The Space-Efficient Core of Vadalog

TL;DR

This paper investigates a space-efficient core of Vadalog, focusing on restricted recursion patterns that maintain tractability and expressiveness in reasoning over knowledge graphs.

Contribution

It identifies that piece-wise linear recursion within warded TGDs offers a practical and space-efficient reasoning formalism without losing essential expressiveness.

Findings

Restricted recursion patterns are common in real-world examples.

Piece-wise linear recursion maintains decidability.

The formalism balances expressiveness and space efficiency.

Abstract

Vadalog is a system for performing complex reasoning tasks such as those required in advanced knowledge graphs. The logical core of the underlying Vadalog language is the warded fragment of tuple-generating dependencies (TGDs). This formalism ensures tractable reasoning in data complexity, while a recent analysis focusing on a practical implementation led to the reasoning algorithm around which the Vadalog system is built. A fundamental question that has emerged in the context of Vadalog is the following: can we limit the recursion allowed by wardedness in order to obtain a formalism that provides a convenient syntax for expressing useful recursive statements, and at the same time achieves space-efficiency? After analyzing several real-life examples of warded sets of TGDs provided by our industrial partners, as well as recent benchmarks, we observed that recursion is often used in a restricted way: the body of a TGD contains at most one atom whose predicate is mutually recursive with a predicate in the head. We show that this type of recursion, known as piece-wise linear in the Datalog literature, is the answer to our main question. We further show that piece-wise linear recursion alone, without the wardedness condition, is not enough as it leads to the undecidability of reasoning. We finally study the relative expressiveness of the query languages based on (piece-wise linear) warded sets of TGDs.