Iosif Lazaridis

Progressive approximate aggregate queries with a multi-resolution tree structure

Answering aggregate queries like SUM, COUNT, MIN, MAX, AVG in an approximate manner is often desirable when the exact answer is not needed or too costly to compute. We present an algorithm for answering such queries in multi-dimensional databases, using selective traversal of a Multi-Resolution Aggregate (MRA) tree structure storing point data. Our approach provides 100% intervals of confidence on the value of the aggregate and works iteratively, coming up with improving quality answers, until some error requirement is satisfied or time constraint as reached. Using the same technique we can also answer aggregate queries exactly and our experiments indicate that even for exact answering the proposed data structure and algorithm are very fast.

Capturing sensor-generated time series with quality guarantees

We are interested in capturing time series generated by small wireless electronic sensors. Battery-operated sensors must avoid heavy use of their wireless radio which is a key cause of energy dissipation. When many sensors transmit, the resources of the recipient of the data are taxed; hence, limiting communication will benefit the recipient as well. We show how time series generated by sensors can be captured and stored in a database system (archive). Sensors compress time series instead of sending them in raw form. We propose an optimal online algorithm for constructing a piecewise constant approximation (PCA) of a time series which guarantees that the compressed representation satisfies an error bound on the L/sub /spl infin// distance. In addition to the capture task, we often want to estimate the values of a time series ahead of time, e.g., to answer real-time queries. To achieve this, sensors may fit predictive models on observed data, sending parameters of these models to the archive. We exploit the interplay between prediction and compression in a unified framework that avoids duplicating effort and leads to reduced communication.

Dynamic Queries over Mobile Objects

Increasingly applications require the storage and retrieval of spatio-temporal information in a database management system. A type of such information is mobile objects, i.e., objects whose location changes continuously with time. Various techniques have been proposed to address problems of incorporating such objects in databases. In this paper, we introduce new query processing techniques for dynamic queries over mobile objects, i.e., queries that are themselves continuously changing with time. Dynamic queries are natural in situational awareness systems when an observer is navigating through space. All objects visible by the observer must be retrieved and presented to her at very high rates, to ensure a high-quality visualization. We show how our proposed techniques offer a great performance improvement over a traditional approach of multiple instantaneous queries.

Querying Mobile Objects in Spatio-Temporal Databases

In dynamic spatio-temporal environments where objects may continuously move in space, maintaining consistent information about the location of objects and processing motion-specific queries is a challenging problem. In this paper, we focus on indexing and query processing techniques for mobile objects. Specifically, we develop a classification of different types of selection queries that arise in mobile environments and explore efficient algorithms to evaluate them. Query processing algorithms are developed for both native space and parametric space indexing techniques. A performance study compares the two indexing strategies for different types of queries.

Approximate selection queries over imprecise data

We examine the problem of evaluating selection queries over imprecisely represented objects. Such objects are used either because they are much smaller in size than the precise ones (e.g., compressed versions of time series), or as imprecise replicas of fast-changing objects across the network (e.g., interval approximations for time-varying sensor readings). It may be impossible to determine whether an imprecise object meets the selection predicate. Additionally, the objects appearing in the output are also imprecise. Retrieving the precise objects themselves (at additional cost) can be used to increase the quality of the reported answer. We allow queries to specify their own answer quality requirements. We show how the query evaluation system may do the minimal amount of work to meet these requirements. Our work presents two important contributions: first, by considering queries with set-based answers, rather than the approximate aggregate queries over numerical data examined in the literature; second, by aiming to minimize the combined cost of both data processing and probe operations in a single framework. Thus, we establish that the answer accuracy/performance tradeoff can be realized in a more general setting than previously seen.

Sensor data collection with expected reliability guarantees

Due to the fragility of small sensors, their finite energy supply and the loss of packets in the wireless channel, reports from sensors may not reach the sink node. In this paper we consider the problem of sensor data collection in the presence of faults in sensor networks. We develop a data collection protocol, which provides expected reliability guarantees while minimizing resource consumption by adaptively adjusting the number of retransmissions based on current network fault conditions.

Distributed Virtual Reality Authoring Interfaces for the WWW: The VR-Shop Case

Electronic commerce is emerging as an important domain of integration and enhancement of more specific technologies and research efforts. It is clear that the role of WWW in this context is a cornerstone as the medium of information dissemination. A trend in e-commerce is to provide to the potential customers the ability to view and “try” the products in a persuasive 3D representation. We have designed and implemented a system for WWW enabled interactive design & visualization of a room, definition of pieces of furniture and placement of domestic appliances. The system conveys a generic approach for distributed creation and update of virtual worlds as means of interaction and information dissemination in an e-commerce context.

Optimization of multi-version expensive predicates

Modern query optimizers need to take into account the performance of expensive user-defined predicates. Existing research has shown how to incorporate such predicates in a traditional cost-based query optimizer. In this paper we deal with the optimization of the expensive predicates themselves, showing how their cost can be reduced by utilizing cheaper, but less accurate, versions of the predicates to pre-filter tuples. We discuss the generalized tuple handling mechanism, which processes tuples along a fixed sequence of versions, as well as adaptive approaches that either split tuple streams into groups, or make routing decisions at the individual tuple level. We identify the lower bound to the problem of evaluating a multi-version selection predicate by an ideal individualized plan (IIP), and develop an optimal generalized plan (OGP). We then show how realistic individualized or grouped schemes can produce an intermediate cost between OGP and IIP, if tuples substantially deviate from the average stream behavior. Our algorithms are tested experimentally, identifying many of the issues that arise whenever multi-version predicates are used.

Fault Tolerant Evaluation of Continuous Selection Queries over Sensor Data

We consider the problem of evaluating continuous selection queries over sensor-generated values in the presence of faults. Small sensors are fragile, have finite energy and memory, and communicate over a lossy medium; hence, tuples produced by them may not reach the querying node, resulting in an incomplete and ambiguous answer, as any of the non-reporting sensors may have produced a tuple which was lost. We develop a protocol, FAult Tolerant Evaluation of Continuous Selection Queries (FATE-CSQ), which guarantees a user-requested level of quality in an efficient manner. When many faults occur, this may not be achievable; in that case, we aim for the best possible answer, under the querys time constraints. FATE-CSQ is designed to be resilient to different kinds of failures. Our design decisions are based on an analytical model of different fault tolerance strategies based on feedback and retransmission. Additionally, we evaluate FATE-CSQ and competing protocols with realistic simulation parameters under a variety of conditions, demonstrating its good performance.

Stedel: a language for interactive spatio-temporal compositions

The two fundamental challenges in Virtual Reality over the WWW are (i) to facilitate the process of creating convincing, non-trivial, highly interactive and maintainable content and (ii) to efficiently deliver this content to the client with QoS guarantees. This paper focuses on the first challenge: we have created a data model for spatio-temporal compositions that allows us to define relative placement of objects in a virtual world by means of their geometric characteristics (Joints). Thus, we model spatial relationships of objects at a higher level of abstraction than simple co-ordinate system transformations. We also define the behavior of objects in a declarative way, by means of Event-Condition-Action rules. Events and Actions can be synthesized by a set of operators for the creation of really complex behavior. Our model has been realized in prototype form in Spatio-Temporal Descriptive Language (STEDEL) that demonstrates the validity and power of our approach in terms of re-usability and authorship effort.