mongodb - Poor lookup aggregation performance

Question

I have two collections

Posts:

{
    "_Id": "1",
    "_PostTypeId": "1",
    "_AcceptedAnswerId": "192",
    "_CreationDate": "2012-02-08T20:02:48.790",
    "_Score": "10",
    ...
    "_OwnerUserId": "6",
    ...
},
...

and users:

{
    "_Id": "1",
    "_Reputation": "101",
    "_CreationDate": "2012-02-08T19:45:13.447",
    "_DisplayName": "Geoff Dalgas",
    ...
    "_AccountId": "2"
},
...

and I want to find users who write between 5 and 15 posts. This is how my query looks like:

db.posts.aggregate([
    {
        $lookup: {
            from: "users", 
            localField: "_OwnerUserId",
            foreignField: "_AccountId", 
            as: "X"
        }
    },  
    {
        $group: {
            _id: "$X._AccountId", 
            posts: { $sum: 1 }
        }
    },   
    {
        $match : {posts: {$gte: 5, $lte: 15}}
    },  
    {
        $sort: {posts: -1 }
    },
    {
        $project : {posts: 1}
    }
])

and it works terrible slow. For 6k users and 10k posts it tooks over 40 seconds to get response while in relational database I get response in a split second. Where's the problem? I'm just getting started with mongodb and it's quite possible that I messed up this query.

Answer 1

from https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/

foreignField Specifies the field from the documents in the from collection. $lookup performs an equality match on the foreignField to the localField from the input documents. If a document in the from collection does not contain the foreignField, the $lookup treats the value as null for matching purposes.

This will be performed the same as any other query.

If you don't have an index on the field _AccountId, it will do a full tablescan query for each one of the 10,000 posts. The bulk of the time will be spent in that tablescan.

db.users.ensureIndex("_AccountId", 1) 

speeds up the process so it's doing 10,000 index hits instead of 10,000 table scans.