Make _ReferenceKeys with lowercase values for the cache so we don't have
    to keep track of quoting

def _make_key(relation):
    """Make _ReferenceKeys with lowercase values for the cache so we don't have
    to keep track of quoting
    """
    return _ReferenceKey(_lower(relation.database),
                         _lower(relation.schema),
                         _lower(relation.identifier))