok': 1.0}
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>>> pprint.pprint(c.bar.command('aggregate', 'bar', pipeline=[{"$collStats": {"count": {}}}], cursor={}, check=False))
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{'code': 40481,
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'codeName': 'Location40481',
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'errmsg': 'Unable to retrieve count in $collStats stage: Database [bar] not '
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'found.',
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'ok': 0.0}
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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.
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