Papers by Simonas Šaltenis
IEEE Transactions on Knowledge and Data Engineering, 2021
So-called spatial web queries retrieve web content representing points of interest, such that the... more So-called spatial web queries retrieve web content representing points of interest, such that the points of interest have descriptions that are relevant to query keywords and are located close to a query location. Two broad categories of such queries exist. The first encompasses queries that retrieve single spatial web objects that each satisfy the query arguments. Most proposals belong to this category. The second category, to which this paper's proposal belongs, encompasses queries that support exploratory user behavior and retrieve sets of objects that represent regions of space that may be of interest to the user. Specifically, the paper proposes a new type of query, the top-k spatial textual cluster retrieval (k-STC) query that returns the top-k clusters that (i) are located close to a query location, (ii) contain objects that are relevant with regard to given query keywords, and (iii) have an object density that exceeds a given threshold. To compute this query, we propose a DBSCAN-based approach and an OPTICS-based approach that rely on on-line density-based clustering and that exploit early stop conditions. Empirical studies on real data sets offer evidence that the paper's proposals can find good quality clusters and are capable of excellent performance.
A location-based service called friend-locator notifies a user if the user is geographically clos... more A location-based service called friend-locator notifies a user if the user is geographically close to any of the user's friends. Services of this kind are getting increasingly popular due to the penetration of GPS in mobile phones, but existing commercial friend-locator services require users to trade their location privacy for quality of service, limiting the attractiveness of the services. The challenge is to develop a communication-efficient solution such that (i) it detects proximity between a user and the user's friends, (ii) any other party is not allowed to infer the location of the user, and (iii) users have flexible choices of their proximity detection distances. To address this challenge, we develop a client-server solution for proximity detection based on an encrypted, grid-based mapping of locations. Experimental results show that our solution is indeed efficient and scalable to a large number of users.
Emerging communication and sensor technologies enable new applications of database technology tha... more Emerging communication and sensor technologies enable new applications of database technology that require database systems to efficiently support very high rates of spatial-index updates. Previous works in this area require the availability of large amounts of main memory, do not exploit all the main memory that is indeed available, or do not support some of the standard index operations.
Managing spatially and temporally referenced data is becoming increasingly important, given the c... more Managing spatially and temporally referenced data is becoming increasingly important, given the continuing advances in wireless communications, sensor technologies, and ubiquitous computing. New types of spatial databases and applications enabled by these technologies require data management technology to store, query, and analyze spatio-temporal data. This volume contains papers presented at the Third Workshop on Spatio-Temporal Database Management, STDBM'06, co-located with the 32nd International Conference on Very Large Data Bases, VLDB 2006. This workshop follows the previous STDBM workshops respectively held in conjunction with VLDB'99 in Edinburgh and VLDB'04 in Toronto. The goal of this workshop is to bring together leading researchers and developers in the area of spatio-temporal databases in order to discuss and exchange novel research ideas and experiences with real world spatio-temporal databases. The workshop received 14 submissions. All papers were evaluated by at least three of the 42 members of the program committee. At the end nine papers of high quality were accepted and are included in these proceedings. We would like to thank the members of the workshop's steering committee for their valuable advice organizing the workshop, and program committee members for making their reviews in due time. We are thankful to our sponsor, Geospatial Information Technology Co. Ltd., for their support. We finally acknowledge the support provided by the VLDB organization, and of the CEUR organization for supporting the on-line publication of the proceedings.
