Details
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Type:
Improvement
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Status: Open
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Priority:
Major - P3
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Resolution: Unresolved
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Affects Version/s: 3.3.9
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Fix Version/s: Backlog
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Component/s: Aggregation Framework
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Labels:
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Sprint:Query 2017-03-27, Query 2017-04-17
Description
Aggregation pipeline sends down to query fields to return which are generated later in the pipeline (as result of "as" in $lookup).
Reproduce:
db.companies.explain().aggregate({$match:{_id:"14"}},{$graphLookup:{from:"companies",startWith:"$_id",connectFromField:"_id",connectToField:"parent",as:"tree",maxDepth:5, depthField:"order"}},{$group:{"_id":"$tree.country"}})
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{
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"waitedMS" : NumberLong(0),
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"stages" : [
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{
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"$cursor" : {
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"query" : {
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"_id" : "14"
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},
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"fields" : {
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"tree.country" : 1,
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"_id" : 1
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},
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"queryPlanner" : {
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"plannerVersion" : 1,
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"namespace" : "d.companies",
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"indexFilterSet" : false,
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"parsedQuery" : {
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"_id" : {
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"$eq" : "14"
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}
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},
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"winningPlan" : {
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"stage" : "PROJECTION",
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"transformBy" : {
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"tree.country" : 1,
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"_id" : 1
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},
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"inputStage" : {
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"stage" : "IDHACK"
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}
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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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"$graphLookup" : {
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"from" : "companies",
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"as" : "tree",
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"connectToField" : "parent",
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"connectFromField" : "_id",
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"startWith" : "$_id",
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"depthField" : "order",
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"maxDepth" : NumberLong(5)
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}
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},
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{
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"$group" : {
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"_id" : "$tree.country"
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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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Note "tree.country" which is coming from the lookup and not original document.
It appears that if project or group needs a field after lookup, there is no filtering out of fields that start with string passed as "as:" to $lookup and it ends up being pushed down into the query.