Gotcha. I think I would lean towards duplicate relational data, just because it feels a bit more sane/secure to benefit from some of the DB features (e.g. FKs,

ON DELETE CASCADE

, etc.). There'd be the risk of data integrity issues if the relations are kept in multiple tables (or even separate DBs), but that might be acceptable. Edit: On second thought, the data integrity risks are probably a reason not to keep relations in two DBs/systems.