Forward XPath rewriting over XML data streams

Abstract

Streaming XML data is often encountered in applications in medical biology. So, how to effectively manage and query XML data streams can be a fundamental issue for the quality of those medical biology applications. With respect to current methods for XPath query evaluation over XML data streams, adoption of some query rewriting techniques are common when encountering some complex query which can cause ambiguities and redundant query cost. Lots of work can be found in database research community to simplify query and remove its redundancy. However, less attention is paid to make use of query rewriting to benefit the query evaluation process of an efficient XML data stream query system. In other words, the query rewriting is not directly for the purpose of simplicity or redundancy removal; whereas it is for the convenience of an efficient query. In this paper, we propose a normalization process which rewrites the Forward XPath expressions for our XML data stream query system. The proposed normalization is expected to normalize all the complex predicates connected with logic operators into twig structures. With this normalization, queries can be transformed into a well-structured one, the formal way of the normalization makes the query language of our XML data stream query system more precise and the twig structure facilitates the query evaluation of our query system.