Cleaned up join promotion in query.combine() with OR

                # the join type for the unused alias.

                self.unref_alias(new_alias)

        # So that we don't exclude valid results in an "or" query combination,

        # So that we don't exclude valid results in an OR query combination,

        # all joins exclusive to either the lhs or the rhs must be converted

        # to an outer join.

        # to an outer join. RHS joins were already set to outer joins above,

        # so check which joins were used only in the lhs query.

        if not conjunction:

            l_tables = set(self.tables)

            r_tables = set(rhs.tables)

            # Update r_tables aliases.

            for alias in change_map:

                if alias in r_tables:

                    # r_tables may contain entries that have a refcount of 0

                    # if the query has references to a table that can be

                    # trimmed because only the foreign key is used.

                    # We only need to fix the aliases for the tables that

                    # actually have aliases.

                    if rhs.alias_refcount[alias]:

                        r_tables.remove(alias)

                        r_tables.add(change_map[alias])

            # Find aliases that are exclusive to rhs or lhs.

            # These are promoted to outer joins.

            outer_tables = (l_tables | r_tables) - (l_tables & r_tables)

            for alias in outer_tables:

                # Again, some of the tables won't have aliases due to

                # the trimming of unnecessary tables.

                if self.alias_refcount.get(alias) or rhs.alias_refcount.get(alias):

                    self.promote_joins([alias], True)

            rhs_used_joins = set(change_map.values())

            to_promote = [alias for alias in self.tables

                          if alias not in rhs_used_joins]

            self.promote_joins(to_promote, True)

        # Now relabel a copy of the rhs where-clause and add it to the current

        # one.