Abstract
The integration of Logic Programming and Relational Databases has many advantages: on one hand, a better flexibility is obtained in the use of relational databases by allowing deductive inference and unification; on the other handy techniques developed within a rich body of literature on databases can be exploited to speed up the execution of logic programs.
The main point of this chapter is to allow Horn clauses to be expressed and computed in terms of the Relational Algebra operators, even in the case of recursive clauses.
Our work builds on and completes the results of Zaniolo who has extended Relational Algebra to deal with unification in non-recursive Horn clauses.
Recursive Horn clauses are considered as set transformations, hence the set of tuples satisfying a predicate defined by a system of (mutually) recursive Horn clauses is an inductively defined set, namely the smallest set closed under these transformations.
Closure sets can be computed by an iterative algorithm traversing a cyclic graph whose nodes are labelled by extended relational algebra operators.
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Aiello, L., Cecchi, C. (1989). Adding a Closure Operator to the Extended Relational Algebra: A Further Step Towards the Integration of Database Techniques and Logic Programming. In: Schmidt, J.W., Thanos, C. (eds) Foundations of Knowledge Base Management. Topics in Information Systems. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-83397-7_9
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DOI: https://doi.org/10.1007/978-3-642-83397-7_9
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