Change the `StandardAttribute` load method to "selectin"

This load method will remove the ``StandardAttribute`` load from the
resource main query and PostgreSQL won't require it to be in the
GROUP BY clause.

Closes-Bug: #2072567

Change-Id: I17a455e19c4bbbc45e26dee88849ab5e0b4b5d65

neutron_lib/db/standard_attr.py

@@ -152,8 +152,11 @@ class HasStandardAttributes(object):
     # pylint: disable=method-hidden
     @declarative.declared_attr
     def standard_attr(cls):
+        # NOTE(ralonsoh): the load method has been updated from "joined" to
+        # "selectin". The first provides more complex queries but it is only
+        # needed one query. This is needed by PostgreSQL (LP#2072567).
         return sa.orm.relationship(StandardAttribute,
-                                   lazy='joined',
+                                   lazy='selectin',
                                    cascade='all, delete-orphan',
                                    single_parent=True,
                                    uselist=False)