Unpreserved Dependencies
Suppose the users now decide that all rinks in the same city have the same capacity:
- city ? capacity
- While trying to remain calm, you realize that:
- This FD doesn’t exist in any single relation, so a join is required to check it each time we add or change a capacity value.
The tradeoff:
- Expensive to check, but
- may not be checked often enough to justify creating a dependency-preserving decomposition.