Currently when run on a shard server, a $lookup will target shards to attach a cursor source regardless of whether the 'from' collection is sharded or unsharded. This is because, if the source collection is sharded and the 'from' collection is unsharded, the $lookup could be running on a non-primary shard and must target the primary (where the 'from' collection lives) to get correct results. However, if the $lookup is running on the primary shard of the database and the 'from' collection is unsharded, it is safe to perform local reads instead of targeting shards.