> 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 }