Johann-Christoph Freytag

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The Web of Linked Data forms a single, globally distributed dataspace. Due to the openness of this dataspace, it is not possible to know in advance all data sources that might be relevant for query answering. This openness poses a new challenge that is not addressed by traditional research on federated query processing. In this paper we present an approach(More)
Today's DBMSs are unable to support the increasing demands of the various applications that would like to use a DBMS. Each kind of application poses new requirements for the DBMS. The Starburst project at IBM's Almaden Research Center aims to extend relational DBMS technology to bridge this gap between applications and the DBMS. While providing a full(More)
We present Stratosphere, an open-source software stack for parallel data analysis. Stratosphere brings together a unique set of features that allow the expressive, easy, and efficient programming of analytical applications at very large scale. Stratosphere’s features include “in situ” data processing, a declarative query language, treatment of user-defined(More)
Cleansing data from impurities is an integral part of data processing and maintenance. This has lead to the development of a broad range of methods intending to enhance the accuracy and thereby the usability of existing data. This paper presents a survey of data cleansing problems, approaches, and methods. We classify the various types of anomalies(More)
Integrated access to information that is spread over multiple, distributed, and heterogeneous sources is an important problem in many sci-entiic and commercial domains. While much work has been done on query processing and choosing plans under cost criteria, very little is known about the important problem of incorporating the information quality aspect(More)
For many information domains there are numerous World Wide Web data sources. The sources vary both in their extension and their intension: They represent different real world entities with possible overlap and provide different attributes of these entities. Mediator-based information systems allow integrated access to such sources by providing a common(More)
Complex queries often contain common or similar subexpressions, either within a single query or among multiple queries submitted as a batch. If so, query execution time can be improved by evaluating a common subexpression once and reusing the result in multiple places. However, current query optimizers do not recognize and exploit similar subexpressions,(More)
The use of semantic knowledge in its various forms has become an important aspect in managing data in database and information systems. In the form of integrity constraints , it has been used intensively in query optimization for some time. Similarly, data integration techniques have utilized semantic knowledge to handle heterogeneity for query processing(More)
Query execution over the Web of Linked Data has attracted much attention recently. A particularly interesting approach is link traversal based query execution which proposes to integrate the traversal of data links into the creation of query results. Hence -in contrast to traditional query execution paradigms- this does not assume a fixed set of relevant(More)
Data flows are a popular abstraction to define dataintensive processing tasks. In order to support a wide range of use cases, many data processing systems feature MapReduce-style user-defined functions (UDFs). In contrast to UDFs as known from relational DBMS, MapReduce-style UDFs have less strict templates. These templates do not alone provide all the(More)