MongoDB为同一查询选择不同的索引

Question

我使用mongodb 2.6。我的一个查询有时会选择一个错误的索引来使用。

我的收藏集RawAssetStatistics有两个索引：\_id.asset_1和\_id.timestamp_-1__id.asset_1。

我的查询：

db.RawAssetStatistics.find({
    "_id.timestamp": { $lte: ISODate("2016-04-14T06:36:00.000Z"), $gte:ISODate("2016-04-14T05:36:00.000Z") },
    "_id.asset":ObjectId('56ec426fdf0f6b4766e93b7f')
}).sort({"_id.timestamp":1})

通常使用\_id.timestamp_-1__id.asset_1并且需要很少的milis才能完成。但有时会使用\_id.asset_1，这会更糟糕。

有没有办法检测为什么Mongo做出这么糟糕的决定。

使用提示解释查询以使用\_id.asset_1：

{
"cursor" : "BtreeCursor _id.asset_1",
"isMultiKey" : false,
"n" : 61,
"nscannedObjects" : 8991,
"nscanned" : 8991,
"nscannedObjectsAllPlans" : 8991,
"nscannedAllPlans" : 8991,
"scanAndOrder" : true,
"indexOnly" : false,
"nYields" : 70,
"nChunkSkips" : 0,
"millis" : 10376,
"indexBounds" : {
    "_id.asset" : [ 
        [ 
            ObjectId("56ec426fdf0f6b4766e93b7f"), 
            ObjectId("56ec426fdf0f6b4766e93b7f")
        ]
    ]
},
"server" : "UNI-DB:27017",
"filterSet" : false,
"stats" : {
    "type" : "SORT",
    "works" : 9055,
    "yields" : 70,
    "unyields" : 70,
    "invalidates" : 89,
    "advanced" : 61,
    "needTime" : 8992,
    "needFetch" : 0,
    "isEOF" : 1,
    "forcedFetches" : 0,
    "memUsage" : 27572,
    "memLimit" : 33554432,
    "children" : [ 
        {
            "type" : "KEEP_MUTATIONS",
            "works" : 8992,
            "yields" : 70,
            "unyields" : 70,
            "invalidates" : 89,
            "advanced" : 61,
            "needTime" : 8930,
            "needFetch" : 0,
            "isEOF" : 1,
            "children" : [ 
                {
                    "type" : "FETCH",
                    "works" : 8992,
                    "yields" : 70,
                    "unyields" : 70,
                    "invalidates" : 89,
                    "advanced" : 61,
                    "needTime" : 8930,
                    "needFetch" : 0,
                    "isEOF" : 1,
                    "alreadyHasObj" : 0,
                    "forcedFetches" : 0,
                    "matchTested" : 61,
                    "children" : [ 
                        {
                            "type" : "IXSCAN",
                            "works" : 8992,
                            "yields" : 70,
                            "unyields" : 70,
                            "invalidates" : 89,
                            "advanced" : 8991,
                            "needTime" : 0,
                            "needFetch" : 0,
                            "isEOF" : 1,
                            "keyPattern" : "{ _id.asset: 1 }",
                            "isMultiKey" : 0,
                            "boundsVerbose" : "field #0['_id.asset']: [ObjectId('56ec426fdf0f6b4766e93b7f'), ObjectId('56ec426fdf0f6b4766e93b7f')]",
                            "yieldMovedCursor" : 0,
                            "dupsTested" : 0,
                            "dupsDropped" : 0,
                            "seenInvalidated" : 0,
                            "matchTested" : 0,
                            "keysExamined" : 8991,
                            "children" : []
                        }
                    ]
                }
            ]
        }
    ]
}
}

使用提示解释查询以使用＆＃34; _id.timestamp_-1__id.asset_1＆＃34;

{
"cursor" : "BtreeCursor _id.timestamp_-1__id.asset_1 reverse",
"isMultiKey" : false,
"n" : 61,
"nscannedObjects" : 61,
"nscanned" : 181,
"nscannedObjectsAllPlans" : 61,
"nscannedAllPlans" : 181,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 1,
"nChunkSkips" : 0,
"millis" : 2,
"indexBounds" : {
    "_id.timestamp" : [ 
        [ 
            ISODate("2016-04-14T05:36:00.000Z"), 
            ISODate("2016-04-14T06:36:00.000Z")
        ]
    ],
    "_id.asset" : [ 
        [ 
            ObjectId("56ec426fdf0f6b4766e93b7f"), 
            ObjectId("56ec426fdf0f6b4766e93b7f")
        ]
    ]
},
"server" : "UNI-DB:27017",
"filterSet" : false,
"stats" : {
    "type" : "FETCH",
    "works" : 182,
    "yields" : 1,
    "unyields" : 1,
    "invalidates" : 0,
    "advanced" : 61,
    "needTime" : 120,
    "needFetch" : 0,
    "isEOF" : 1,
    "alreadyHasObj" : 0,
    "forcedFetches" : 0,
    "matchTested" : 0,
    "children" : [ 
        {
            "type" : "IXSCAN",
            "works" : 182,
            "yields" : 1,
            "unyields" : 1,
            "invalidates" : 0,
            "advanced" : 61,
            "needTime" : 120,
            "needFetch" : 0,
            "isEOF" : 1,
            "keyPattern" : "{ _id.timestamp: -1, _id.asset: 1 }",
            "isMultiKey" : 0,
            "boundsVerbose" : "field #0['_id.timestamp']: [new Date(1460612160000), new Date(1460615760000)], field #1['_id.asset']: [ObjectId('56ec426fdf0f6b4766e93b7f'), ObjectId('56ec426fdf0f6b4766e93b7f')]",
            "yieldMovedCursor" : 0,
            "dupsTested" : 0,
            "dupsDropped" : 0,
            "seenInvalidated" : 0,
            "matchTested" : 0,
            "keysExamined" : 181,
            "children" : []
        }
    ]
}
}