MongoDB $lookup pipeline: does this use indexes?

Question

I have a query that makes use of the pipeline feature of $lookup and it also uses $expr. Well, it works, but the performance is not great. It looks up stuff in a collection with about 4000 docs and joins 2 other collections (using $lookup blocks). It takes about 2000ms to run, even though there are only a few thousand documents in each collection.

The query looks something like this:

            {
                $match: {
                   language: 'str'
                }
            },
            {
                $lookup: {
                    from: 'somecollection',
                    let: { someId: '$someId' },
                    pipeline: [
                        {
                            $match: {
                                $expr: {
                                    $and: [
                                        {
                                            $eq: [
                                                '$_id',
                                                '$$someId'
                                            ]
                                        },
                                        {
                                            $gte: ['$field',value]
                                        },
                                        {
                                            $lte: ['$field2',value]
                                        }
                                       ....
                                       // some more conditions..

                                    ]
                                }
                            }
                        }
                    ]

Running explain() on this provides info about the first $match block only. But how can figure out whether the $expr in the pipeline is using an index or not?

I tried to add indexes to all fields used in the pipeline, and I also tried to create a compound index, but I couldn't make it any faster.

How could i improve performance?

The structure of my query:

match (filter by language),
lookup (col1 join)
lookup (col2 join)
project,
sort

I'm using Mongo 4.0

EDIT:

Actually, can the ops in the pipeline use an index or not? I've heard that only $eq is able to use indexes, the other ops can't. Is this true???

Thanks in advance!

Answer 1

It is supposed to, but there have been some bugs reported (and fixed) related to this:

https://jira.mongodb.org/browse/SERVER-31760

Make sure you're using the latest version and experiment.