data range is from "2015-10-28T16:00:00Z" to "2015-10-29T08:00:00Z".

> db.order.getIndexes();
"key" : {
	"order_status.update_time" : -1,
	"order_status.status" : -1
},

> db.order.find({ "order_status" : { "$elemMatch" : { "update_time" : { "$gte" : ISODate("2015-10-29T07:00:00Z"), "$lt" : ISODate("2015-10-29T08:00:00Z") }, "status" : 2 } } }).explain();
"indexBounds" : {
	"order_status.update_time" : [
		"[new Date(9223372036854775807), new Date(1446102000000)]"
	],
	"order_status.status" : [
		"[2.0, 2.0]"
	]
}

> db.order.find({ "order_status" : { "$elemMatch" : { "update_time" : { "$gte" : ISODate("2015-10-28T19:00:00Z"), "$lt" : ISODate("2015-10-28T20:00:00Z") }, "status" : 2 } } }).explain();
"indexBounds" : {
	"order_status.update_time" : [
		"[new Date(9223372036854775807), new Date(1446058800000)]"
	],
	"order_status.status" : [
		"[2.0, 2.0]"
	]
}

> db.order.getIndexes();
"key" : {
	"order_status.update_time" : 1,
	"order_status.status" : 1
},

> db.order.find({ "order_status" : { "$elemMatch" : { "update_time" : { "$gte" : ISODate("2015-10-28T19:00:00Z"), "$lt" : ISODate("2015-10-28T20:00:00Z") }, "status" : 2 } } }).explain();
"indexBounds" : {
	"order_status.update_time" : [
		"(true, new Date(1446062400000))"
	],
	"order_status.status" : [
		"[2.0, 2.0]"
	]
}

> db.order.find({ "order_status" : { "$elemMatch" : { "update_time" : { "$gte" : ISODate("2015-10-29T07:00:00Z"), "$lt" : ISODate("2015-10-29T08:00:00Z") }, "status" : 2 } } }).explain();
"indexBounds" : {
	"order_status.update_time" : [
		"[new Date(1446102000000), new Date(9223372036854775807)]"
	],
	"order_status.status" : [
		"[2.0, 2.0]"
	]
}