  We study the problem of deciding satisfiability of first order logic queries
over views, our aim being to delimit the boundary between the decidable and the
undecidable fragments of this language. Views currently occupy a central place
in database research, due to their role in applications such as information
integration and data warehousing. Our main result is the identification of a
decidable class of first order queries over unary conjunctive views that
generalises the decidability of the classical class of first order sentences
over unary relations, known as the Lowenheim class. We then demonstrate how
various extensions of this class lead to undecidability and also provide some
expressivity results. Besides its theoretical interest, our new decidable class
is potentially interesting for use in applications such as deciding implication
of complex dependencies, analysis of a restricted class of active database
rules, and ontology reasoning.
