Jensen, Christian S.

Abstract: This paper considers the problem of efficiently maintaining a clustering of a dynamic set of data points that move continuously in two-dimensional Euclidean space. This problem has received little attention and introduces new challenges to clustering. The paper proposes a new scheme that is capable of incrementally clustering moving objects. This proposal employs a notion of object dissimilarity that considers object movement across a period of time, and it employs clustering features that can be maintained efficiently in incremental fashion. In the proposed scheme, a quality measure for incremental clusters is used for identifying clusters that are not compact enough after certain insertions and deletions. An extensive experimental study shows that the new scheme performs significantly faster than traditional ones that frequently rebuild clusters. The study also shows that the new scheme is effective in preserving the quality of moving-object clusters.

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Abstract: Multiple representation of geographic information occurs when a real-world entity is represented more than once in the same or different databases. This occurs frequently in practice, and it invariably results in the occurrence of inconsistencies among the different representations of the same entity. In this paper, we propose an approach to the modeling of multiple represented entities, which is based on the relationships among the entities and their representations. Central to our approach is the Multiple Representation Schema Language that, by intuitive and declarative means, is used to specify rules that match objects representing the same entity, maintain consistency among these representations, and restore consistency if necessary. The rules configure a Multiple Representation Management System, the aim of which is to manage multiple representations over a number of autonomous federated databases. We present a graphical and a lexical binding to the schema language. The graphical binding is built on an extension to the Unified Modeling Language and the Object Constraint Language. We demonstrate that it is possible to implement the constructs of the schema language in the object-relational model of a commercial RDBMS.

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Abstract: This paper assumes an Internet-service scenario where objects that have not reported their position within a specified duration of time are expected to no longer be interested in, or of interest to, the service. Due to the possibility of many “expiring” objects, a highly dynamic database results. The paper presents an R-tree based technique for the indexing of the current positions of such objects. Different types of bounding regions are studied, and new algorithms are provided for maintaining the tree structure. Performance experiments indicate that, when compared to the approach where the objects are not assumed to expire, the new indexing technique can improve search performance by a factor of two or more without sacrificing update performance.

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Abstract: The current plethora of diverse and incompatible temporal data models has an impeding effect on the design of a consensus temporal data model. A single data model is highly desirable, both to the temporal database community and to the database user community at large. It is our contention that the simultaneous foci on the modeling, presentation, representation and querying of temporal data have been a major cause of the proliferation of models. We advocate instead a separation of concerns. As the next step, we propose a data model for the single, central task of temporal data modeling. In this model, tuples are stamped with bitemporal elements, i.e., sets of pairs of valid and transaction time chonons. This model has no intension of being suitable for the other tasks, where existing models may perhaps be more appropriate. However, this model does capture time-varying data in a natural way.

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Abstract: This paper describes a new, conceptual temporal data model that better captures the time-dependent semantics of the data while permitting multiple data models at the representation level. This conceptual model effectively moves the disticetion between the various existing data models from a semantic basis to a physical, performance-relevant basis. We define a conceptual notion of a bitemporal relation where tuples are stamped with sets of two-dimensionalchronons in transaction-time/valid-time space. Next, we describe three representation schemes: a tuple-timestamped 1NF representation, a backlog relation composed of 1NF timestamped change requests, and a non-1NF attribute value-timestamped representation. We further investigate several variants of these representations. (distinguishes between data presentation and data storage issues)

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