Worst-case optimal algorithm for XPath evaluation over XML streams

  • Authors:

  • Prakash Ramanan

  • Affiliations:

  • EECS Department, Wichita State University, 1845 N Fairmount Ave, Wichita, KS 67260-0083, United States

  • Venue:
  • Journal of Computer and System Sciences
  • Year:
  • 2009

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Abstract

We consider the XPath evaluation problem: Evaluate an XPath query Q on a streaming XML document D; i.e., determine the set Q(D) of document elements selected by Q. We mainly consider Conjunctive XPath queries that involve only the child and descendant axes. Previously known in-memory algorithms for this problem use O(|D|) space and O(|Q||D|) time. Several previously known algorithms for the streaming version use @W(d^n) space and @W(d^n|D|) time in the worst case; d denotes the depth of D, and n denotes the number of location steps in Q. Their exponential space requirement could well exceed the O(|D|) space used by the in-memory algorithms. We present an efficient algorithm that uses O(d|Q|+nc) space and O((|Q|+dn)|D|) time in the worst case; c denotes the maximum number of elements of D that can be candidates for output, at any one instant. For some worst case Q and D, the memory space used by our algorithm matches our lower bound proved in a different paper; so, our algorithm uses optimal memory space in the worst case.