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Type:
Bug
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Resolution: Unresolved
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Priority:
Critical - P2
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None
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Affects Version/s: None
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Component/s: Querying
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Query Execution
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ALL
A fun fact about our query language which I recently learned is that {$ne: null} doesn't necessarily return a subset of what {$exists: true} returns:
db.c.insert({a: [1, {c: 1}]});
db.c.insert({a: [1]});
db.c.find({"a.b": {$exists: true}}) // Returns nothing.
db.c.find({"a.b": {$ne: null}}) // Returns {a: [1]} !!!!
When we create a partial index using a filter {$exists: true}, we do not respect these semantics for {$ne: null}:
db.d.drop();
db.d.createIndex(
{ "a.b": 1 },
{ partialFilterExpression: { "a.b": { $exists: true } } }
);
db.d.insert({a: [1]});
const query = {"a.b": {$ne: null}};
print("Running the query " + tojson(query) + " with a partial index present");
print("Results are " + tojson(db.d.find(query).toArray())); // (1)
print("Dropping the index");
db.d.dropIndexes();
print("Results from the same query are now " + tojson(db.d.find(query).toArray())); // (2)
The results from (1) will be different than those from (2).
I think the bug is here. This code basically says "{$ne: null} is a subset of {$exists true}".
- related to
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SERVER-36681 Change {$ne: null} semantics to be more intuitive
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- Backlog
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