Block as a value for SQL over NoSQL

Abstract

This paper presents Zidian, a middleware for key-value (KV) stores to speed up SQL query evaluation over NoSQL. As opposed to common practice that takes a tuple id or primary key as key and the entire tuple as value, Zidian proposes a block-as-a-value model BaaV. BaaV represents a relation as keyed blocks ( k, B ), where k is a key of a block (a set) B of partial tuples. We extend relational algebra to BaaV. We show that under BaaV, Zidian substantially reduces data access and communication cost. We provide characterizations (sufficient and necessary conditions) for (a) result-preserving queries, i.e., queries covered by available BaaV stores, (b) scan-free queries, i.e., queries that can be evaluated without scanning any table, and (c) bounded queries, i.e., queries that can be answered by accessing a bounded amount of data. We show that in parallel processing, Zidian guarantees (a) no scans for scan-free queries, (b) bounded communication cost for bounded queries; and (c) parallel scalability, i.e., speed up when adding processors. Moreover, Zidian can be plugged into existing SQL-over-NoSQL systems and retains horizontal scalability. Using benchmark and real-life data, we empirically verify that Zidian improves existing SQL-over-NoSQL systems by 2 orders of magnitude on average.