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The Global ETD Search service is a free service for researchers
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Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
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Showing 21 to 30 of 31 (0.016 seconds)| 21 |
Practical Query Rewriting for DL-Lite with Numerical Predicates: Extended VersionAlrabbaa, Christian, Koopmann, Patrick, Turhan, Anni-Yasmin 20 June 2022 (has links)
We present a method for answering ontology-mediated queries for DL-Lite extended with a concrete domain, where we allow concrete domain predicates to be used in the query as well. Our method is based on query rewriting, a well-known technique for ontology-based query answering (OBQA), where the knowledge provided by the ontology is compiled into the query so that the rewritten query can be evaluated directly over a database. This technique reduces the problem of query answering w.r.t. an ontology to query evaluation over a database instance. Specifically, we consider members of the DL-Lite family extended with unary and binary concrete domain predicates over the real numbers. While approaches for query rewriting DL-Lite with these concrete domain have been investigated theoretically, these approaches use a combined approach in which also the data is processed, and require the concrete domain values occurring in the data to be known in advance, which makes the procedure data-dependent. In contrast, we show how rewritings can be computed in a data-independent fashion.
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On Implementing Temporal Query Answering in DL-LiteThost, Veronika, Holste, Jan, Özçep, Özgür 20 June 2022 (has links)
Ontology-based data access augments classical query answering over fact bases by adopting the open-world assumption and by including domain knowledge provided by an ontology. We implemented temporal query answering w.r.t. ontologies formulated in the Description Logic DL-Lite. Focusing on temporal conjunctive queries (TCQs), which combine conjunctive queries via the operators of propositional linear temporal logic, we regard three approaches for answering them: an iterative algorithm that considers all data available; a window-based algorithm; and a rewriting approach, which translates the TCQs to be answered into SQL queries. Since the relevant ontological knowledge is already encoded into the latter queries, they can be answered by a standard database system. Our evaluation especially shows that implementations of both the iterative and the window-based algorithm answer TCQs within a few milliseconds, and that the former achieves a constant performance, even if data is growing over time.
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Temporal Query Answering in DL-Lite with NegationBorgwardt, Stefan, Thost, Veronika 20 June 2022 (has links)
Ontology-based query answering augments classical query answering in databases by adopting the open-world assumption and by including domain knowledge provided by an ontology. We investigate temporal query answering w.r.t. ontologies formulated in DL-Lite, a family of description logics that captures the conceptual features of relational databases and was tailored for efficient query answering. We consider a recently proposed temporal query language that combines conjunctive queries with the operators of propositional linear temporal logic (LTL). In particular, we consider negation in the ontology and query language, and study both data and combined complexity of query entailment.
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Relaxation of Subgraph Queries Delivering Empty ResultsVasilyeva, Elena, Thiele, Maik, Mocan, Adrian, Lehner, Wolfgang 16 September 2022 (has links)
Graph databases with the property graph model are used in multiple domains including social networks, biology, and data integration. They provide schema-flexible storage for data of a different degree of a structure and support complex, expressive queries such as subgraph isomorphism queries. The exibility and expressiveness of graph databases make it difficult for the users to express queries correctly and can lead to unexpected query results, e.g. empty results. Therefore, we propose a relaxation approach for subgraph isomorphism queries that is able to automatically rewrite a graph query, such that the rewritten query is similar to the original query and returns a non-empty result set. In detail, we present relaxation operations applicable to a query, cardinality estimation heuristics, and strategies for prioritizing graph query elements to be relaxed. To determine the similarity between the original query and its relaxed variants, we propose a novel cardinality-based graph edit distance. The feasibility of our approach is shown by using real-world queries from the DBpedia query log.
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fAST Refresh using Mass Query OptimizationLehner, Wolfgang, Cochrane, Bobbie, Pirahesh, Hamid, Zaharioudakis, Markos 02 June 2022 (has links)
Automatic summary tables (ASTs), more commonly known as materialized views, are widely used to enhance query performance, particularly for aggregate queries. Such queries access a huge number of rows to retrieve aggregated summary data while performing multiple joins in the context of a typical data warehouse star schema. To keep ASTs consistent with their underlying base data, the ASTs are either immediately synchronized or fully recomputed. This paper proposes an optimization strategy for simultaneously refreshing multiple ASTs, thus avoiding multiple scans of a large fact table (one pass for AST computation). A query stacking strategy detects common sub-expressions using the available query matching technology of DB2. Since exact common sub-expressions are rare, the novel query sharing approach systematically generates common subexpressions for a given set of 'related' queries, considering different predicates, grouping expressions, and sets of base tables. The theoretical framework, a prototype implementation of both strategies in the IBM DB2 UDB/UWO database system, and performance evaluations based on the TPC/R data schema are presented in this paper.
