An Inflationary Fixed Point Operator in XQuery

TL;DR

This paper introduces inflationary fixed points in XQuery, enabling controlled recursion that captures complex semantics like Regular XPath, with efficient evaluation methods and practical benefits demonstrated through experiments.

Contribution

It presents a new recursion operator in XQuery, analyzes its properties, and shows how it can be efficiently evaluated under certain conditions.

Findings

Inflationary fixed points can capture Regular XPath semantics.

Efficient evaluation is possible when recursive expressions are distributive.

XQuery processors benefit from the proposed evaluation method.

Abstract

We introduce a controlled form of recursion in XQuery, inflationary fixed points, familiar in the context of relational databases. This imposes restrictions on the expressible types of recursion, but we show that inflationary fixed points nevertheless are sufficiently versatile to capture a wide range of interesting use cases, including the semantics of Regular XPath and its core transitive closure construct. While the optimization of general user-defined recursive functions in XQuery appears elusive, we will describe how inflationary fixed points can be efficiently evaluated, provided that the recursive XQuery expressions exhibit a distributivity property. We show how distributivity can be assessed both, syntactically and algebraically, and provide experimental evidence that XQuery processors can substantially benefit during inflationary fixed point evaluation.