Translating QQL into Monoid Comprehensions : Stuck with Nested Loops? ; Revised Version

Abstract

This work tries to employ the monoid comprehension calculus - which has proven to be an adequate framework to capture the semantics of modern object query languages featuring a family of collection types like sets, bags, and lists - in a twofold manner: First, serving as a target language for the translation of ODMG OQL queries. We review work done in this field and also give comprehension calculus equivalents for the recently introduced OQL 1.2 concepts.Second, we use monoid comprehensions as the formalism in which we try to find efficient execution methods working on a rich set of physical structures (including indices, vertical and horizontal decomposition, etc.). The main problem coming up here is the "nested-loop nature" of the calculus expressions. While these loop-based semantics for evaluating comprehensions at least provide a way for executing OQL queries, their execution is almost always much less efficient than alternative physical algorithms of the database engine (e.g. different types of joins).We are able to capture these efficient algorithms in the comprehension formalism, too. Furthermore, comprehensions are amenable to pattern-matching driven rewriting. This paper studies query rewrite optimization using monoid comprehensions and shows that efficient implementation plans can be derived in this framework.