Dependency Preserving Decomposition
Consider CSJDPQV, C is key, JP C and SD P.
- BCNF decomposition: CSJDQV and SDP
- Problem: Checking JP C requires a join!
Dependency preserving decomposition (Intuitive):
- If R is decomposed into X, Y and Z, and we enforce the FDs that hold on X, on Y and on Z, then all FDs that were given to hold on R must also hold.
Projection of set of FDs F: If R is decomposed into X, ... projection of F onto X (denoted FX ) is the set of FDs U V in F+ (closure of F ) such that U, V are in X.