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Type:
Improvement
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Resolution: Done
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Priority:
Major - P3
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Affects Version/s: None
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Component/s: None
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Query Optimization
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Inequalities always appear to be estimated as if all documents match.
To reproduce:
db.foo.drop(); docs = []; for (let i = 1; i <= 100000; i++) { docs.push({a:i % 2}); } db.foo.insertMany(docs); db.foo.createIndex({a:1}); db.foo.find({"a":{"$ne":1}}).count(); db.foo.find({"a":{"$ne":1}}).explain().queryPlanner.winningPlan.cardinalityEstimate;
returns:
Enterprise test> db.foo.find({"a":{"$ne":1}}).count();
50000
Enterprise test> db.foo.find({"a":{"$ne":1}}).explain().queryPlanner.winningPlan.cardinalityEstimate;
100000