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.