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Queries that require a blocking sort always fetch the full document from the collection to retrieve the sort key, even if the query plan includes an index scan over an index that has the sort key as a part of its key pattern. These queries should be covered instead.
To reproduce:
> db.foo.drop()
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true
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> db.foo.ensureIndex({a: 1, b: 1})
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> db.foo.find({a: {$gt: 0}}, {a: 1, b: 1, _id: 0}).sort({b: 1}).explain(true).stats
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{
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"stage" : "PROJECTION",
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"nReturned" : 0,
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"executionTimeMillis" : 2,
|
"works" : 4,
|
"advanced" : 0,
|
"needTime" : 0,
|
"isEOF" : 1,
|
"invalidates" : 0,
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"transformBy" : {
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"_id" : 0,
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"a" : 1,
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"b" : 1
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},
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"inputStage" : {
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"stage" : "SORT",
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"nReturned" : 0,
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"executionTimeMillis" : 2,
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"works" : 4,
|
"advanced" : 0,
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"needTime" : 2,
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"isEOF" : 1,
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"invalidates" : 0,
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"sortPattern" : {
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"b" : 1
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},
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"memUsage" : 0,
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"memLimit" : 33554432,
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"inputStage" : {
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"stage" : "KEEP_MUTATIONS",
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"nReturned" : 0,
|
"executionTimeMillis" : 0,
|
"works" : 2,
|
"advanced" : 0,
|
"needTime" : 1,
|
"isEOF" : 1,
|
"invalidates" : 0,
|
"inputStage" : {
|
"stage" : "FETCH",
|
"nReturned" : 0,
|
"executionTimeMillis" : 0,
|
"works" : 2,
|
"advanced" : 0,
|
"needTime" : 1,
|
"isEOF" : 1,
|
"invalidates" : 0,
|
"docsExamined" : 0,
|
"alreadyHasObj" : 0,
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"inputStage" : {
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"stage" : "IXSCAN",
|
"nReturned" : 0,
|
"executionTimeMillis" : 0,
|
"works" : 1,
|
"advanced" : 0,
|
"needTime" : 1,
|
"isEOF" : 1,
|
"invalidates" : 0,
|
"keysExamined" : 0,
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"keyPattern" : {
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"a" : 1,
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"b" : 1
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},
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"isMultiKey" : false,
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"indexBounds" : "field #0['a']: (0.0, inf.0], field #1['b']: [MinKey, MaxKey]",
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"dupsTested" : 0,
|
"dupsDropped" : 0,
|
"seenInvalidated" : 0,
|
"matchTested" : 0
|
}
|
}
|
}
|
}
|
}
|
|