ref: _fact-covered-query-sharded-collection
content: |
  An index cannot {{cover}} a query on a :term:`sharded <shard>` collection
  when run against a :binary:`~bin.mongos` if the index does not contain the
  shard key, with the following exception for the ``_id`` index: If a
  query on a sharded collection only specifies a condition on the ``_id``
  field and returns only the ``_id`` field, the ``_id`` index can cover
  the query when run against a :binary:`~bin.mongos` even if the ``_id``
  field is not the shard key.

  .. versionchanged:: 3.0

     In previous versions, an index cannot :ref:`cover <covered-queries>`
     a query on a :term:`sharded <shard>` collection when run against a
     :binary:`~bin.mongos`.
...