Lehner, Wolfgang, Ruf, Thomas, Teschke, Michael
13 September 2022
Statistical and scientific computing applications exhibit characteristics that are fundamentally different from classical database system application domains. The CROSS-DB data model presented in this paper is optimized for use in such applications by providing advanced data modelling methods and application-oriented query facilities, thus providing a framework for optimized data management procedures. CROSS-DB (which stands for Classification-oriented, Redundancy-based Optimization of Statistical and Scientific DataBases) is based on a multidimensional data view. The model differs from other approaches by o~ering two complementary rnechanisrnsfor structuring qualifying information, classification and feature description. Using these mechanisms results in a normalized, low-dimensional database schema which ensures both, modelling uniqueness and understandability while providing enhanced modelling flexibility.
Onder, Ibrahim Seckin
01 January 2010
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Increasing performance of computers and ability to connect computers with high speed communication networks make distributed databases systems an attractive research area. In this study, we evaluate communication and data processing capabilities of a HPC machine. We calculate accurate cost formulas for high volume data communication between processing nodes and experimentally measure sorting times. A left deep query plan executer has been implemented and experimentally used for executing plans generated by two different genetic algorithms for a distributed database environment using message passing paradigm to prove that a parallel system can provide scalable performance by increasing the number of nodes used for storing database relations and processing nodes. We compare the performance of plans generated by genetic algorithms with optimal plans generated by exhaustive search algorithm. Our results have verified that optimal plans are better than those of genetic algorithms, as expected.
Improving The Communication Performance Of I/O Intensive And Communication Intensive Application In Cluster Computer SystemsKumar, V Santhosh 10 1900 (has links)
Cluster computer systems assembled from commodity off-the-shelf components have emerged as a viable and cost-effective alternative to high-end custom parallel computer systems.In this thesis, we investigate how scalable performance can be achieved for database systems on clusters. In this context we specﬁcally considered database query processing for evaluation of botlenecks and suggest optimization techniques for obtaining scalable application performance. First we systematically demonstrated that in a large cluster with high disk bandwidth, the processing capability and the I/O bus bandwidth are the two major performance bottlenecks in database systems. To identify and assess bottlenecks, we developed a Petri net model of parallel query execution on a cluster. Once identiﬁed and assessed,we address the above two performance bottlenecks by offoading certain application related tasks to the processor in the network interface card. Offoading application tasks to the processor in the network interface cards shifts the bottleneck from cluster processor to I/O bus. Further, we propose a hardware scheme,network attached disk ,and a software scheme to achieve a balanced utilization of re-sources like host processor, I/O bus, and processor in the network interface card. The proposed schemes result in a speedup of upto 1.47 compared to the base scheme, and ensures scalable performance upto 64 processors. Encouraged by the beneﬁts of ofﬂoading application tasks to network processors, we explore the possibilities of performing the bloom ﬁlter operations in network processors. We combine ofﬂoading bloom ﬁlter operations with the proposed hardware schemes to achieve upto 50% reduction in execution time. The later part of the thesis provides introductory experiments conducted in Community At-mospheric Model(CAM), a large scale parallel application used for global weather and climate prediction. CAM is a communication intensive application that involves collective communication of large messages. In our limited experiment, we identiﬁed CAM to see the effect of compression techniques and ofﬂoading techniques (as formulated for database) on the performance of communication intensive applications. Due to time constraint, we considered only the possibility of compression technique for improving the application performance. However, ofﬂoading technique could be taken as a full-ﬂedged research problem for further investigation In our experiment, we found compression of messages reduces the message latencies, and hence improves the execution time and scalability of the application. Without using compression techniques, performance measured on 64 processor cluster resulted in a speed up of only 15.6. While lossless compression retains the accuracy and correctness of the program, it does not result in high compression. We therefore propose lossy compression technique which can achieve a higher compression, yet retain the accuracy and numerical stability of the application while achieving a scalable performance. This leads to speedup of 31.7 on 64 processors compared to a speedup of 15.6 without message compression. We establish that the accuracy within prescribed limit of variation and numerical stability of CAM is retained under lossy compression.
