Performance comparison of point and spatial access methods

In the past few years a large number of multidimensional point access methods, also called multiattribute index structures, has been suggested, all of them claiming good performance. Since no performance comparison of these structures under arbitrary (strongly correlated nonuniform, short "ugly") data distributions and under various types of queries has been performed, database researchers and designers were hesitant to use any of these new point access methods. As shown in a recent paper, such point access methods are not only important in traditional database applications. In new applications such as CAD/CIM and geographic or environmental information systems, access methods for spatial objects are needed. As recently shown such access methods are based on point access methods in terms of functionality and performance. Our performance comparison naturally consists of two parts. In part I we w i l l compare multidimensional point access methods, whereas in part I I spatial access methods for rectangles will be compared. In part I we present a survey and classification of existing point access methods. Then we carefully select the following four methods for implementation and performance comparison under seven different data files (distributions) and various types of queries: the 2-level grid file, the BANG file, the hB-tree and a new scheme, called the BUDDY hash tree. We were surprised to see one method to be the clear winner which was the BUDDY hash tree. It exhibits an at least 20 % better average performance than its competitors and is robust under ugly data and queries. In part I I we compare spatial access methods for rectangles. After presenting a survey and classification of existing spatial access methods we carefully selected the following four methods for implementation and performance comparison under six different data files (distributions) and various types of queries: the R-tree, the BANG file, PLOP hashing and the BUDDY hash tree. The result presented two winners: the BANG file and the BUDDY hash tree. This comparison is a first step towards a standardized testbed or benchmark. We offer our data and query files to each designer of a new point or spatial access method such that he can run his implementation in our testbed

In-medium operator product expansion for heavy-light-quark pseudoscalar mesons

The operator product expansion (OPE) for heavy-light-quark pseudoscalar mesons (D-mesons and B-mesons) in medium is determined, both for a moving meson with respect to the surrounding medium as well as for a meson at rest. First of all, the OPE is given in terms of normal-ordered operators up to mass dimension 5, and the mass of the heavy-quark and the mass of the light-quark are kept finite. The Wilson coefficients of such an expansion are infrared (IR) divergent in the limit of a vanishing light-quark mass. A consistent separation of scales necessitates an OPE in terms of non-normal-ordered operators, which implies operator mixing, where the IR-divergences are absorbed into the operators. It is shown that the Wilson coefficients of such an expansion are IR-stable, and the limit of a vanishing light-quark mass is perfomed. Details of the major steps for the calculation of the Wilson coefficients are presented. By a comparison with previous results obtained by other theoretical groups we have found serious disagreements.Comment: 51 pages, 3 figure

A Ritual Approach to Deterrence

How can ritual help to elucidate the symbolism of deterrence? Traditional deterrence scholarship tends to overlook the active role of deterring actors in creating and redefining the circumstances to which they are allegedly only reacting. In order to address the weight of deterrence as a symbol, collective representation and strategic repertoire, this paper proposes to rethink deterrence as a performative and generative (ontological) security practice with ritual features, functions and effects. Riding the ‘fourth wave’ of deterrence studies, the paper posits a novel theoretical account for understanding the mediation of ambiguity in extended deterrence. Drawing on the example of NATO’s Enhanced Forward Presence (eFP) in the Alliance’s eastern flank, the proposed conceptualization of extended deterrence as an interaction ritual chain in allied defence, solidarity and community-building offers new insights about the deterrence and collective identity nexus. The case of eFP throws into stark relief the ontological security significance of ritualizing allied deterrence. Lacking social and political consensus about the severity of a threat in question, the ritualization of deterrence serves as a potent valve for communicating the credibility and commitment in an extended deterrence situation, along with answering the identity demands of a collective actor

Indexing Dense Nested Metric Spaces for Efficient Similarity Search ⋆

Abstract. Searching in metric spaces is a very active field since it offers methods for indexing and searching by similarity in collections of unstructured data. These methods select some objects of the collection as reference objects to build the indexes. It has been shown that the way the references are selected affects the search performance, and several algorithms for good reference selection have been proposed. Most of them assume the space to have a reasonably regular distribution. However, in some spaces the objects are grouped in small dense clusters that can make these methods perform worse than a random selection. In this paper, we propose a new method able to detect these situations and adapt the structure of the index to them. Our experimental evaluation shows that our proposal is more efficient than previous approaches when using the same amount of memory.