A synthetic key is adding an additional dataum - so any problem that could be solved with the original data can be solved by the new data too (worst case, ignor

A synthetic key is adding an additional dataum - so any problem that could be solved with the original data can be solved by the new data too (worst case, ignore the primary key). That is one of the best reasons to add a proper index - there is almost literally no downside. We're talking maybe a few bytes/record and arguably a trivial amount of administrative overhead. Bearing that in mind: 1. Add a unique constraint on the natural key. 2. If the accuracy of joining by natural key is acceptable, join based on it. If that doesn't work, then a table design without a synthetic key wouldn't be fit for purpose either.