samplingCE: cardinalityEstimate is incorrect for $ne

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    • Type: Improvement
    • Resolution: Unresolved
    • Priority: Blocker - P1
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    • Affects Version/s: None
    • Component/s: None
    • Query Optimization
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      samplingCE returns the correct numKeysEstimate for $ne but the wrong cardinalityEstimate. So the top-level FETCH has an almost-zero cost, so a bad plan may win.

      To reproduce:

      db.foo.drop();
db.adminCommand({setParameter: 1, planRankerMode: "samplingCE"});
db.foo.createIndex({a:1});
db.foo.insert({a: "abc"});
# This returns 1 row
db.foo.aggregate([{ '$match': { 'a': { '$ne': null }}}]);
# This returns cardinalityEstimate = 0
db.foo.aggregate([{ '$match': { 'a': { '$ne': null }}}]).explain().queryPlanner.winningPlan;
...
    cardinalityEstimate: 0,
    numKeysEstimate: 1,
...

      This is not an off-by-one issue , it is observed with various dataset sizes and selectivites. Contrast with histogramCE:

          cardinalityEstimate: 1,
    numKeysEstimate: 1,

      git revision 068f5a66c02dc39afe1958556ae5883b34d29f41

            Assignee:
            Kartal Kaan Bozdogan
            Reporter:
            Philip Stoev
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              Created:
              Updated: