SERVER-19355 introduced a sortKey $meta projection which mongos will issue to mongod in order to accomplish a sort. This meta-projection instructs mongod to add the sort key it used to each document in the result set; mongos can then use this sort key to perform its sorted merge.

Currently specifying a sortKey $meta projection will always cause the query planner to generated non-covered plans. This means that the find/getMore commands path on mongos currently cannot use index-only query execution on the shards if there is a user-specified sort. In order to support covered sort queries, we would need to allow sortKey $meta projection to work on index key data in addition to fetched data.