Details
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Bug
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Status: Closed
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Major - P3
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Resolution: Fixed
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None
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Fully Compatible
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ALL
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v5.2, v5.1, v5.0, v4.4, v4.2, v4.0, v3.6
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Query Optimization 2021-06-28, QO 2021-09-20, QO 2021-10-04, QO 2021-10-18, QO 2021-11-01, QO 2021-11-15, QO 2021-11-29
Description
When indexing an array element by position (ex: {"a.0"}) if the value at the indexed position is an array, the index won't be multikey and can effect the result of a query if the index is selected as the winning plan.
db.version()
|
// => 4.4.6
|
db.foo.drop();
|
db.foo.insert(
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{ a:
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[
|
[ "b" ], |
[ "c" ] |
]
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});
|
|
|
print(db.foo.count({ "a.0": { $elemMatch: { $in: ["b"] } } })); |
// => 1
|
|
|
/*
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"inputStage" : {
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"stage" : "COLLSCAN",
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"filter" : {
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"a.0" : {
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"$elemMatch" : {
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"$eq" : "b"
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}
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}
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},
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"nReturned" : 1.0,
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"executionTimeMillisEstimate" : 0.0,
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"works" : 3.0,
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"advanced" : 1.0,
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"needTime" : 1.0,
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"needYield" : 0.0,
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"saveState" : 0.0,
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"restoreState" : 0.0,
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"isEOF" : 1.0,
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"direction" : "forward",
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"docsExamined" : 1.0
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}
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*/ |
Now if the following index is created and the operation run again the results are different:
db.foo.createIndex({ "a.0": 1 }) |
print(db.foo.count({ "a.0": { $elemMatch: { $in: ["b"] } } })); |
// => 0
|
|
|
/*
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"inputStage" : {
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"stage" : "FETCH",
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"filter" : {
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"a.0" : {
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"$elemMatch" : {
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"$eq" : "b"
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}
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}
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},
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"inputStage" : {
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"stage" : "IXSCAN",
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"keyPattern" : {
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"a.0" : 1.0
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},
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"indexName" : "a.0_1",
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"isMultiKey" : false,
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"multiKeyPaths" : {
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"a.0" : [
|
|
|
]
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},
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"isUnique" : false,
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"isSparse" : false,
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"isPartial" : false,
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"indexVersion" : 2.0,
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"direction" : "forward",
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"indexBounds" : {
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"a.0" : [
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"[\"b\", \"b\"]"
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]
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}
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}
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}
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*/ |