Matching twigs in probabilistic XML

  • Authors:

  • Benny Kimelfeld;Yehoshua Sagiv

  • Affiliations:

  • The Hebrew University of Jerusalem, Edmond J. Safra Campus Jerusalem, Israel;The Hebrew University of Jerusalem, Edmond J. Safra Campus Jerusalem, Israel

  • Venue:
  • VLDB '07 Proceedings of the 33rd international conference on Very large data bases
  • Year:
  • 2007

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Abstract

Evaluation of twig queries over probabilistic XML is investigated. Projection is allowed and, in particular, a query may be Boolean. It is shown that for a well-known model of probabilistic XML, the evaluation of twigs with projection is tractable under data complexity (whereas in other probabilistic data models, projection is intractable). Under query-and-data complexity, the problem becomes intractable even without projection (and for rather simple twigs and data). In earlier work on probabilistic XML, answers are always complete. However, there is often a need to produce partial answers because XML data may have missing sub-elements and, furthermore, complete answers may be deemed irrelevant if their probabilities are too low. It is shown how to define a semantics that provides partial answers that are maximal with respect to a probability threshold, which is specified by the user. For this semantics, it is shown how to efficiently evaluate twigs, even under query-and-data complexity if there is no projection.