Details
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Improvement
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Status: Backlog
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Major - P3
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Resolution: Unresolved
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2.6.0
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None
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Query Optimization
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Fully Compatible
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Query 2017-03-27, Query 2017-04-17, QO 2022-10-03
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(copied to CRM)
Description
Even though the user explicitly tells us to include specific fields, we can see that they are not actually being used, so doesn't it make sense to optimize $project away the same as if it wasn't there?
db.t1.aggregate([ {$match:{username:/^user8/}}, {$project:{username:1}},{$group:{_id:1,count:{$sum:1}}}],{explain:true})
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{
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"stages" : [
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{
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"$cursor" : {
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"query" : {
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"username" : /^user8/
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},
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"fields" : {
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"username" : 1,
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"_id" : 1
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},
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"plan" : {
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"cursor" : "BtreeCursor username_1",
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"isMultiKey" : false,
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"scanAndOrder" : false,
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"indexBounds" : {
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"username" : [
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[
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"user8",
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"user9"
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],
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[
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/^user8/,
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/^user8/
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]
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]
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},
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"allPlans" : [
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{
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"cursor" : "BtreeCursor username_1",
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"isMultiKey" : false,
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"scanAndOrder" : false,
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"indexBounds" : {
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"username" : [
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[
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"user8",
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"user9"
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],
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[
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/^user8/,
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/^user8/
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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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{
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"$project" : {
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"username" : true
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}
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},
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{
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"$group" : {
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"_id" : {
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"$const" : 1
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},
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"count" : {
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"$sum" : {
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"$const" : 1
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}
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}
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}
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}
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]
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Without the (needless) $project
db.t1.aggregate([ {$match:{username:/^user8/}}, {$group:{_id:1,count:{$sum:1}}}],{explain:true})
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{
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"stages" : [
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{
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"$cursor" : {
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"query" : {
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"username" : /^user8/
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},
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"fields" : {
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"_id" : 0,
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"$noFieldsNeeded" : 1
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},
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"plan" : {
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"cursor" : "BtreeCursor username_1",
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"isMultiKey" : false,
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"scanAndOrder" : false,
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"indexBounds" : {
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"username" : [
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[
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"user8",
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"user9"
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],
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[
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/^user8/,
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/^user8/
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]
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]
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},
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"allPlans" : [
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{
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"cursor" : "BtreeCursor username_1",
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"isMultiKey" : false,
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"scanAndOrder" : false,
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"indexBounds" : {
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"username" : [
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[
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"user8",
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"user9"
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],
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[
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/^user8/,
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/^user8/
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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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{
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"$group" : {
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"_id" : {
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"$const" : 1
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},
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"count" : {
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"$sum" : {
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"$const" : 1
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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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Attachments
Issue Links
- is duplicated by
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- Closed
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- Closed
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- related to
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- Backlog
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- Backlog
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- Closed
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