Description
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]}}})
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"stages" : [
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{
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"$cursor" : {
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"query" : {
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},
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"queryPlanner" : {
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"plannerVersion" : 1,
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"namespace" : "tpcds10.foo",
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"indexFilterSet" : false,
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"parsedQuery" : {
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},
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"winningPlan" : {
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"stage" : "EOF"
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},
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"rejectedPlans" : [ ]
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}
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}
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},
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{
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"$lookup" : {
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"from" : "b",
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"as" : "a",
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"localField" : "x",
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"foreignField" : "y"
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}
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},
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{
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"$match" : {
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"$and" : [
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{
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"a.z" : {
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"$eq" : 10
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}
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},
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{
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"$expr" : {
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"$gt" : [
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"$x",
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{
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"$const" : 5
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}
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]
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}
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}
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]
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}
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}
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],
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"ok" : 1
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}
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Scope of changes
Impact to Other Docs
MVP (Work and Date)
Resources (Scope or Design Docs, Invision, etc.)
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