db.collections.find({'$where': "this.x > this.y"}, {'name': 1})

这样还能按照x y 的差值排序

db.collection.aggregate(
    [  
      {
        $project : {
           _id: '$name',
           val: { $subtract : [ "$x", "$y" ] }，
           x: '$x',
           y: '$y'
        }
      },
      {$match: {val: {$gt: 0}}},
      {$sort: { val: -1 }}
   ]
)