Dependency Preserving Decompositions (Contd.)
Decomposition of R into X and Y is dependency preserving if (FX union FY ) + = F +
- i.e., if we consider only dependencies in the closure F + that can be checked in X without considering Y, and in Y without considering X, these imply all dependencies in F +.
Important to consider F +, not F, in this definition:
- ABC, A B, B C, C A, decomposed into AB and BC.
- Is this dependency preserving? Is C A preserved?????
Dependency preserving does not imply lossless join:
- ABC, A B, decomposed into AB and BC.
And vice-versa! (Example?)