Fixed -- Doc'd that admin changelist may add pk to ordering.
@@ -1037,6 +1037,17 @@ subclass::
If you need to specify a dynamic order (for example depending on user or
language) you can implement a :meth:`~ModelAdmin.get_ordering` method.
+ .. admonition:: Performance considerations with ordering and sorting
+
+ To ensure a deterministic ordering of results, the changelist adds
+ ``pk`` to the ordering if it can't find a single or unique together set
+ of fields that provide total ordering.
+
+ For example, if the default ordering is by a non-unique ``name`` field,
+ then the changelist is sorted by ``name`` and ``pk``. This could
+ perform poorly if you have a lot of rows and don't have an index on
+ ``name`` and ``pk``.
+
.. attribute:: ModelAdmin.paginator
The paginator class to be used for pagination. By default,
