Efficient pagination of MongoDB aggregation?

Question

For efficiency, the Mongo documentation recommends that limit statements immediately follow sort statements, thus ending up with the somewhat nonsensical:

 colle         


        

Answer 1

MongoDB recommendation of $sort preceding $limit is absolutely true as when it happens it optimizes the memory required to do the operation for top n results.

It just that the solution you proposes doesn't fit your use case, which is pagination.

You can modify your query to to get the benefit from this optimization.

collection.aggregate([
  {
    $unwind: u
  }, 
  {
    $group: g
  },
  {
    $match: f
  }, 
  {
    $sort: s
  }, 
  {
    $limit: l+p
  },
  { 
    $skip: p
  }
]);

or for find query

 collection.find(f).sort(s).limit(l+p).skip(p)

Though, as you can see the with big pagination the memory will grow more and more even with this optimization.