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  1. Core Server
  2. SERVER-15958

The "isMultiKey" value is not correct in the output of aggregation explain plan

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      > db.version()
2.6.5
> db.test.insert({x:1, y:[2,3]})
WriteResult({ "nInserted" : 1 })
> db.test.ensureIndex({y:1})
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}
> db.test.find({y:2}).explain()
{
	"cursor" : "BtreeCursor y_1",
	"isMultiKey" : true,
	"n" : 1,
	"nscannedObjects" : 1,
	"nscanned" : 1,
	"nscannedObjectsAllPlans" : 1,
	"nscannedAllPlans" : 1,
	"scanAndOrder" : false,
	"indexOnly" : false,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"millis" : 0,
	"indexBounds" : {
		"y" : [
			[
				2,
				2
			]
		]
	},
	"server" : "LindaQins-MacBook-Pro.local:22222",
	"filterSet" : false
}
> db.test.aggregate([{$match:{y:2}}], {explain:true})
{
	"stages" : [
		{
			"$cursor" : {
				"query" : {
					"y" : 2
				},
				"plan" : {
					"cursor" : "BtreeCursor ",
					"isMultiKey" : false,
					"scanAndOrder" : false,
					"indexBounds" : {
						"y" : [
							[
								2,
								2
							]
						]
					},
					"allPlans" : [
						{
							"cursor" : "BtreeCursor ",
							"isMultiKey" : false,
							"scanAndOrder" : false,
							"indexBounds" : {
								"y" : [
									[
										2,
										2
									]
								]
							}
						}
					]
				}
			}
		}
	],
	"ok" : 1
}
      Show
      > db.version() 2.6.5 > db.test.insert({x:1, y:[2,3]}) WriteResult({ "nInserted" : 1 }) > db.test.ensureIndex({y:1}) { "createdCollectionAutomatically" : false , "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } > db.test.find({y:2}).explain() { "cursor" : "BtreeCursor y_1" , "isMultiKey" : true , "n" : 1, "nscannedObjects" : 1, "nscanned" : 1, "nscannedObjectsAllPlans" : 1, "nscannedAllPlans" : 1, "scanAndOrder" : false , "indexOnly" : false , "nYields" : 0, "nChunkSkips" : 0, "millis" : 0, "indexBounds" : { "y" : [ [ 2, 2 ] ] }, "server" : "LindaQins-MacBook-Pro.local:22222" , "filterSet" : false } > db.test.aggregate([{$match:{y:2}}], {explain: true }) { "stages" : [ { "$cursor" : { "query" : { "y" : 2 }, "plan" : { "cursor" : "BtreeCursor " , "isMultiKey" : false , "scanAndOrder" : false , "indexBounds" : { "y" : [ [ 2, 2 ] ] }, "allPlans" : [ { "cursor" : "BtreeCursor " , "isMultiKey" : false , "scanAndOrder" : false , "indexBounds" : { "y" : [ [ 2, 2 ] ] } } ] } } } ], "ok" : 1 }

      The $match aggregation pipeline uses a multikey index, however, "isMultiKey" is false in the output of aggregation explain plan.

      Also, the index being used ("y_1") is missing in the field ("cursor" : "BtreeCursor "). It would be nice if the output can be ("cursor" : "BtreeCursor y_1").

            Assignee:
            david.storch@mongodb.com David Storch
            Reporter:
            linda.qin@mongodb.com Linda Qin
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              Created:
              Updated:
              Resolved: