Towards Tractability of the Diversity of Query Answers: Ultrametrics to the Rescue

TL;DR

This paper explores the use of ultrametrics to efficiently find diverse query answer subsets, overcoming previous intractability issues, and provides polynomial algorithms under certain conditions for practical query evaluation.

Contribution

It introduces sufficient conditions on diversity functions based on ultrametrics that enable polynomial-time algorithms for constructing diverse answer subsets.

Findings

Polynomial-time algorithms for diverse answer sets under ultrametric conditions

Conditions satisfied by all literature-diversity functions considered

Efficient algorithms for acyclic conjunctive query answer diversity

Abstract

The set of answers to a query may be very large, potentially overwhelming users when presented with the entire set. In such cases, presenting only a small subset of the answers to the user may be preferable. A natural requirement for this subset is that it should be as diverse as possible to reflect the variety of the entire population. To achieve this, the diversity of a subset is measured using a metric that determines how different two solutions are and a diversity function that extends this metric from pairs to sets. In the past, several studies have shown that finding a diverse subset from an explicitly given set is intractable even for simple metrics (like Hamming distance) and simple diversity functions (like summing all pairwise distances). This complexity barrier becomes even more challenging when trying to output a diverse subset from a set that is only implicitly given such as the query answers of a query and a database. Until now, tractable cases have been found only for restricted problems and particular diversity functions. To overcome these limitations, we focus on the notion of ultrametrics, which have been widely studied and used in many applications. Starting from any ultrametric $d$ and a diversity function $\delta$ extending $d$, we provide sufficient conditions over $\delta$ for having polynomial-time algorithms to construct diverse answers. To the best of our knowledge, these conditions are satisfied by all diversity functions considered in the literature. Moreover, we complement these results with lower bounds that show specific cases when these conditions are not satisfied and finding diverse subsets becomes intractable. We conclude by applying these results to the evaluation of conjunctive queries, demonstrating efficient algorithms for finding a diverse subset of solutions for acyclic conjunctive queries when the attribute order is used to measure diversity.