A New Characterisation of Information Capacity Equivalence in Nested Relations

Nested relations are an important subclass of object-relational systems that are now being widely used in industry. Also, a subclass of nested relations, partitioned normal form (PNF) relations, is important because the semantics and manipulation of these relations are more transparent than general nested relations. In this paper we address the question of determining when every PNF relation stored under one nested relation scheme can be transformed into another PNF relation stored under a different nested relation scheme without loss of information, referred to as the two schemes having equivalent information capacity. This issue is important in many database application areas such as view processing, schema integration and schema evolution. The main result of the paper provides a new characterisation of information capacity equivalence for single nested schemes. It shows that two schemes are information capacity equivalent if and only if the two sets of multivalued dependencies induced by the two corresponding scheme trees are equivalent.