natural disables index use

source/reference/method/cursor.sort.txt

@@ -180,6 +180,8 @@ The query returns the following documents, ordered first by the
    { "_id" : 2, "item" : { "category" : "cookies", "type" : "chocolate chip" }, "amount" : 50 }
    { "_id" : 3, "item" : { "category" : "cookies", "type" : "chocolate chip" }, "amount" : 15 }
 
+.. _return-storage-order:
+
 Return in Storage Order
 -----------------------
 
@@ -191,6 +193,8 @@ documents relocate because of :ref:`document growth due to updates
 <data-model-document-growth>` or remove operations free up space which
 are then taken up by newly inserted documents.
 
+.. include:: /includes/fact-natural-parameter.rst
+
 Consider the sequence of insert operations to the ``trees`` collection:
 
 .. code-block:: javascript

source/reference/operator/meta/natural.txt

@@ -4,11 +4,14 @@ $natural
 
 .. default-domain:: mongodb
 
+Definition
+----------
+
 .. operator:: $natural
 
   Use the :operator:`$natural` operator to use :term:`natural order` for
   the results of a sort operation. Natural order refers to the
-  order of documents in the file on disk.
+  :ref:`storage order <return-storage-order>` of documents in the file on disk.
 
   The :operator:`$natural` operator uses the following syntax to return
   documents in the order they exist on disk:
@@ -17,13 +20,116 @@ $natural
 
      db.collection.find().sort( { $natural: 1 } )
 
-  Use ``-1`` to return documents in the reverse order as they occur on
-  disk:
+Behavior
+--------
+
+On a sharded collection the :operator:`$natural` operator returns a
+collection scan sorted in :ref:`storage order<return-storage-order>`, the
+order the database inserts and stores documents on disk.
+
+.. include:: /includes/fact-natural-parameter.rst
+
+.. include:: /includes/fact-natural-sort-order-text-query-restriction.rst
+
+Examples
+--------
+
+Reverse Order
+~~~~~~~~~~~~~
+
+Use ``-1`` to return documents in the reverse order as they occur on disk:
+
+.. code-block:: javascript
+
+   db.collection.find().sort( { $natural: -1 } )
+
+Natural Order Comparison
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+In this scenario:
+
+- Create an extra index as { normal: 1 }.
+
+- Insert relevant objects with ``_id`` and ``normal`` values, for example,
+  a string type object created with:
 
   .. code-block:: javascript
 
-     db.collection.find().sort( { $natural: -1 } )
+     db.coll.insert( { _id: "01", normal: "01" } )
+
+- Use a different type for member values for each distinct object, with the
+  same values in each object.
+
+- Use ``.find().sort().explain()`` for all operations.
+
+This scenario returns these results when using
+:method:`~db.collection.find()` for different ``_id`` values; sorting with
+the :operator:`$natural` operator, ``_id`` index, and ``normal`` index;
+and a description of the :method:`~cursor.explain()` method output:
+
+.. list-table::
+   :header-rows: 2
+   :widths: 20 25 25 25
+
+   * - 
+     - 
+     - :method:`~cursor.sort()`
+     - 
+
+   * - :method:`~db.collection.find()`
+     - :operator:`$natural`:1
+     - :term:`_id`:1
+     - normal:1
+
+   * - :term:`_id`::doc:`ObjectId() </reference/bson-types>`
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+   * - :term:`_id`::doc:`Object() </reference/bson-types>`
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+   * - :term:`_id`::doc:`string() </reference/bson-types>`
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+   * - :term:`_id`::doc:`integer() </reference/bson-types>`
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+   * - :term:`_id`::doc:`BinData() </reference/bson-types>`
+
+     - :method:`~cursor.explain()` scanned entire collection
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+   * - normal:(any query)
+
+     - :method:`~cursor.explain()` scanned entire collection
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
+
+     - :method:`~cursor.explain()` used :term:`B-Tree` cursor
 
-  .. include:: /includes/fact-natural-sort-order-text-query-restriction.rst
+Additional Information
+----------------------
 
-  .. seealso:: :method:`cursor.sort()`
+:method:`cursor.sort()`