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Type:
Bug
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Resolution: Fixed
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Priority:
Major - P3
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Affects Version/s: None
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Component/s: Querying
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Labels:
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Fully Compatible
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ALL
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Query 2020-11-30, Query 2020-12-14
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.
- related to
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- Closed
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