XPath Satisfiability with Parent Axes or Qualifiers Is Tractable under Many of Real-World DTDs

TL;DR

This paper introduces MRW-DTDs, a new subclass of DTDs that enables polynomial-time XPath satisfiability checking with parent axes or qualifiers, covering most real-world DTDs and expanding tractability.

Contribution

The paper proposes MRW-DTDs, a subclass of DTDs that extends tractability of XPath satisfiability to include parent axes and qualifiers, improving upon previous RW-DTDs.

Findings

MRW-DTDs cover 24 of 27 real-world DTDs.

XPath satisfiability with parent axes is tractable under MRW-DTDs.

XPath satisfiability with qualifiers is tractable under MRW-DTDs.

Abstract

This paper aims at finding a subclass of DTDs that covers many of the real-world DTDs while offering a polynomial-time complexity for deciding the XPath satisfiability problem. In our previous work, we proposed RW-DTDs, which cover most of the real-world DTDs (26 out of 27 real-world DTDs and 1406 out of 1407 DTD rules). However, under RW-DTDs, XPath satisfiability with only child, descendant-or-self, and sibling axes is tractable. In this paper, we propose MRW-DTDs, which are slightly smaller than RW-DTDs but have tractability on XPath satisfiability with parent axes or qualifiers. MRW-DTDs are a proper superclass of duplicate-free DTDs proposed by Montazerian et al., and cover 24 out of the 27 real-world DTDs and 1403 out of the 1407 DTD rules. Under MRW-DTDs, we show that XPath satisfiability problems with (1) child, parent, and sibling axes, and (2) child and sibling axes and qualifiers are both tractable, which are known to be intractable under RW-DTDs.