Deep Learning Models for Selectivity Estimation of Multi-Attribute Queries

Selectivity estimation - the problem of estimating the result size of queries - is a fundamental problem in databases. Accurate estimation of query selectivity involving multiple correlated attributes is especially challenging. Poor cardinality estimates could result in the selection of bad plans by the query optimizer. Recently, deep learning has been applied to this problem with promising results. However, many of the proposed approaches often struggle to provide accurate results for multi attribute queries involving large number of predicates and with low selectivity. In this paper, we propose two complementary approaches that are effective for this scenario. Our first approach models selectivity estimation as a density estimation problem where one seeks to estimate the joint probability distribution from a finite number of samples. We leverage techniques from neural density estimation to build an accurate selectivity estimator. The key idea is to decompose the joint distribution into a set of tractable conditional probability distributions such that they satisfy the autoregressive property. Our second approach formulates selectivity estimation as a supervised deep learning problem that predicts the selectivity of a given query. We describe how to extend our algorithms for range queries. We also introduce and address a number of practical challenges arising when adapting deep learning for relational data. These include query/data featurization, incorporating query workload information in a deep learning framework and the dynamic scenario where both data and workload queries could be updated. Our extensive experiments with a special emphasis on queries with a large number of predicates and/or small result sizes demonstrates that our proposed techniques provide fast and accurate selective estimates with minimal space overhead.