XPath Whole Query Optimization

TL;DR

This paper introduces a novel approach to XPath query optimization using tree automata, enabling efficient skipping of irrelevant subtrees and internal nodes, significantly improving query execution speed over compressed XML indexes.

Contribution

It proposes a new framework leveraging tree automata for fine-grained XPath query optimization, including methods to approximate automaton runs and techniques to evaluate automata efficiently.

Findings

Significant reduction in query execution time.

Effective skipping of irrelevant subtrees and internal nodes.

Validation through extensive experiments.

Abstract

Previous work reports about SXSI, a fast XPath engine which executes tree automata over compressed XML indexes. Here, reasons are investigated why SXSI is so fast. It is shown that tree automata can be used as a general framework for fine grained XML query optimization. We define the "relevant nodes" of a query as those nodes that a minimal automaton must touch in order to answer the query. This notion allows to skip many subtrees during execution, and, with the help of particular tree indexes, even allows to skip internal nodes of the tree. We efficiently approximate runs over relevant nodes by means of on-the-fly removal of alternation and non-determinism of (alternating) tree automata. We also introduce many implementation techniques which allows us to efficiently evaluate tree automata, even in the absence of special indexes. Through extensive experiments, we demonstrate the impact of the different optimization techniques.