XML design for relational storage

Design principles for XML schemas that eliminate redun-dancies and avoid update anomalies have been studied re-cently. Several normal forms, generalizing those for rela-tional databases, have been proposed. All of them, how-ever, are based on the assumption of a native XML storage, while in practice most of XML data is stored in relational databases. In this paper we study XML design and normalization for relational storage of XML documents. To be able to re-late and compare XML and relational designs, we use an information-theoretic framework that measures information content in relations and documents, with higher values cor-responding to lower levels of redundancy. We show that most common relational storage schemes preserve the no-tion of being well-designed (i.e., anomalies- and redundancy-free). Thus, existing XML normal forms guarantee well-designed relational storages as well. We further show that if this perfect option is not achievable, then a slight restriction on XML constraints guarantees a “second-best ” relational design, according to possible values of the information-theoretic measure. We finally consider an edge-based re-lational representation of XML documents, and show that while it has similar information-theoretic properties with other relational representations, it can behave significantly worse in terms of enforcing integrity constraints