[1.5.X] Fixed #15361 - Documented performance considerations for QuerySet.get()

Thanks mmcnickle for the patch.

Backport of feaf9f279a from master

docs/topics/db/optimization.txt

@@ -132,6 +132,41 @@ Write your own :doc:`custom SQL to retrieve data or populate models
 </topics/db/sql>`. Use ``django.db.connection.queries`` to find out what Django
 is writing for you and start from there.
 
+Retrieve individual objects using a unique, indexed column
+==========================================================
+
+There are two reasons to use a column with
+:attr:`~django.db.models.Field.unique` or
+:attr:`~django.db.models.Field.db_index` when using
+:meth:`~django.db.models.query.QuerySet.get` to retrieve individual objects.
+First, the query will be quicker because of the underlying database index.
+Also, the query could run much slower if multiple objects match the lookup;
+having a unique constraint on the column guarantees this will never happen.
+
+So using the :ref:`example Weblog models <queryset-model-example>`::
+
+  >>> entry = Entry.objects.get(id=10)
+
+will be quicker than:
+
+  >>> entry = Entry.object.get(headline="News Item Title")
+
+because ``id`` is indexed by the database and is guaranteed to be unique.
+
+Doing the following is potentially quite slow:
+
+  >>> entry = Entry.objects.get(headline__startswith="News")
+
+First of all, `headline` is not indexed, which will make the underlying
+database fetch slower.
+
+Second, the lookup doesn't guarantee that only one object will be returned.
+If the query matches more than one object, it will retrieve and transfer all of
+them from the database. This penalty could be substantial if hundreds or
+thousands of records are returned. The penalty will be compounded if the
+database lives on a separate server, where network overhead and latency also
+play a factor.
+
 Retrieve everything at once if you know you will need it
 ========================================================