mongodb联表查询可以使用mongoose的pupulate()，也可以使用聚合$lookup,如果使用两次单表查询：

//one
{
   _id: "joe",
   name: "Joe Bookreader"
}
//two
{
   patron_id: "joe",
   street: "123 Fake Street",
   city: "Faketon",
   state: "MA",
   zip: "12345"
}
{
   patron_id: "joe",
   street: "1 Some Other Street",
   city: "Boston",
   state: "MA",
   zip: "12345"
}

db.ones.findOne({_id:'joe'))
db.twos.find({patron_id:'joe'})

执行两次单表查询，_id,patron_id都有索引,patron_id对应为joe数量为个位数，索引粒度很低，能过滤很多数据，那么这两次查询是不是也应该很快（都在内存里），这和$lookup查询比起来谁的效率会好一些？聚合$lookup查询是不是内部也是这样做了两次find查询,foreignField对应键上有无索引应该很影响查询效率吧？

db.ones.aggregate([
    {
      $lookup:
        {
          from: "twos",
          localField: "_id",
          foreignField: "patron_id",
          as: "arrs"
        }
   },
   { $match : {_id:'joe') }
])