Unordered Tree Matching and Strict Unordered Tree Matching: The Evaluation of Tree Pattern Queries

In this paper, we consider two kinds of unordered tree matchings for evaluating tree pattern queries in XML databases. For the first kind of unordered tree matching, we propose a new algorithm, which runs in O(|D| |Q|) time, where Q is a tree pattern and D is a largest data stream associated with a node of Q. It can also be adapted to an indexing environment with XB-trees being used to speed up disk access. Experiments have been conducted, showing that the new algorithm is promising. For the second of tree matching, the so-called strict unordered tree matching, we show that the problem is NP-complete by a reduction from the satisfiability problem.