Based on this community post: https://www.mongodb.com/community/forums/t/clustered-collections-as-lookup-target/246893

The user is worried we are not using clustered index, because in explain output for $lookup they see a lot of collectionScans and no indexes used.

It happens because internally clustered index scan is a collection scan and not an index scan.

We should improve explain output by showing explicitly that clustered index is used.