Advances in Spatial and Temporal Databases
Mobile services is emerging as an important application area for spatio-temporal database managem... more Mobile services is emerging as an important application area for spatio-temporal database management technologies. Service users are often constrained to a spatial network, e.g., a road network, through which points of interest, termed data points, are accessible. Queries that implement services will often concern data points of some specific type, e.g., Thai restaurants or art museums. As a result, the relatively few data points are relevant to a query in comparison to the number of network edges, meaning that queries, e.g., k nearest-neighbor queries, must access large portions of the network. Existing query processing techniques pre-compute distances between data points and network vertices for improving the performance. However, precomputation becomes problematic when the network or data points must be updated, possibly concurrently with the querying; and if the data points are moving, the existing techniques are inapplicable. In addition, multiple pre-computed structures must be maintained-one for each type of data point. We propose a versatile pre-computation approach for spatial network data. This approach uses a grid for pre-computing a simplified network. The above-mentioned shortcomings are avoided by making the pre-computed data independent of the data points. Empirical performance studies show that the structure is competitive with respect to the existing, more specialized techniques.
Proceedings of the 6th international conference on Mobile data management, 2005
Although significant effort has been put into the development of efficient spatio-temporal indexi... more Although significant effort has been put into the development of efficient spatio-temporal indexing techniques for moving objects, little attention has been given to the development of techniques that efficiently support queries about the past, present, and future positions of objects. The provisioning of such techniques is challenging, both because of the nature of the data, which reflects continuous movement, and because of the types of queries to be supported. This paper proposes the BB x-index structure, which indexes the positions of moving objects, given as linear functions of time, at any time. The index stores linearized moving-object locations in a forest of B +-trees. The index supports queries that select objects based on temporal and spatial constraints, such as queries that retrieve all objects whose positions fall within a spatial range during a set of time intervals. Empirical experiments are reported that offer insight into the query and update performance of the proposed technique.
Lecture Notes in Computer Science, 2005
For additional information, see the DB TECH REPORTS homepage: www.cs.aau.dk/DBTR. Any software ma... more For additional information, see the DB TECH REPORTS homepage: www.cs.aau.dk/DBTR. Any software made available via DB TECH REPORTS is provided "as is" and without any express or implied warranties, including, without limitation, the implied warranty of merchantability and fitness for a particular purpose. The DB TECH REPORTS icon is made from two letters in an early version of the Rune alphabet, which was used by the Vikings, among others. Runes have angular shapes and lack horizontal lines because the primary storage medium was wood, although they may also be found on jewelry, tools, and weapons. Runes were perceived as having magic, hidden powers. The first letter in the logo is "Dagaz," the rune for day or daylight and the phonetic equivalent of "d." Its meanings include happiness, activity, and satisfaction. The second letter is "Berkano," which is associated with the birch tree. Its divinatory meanings include health, new beginnings, growth, plenty, and clearance. It is associated with Idun, goddess of Spring, and with fertility. It is the phonetic equivalent of "b."
Proceedings International Database Engineering and Applications Symposium
With the proliferation of wireless communications and the rapid advances in technologies for trac... more With the proliferation of wireless communications and the rapid advances in technologies for tracking the positions of continuously moving objects, algorithms for efficiently answering queries about large numbers of moving objects increasingly are needed. One such query is the reverse nearest neighbor (RNN) query that returns the objects that have a query object as their closest object. While algorithms have been proposed that compute RNN queries for non-moving objects, there have been no proposals for answering RNN queries for continuously moving objects. Another such query is the nearest neighbor (NN) query, which has been studied extensively and in many contexts. Like the RNN query, the NN query has not been explored for moving query and data points. This paper proposes an algorithm for answering RNN queries for continuously moving points in the plane. As a part of the solution to this problem and as a separate contribution, an algorithm for answering NN queries for continuously moving points is also proposed. The results of performance experiments are reported.
A privacy-aware proximity detection service determines if two mobile users are close to each othe... more A privacy-aware proximity detection service determines if two mobile users are close to each other without requiring them to disclose their exact locations. Existing proposals for such services provide weak privacy, give low accuracy guarantees, incur high communication costs, or lack flexibility in user preferences. We address these shortcomings with a client-server solution for proximity detection, based on encrypted, multi-level partitions of the spatial domain. Our service notifies a user if any friend users enter the user's specified area of interest, called the vicinity region. This region, in contrast to related work, can be of any shape and can be flexibly changed on the fly. Encryption and blind evaluation on the server ensures strong privacy, while low communication costs are achieved by an adaptive location-update policy. Experimental results show that the flexible functionality of the proposed solution is provided with low communication cost.
