Avoid unnecessary joins in InstanceGroup.get_hosts

The InstanceList.get_by_filters query is joining on
info_cache, security_groups, metadata and system_metadata
because of how instance_get_all_by_filters_sort in the DB API
works if columns_to_join (expected_attrs) is None. The get_hosts
method only cares about the instance.host value of its members
so those joins are unnecessarily expensive.

This change simply passes expected_attrs=[] to get_by_filters
to avoid the joins. A follow up change can further optimize this
code by adding a new query method to just get the host values
for a list of instance uuids, so a TODO is left in place for that.
Note that a new query method would need to be remotable and thus
not something we can backport.

Change-Id: I53d4b38d12404a1641f667c537404effa837a83d
Partial-Bug: #1830234
(cherry picked from commit 15ccf2ddfb)

nova/objects/instance_group.py

@@ -497,8 +497,15 @@ class InstanceGroup(base.NovaPersistentObject, base.NovaObject,
         if exclude:
             filter_uuids = set(filter_uuids) - set(exclude)
         filters = {'uuid': filter_uuids, 'deleted': False}
+        # Pass expected_attrs=[] to avoid unnecessary joins.
+        # TODO(mriedem): This is pretty inefficient since all we care about
+        # are the hosts. We could optimize this with a single-purpose SQL query
+        # like:
+        #   SELECT host FROM instances WHERE deleted=0 AND host IS NOT NULL
+        #       AND uuid IN ($filter_uuids) GROUP BY host;
         instances = objects.InstanceList.get_by_filters(self._context,
-                                                        filters=filters)
+                                                        filters=filters,
+                                                        expected_attrs=[])
         return list(set([instance.host for instance in instances
                          if instance.host]))
 

nova/tests/unit/objects/test_instance_group.py

@@ -280,7 +280,8 @@ class _TestInstanceGroupObject(object):
             'deleted': False
         }
         mock_il_get.assert_called_once_with(self.context,
-                                            filters=expected_filters)
+                                            filters=expected_filters,
+                                            expected_attrs=[])
         self.assertEqual(2, len(hosts))
         self.assertIn('host1', hosts)
         self.assertIn('host2', hosts)
@@ -293,7 +294,8 @@ class _TestInstanceGroupObject(object):
             'deleted': False
         }
         mock_il_get.assert_called_once_with(self.context,
-                                            filters=expected_filters)
+                                            filters=expected_filters,
+                                            expected_attrs=[])
 
     def test_obj_make_compatible(self):
         obj = objects.InstanceGroup(self.context, **_INST_GROUP_OBJ_VALS)