A practical algorithm for evaluating database queries
John H. M. de Vet · Software Practice and Experience · 1989
Abstract This paper describes an algorithm for evaluating database queries represented as expressions in a logical language. Such a database query expression can be evaluated efficiently by focusing on the variable dependencies. The algorithm recursively computes the values of subexpressions to evaluate the input expression, but it avoids re‐evaluation of those subexpressions whose values are not affected by new variable assignments. The input expression is internally structured as a directed acyclic graph. Two additional techniques to improve efficiency of the evaluation are discussed: transformations of the input expression and special primitive database operations. Finally, its implementation in the natural language question‐answering system SPICOS is described.