preferred_server doesn't work with multi-table queries

Given the following query:

SELECT a.data, b.data FROM shared.t1 AS a JOIN private.t1 AS b ON (a.id = b.id);

If the database shared is located on all servers but private is located only on one server, the query will be routed to the preferred server. This happens because the shard location information contains only the chosen location, not the actual locations where the database exists. The act of choosing the shard location is done too early when there is a conflict and as a result of this the query is routed to the preferred server.