Problems with Decompositions
There are three potential problems to consider:
- Some queries become more expensive.
- e.g., find employee and department names
- Given instances of the decomposed relations, we may not be able to reconstruct the corresponding instance of the original relation!
- First 3NF decomposition attempt
- Checking some dependencies may require joining the instances of the decomposed relations.
- BCNF decomposition example
Tradeoff: Must consider these issues vs. redundancy.