Optimizing object queries using an effective calculus

Abstract

Object-oriented databases (OODBs) provide powerful data abstractions and modeling facilities, but they generally lack a suitable framework for query processing and optimization. The development of an effective query optimizer is one of the key factors for OODB systems to successfully compete with relational systems, as well as to meet the performance requirements of many nontraditional applications. We propose an effective framework with a solid theoretical basis for optimizing OODB query languages. Our calculus, called the monoid comprehension calculus, captures most features of ODMG OQL, and is a good basis for expressing various optimization algorithms concisely. This article concentrates on query unnesting (also known as query decorrelation), an optimization that, even though it improves performance considerably, is not treated properly (if at all) by most OODB systems. Our framework generalizes many unnesting techniques proposed recently in the literature, and is capable of removing any form of query nesting using a very simple and efficient algorithm. The simplicity of our method is due to the use of the monoid comprehension calculus as an intermediate form for OODB queries. The monoid comprehension calculus treats operations over multiple collection types, aggregates, and quantifiers in a similar way, resulting in a uniform method of unnesting queries, regardless of their type of nesting.

The Tripod spatio-historical data model

“…Such queries are mapped to a spatio-historical calculus and then to a spatio-historical algebra. These mappings provide several opportunities for optimization using rewrite rules that are extensions to the techniques used by Fegaras and Maier for optimising object query languages [12]. Examples of these mappings and of the Tripod spatio-historical OQL have been reported in [15].…”

Three Cases for Query Decorrelation in XQuery

“…Note that although the order is destroyed on authors, both expressions produce the titles of each author in document order, as is required by the XQuery semantics for this query. While the unnesting algorithm published in [9,10] is able to unnest many more nested expressions, the resulting query does not preserve order (a hash join operator is used). In [25] unnesting is described rather informally, making it difficult to apply the technique in a general context.…”

“…With the advent of object-oriented databases and their query languages, unnesting once again attracted some attention [5,6,10,[29][30][31]. In contrast to the relational unnesting strategies, which performed unnesting at the (extended) SQL source level, most researchers from the object-oriented area preferred to describe unnesting techniques at the algebraic level.…”

“…Hence, they are only applicable to queries that do not have to retain order. Fegaras and Maier describe an alternative approach in which queries are translated into a monoid comprehension calculus representation [10]. The actual unnesting of the queries is done in terms of this calculus.…”

An Experimental Performance Evaluation of Join Algorithms for Parallel Object Databases

“…In Polar, OQL queries are compiled into parallel query execution plans (PQEPs) expressed in an object algebra based on that of [5]. All the algebraic operators are implemented as iterators [7].…”