Cem Evrendilek

Multidatabase Query Optimization

A multidatabase system (MDBS) allows the users to simultaneously access heterogeneous,and autonomous databases using an integrated schema and a single global query language. The query optimization problem in MDBSs is quite different from the query optimization problem in distributed homogeneous databases due to schema heterogeneity and autonomy of local database systems. In this work, we consider the optimization of query distribution in case of data replication and the optimization of intersite joins, that is, the join of the results returned by the local sitesin response to the global subqueries. The algorithms presented for the optimization of intersite joins try to maximize the parallelism in execution and take the federated nature of the problem into account. It has also been shown through a comparativeperformance study that the proposed intersite join optimization algorithms are efficient.The approach presented can easily be generalized to any operation required for intersite query processing.The query optimization scheme presentedin this paper is being implemented within the scopeof a multidatabase system which is based on OMG‘sobject management architecture.

GPS-free directional localization via dual wireless radios

Location discovery, especially in mobile environments, has recently become the key component of many applications. Accurate location discovery, particularly in safety critical applications using autonomous robots or unmanned vehicles, however, is still an open problem. Existing popular methods either heavily rely on the use of global positioning systems (GPS) which do not readily lend themselves for use for the majority of applications where precision is of primary concern or are not suitable for ad-hoc deployments. In this paper, we propose a novel directional localization algorithm, called dual wireless radio localization (DWRL), which performs accurate node localizations in the plane using only distances between nodes, without the use of a GPS or nodes with known positions (anchors). The main novelty of DWRL is the use of an additional radio per node to support directional localization in static networks. To the best of our knowledge, this is the first time dual radios are employed in a localization setting. Existence of the dual radios on board enables DWRL algorithm to perform directional localization, which is not possible with existing single radio systems in static networks. We present the practical and theoretical benefits of the use of an additional radio per node in detail, test our algorithm under excessive synthetic and real-world noise scenarios, and show that DWRL algorithm is robust enough to perform directional localization even in high noise environments.

On the Complexity of Trilateration with Noisy Range Measurements

Recent developments, especially in wireless and mobile networks, have enabled the use of location based services in many application areas. Accurate location discovery, however, is still an open problem. A widely used and practical localization method is trilateration. However, trilateration works best when exact range measurements are available, which is not apparently the case in real-world due to device errors or environmental noise. In this paper, localization through trilateration when the distance measurements are imprecise, is shown to be NP-complete. Moreover, we also prove that no matter how small the ranging errors get, the problem is still intractable. This result alone justifies the need for new models for localization which are robust enough to operate even in noisy environments.

MoodView: an advanced graphical user interface for OODBMSs

OODBMSs need more than declarative query languages and programming languages as their interfaces since they are designed and implemented for complex applications requiring more advanced and easy to use visual interfaces. We have developed a complete programming environment for this purpose, called MoodView. MoodView translates all the user actions performed through its graphical interface to SQL statements and therefore it can be ported onto any object-oriented database systems using SQL. MoodView provides the database programmer with tools and functionalities for every phase of object oriented database application development. Current version of MoodView allows a database user to design, browse, and modify database schema interactively and to display class inheritance hierarchy as a directed acyclic graph. MoodView can automatically generate graphical displays for complex and multimedia database objects which can be updated through the object browser. Furthermore, a database administration tool, a full screen text-editor, a SQL based query manager, and a graphical indexing tool for the spatial data, i.e., R Trees are also implemented.

Covering oriented points in the plane with orthogonal polygons is NP-complete

We address the problem of covering points with orthogonal polygons. Specifically, given a set of n grid-points in the plane each designated in advance with either a horizontal or vertical reading, we investigate the existence of an orthogonal polygon covering these n points in such a way that each edge of the polygon covers exactly one point and each point is covered by exactly one edge with the additional requirement that the reading associated with each point dictates whether the edge covering it is to be horizontal or vertical. We show that this problem is NP-complete.

Covering points with orthogonally convex polygons

In this paper, we address the problem of covering points with orthogonally convex polygons. In particular, given a point set of size n on the plane, we aim at finding if there exists an orthogonally convex polygon such that each edge of the polygon covers exactly one point and each point is covered by exactly one edge. We show that if such a polygon exists, it may not be unique. We propose an O(nlogn) algorithm to construct such a polygon if it exists, or else report the non-existence in the same time bound. We also extend our algorithm to count all such polygons without hindering the overall time complexity. Finally, we show how to construct all k such polygons in O(nlogn+kn) time. All the proposed algorithms are fast and practical.

Covering points with orthogonal polygons

We address the problem of covering points with orthogonal polygons. Specifically, given a set of n points in the plane, we investigate the existence of an orthogonal polygon such that there is a one-to-one correspondence between the points and the edges of the polygon. In an earlier paper, we have shown that constructing such a covering with an orthogonally convex polygon, if any, can be done in O(nlogn) time. In case an orthogonally convex polygon cannot cover the point set, we show in this paper that the problem of deciding whether such a point set can be covered with any orthogonal polygon is NP-complete. The problem remains NP-complete even if the orientations of the edges covering each point are specified in advance as part of the input.

Task assignment in tree-like hierarchical structures

Many large organizations, such as corporations, are hierarchical by nature. In hierarchical organizations, each entity, except the root, is a sub-part of another entity. In this paper, we study the task assignment problem to the entities of a tree-like hierarchical organization. The inherent tree structure introduces an interesting and challenging constraint to the standard assignment problem. Given a tree rooted at a designated node, a set of tasks, and a real-valued function denoting the weight of assigning a node to a task, the Maximum Weight Tree Matching (MWTM) problem aims at finding a maximum weight matching in such a way that no tasks are left unassigned, and none of the ancestors of an already assigned node is allowed to engage in an assignment. When a task is assigned to an entity in a hierarchical organization, the whole entity including its children becomes responsible from the execution of that particular task. In other words, if an entity has been assigned to a task, neither its descendants nor its ancestors can be assigned to any task. In the paper, we formally introduce MWTM, and prove its NP-hardness. We also propose and experimentally validate an effective heuristic solution based on iterative rounding of a linear programming relaxation for MWTM.

Covering points with minimum/maximum area orthogonally convex polygons

In this paper, we address the problem of covering a given set of points on the plane with minimum and/or maximum area orthogonally convex polygons. It is known that the number of possible orthogonally convex polygon covers can be exponential in the number of input points. We propose, for the first time, an O ( n 2 ) algorithm to construct either the maximum or the minimum area orthogonally convex polygon if it exists, else report the non-existence in O ( n log ? n ) . We examine maximum area coverings of point sets using orthogonally convex polygons.If no such covering exists, we report in O ( n log ? n ) time.If covering exists, we build it in O ( n 2 ) time.Minimum area coverings can be constructed with very minor modificationsOur approach uses dynamic programming with memoization.