PAQO: Preference-aware query optimization for decentralized database systems

The declarative nature of SQL has traditionally been a major strength. Users simply state what information they are interested in, and the database management system determines the best plan for retrieving it. A consequence of this model is that should a user ever want to specify some aspect of how their queries are evaluated (e.g., a preference to read data from a specific replica, or a requirement for all joins to be performed by a single server), they are unable to. This can leave database administrators shoehorning evaluation preferences into database cost models. Further, for distributed database users, it can result in query evaluation plans that violate data handling best practices or the privacy of the user. To address such issues, we have developed a framework for declarative, user-specified constraints on the query optimization process and implemented it within PosgreSQL. Our Preference-Aware Query Optimizer (PAQO) upholds both strict requirements and partially ordered preferences that are issued alongside of the queries that it processes. In this paper, we present the design of PAQO and thoroughly evaluate its performance.