On testing satisfiability of tree pattern queries

  • Authors:

  • Laks V. S. Lakshmanan;Ganesh Ramesh;Hui Wang;Zheng Zhao

  • Affiliations:

  • Department of Computer Science, University of British Columbia;Department of Computer Science, University of British Columbia;Department of Computer Science, University of British Columbia;Department of Computer Science, University of British Columbia

  • Venue:
  • VLDB '04 Proceedings of the Thirtieth international conference on Very large data bases - Volume 30
  • Year:
  • 2004

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Abstract

XPath and XQuery (which includes XPath as a sublanguage) are the major query languages for XML. An important issue arising in efficient evaluation of queries expressed in these languages is satisfiability, i.e., whether there exists a database, consistent with the schema if one is available, on which the query has a non-empty answer. Our experience shows satisfiability check can effect substantial savings in query evaluation. We systematically study satisfiability of tree pattern queries (which capture a useful fragment of XPath) together with additional constraints, with or without a schema. We identify cases in which this problem can be solved in polynomial time and develop novel efficient algorithms for this purpose. We also show that in several cases, the problem is NP-complete. We ran a comprehensive set of experiments to verify the utility of satisfiability check as a preprocessing step in query processing. Our results show that this check takes a negligible fraction of the time needed for processing the query while often yielding substantial savings.