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Type:
Bug
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Resolution: Duplicate
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Priority:
Major - P3
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None
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Affects Version/s: None
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Component/s: None
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None
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Query Execution
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ALL
Consider the collection
db.test.drop();
db.test.insertMany([{a: 1}, {a: null}, {}]);
In classic engine find on {a: {$gte: MinKey}} will return all documents,
but if we force SBE (or use $match - $group pipeline in default configuration), it will not return an empty document:
rs0 [direct: primary] test> db.test.find({a: {$gte: MinKey}}) // classic
[
{ _id: ObjectId('67164434cb760047b0d31db1'), a: 1 },
{ _id: ObjectId('67164434cb760047b0d31db2'), a: null },
{ _id: ObjectId('67164434cb760047b0d31db3') }
]
rs0 [direct: primary] test> db.adminCommand({setParameter: 1, internalQueryFrameworkControl: "trySbeEngine"})
rs0 [direct: primary] test> db.test.find({ a: { $gte: MinKey } }) // sbe
[
{ _id: ObjectId('67164434cb760047b0d31db1'), a: 1 },
{ _id: ObjectId('67164434cb760047b0d31db2'), a: null }
]
rs0 [direct: primary] test> db.test.createIndex({a: 1})
a_1
rs0 [direct: primary] test> db.test.find({ a: { $gte: MinKey } }). // sbe with index
[
{ _id: ObjectId('67164434cb760047b0d31db2'), a: null },
{ _id: ObjectId('67164434cb760047b0d31db3') },
{ _id: ObjectId('67164434cb760047b0d31db1'), a: 1 }
]
The empty document is skipped in collection scan because {$gte: MinKey} is translated into exists() in stage builders:
filter {traverseF(s1, lambda(l2.0) { exists(move(l2.0)) }, true)}
and in empty document field "a" doesn't exist.
But in case of index scan the predicate is absorbed into index bounds and works as expected.
This predicate itself is pretty strange and only comes around as work-around for some issues to make sure the index is covering the query. Example: SERVER-12769
- duplicates
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- Needs Scheduling
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