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Type:
Task
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Resolution: Fixed
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Priority:
Major - P3
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Affects Version/s: None
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Labels:None
Description
Marking this for documentation changes since I noticed we document that we only use indexes for equality predicates in the 'let' parameter documentation for $lookup:
https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#lookup-join-let
After this ticket, any comparison inside $expr is eligible for index use, if the right index exists: $eq, $lt, $lte, $gt, $gte
Description of Linked Ticket
When aggregation operators such as $gt/$gte/$lt/$lte are used within $expr, we will not rewrite to match expression. This is because they would not be eligible to use indexes due to type bracketing differences between match and aggregation. We should consider adding regardless, as the rewrite would make these expressions eligible for aggregation match optimization, such as repositioning a comparison expression in the pipeline.
The following is an example of where this would be useful. Ideally we would rewrite the {$expr": {"$gt" : [ "$x",
{"$const": 5}]}} and move before the $lookup stage as an optimization.
db.foo.explain().aggregate({$lookup:{as:"a",from:"b",localField:"x",foreignField:"y"}},{$unwind:"$a"},{$match:{"a.z":10,$expr:{$gt:["$x",5]}}})
"stages" : [
{
"$cursor" : {
"query" : {
},
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "tpcds10.foo",
"indexFilterSet" : false,
"parsedQuery" : {
},
"winningPlan" : {
"stage" : "EOF"
},
"rejectedPlans" : [ ]
}
}
},
{
"$lookup" : {
"from" : "b",
"as" : "a",
"localField" : "x",
"foreignField" : "y"
}
},
{
"$match" : {
"$and" : [
{
"a.z" : {
"$eq" : 10
}
},
{
"$expr" : {
"$gt" : [
"$x",
{
"$const" : 5
}
]
}
}
]
}
}
],
"ok" : 1
}
Scope of changes
Impact to Other Docs
MVP (Work and Date)
Resources (Scope or Design Docs, Invision, etc.)
- documents
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SERVER-39943 Create match expressions for aggregation $gt/$gte/$lt/$lte and add to $expr rewrite
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- Closed
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