Tail Recursion Elimination in Deductive Databases

Abstract

We consider an optimization technique for deductive and relational databases. The optimization technique is an extension of the magic templates rewriting, and it can improve the performance of query evaluation by not materializing the extension of intermediate views. Standard relational techniques, such as unfolding embedded view definitions, do not apply to recursively defined views, and so alternative techniques are necessary. We demonstrate the correctness of our rewriting. We define a class of "nonrepeating" view definitions, and show that for certain queries our rewriting performs at least as well as magic templates on nonrepeating views, and often much better. A syntactically recognizable property, called "weak right-linearity," is proposed. Weak right-linearity is a sufficient condition for nonrepetition, and is more general than right-linearity. Our technique gives the same benefits as right-linear evaluation of right-linear views, while applying to a significantly more general class of views.