Yu-Ling Hsueh

Efficient Updates for Continuous Skyline Computations

We address the problem of maintaining continuous skyline queriesefficiently over dynamic objects with ddimensions. Skyline queries are an important new search capability for multi-dimensional databases. In contrast to most of the prior work, we focus on the unresolved issue of frequent data object updates. In this paper we propose the ESCalgorithm, an E fficient update approach for S kyline C omputations, which creates a pre-computed second skylineset that facilitates an efficient and incremental skyline update strategy and results in a quicker response time. With the knowledge of the second skylineset, ESCenables (1) to efficiently find the substitute skyline points from the second skylineset only when removing or updating a skyline point (which we call a first skyline point) and (2) to delegate the most time-consuming skyline update computation to another independent procedure, which is executed after the complete updated query result is reported. We leverage the basic idea of the traditional BBSskyline algorithm for our novel design of a two-threaded approach. The first skyline can be replenished quickly from a small set of second skylines - hence enabling a fast query response time - while de-coupling the computationally complex maintenance of the second skyline. Furthermore, we propose the Approximate Exclusive Data Regionalgorithm (AEDR) to reduce the computational complexity of determining a candidate set for second skyline updates. In this paper, we evaluate the ESCalgorithm through rigorous simulations and compare it with existing techniques. We present experimental results to demonstrate the performance and utility of our novel approach.

Approximate continuous k nearest neighbor queries for continuous moving objects with pre-defined paths

Continuous K nearest neighbor queries (C-KNN) on moving objects retrieve the K nearest neighbors of all points along a query trajectory. In existing methods, the cost of retrieving the exact C-KNN data set is expensive, particularly in highly dynamic spatio-temporal applications. The cost includes the location updates of the moving objects when the velocities change over time and the number of continuous KNN queries posed by the moving object to the server. In some applications (e.g., finding my nearest taxies while I am moving), obtaining the perfect result set is not necessary. For such applications, we introduce a novel technique, AC-KNN, that approximates the results of the classic C-KNN algorithm, but with efficient updates and while still retaining a competitive accuracy. We evaluate the AC-KNN technique through simulations and compare it with a traditional approach. Experimental results are presented showing the utility of our new approach.

Partition-based lazy updates for continuous queries over moving objects

Continuous spatial queries posted within an environment of moving objects produce as their results a time-varying set of objects. In the most ambitious case both queries and data objects are dynamic, making it very challenging to find an efficient query evaluation strategy. The significant overhead related to frequent location updates from moving objects often results in poor performance. The most advanced existing techniques use the concept of simple geometric safe regions to delay or avoid location updates. We introduce a Partition-based Lazy Update (PLU) algorithm that elevates this idea further by adopting Location Information Tables (LIT) which (a) allow each moving object to estimate possible query movements and issue a location update only when it may affect any query results and (b) enable smart server probing that results in fewer messages. Among the significant advantages, our technique performs well even in very highly dynamic environments (with up to 100% mobility) where many other techniques deteriorate. PLU can be efficiently implemented and we demonstrate its query performance improvement of up to 28% over the current state-of-the-art.

SkyEngine: Efficient Skyline search engine for Continuous Skyline computations

Skyline query processing has become an important feature in multi-dimensional, data-intensive applications. Such computations are especially challenging under dynamic conditions, when either snapshot queries need to be answered with short user response times or when continuous skyline queries need to be maintained efficiently over a set of objects that are frequently updated. To achieve high performance, we have recently designed the ESC algorithm, an Efficient update approach for Skyline Computations. ESC creates a pre-computed candidate skyline set behind the first skyline (a “second line of defense,” so to speak) that facilitates an incremental, two-stage skyline update strategy which results in a quicker query response time for the user. Our demonstration presents the two-threaded SkyEngine system that builds upon and extends the base-features of the ESC algorithm with innovative, user-oriented functionalities that are termed SkyAlert and AutoAdjust. These functions enable a data or service provider to be informed about and gain the opportunity of automatically promoting its data records to remain part of the skyline, if so desired. The SkyEngine demonstration includes both a server and a web browser based client. Finally, the SkyEngine system also provides visualizations that reveal its internal performance statistics.

Adaptive Safe Regions for Continuous Spatial Queries over Moving Objects

Continuous spatial queries retrieve a set of time-varying objects continuously during a given period of time. However, monitoring moving objects to maintain the correctness of the query results often incurs frequent location updates from these moving objects. To address this problem, existing solutions propose lazy updates, but such techniques generally avoid only a small fraction of all unnecessary location updates because of their basic approach (e.g., safe regions, time or distance thresholds). In this paper, we introduce an Adaptive Safe Region (ASR) technique that retrieves an adjustable safe region which is continuously reconciled with the surrounding dynamic queries. In addition, we design a framework that supports multiple query types (e.g., range and c-k NN queries). In this framework, our query re-evaluation algorithms take advantage of ASRs and issue location probes only to the affected data objects. Simulation results confirm that the ASR concept improves scalability and efficiency over existing methods by reducing the number of updates.

