// Set up collections for a $graphLookup. > db.foo.drop() true > db.foo.insert({_id: 1}) WriteResult({ "nInserted" : 1 }) > db.bar.drop() true > db.bar.insert([{vertexId: 0, nextVertexId: 1}, {vertexId: 1, nextVertexId: 2}, {vertexId: 2}]) BulkWriteResult({ "writeErrors" : [ ], "writeConcernErrors" : [ ], "nInserted" : 3, "nUpserted" : 0, "nMatched" : 0, "nModified" : 0, "nRemoved" : 0, "upserted" : [ ] }) // Create two indexes so that a plan will be cached. The cached plan is the only evidence that $graphLookup is using an index, and if there is only one plan available, it will not be cached. > db.bar.ensureIndex({vertexId: 1}) { "createdCollectionAutomatically" : false , "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } > db.bar.ensureIndex({vertexId: -1, _id: 1}) { "createdCollectionAutomatically" : false , "numIndexesBefore" : 2, "numIndexesAfter" : 3, "ok" : 1 } // There are no plans for this query yet, we will compare the cache after the $graphLookup for proof that it used an index. > db.runCommand({planCacheListPlans: 'bar' , query: {$and: [{vertexId: {$ in : [1]}}]}}) { "plans" : [ ], "ok" : 1 } // There are no index accesses yet. > db.bar.aggregate([{$indexStats: {}}, {$project: {key: 0, host: 0}}]).table() ╔═══════════════════╤═════════════════════════════════════════════════════════════════╗ ║ name │ accesses ║ ║ │ ┌───────────────┬──────────────────────────────────────╢ ║ │ │ accesses.ops │ accesses.since ║ ╠═══════════════════╪══════════╪═══════════════╪══════════════════════════════════════╣ ║ vertexId_-1__id_1 │ │ NumberLong(0) │ Thu Dec 01 2016 12:44:11 GMT-0500 (E ║ ║ │ │ │ ST) ║ ╟───────────────────┼──────────┼───────────────┼──────────────────────────────────────╢ ║ _id_ │ │ NumberLong(0) │ Thu Dec 01 2016 12:43:55 GMT-0500 (E ║ ║ │ │ │ ST) ║ ╟───────────────────┼──────────┼───────────────┼──────────────────────────────────────╢ ║ vertexId_1 │ │ NumberLong(0) │ Thu Dec 01 2016 12:44:03 GMT-0500 (E ║ ║ │ │ │ ST) ║ ╚═══════════════════╧══════════╧═══════════════╧══════════════════════════════════════╝ // Do the $graphLookup. > db.foo.aggregate([{$graphLookup: {from: "bar" , startWith: "$_id" , connectFromField: "nextVertexId" , connectToField: "vertexId" , as: "results" }}]) { "_id" : 1, "results" : [ { "_id" : ObjectId( "5840615bca13c1414d96ceb3" ), "vertexId" : 2 }, { "_id" : ObjectId( "5840615bca13c1414d96ceb2" ), "vertexId" : 1, "nextVertexId" : 2 } ] } // Still no index accesses! > db.bar.aggregate([{$indexStats: {}}, {$project: {key: 0, host: 0}}]).table() ╔═══════════════════╤═════════════════════════════════════════════════════════════════╗ ║ name │ accesses ║ ║ │ ┌───────────────┬──────────────────────────────────────╢ ║ │ │ accesses.ops │ accesses.since ║ ╠═══════════════════╪══════════╪═══════════════╪══════════════════════════════════════╣ ║ vertexId_-1__id_1 │ │ NumberLong(0) │ Thu Dec 01 2016 12:44:11 GMT-0500 (E ║ ║ │ │ │ ST) ║ ╟───────────────────┼──────────┼───────────────┼──────────────────────────────────────╢ ║ _id_ │ │ NumberLong(0) │ Thu Dec 01 2016 12:43:55 GMT-0500 (E ║ ║ │ │ │ ST) ║ ╟───────────────────┼──────────┼───────────────┼──────────────────────────────────────╢ ║ vertexId_1 │ │ NumberLong(0) │ Thu Dec 01 2016 12:44:03 GMT-0500 (E ║ ║ │ │ │ ST) ║ ╚═══════════════════╧══════════╧═══════════════╧══════════════════════════════════════╝ // But it did use the index! > db.runCommand({planCacheListPlans: 'bar' , query: {$and: [{vertexId: {$ in : [1]}}]}}) { "plans" : [ { "details" : { "solution" : "(index-tagged expression tree: tree=Leaf { vertexId: 1.0 }, pos: 0, can combine? 1\n)" }, "reason" : { "score" : 1.5003000000000002, "stats" : { "stage" : "FETCH" , "nReturned" : 1, "executionTimeMillisEstimate" : 0, "works" : 2, "advanced" : 1, "needTime" : 0, "needYield" : 0, "saveState" : 0, "restoreState" : 0, "isEOF" : 1, "invalidates" : 0, "docsExamined" : 1, "alreadyHasObj" : 0, "inputStage" : { "stage" : "IXSCAN" , "nReturned" : 1, "executionTimeMillisEstimate" : 0, "works" : 2, "advanced" : 1, "needTime" : 0, "needYield" : 0, "saveState" : 0, "restoreState" : 0, "isEOF" : 1, "invalidates" : 0, "keyPattern" : { "vertexId" : 1 }, "indexName" : "vertexId_1" , "isMultiKey" : false , "isUnique" : false , "isSparse" : false , "isPartial" : false , "indexVersion" : 2, "direction" : "forward" , "indexBounds" : { "vertexId" : [ "[1.0, 1.0]" ] }, "keysExamined" : 1, "seeks" : 1, "dupsTested" : 0, "dupsDropped" : 0, "seenInvalidated" : 0 } } }, "feedback" : { "nfeedback" : 1, "scores" : [ { "score" : 1.5003 } ] }, "filterSet" : false }, { "details" : { "solution" : "(index-tagged expression tree: tree=Leaf { vertexId: -1.0, _id: 1.0 }, pos: 0, can combine? 1\n)" }, "reason" : { "score" : 1.5003000000000002, "stats" : { "stage" : "FETCH" , "nReturned" : 1, "executionTimeMillisEstimate" : 0, "works" : 2, "advanced" : 1, "needTime" : 0, "needYield" : 0, "saveState" : 0, "restoreState" : 0, "isEOF" : 1, "invalidates" : 0, "docsExamined" : 1, "alreadyHasObj" : 0, "inputStage" : { "stage" : "IXSCAN" , "nReturned" : 1, "executionTimeMillisEstimate" : 0, "works" : 2, "advanced" : 1, "needTime" : 0, "needYield" : 0, "saveState" : 0, "restoreState" : 0, "isEOF" : 1, "invalidates" : 0, "keyPattern" : { "vertexId" : -1, "_id" : 1 }, "indexName" : "vertexId_-1__id_1" , "isMultiKey" : false , "isUnique" : false , "isSparse" : false , "isPartial" : false , "indexVersion" : 2, "direction" : "forward" , "indexBounds" : { "vertexId" : [ "[1.0, 1.0]" ], "_id" : [ "[MinKey, MaxKey]" ] }, "keysExamined" : 1, "seeks" : 1, "dupsTested" : 0, "dupsDropped" : 0, "seenInvalidated" : 0 } } }, "feedback" : { }, "filterSet" : false } ], "ok" : 1 }