$group can still use a distinct scan on a sparse index if an alternative index is available

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    • Type: Bug
    • Resolution: Fixed
    • Priority: Major - P3
    • 8.3.0-rc0, 8.2.2
    • Affects Version/s: None
    • Component/s: None
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    • Query Optimization
    • Fully Compatible
    • ALL
    • v8.2, v8.0, v7.0
    • Hide 
      db.test.drop()
db.test.insert({b: 5})
db.test.createIndex({a: 1}, {sparse: true})
db.test.createIndex({a: 1, b: 1})
db.test.explain().aggregate({$group: {_id: "$a"}})
// shows plan using distinct scan over sparse index, no rejected plans
      Show
      db.test.drop() db.test.insert({b: 5}) db.test.createIndex({a: 1}, {sparse: true }) db.test.createIndex({a: 1, b: 1}) db.test.explain().aggregate({$group: {_id: "$a" }}) // shows plan using distinct scan over sparse index, no rejected plans
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      It seems there is a case that SERVER-110423 did not fix. If there is an alternative index available then we can still use the sparse index. Note that if the compound index was not there then we would correctly use a collscan.

            Assignee:
            Militsa Sotirova
            Reporter:
            Militsa Sotirova
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            Watchers:
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              Created:
              Updated:
              Resolved: