if inscribe
This is the data I showed with explain.

> db.users.explain("allPlansExecution").find({username:'user101'})
 {
 "queryPlanner" : {
 "plannerVersion" : 1,
 "namespace" : "test.users",
 "indexFilterSet" : false,
 "parsedQuery" : {
 "username" : {
 "$eq" : "user101"
 }
 },
 "winningPlan" : {
 "stage" : "FETCH",
 "inputStage" : {
 "stage" : "IXSCAN",
 "keyPattern" : {
 "username" : 1
 },
 "indexName" : "username_1",
 "isMultiKey" : false,
 "direction" : "forward",
 "indexBounds" : {
 "username" : [
 "[\"user101\", \"user101\"]"
 ]
 }
 }
 },
 "rejectedPlans" : [ ]
 },
 "executionStats" : {
 "executionSuccess" : true,
 "nReturned" : 1,
 "executionTimeMillis" : 0,
 "totalKeysExamined" : 1,
 "totalDocsExamined" : 1,
 "executionStages" : {
 "stage" : "FETCH",
 "nReturned" : 1,
 "executionTimeMillisEstimate" : 0,
 "works" : 2,
 "advanced" : 1,
 "needTime" : 0,
 "needFetch" : 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,
 "needFetch" : 0,
 "saveState" : 0,
 "restoreState" : 0,
 "isEOF" : 1,
 "invalidates" : 0,
 "keyPattern" : {
 "username" : 1
 },
 "indexName" : "username_1",
 "isMultiKey" : false,
 "direction" : "forward",
 "indexBounds" : {
 "username" : [
 "[\"user101\", \"user101\"]"
 ]
 },
 "keysExamined" : 1,
 "dupsTested" : 0,
 "dupsDropped" : 0,
 "seenInvalidated" : 0,
 "matchTested" : 0
 }
 },
 "allPlansExecution" : [ ]
 },
 "serverInfo" : {
 "host" : "mongo2",
 "port" : 27017,
 "version" : "3.0.0",
 "gitVersion" : "a841fd6394365954886924a35076691b4d149168"
 },
 "ok" : 1
 }