def __call__(self, server_descriptions):
        secondaries = secondary_with_tags_server_selector(
            self.tag_sets,
            server_descriptions)

        if secondaries:
            return secondaries
        else:
            return writable_server_selector(server_descriptions)
Outer joins t1->t2,t3, where on t1, t2 and t3.
dataset/ETHPy150Open mongodb/mongo-python-driver/pymongo/read_preferences.py/SecondaryPreferred.__call__--crossover with--dataset/ETHPy150Open zzzeek/sqlalchemy/test/sql/test_query.py/JoinTest.test_outerjoin_where_x2_t1t2t3