Poor lookup aggregation performance

Question

I have two collections

Posts:

{
    \"_Id\": \"1\",
    \"_PostTypeId\": \"1\",
    \"_AcceptedAnswerId\": \"192\",
    \"_CreationDate\": \"2012-02-         


        

Answer 1

as long as you're going to group by user _AccountId, you should do the $group first by _OwnerUserId then lookup only after filtering accounts having 10 this will reduce lookups:

db.posts.aggregate([{
    $group: {
      _id: "$_OwnerUserId",
      postsCount: {
        $sum: 1
      },
      posts: {
        $push: "$$ROOT"
      } //if you need to keep original posts data
    }
  },
  {
    $match: {
      postsCount: {
        $gte: 5,
        $lte: 15
      }
    }
  },
  {
    $lookup: {
      from: "users",
      localField: "_id",
      foreignField: "_AccountId",
      as: "X"
    }
  },
  {
    $unwind: "$X"
  },
  {
    $sort: {
      postsCount: -1
    }
  },
  {
    $project: {
      postsCount: 1,
      X: 1
    }
  }
])