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Type:
Bug
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Resolution: Unresolved
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Priority:
Major - P3
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None
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Affects Version/s: None
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Component/s: Query Execution
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None
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Query Optimization
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ALL
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QO 2022-09-19, QO 2023-08-07, QO 2023-08-21
As of DRIVERS-501 the driver CRUD specification defines a pipeline to be used to count documents without resorting to the count command (due to potential inaccurate counts without query predicates).
db.foo.drop();
db.foo.insertMany([{},{},{},{},{}]);
// Driver CRUD spec definition for countDocuments ////////////////////
filter = {}
skip = null
limit = null
pipeline = [{'$match': filter}]
if (skip) {
pipeline.push({'$skip': skip})
}
if (limit) {
pipeline.push({'$limit': limit})
}
pipeline.push({'$group': {'_id': 1, 'n': {'$sum': 1}}})
//////////////////////////////////////////////////////////////////////
db.foo.explain("executionStats").aggregate(pipeline)
Without specifying a filter the empty $match stage would result in a COLLSCAN plan winning, though a COUNT_SCAN should be appropriate.
/* "nReturned": 5, "executionTimeMillis": 0, "totalKeysExamined": 0, "totalDocsExamined": 5, "executionStages": { "stage": "COLLSCAN", */
This could be worked around by unshifting a {$sort: { _id: 1 }} stage to the pipeline, however after discussing with david.storch@mongodb.com this may be addressable server-side.
- causes
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- Closed
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- depends on
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SERVER-23406 index scan is slower than full collection scan in some scenarios
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- Closed
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- is related to
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- Backlog
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- Needs Scheduling
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