Imposed-functional dependencies inducing horizontal decompositions
A new decomposition theory for functional dependencies in the Relational Database Model is given. It uses a horizontal decomposition of a relation into two disjoint subrelations, of which the union is the given relation. This horizontal decomposition is based on a new constraint, the imposed-functional dependency (ifd), of which the conditional-functional dependency (cfd), introduced in a previous work, is a special case.
Functional dependencies can be expressed as special cfd's, (which themselves are special cases of ifd's). The horizontal decomposition induces another constraint: the afunctional dependency. The membership problem for mixed ifd's and ad's is solved, and a complete set of inference rules is given. The inheritance problem, i.e. the problem of determining which dependencies hold in the subrelations that result from a horizontal decomposition step, is shown to be solvable in polynomial time.
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