Online Feedback for Nested Aggregate Queries with Multi-Threading

Abstract

In this paper, we study the progressive evaluation of nested queries with aggregates (i.e., the inner query block is an aggregate query), where users are provided progressively with (approximate) answers as the inner query block is being evaluated. We propose an incremental evaluation strategy to present answers that are certainly in the final answer space first, before presenting those whose validity may be affected as the inner query aggregates are refined. We also propose a multithreaded model in evaluating such queries: the outer query is assigned to a thread, and the inner query is assigned to another thread. The time-sliced across the two subqueries is nondeterministic in the sense that the user controls the relative rate at which these subqueries are being evaluated. We implemented a prototype system using JAVA, and evaluated our system. Our results show the effectiveness of the proposed mechanisms in providing online feedback that reduces the initial waiting time of users significantly without sacrificing on the quality of the answers.