By that do you mean, should not vary in the data returned or should not break? Personally I agree that changing a constraint shouldn't alter the data returned.

By that do you mean, should not vary in the data returned or should not break? Personally I agree that changing a constraint shouldn't alter the data returned. But I'm happy enough if it breaks in a clear and verifiable manner. There are plenty of other situations where adding a constraint will cause existing SQL (if not queries) to break so its not really that much of a change.