Refs #16614 -- Made QuerySet.iterator() use server-side cursors on PostgreSQL.

Thanks to Josh Smeaton for the idea of implementing server-side cursors
in PostgreSQL from the iterator method, and Anssi Kääriäinen and Kevin
Turner for their previous work. Also Simon Charette and Tim Graham for
review.

tests/backends/test_postgresql.py

@@ -0,0 +1,54 @@
+import unittest
+from collections import namedtuple
+
+from django.db import connection
+from django.test import TestCase
+
+from .models import Person
+
+
+@unittest.skipUnless(connection.vendor == 'postgresql', "Test only for PostgreSQL")
+class ServerSideCursorsPostgres(TestCase):
+    cursor_fields = 'name, statement, is_holdable, is_binary, is_scrollable, creation_time'
+    PostgresCursor = namedtuple('PostgresCursor', cursor_fields)
+
+    @classmethod
+    def setUpTestData(cls):
+        Person.objects.create(first_name='a', last_name='a')
+        Person.objects.create(first_name='b', last_name='b')
+
+    def inspect_cursors(self):
+        with connection.cursor() as cursor:
+            cursor.execute('SELECT {fields} FROM pg_cursors;'.format(fields=self.cursor_fields))
+            cursors = cursor.fetchall()
+        return [self.PostgresCursor._make(cursor) for cursor in cursors]
+
+    def test_server_side_cursor(self):
+        persons = Person.objects.iterator()
+        next(persons)  # Open a server-side cursor
+        cursors = self.inspect_cursors()
+        self.assertEqual(len(cursors), 1)
+        self.assertIn('_django_curs_', cursors[0].name)
+        self.assertFalse(cursors[0].is_scrollable)
+        self.assertFalse(cursors[0].is_holdable)
+        self.assertFalse(cursors[0].is_binary)
+
+    def test_server_side_cursor_many_cursors(self):
+        persons = Person.objects.iterator()
+        persons2 = Person.objects.iterator()
+        next(persons)  # Open a server-side cursor
+        next(persons2)  # Open a second server-side cursor
+        cursors = self.inspect_cursors()
+        self.assertEqual(len(cursors), 2)
+        for cursor in cursors:
+            self.assertIn('_django_curs_', cursor.name)
+            self.assertFalse(cursor.is_scrollable)
+            self.assertFalse(cursor.is_holdable)
+            self.assertFalse(cursor.is_binary)
+
+    def test_closed_server_side_cursor(self):
+        persons = Person.objects.iterator()
+        next(persons)  # Open a server-side cursor
+        del persons
+        cursors = self.inspect_cursors()
+        self.assertEqual(len(cursors), 0)