Evaluation of Spatial Keyword Queries with Partial Result Support on Spatial Networks

Numerous geographic information system applications need to retrieve spatial objects which bear user specified keywords close to a given location. In this research, we present efficient approaches to answer spatial keyword queries on spatial networks. In particular, we formally introduce definitions of Spatial Keyword k Nearest Neighbor (SKkNN) and Spatial Keyword Range (SKR) queries. Then, we present a framework of a spatial keyword query evaluation system which is comprised of Keyword Constraint Filter (KCF), Keyword and Spatial Refinement (KSR), and the spatial keyword ranker. KCF employs an inverted index to calculate keyword relevancy of spatial objects, and KSR refines intermediate results by considering both spatial and keyword constraints with the spatial keyword ranker. In addition, we design novel algorithms for evaluating SKkNN and SKR queries. These algorithms employ the inverted index technique, shortest path search algorithms, and network Voronoi diagrams. Our extensive simulations show that the proposed SKkNN and SKR algorithms can answer spatial keyword queries effectively and efficiently.

Efficient location updates for continuous queries over moving objects

The significant overhead related to frequent location updates from moving objects often results in poor performance. As most of the location updates do not affect the query results, the network bandwidth and the battery life of moving objects are wasted. Existing solutions propose lazy updates, but such techniques generally avoid only a small fraction of all unnecessary location updates because of their basic approach (e.g., safe regions, time or distance thresholds). Furthermore, most prior work focuses on a simplified scenario where queries are either static or rarely change their positions. In this study, two novel efficient location update strategies are proposed in a trajectory movement model and an arbitrary movement model, respectively. The first strategy for a trajectory movement environment is the Adaptive Safe Region (ASR) technique that retrieves an adjustable safe region which is continuously reconciled with the surrounding dynamic queries. The communication overhead is reduced in a highly dynamic environment where both queries and data objects change their positions frequently. In addition, we design a framework that supports multiple query types (e.g., range and c-kNN queries). In this framework, our query re-evaluation algorithms take advantage of ASRs and issue location probes only to the affected data objects, without flooding the system with many unnecessary location update requests. The second proposed strategy for an arbitrary movement environment is the Partition-based Lazy Update (PLU, for short) algorithm that elevates this idea further by adopting Location Information Tables (LITs) which (a) allow each moving object to estimate possible query movements and issue a location update only when it may affect any query results and (b) enable smart server probing that results in fewer messages. We first define the data structure of an LIT which is essentially packed with a set of surrounding query locations across the terrain and discuss the mobile-side and server-side processes in correspondence to the utilization of LITs. Simulation results confirm that both the ASR and PLU concepts improve scalability and efficiency over existing methods.

Caching support for skyline query processing with partially-ordered domains

Existing methods have addressed the issue of handling each individual skyline query performed on data sets with partially ordered domains. However, it is still very challenging to process such queries for on-line applications with low response time. In this paper, we introduce a cache-based framework, called CSS, for further reducing the query processing time to support high-responsive applications. Skyline queries that were previously processed with user preferences similar to those of the current query contribute useful candidate result points. Hence, the answered queries are cached with both their results and user preferences such that the query processor can rapidly retrieve the result for a new query only from the result sets of selected queries with compatible user preferences. We introduce a similarity measure that establishes the level of similarity between the user preferences of a new query and a cached query; hence the system can start with the most similar candidates. Furthermore, if a new query is only partially answerable from the cache, then the query processor utilizes the partial result sets and performs less expensive constraint skyline queries guided by violated preferences. Furthermore, we introduce two access methods for cached queries indexed by their user preferences to only access a set of relevant cached queries for similarity measures. Extensive experiments are presented to demonstrate the performance and utility of our novel approach.

An efficient approach to finding potential products continuously

Skyline points and queries are important in the context of processing datasets with multiple dimensions. As skyline points can be viewed as representing marketable products that are useful for clients and business owners, one may also consider non-skyline points that are highly competitive with the current skyline points. We address the problem of continuously finding such potential products from a dynamic d-dimensional dataset, and formally define a potential product and its upgrade promotion cost. In this paper, we propose the CP-Sky algorithm, an efficient approach for continuously evaluating potential products by utilizing a second-order skyline set, which consists of candidate points that are closest to regular skyline points (also termed the first-order skyline set), to facilitate efficient computations and updates for potential products. With the knowledge of the second-order skyline set, CP-Sky enables the system to (1) efficiently find substitute skyline points from the second-order skyline set only if a first-order skyline point is removed, and (2) continuously retrieve the top-k potential products. Within this context, the Approximate Exclusive Dominance Region algorithm (AEDR) is proposed to reduce the computational complexity of determining a candidate set for second-order skyline updates over a dynamic data set without affecting the result accuracy. Additionally, we extend the CP-Sky algorithm to support the computations of top-k potential products. Finally, we present experimental results on data sets with various distributions to demonstrate the performance and utility of our approach. HighlightsAn efficient approach to solving the problem of continuously potential products from a dynamic d-dimensional dataset.Efficiently finding substitute skyline points from the second-order skyline set only if a first-order skyline point is removed.The Approximate Exclusive Dominance Region algorithm (AEDR) is proposed to reduce the computational complexity of determining a candidate set for second-order skyline updates over a dynamic data set.Numerous experiments with various distributions indicating that our proposed algorithm outperform existing approaches when continuously finding potential products.

PLUS: A Message-Efficient Prototype for Location-Based Applications

The PLUS system is designed to efficiently track moving object locations on a road network and execute continuous spatial queries in support of location-based services. PLUS implements a novel lazy position update mechanism that significantly reduces the communication overhead and server indexing load related to frequent location updates in moving object and moving query scenarios. The contribution of this demo is to present how the lazy position update scheme can achieve message-efficiency under various conditions which can be interactively set via user-selectable parameters in a graphical user interface.