samplingCE overestimates cardinality for $elemMatch

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    • Type: Bug
    • Resolution: Unresolved
    • Priority: Major - P3
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    • Affects Version/s: None
    • Component/s: None
    • Query Optimization
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      db.adminCommand({ setParameter: 1, internalQueryPlanRanker: "costBased"})
db.a.drop()
db.a.createIndex({"a.b":1, "a.c": 1})
db.a.insertOne({a: [{b: 0, c: 1}, {b:1, c: 0}]})
db.a.explain("executionStats").find({a: {$elemMatch: {b:1, c: 1}}}).finish().executionStats.executionStages.inputStage

      Gives:

        cardinalityEstimate: 1,
  estimatesMetadata: { ceSource: 'Code' },
  numKeysEstimate: 1,
  nReturned: 0,
  keysExamined: 0,
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      db.adminCommand({ setParameter: 1, internalQueryPlanRanker: "costBased" }) db.a.drop() db.a.createIndex({ "a.b" :1, "a.c" : 1}) db.a.insertOne({a: [{b: 0, c: 1}, {b:1, c: 0}]}) db.a.explain( "executionStats" ).find({a: {$elemMatch: {b:1, c: 1}}}).finish().executionStats.executionStages.inputStage Gives: cardinalityEstimate: 1, estimatesMetadata: { ceSource: 'Code' }, numKeysEstimate: 1, nReturned: 0, keysExamined: 0,
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      SERVER-98371 introduced the MultiKeyDottedPathIterator, which iterates the nested fields in the shape of a multikey index independently of one another, losing the coherence required for $elemMatch.

      Found by 6.1 Sol.

            Assignee:
            Unassigned
            Reporter:
            Kartal Kaan Bozdogan
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              Created:
              Updated: