Flexible query facilities for heterogeneous semi-structured data.

Abstract

This dissertation studies flexible query facilities for semi-structured data in a heterogeneous environment, with a focus on XML databases. The popularity of XML naturally follows from the needs of querying XML documents from a wide spectrum of users. Although formal database query languages such as XQuery can provide precise access to XML data, the challenges of querying XML using such rigid formal database languages---i.e., requiring users to have perfect knowledge of database schema, query language syntax and query semantics---leads to the requests for flexible yet accurate query facilities over XML documents. This dissertation discusses a two-part solution for supporting flexible queries over XML documents: (a) Schema-Free XQuery that allows database queries to be specified with limited or even no schema knowledge and (b) Natural Language Interface for Querying XML (NaLIX) that can translate database queries with complex semantics in plain English into Schema-Free XQuery expressions. NaLIX enables users to pose complex database queries in plain English without knowing any formal query language or underlying database schema. Iterative user search is also supported in NaLIX by allowing queries to be stated with respect to previous queries. NaLIX does not depend on any domain knowledge. However, it can be improved further by automatically learning domain information. We also present a novel stack-based algorithm and cost-based optimization techniques to allow these techniques, Schema-Free XQuery and NaLIX, to be implemented efficiently. In addition, we report experimental results that validate the proposed solution. Finally, we discuss how our solution improves the state-of-art through comparison with previous work.