Convolution Sum-Product Queries

Abstract: We study query evaluation for an extension of sum-product queries (SPQ) that allows atoms with linear combinations of variables (e.g. $A(2X+3Y-Z, Y+Z)$), which we call convolution sum-product queries (CSPQs). These queries arise in both practical settings (e.g. image processing workloads) and theoretical ones (e.g. the $(\min,+)$-convolution and $k$-SUM conjectures), and capture SPQs with added linear equality constraints. While prior work has considered the impact of linear in- and dis-equality constraints on query evaluation, the techniques developed in that setting are asymptotically sub-optimal for CSPQs. To address this, we describe several evaluation algorithms for CSPQs that leverage linear algebra techniques like rank analysis, quotient spaces, and variable substitution. First, we adapt Worst-Case Optimal Joins to CSPQs, achieving a runtime similar in spirit to that for conjunctive queries. Then, we extend the definition of tree decompositions (TDs) to CSPQs, and describe a factorized execution. In this extension, linear combinations are first-class citizens and play the same role as variables in traditional TDs. We define three width measures that bound the complexity of this execution with respect to the size of the domain values, of the active domain, and of the relation's support. Lastly, we show how these methods can be applied to arbitrary fields beyond the rationals.