Why does this explain indicate an index was not used? - mongodb

Question

Given the following query:
var exp = db.messages.explain('executionStats')
exp.find( { 'headers.Date' : { '$gt' : new Date(2001,3,1) } }, { 'headers.From' : 1, '_id' : 0 } ).sort( { 'headers.From' : 1 } )
That produces the following output:
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "enron.messages",
"indexFilterSet" : false,
"parsedQuery" : {
"headers.Date" : {
"$gt" : ISODate("2001-04-01T05:00:00Z")
}
},
"winningPlan" : {
"stage" : "PROJECTION",
"transformBy" : {
"headers.From" : 1,
"_id" : 0
},
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"headers.Date" : {
"$gt" : ISODate("2001-04-01T05:00:00Z")
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"headers.From" : 1
},
"indexName" : "headers.From_1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"headers.From" : [
"[MinKey, MaxKey]"
]
}
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 83057,
"executionTimeMillis" : 726,
"totalKeysExamined" : 120477,
"totalDocsExamined" : 120477,
"executionStages" : {
"stage" : "PROJECTION",
"nReturned" : 83057,
"executionTimeMillisEstimate" : 690,
"works" : 120478,
"advanced" : 83057,
"needTime" : 37420,
"needFetch" : 0,
"saveState" : 941,
"restoreState" : 941,
"isEOF" : 1,
"invalidates" : 0,
"transformBy" : {
"headers.From" : 1,
"_id" : 0
},
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"headers.Date" : {
"$gt" : ISODate("2001-04-01T05:00:00Z")
}
},
"nReturned" : 83057,
"executionTimeMillisEstimate" : 350,
"works" : 120478,
"advanced" : 83057,
"needTime" : 37420,
"needFetch" : 0,
"saveState" : 941,
"restoreState" : 941,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 120477,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 120477,
"executionTimeMillisEstimate" : 60,
"works" : 120477,
"advanced" : 120477,
"needTime" : 0,
"needFetch" : 0,
"saveState" : 941,
"restoreState" : 941,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"headers.From" : 1
},
"indexName" : "headers.From_1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"headers.From" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 120477,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0,
"matchTested" : 0
}
}
}
},
"serverInfo" : {
"host" : "dpercy-mac-air.local",
"port" : 27017,
"version" : "3.0.1",
"gitVersion" : "534b5a3f9d10f00cd27737fbcd951032248b5952"
},
"ok" : 1
}
What in the explain says that the query did NOT use an index? From my understanding the following section implies that an index was used because the winning plan has an IXSCAN stage in it:
"winningPlan" : {
"stage" : "PROJECTION",
"transformBy" : {
"headers.From" : 1,
"_id" : 0
},
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"headers.Date" : {
"$gt" : ISODate("2001-04-01T05:00:00Z")
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"headers.From" : 1
},
"indexName" : "headers.From_1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"headers.From" : [
"[MinKey, MaxKey]"
]
}
}
}
But I'm being told that this query did not use an index. Why is that?