What is dependency preservation?

Answer 1

Given a set of dependencies F on R, the projection of F on Ri, denoted by

'lTR(F) where Ri is a subset of R, is the set of dependencies X ---.. Y in P+ such that the

attributes in X U Yare all contained in Ri• Hence, the projection of F on each relation

schema Ri in the decomposition D is the set of functional dependencies in P+, the closure

of F, such that all their left- and right-hand-side attributes are in Ri• We say that a

decomposition D '= {R[, Rz, ... , Rm} of R is dependency-preserving with respect to F if

the union of the projections of F on each Ri in D is equivalent to F; that is,

(('lTR (F» U ... U ('lTR (F)W '= P+