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We provide new techniques for the analysis of the expressive power of query languages for nested collections. These languages may use set or bag semantics and may be further complicated by the presence of aggregate functions. We exhibit certain classes of graphics and prove that properties of these graphics that can be tested in such languages are either finite or cofinite. This result settles that conjectures of Grumbach, Milo, and Paredaens that parity test, transitive closure, and balanced binary tree test are not expressible in bah languages like BALG of Grumbach and Milo and BQL of Libkin and Wong. Moreover, it implies that many recursive queries, including simple ones like test for a chain, cannot be expressed in a nested relational language even when aggregate functions are available. In an attempt to generalize the finite-cofiniteness result, we study the bounded degree property which says that the number of distinct in- and out-degrees in the output of a graph query does not depend on the size of the input if the input is "simple." We show that such a property implies a number of inexpressibility results in a uniform fashion. We then prove the bounded degree property for the nested relational language.
Leonid Libkin and Limsoon Wong, "A Bounded Degree Property and Finite-Cofiniteness of Graph Queries", . December 1993.
Date Posted: 13 July 2007