On Preservation under Homomorphisms and Unions of Conjunctive Queries

Unions of conjunctive queries, also known as select-project-join-union queries, are the most frequently asked queries in relational database systems. These queries are definable by existential positive first-order formulas and are preserved under homomorphisms. A classical result of mathematical logic asserts that the existential positive formulas are the only first-order formulas (up to logical equivalence) that are preserved under homomorphisms on all structures, finite and infinite. After resisting resolution for a long time, it was eventually shown that, unlike other classical preservation theorems, the homomorphism-preservation theorem holds for the class of all finite structures. In this paper, we show that the homomorphism-preservation theorem holds also for several restricted classes of finite structures of interest in graph theory and database theory. Specifically, we show that this result holds for all classes of finite structures of bounded degree, all classes of finite str...

• Type: Conference
• Year: 2004
• Where: PODS
• Authors: Albert Atserias, Anuj Dawar, Phokion G. Kolaitis
• Keywords: Database, Existential Positive Formulas, Finite Structures, PODS 2004, Positive First-order Formulas