Using $gt instead of $ne

gistfile1.js

> db.users.find({last_logged_in : {'$gt': null}}).explain()                          
{
	"cursor" : "BtreeCursor last_logged_in_1",
	"startKey" : {
		"last_logged_in" : null
	},
	"endKey" : {
		"last_logged_in" : {
			"$maxElement" : 1
		}
	},
	"nscanned" : 536361,
	"n" : 0,
	"millis" : 352,
	"allPlans" : [
		{
			"cursor" : "BtreeCursor last_logged_in_1",
			"startKey" : {
				"last_logged_in" : null
			},
			"endKey" : {
				"last_logged_in" : {
					"$maxElement" : 1
				}
			}
		}
	]
}