The VLDB Journal The International Journal on Very Large Data Bases, 2002
Real-world entities are inherently spatially and temporally referenced, and database applications... more Real-world entities are inherently spatially and temporally referenced, and database applications increasingly exploit databases that record the past, present, and anticipated future locations of entities, e.g., the residences of customers obtained by the geo-coding of addresses. Indices that efficiently support queries on the spatio-temporal extents of such entities are needed. However, past indexing research has progressed in largely separate spatial and temporal streams. Adding time dimensions to spatial indices, as if time were a spatial dimension, neither supports nor exploits the special properties of time. On the other hand, temporal indices are generally not amenable to extension with spatial dimensions. This paper proposes the first efficient and versatile index for a general class of spatio-temporal data: the discretely changing spatial aspect of an object may be a point or may have an extent; both transaction time and valid time are supported, and a generalized notion of the current time, now, is accommodated for both temporal dimensions. The index is based on the R *-tree and provides means of prioritizing space versus time, which enables it to adapt to spatially and temporally restrictive queries. Performance experiments are reported that evaluate pertinent aspects of the index.
The VLDB Journal, 2006
With the continued proliferation of wireless communications and advances in positioning technolog... more With the continued proliferation of wireless communications and advances in positioning technologies, algorithms for efficiently answering queries about large populations of moving objects are gaining interest. This paper proposes algorithms for k nearest and reverse k nearest neighbor queries on the current and anticipated future positions of points moving continuously in the plane. The former type of query returns k objects nearest to a query object for each time point during a time interval, while the latter returns the objects that have a specified query object as one of their k closest neighbors, again for each time point during a time interval. In addition, algorithms for so-called persistent and continuous variants of these queries are provided. The algorithms are based on the indexing of object positions represented as linear functions of time. The results of empirical performance experiments are reported.
Rapid, sustained advances in key computing technologies combine to enable a new class of computin... more Rapid, sustained advances in key computing technologies combine to enable a new class of computing services that aim to meet needs of mobile users. These ubiquitous and intelligent services adapt to each user's particular preferences and current circumstances---they are personalized. The services exploit data available from multiple sources, including data on past interactions with the users, data accessible via the Internet, and data obtained from sensors. The user's geographical location is particularly central to ...
History, 1999
Real-world objects are inherently spatially and temporally referenced, and many database applicat... more Real-world objects are inherently spatially and temporally referenced, and many database applications rely on databases that record the past, present, and anticipated future locations of, eg, people or land parcels. As a result, indices that efficiently support queries on the spatio-temporal extents of objects are needed. In contrast, past indexing research has progressed in largely separate spatial and temporal streams. In the former, focus has been on one-, two-, or three-dimensional space; and in the latter, focus has been on one or both ...
Proceedings of Scandinavian GIS, Jun 1, 2001
Abstract. We are heading rapidly towards a global computing and information infrastructure that w... more Abstract. We are heading rapidly towards a global computing and information infrastructure that will contain billions of wirelessly connected devices, many of which will offer so-called location-based services to their mobile users always and everywhere. Indeed, users will soon take ubiquitous wireless access to information and services for granted. This scenario is made possible by the rapid advances in the underlying hardware technologies, which continue to follow variants of Moore's Law.
Time-Center TR-33
Access Structures for Moving Points Mario A. Nascimento, Jefferson RO Silva and Yannis Theodoridi... more Access Structures for Moving Points Mario A. Nascimento, Jefferson RO Silva and Yannis Theodoridis September 8, 1998 TR- 33 ... Title Access Structures for Moving Points Copyright c 1998 Mario A. Nascimento, Jefferson RO Silva and Yannis Theodoridis. All rights reserved. ...
