XML schema refinement through redundancy detection and normalization

Abstract

As XML becomes increasingly popular, XML schema design has become an increasingly important issue. One of the central objectives of good schema design is to avoid data redundancies: redundantly stored information can lead not just only to a higher data storage cost but also to increased costs for data transfer and data manipulation. Furthermore, such data redundancies can lead to potential update anomalies, rendering the database inconsistent. One strategy to avoid data redundancies is to design redundancy-free schema from the start on the basis of known functional dependencies. We observe that XML databases are often “casually designed” and XML FDs may not be determined in advance. Under such circumstances, discovering XML data redundancies from the data itself becomes necessary and is an integral part of the schema refinement (or re-design) process. We present the design and implementation of the first system, DiscoverXFD, for efficient discovery of XML data redundancies. It employs a novel XML data structure and introduces a new class of partition-based algorithms. The XML data redundancies are defined on the basis of a new notion of XML functional dependency (XML FD) that (1) extends previous notions by incorporating set elements into the XML FD specification, and (2) maintains tuple-based semantics through the novel concept of Generalized Tree Tuple (GTT). Using this comprehensive XML FD notion, we introduce a new normal form (GTT-XNF) for XML documents, and provide comprehensive comparisons with previous studies. Given the set of data redundancies (in the form of redundancy-indicating XML FDs) discovered by DiscoverXFD, we describe a normalization algorithm for converting any original XML schema into one in GTT-XNF.