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Demonstrating Efficient Query Processing in Heterogeneous EnvironmentsKarnagel, Tomas, Hille, Matthias, Ludwig, Mario, Habich, Dirk, Lehner, Wolfgang, Heimel, Max, Markl, Volker 30 June 2022 (has links)
The increasing heterogeneity in hardware systems gives developers many opportunities to add more functionality and computational power to the system. As a consequence, modern database systems will need to be able to adapt to a wide variety of heterogeneous architectures. While porting single operators to accelerator architectures is well-understood, a more generic approach is needed for the whole database system. In prior work, we presented a generic hardware-oblivious database system, where the operators can be executed on the main processor as well as on a large number of accelerator architectures. However, to achieve fully heterogeneous query processing, placement decisions are needed for the database operators. We enhance the presented system with heterogeneity-aware operator placement (HOP) to take a major step towards designing a database system that can efficiently exploit highly heterogeneous hardware environments. In this demonstration, we are focusing on the placement-integration aspect as well as presenting the resulting database system.
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Generating What-If Scenarios for Time Series DataKegel, Lars, Hahmann, Martin, Lehner, Wolfgang 18 August 2022 (has links)
Time series data has become a ubiquitous and important data source in many application domains. Most companies and organizations strongly rely on this data for critical tasks like decision-making, planning, predictions, and analytics in general. While all these tasks generally focus on actual data representing organization and business processes, it is also desirable to apply them to alternative scenarios in order to prepare for developments that diverge from expectations or assess the robustness of current strategies. When it comes to the construction of such what-if scenarios, existing tools either focus on scalar data or they address highly specific scenarios. In this work, we propose a generally applicable and easy-to-use method for the generation of what-if scenarios on time series data. Our approach extracts descriptive features of a data set and allows the construction of an alternate version by means of filtering and modification of these features.
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Temporal Conjunctive Queries in Expressive DLs with Non-simple RolesBaader, Franz, Borgwardt, Stefan, Lippmann, Marcel 20 June 2022 (has links)
In Ontology-Based Data Access (OBDA), user queries are evaluated over a set of facts under the open world assumption, while taking into account background knowledge given in the form of a Description Logic (DL) ontology. Motivated by situation awareness applications, temporal conjunctive queries (TCQs) have recently been proposed as a useful extension of traditional OBDA to support the processing of temporal information. This paper extends the existing complexity analysis of TCQ entailment to very expressive DLs underlying the OWL 2 standard, and in contrast to previous work also allows for queries containing transitive roles. / This is an extended version of the paper “Temporal Conjunctive Queries in Expressive Description Logics with Transitive Roles”, published in the Proceedings of the 28th Australasian Joint Conference on Artificial Intelligence (AI’15).
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Ultraschnelle optoelektronische und Materialeigenschaften von Stickstoff-haltigem GaAsSinning, Steffen 03 March 2006 (has links) (PDF)
This work summarizes properties of nitrogen containing GaAs, which are relevant for optoelectronic application and allow a deeper insight in the physics of this material. In the first part the dependence of the banggap energy of nitrogen implanted GaAs on several process parameters (implanted nitrogen concentration, implantation temperature, annealing duration and temperature) is investigated. The second part focuses on the relaxation dynamics of highly excited carriers. For this, the carrier relaxation dynamics in nitrogen implanted GaAs, in epitaxially grown GaAsN and in (pure) GaAs are investigated by means of pump probe measurements on a femtosecond time scale. The comparision of experimental results to calculated scattering rates leads to relevant informations of scattering mechanisms and electronic properties. / Diese Arbeit widmet sich Eigenschaften von Stickstoff-haltigem Gallium-Arsenid, die sowohl für das physikalische Verständnis als auch für optoelektronische Anwendungen dieses Materials relevant sind. Im ersten Teil dieser Arbeit wird die Abhängigkeit der Bandlücken-Energie von verschiedenen Prozess-Parametern (Stickstoffkonzentration, Implantationstemperatur, Ausheildauer und -temperatur) in Stickstoff-implantiertem GaAs untersucht. Der zweite Teil konzentriert sich auf die Relaxationsdynamik hoch angeregter Ladungsträger. Neben dem oben bereits angesprochenen Material wird in Anrege-Abfrage-Experimenten mit Femtosekunden-Zeitauflösung zusätzlich epitaktisch gewachsenes GaAsN und (Stickstoff-freies) GaAs untersucht. Die Berechnung der Streuraten und der Vergleich mit experimentell gewonnenen Daten liefert wesentliche Informationen über beteiligte Steumechanismen und elektronische Eigenschaften.
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Photo-magnonics in two-dimensional antidot latticesLenk, Benjamin 12 December 2012 (has links)
No description available.
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