Estimating Predicate Selectivities in a NoSQL Database Service

An estimate of the number of items in a database that satisfy an equality or range predicate is useful for several tasks, such as cost-based query optimization, provisioning of system resources, and determining the financial costs of using database services. In a traditional database system, such estimates are computed and used internally by the system and have been well studied. In contrast, such estimates have not received much attention in the context of a cloud-based database service, where they must be computed by the application that uses the service using only the limited features of the interface provided by the service. This paper motivates and formulates the selectivity-estimation problem for database services. It describes the characteristics of this problem that distinguish it from the analogous problem in traditional database systems. It outlines some subproblems and methods to address them. It provides a method for estimating selectivities based on random sampling along with some experimental results.