mongodb criando indice para custom field

mongo_shell

> db.contacts.find({"custom_fields.gender":"fem"}).explain()
{
	"cursor" : "BasicCursor",
	"nscanned" : 1002,
	"nscannedObjects" : 1002,
	"n" : 244,
	"indexBounds" : {
		
	}
}
> db.contacts.ensureIndex({"custom_fields.gender": 1})
> db.contacts.find({"custom_fields.gender":"fem"}).explain()
{
	"cursor" : "BtreeCursor custom_fields.gender_1",
	"nscanned" : 482,
	"nscannedObjects" : 482,
	"n" : 244,
	"indexBounds" : {
		"custom_fields.gender" : [
			[
				"fem",
				"fem"
			]
		]
	}
}