Parallel-Correctness and Transferability for Conjunctive Queries

TL;DR

This paper introduces a correctness condition called parallel-correctness for evaluating conjunctive queries in distributed systems, analyzing its complexity and transferability across different distribution policies to optimize communication rounds.

Contribution

It formalizes parallel-correctness for conjunctive queries, studies its complexity, and explores transferability between queries and distribution policies like the Hypercube.

Findings

Parallel-correctness is a key condition for query evaluation in distributed systems.

The complexity of checking parallel-correctness varies with query and policy.

Transferability of parallel-correctness depends on query structure and distribution policy.

Abstract

A dominant cost for query evaluation in modern massively distributed systems is the number of communication rounds. For this reason, there is a growing interest in single-round multiway join algorithms where data is first reshuffled over many servers and then evaluated in a parallel but communication-free way. The reshuffling itself is specified as a distribution policy. We introduce a correctness condition, called parallel-correctness, for the evaluation of queries w.r.t. a distribution policy. We study the complexity of parallel-correctness for conjunctive queries as well as transferability of parallel-correctness between queries. We also investigate the complexity of transferability for certain families of distribution policies, including, for instance, the Hypercube distribution.