ACM Transactions on Database Systems, 2006
With the proliferation of wireless communications and geo-positioning, e-services are envisioned ... more With the proliferation of wireless communications and geo-positioning, e-services are envisioned that exploit the positions of a set of continuously moving users to provide context-aware functionality to each individual user. Because advances in disk capacities continue to outperform Moore's Law, it becomes increasingly feasible to store online all the position information obtained from the moving e-service users. With the much slower advances in I/O speeds and many concurrent users, indexing techniques are of the essence in this scenario.Existing indexing techniques come in two forms. Some techniques capture the position of an object up until the time of the most recent position sample, while other techniques represent an object's position as a constant or linear function of time and capture the position from the current time and into the (near) future. This article offers an indexing technique capable of capturing the positions of moving objects at all points in time. The ...
Proceedings of the 17th ACM SIGSPATIAL International Conference on Advances in Geographic Information Systems, 2009
New application areas, such as location-based services, rely on the efficient management of large... more New application areas, such as location-based services, rely on the efficient management of large collections of mobile objects. Maintaining accurate, up-to-date positions of these objects results in massive update loads that must be supported by spatial indexing structures and main-memory indexes are usually necessary to provide high update performance. Traditionally, the R-tree and its variants were used for indexing spatial data, but most of the recent research assumes that a simple, uniform grid is the best choice for managing moving objects in main memory. We perform an extensive experimental study to compare the two approaches on modern hardware. As the result of numerous design-and-experiment iterations, we propose the update-and query-efficient variants of the R-tree and the grid. The experiments with these indexes reveal a number of interesting insights. First, the coupling of a spatial index, grid or R-tree, with a secondary index on object IDs boosts the update performance significantly. Next, the Rtree, when combined with such a secondary index, can provide update performance competitive with the grid. Finally, the grid can compete with the R-tree in terms of the query performance and it is surprisingly robust to varying parameters of the workloads. In summary, the study shows that, in most cases, the choice of the index boils down to the issues such as the ease of implementation or the support for spatially extended objects.
Engineering Economics, 2016
Non-performing loan (NPL) problems remain urgent, or even threatening in some countries, despite ... more Non-performing loan (NPL) problems remain urgent, or even threatening in some countries, despite relatively long-lasting macroeconomic recovery since the 2008-2010 crisis. NPL handling tools and methods currently used by the banks are not efficient enough to significantly reduce the NPL ratio and keep it within safe and sustainable limits. This paper analyses the current state and causes of NPLs in retail banking and the ways of improving their handling efficiency. An idea that retail banks should not only focus on administrative handling methods, but also actively promote financial awareness and especially rational consumer behaviour of clients as measures preventing the NPL, is presented. The idea is based on a synthesis of research results performed within three segments, which shape the background of the problem: first, the current NPL situation in retail banking, theoretical framework and methods of their control, second, the budgetary and borrower performance of households and contemporary trends within consumer behaviour and, third, the NPL determinants, evaluation and comparison of their impact. Recommendations for retail banks to invest into preventive NPL reducing measures and run them parallel to the currently used "follow-up" type NPL handling tools are developed. The recommendations are supported by simulation of payback potential of investment by using specially developed algorithm. Research methods used: comparative analysis, processing of statistical data, expert evaluation, mathematical analysis.
Lecture Notes in Computer Science, 2006
This paper concerns the efficient processing of multiple k nearest neighbor queries in a road-net... more This paper concerns the efficient processing of multiple k nearest neighbor queries in a road-network setting. The assumed setting covers a range of scenarios such as the one where a large population of mobile service users that are constrained to a road network issue nearest-neighbor queries for points of interest that are accessible via the road network. Given multiple k nearest neighbor queries, the paper proposes progressive techniques that selectively cache query results in main memory and subsequently reuse these for query processing. The paper initially proposes techniques for the case where an upper bound on k is known a priori and then extends the techniques to the case where this is not so. Based on empirical studies with real-world data, the paper offers insight into the circumstances under which the different proposed techniques can be used with advantage for multiple k nearest neighbor query processing.
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Papers by Simonas Šaltenis