为什么indexOnly == false

Question

我有一个索引的集合：

{
    "UserId" : 1,
    "ShareId" : 1,
    "ParentId" : 1,
    "DeletedDate" : 1
}

如果我查询：

db.Files.find({ "UserId" : ObjectId("5450d837f32a1e098c844e2a"),
    "ShareId" : ObjectId("5450d879f32a1e098c844e94"),
    "ParentId" : ObjectId("5450d8af6a092a0b74a44026"),
    "DeletedDate":null},
    {_id:0, ShareId:1}).explain()   

输出表示"indexOnly" : false:

{
"cursor" : "BtreeCursor UserId_1_ShareId_1_ParentId_1_DeletedDate_1",
"isMultiKey" : false,
"n" : 2120,
"nscannedObjects" : 2120,
"nscanned" : 2120,
"nscannedObjectsAllPlans" : 2318,
"nscannedAllPlans" : 2320,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 21,
"nChunkSkips" : 0,
"millis" : 42,
"indexBounds" : {
    "UserId" : [ 
        [ 
            ObjectId("5450d837f32a1e098c844e2a"), 
            ObjectId("5450d837f32a1e098c844e2a")
        ]
    ],
    "ShareId" : [ 
        [ 
            ObjectId("5450d879f32a1e098c844e94"), 
            ObjectId("5450d879f32a1e098c844e94")
        ]
    ],
    "ParentId" : [ 
        [ 
            ObjectId("5450d8af6a092a0b74a44026"), 
            ObjectId("5450d8af6a092a0b74a44026")
        ]
    ],
    "DeletedDate" : [ 
        [ 
            null, 
            null
        ]
    ]
},
"server" : "mongowecntprod:27017",
"filterSet" : false,
"stats" : {
    "type" : "PROJECTION",
    "works" : 2124,
    "yields" : 21,
    "unyields" : 21,
    "invalidates" : 0,
    "advanced" : 2120,
    "needTime" : 0,
    "needFetch" : 2,
    "isEOF" : 1,
    "children" : [ 
        {
            "type" : "KEEP_MUTATIONS",
            "works" : 2124,
            "yields" : 21,
            "unyields" : 21,
            "invalidates" : 0,
            "advanced" : 2120,
            "needTime" : 1,
            "needFetch" : 2,
            "isEOF" : 1,
            "children" : [ 
                {
                    "type" : "FETCH",
                    "works" : 2124,
                    "yields" : 21,
                    "unyields" : 21,
                    "invalidates" : 0,
                    "advanced" : 2120,
                    "needTime" : 1,
                    "needFetch" : 2,
                    "isEOF" : 1,
                    "alreadyHasObj" : 0,
                    "forcedFetches" : 0,
                    "matchTested" : 2120,
                    "children" : [ 
                        {
                            "type" : "IXSCAN",
                            "works" : 2121,
                            "yields" : 21,
                            "unyields" : 21,
                            "invalidates" : 0,
                            "advanced" : 2120,
                            "needTime" : 1,
                            "needFetch" : 0,
                            "isEOF" : 1,
                            "keyPattern" : "{ UserId: 1, ShareId: 1, ParentId: 1, DeletedDate: 1 }",
                            "isMultiKey" : 0,
                            "boundsVerbose" : "field #0['UserId']: [ObjectId('5450d837f32a1e098c844e2a'), ObjectId('5450d837f32a1e098c844e2a')], field #1['ShareId']: [ObjectId('5450d879f32a1e098c844e94'), ObjectId('5450d879f32a1e098c844e94')], field #2['ParentId']: [ObjectId('5450d8af6a092a0b74a44026'), ObjectId('5450d8af6a092a0b74a44026')], field #3['DeletedDate']: [null, null]",
                            "yieldMovedCursor" : 0,
                            "dupsTested" : 0,
                            "dupsDropped" : 0,
                            "seenInvalidated" : 0,
                            "matchTested" : 0,
                            "keysExamined" : 2120,
                            "children" : []
                        }
                    ]
                }
            ]
        }
    ]
}
}

但如果我在没有DeletedDate的情况下进行查询：

db.Files.find({ "UserId" : ObjectId("5450d837f32a1e098c844e2a"),
    "ShareId" : ObjectId("5450d879f32a1e098c844e94"),
    "ParentId" : ObjectId("5450d8af6a092a0b74a44026")},
    {_id:0, ShareId:1}).explain()

然后"indexOnly"为true。

如何将第一个查询更改为制作indexOnly=true？