Structures for Manipulating Proposed

Support for virtual states and deltas between them is useful for a variety of database applications, including hypothetical database access, version management, simulation, and active databases. The Heraclitus paradigm elevates delta values to be \\rst-class citizens" in database programming languages, so that they can be explicitly created, accessed and manipulated. A fundamental issue concerns the trade-oo between the \accuracy" or \robustness" of a form of delta representation, and the ease of access and manipulation of that form. At one end of the spectrum, code-blocks could be used to represent delta values, resulting in a more accurate capture of the intended meaning of a proposed update, at the cost of more expensive access and manipulation. In the context of object-oriented databases, another point on the spectrum is \attribute-granularity" deltas which store the net changes to each modiied attribute value of modiied objects. This paper introduces a comprehensive framework for specifying a broad array of forms for representing deltas for complex value types (tuple, set, bag, list, o-set and dictionary). In general, the granularity of such deltas can be arbitrarily deep within the complex value structure. Applications of this framework in connection with hypothetical access to, and \merging" of, proposed updates are discussed.