Dependency Preservation Notes, Lecture notes of Database Management Systems (DBMS)

Download Dependency Preservation Notes and more Lecture notes Database Management Systems (DBMS) in PDF only on Docsity!

Adathazis-kezelés, (4 eldadas: Relico felbontasai (dekomponalis)) 1 Dependency preservation Getting lossless decomposition is necessary. But of course, we also want to keep dependencies, since losing a dependency means, that the corresponding constraint can be check only through natural join of the appropriate resultant relation in the decomposition. This would be very expensive, $0, our aim is to get a lossless dependency preserving decomposition. Example: R=(A, B, C), F=E{A>B, BSC} Decomposition of R: RI=(A, C) R2=(B, C) Does this decomposition preserve the given dependencies? Solution: In RI the following dependencies hold: = Fl’°={A9A,C3C, ADC, AC>AC} In R2 the following dependencies hold: F2’= {B3>B, C3C, BOC, BC>BC} The set of nontrivial dependencies hold on RI and R2: F':= {BOC, ADC} AB can not be derived from F’, so this decomposition is NOT dependency preserving. Tempus 8_JEP-12435-97 Készitette: Bércesné Novak Agnes