Woltmann, Lucas, Hartmann, Claudio, Thiele, Maik, Habich, Dirk, Lehner, Wolfgang
14 June 2022
Cardinality estimation is a fundamental task in database query processing and optimization. Unfortunately, the accuracy of traditional estimation techniques is poor resulting in non-optimal query execution plans. With the recent expansion of machine learning into the field of data management, there is the general notion that data analysis, especially neural networks, can lead to better estimation accuracy. Up to now, all proposed neural network approaches for the cardinality estimation follow a global approach considering the whole database schema at once. These global models are prone to sparse data at training leading to misestimates for queries which were not represented in the sample space used for generating training queries. To overcome this issue, we introduce a novel local-oriented approach in this paper, therefore the local context is a specific sub-part of the schema. As we will show, this leads to better representation of data correlation and thus better estimation accuracy. Compared to global approaches, our novel approach achieves an improvement by two orders of magnitude in accuracy and by a factor of four in training time performance for local models.
Lehner, Wolfgang, Rösch, Philipp
22 April 2022
With the amount of data in current data warehouse databases growing steadily, random sampling is continuously gaining in importance. In particular, interactive analyses of large datasets can greatly benefit from the significantly shorter response times of approximate query processing. Typically, those analytical queries partition the data into groups and aggregate the values within the groups. Further, with the commonly used roll-up and drill-down operations a broad range of group-by queries is posed to the system, which makes the construction of highly-specialized synopses difficult. In this paper, we propose a general-purpose sampling scheme that is biased in order to answer group-by queries with high accuracy. While existing techniques focus on the size of the group when computing its sample size, our technique is based on its standard deviation. The basic idea is that the more homogeneous a group is, the less representatives are required in order to give a good estimate. With an extensive set of experiments, we show that our approach reduces both the estimation error and the construction cost compared to existing techniques.
Klein, Anja, Gemulla, Rainer, Rösch, Philipp, Lehner, Wolfgang
10 November 2022
Although approximate query processing is a prominent way to cope with the requirements of data analysis applications, current database systems do not provide integrated and comprehensive support for these techniques. To improve this situation, we propose an SQL extension---called SQL/S---for approximate query answering using random samples, and present a prototypical implementation within the engine of the open-source database system Derby---called Derby/S. Our approach significantly reduces the required expert knowledge by enabling the definition of samples in a declarative way; the choice of the specific sampling scheme and its parametrization is left to the system. SQL/S introduces new DDL commands to easily define and administrate random samples subject to a given set of optimization criteria. Derby/S automatically takes care of sample maintenance if the underlying dataset changes. Finally, samples are transparently used during query processing, and error bounds are provided. Our extensions do not affect traditional queries and provide the means to integrate sampling as a first-class citizen into a DBMS.
Schmidt, Sven, Legler, Thomas, Schär, Sebastian, Lehner, Wolfgang
08 August 2023
Processing data streams with Quality-of-Service (QoS) guarantees is an emerging area in existing streaming applications. Although it is possible to negotiate the result quality and to reserve the required processing resources in advance, it remains a challenge to adapt the DSMS to data stream characteristics which are not known in advance or are difficult to obtain. Within this paper we present the second generation of our QStream DSMS which addresses the above challenge by using a real-time capable operating system environment for resource reservation and by applying an adaptation mechanism if the data stream characteristics change spontaneously.
Albrecht, Jens, Hümmer, Wolfgang, Lehner, Wolfgang, Schlesinger, Lutz
10 January 2023
Materialized summary tables and cached query results are frequently used for the optimization of aggregate queries in a data warehouse. Query rewriting techniques are incorporated into database systems to use those materialized views and thus avoid the access of the possibly huge raw data. A rewriting is only possible if the query is derivable from these views. Several approaches can be found in the literature to check the derivability and find query rewritings. The specific application scenario of a data warehouse with its multidimensional perspective allows the consideration of much more semantic information, e.g. structural dependencies within the dimension hierarchies and different characteristics of measures. The motivation of this article is to use this information to present conditions for derivability in a large number of relevant cases which go beyond previous approaches.
Luong, Johannes, Habich, Dirk, Lehner, Wolfgang
13 June 2022
Data driven organizations gather information on various aspects of their endeavours and analyze that information to gain valuable insights or to increase automatization. Today, these organizations can choose from a wealth of specialized analytical libraries and platforms to meet their functional and non-functional requirements. Indeed, many common application scenarios involve the combination of multiple such libraries and platforms in order to provide a holistic perspective. Due to the scattered landscape of specialized analytical tools, this integration can result in complex and hard to evolve applications. In addition, the necessary movement of data between tools and formats can introduce a serious performance penalty. In this article we present a unified programming environment for analytical applications. The environment includes AL, a programming language that combines concepts of various common analytical domains. Further, the environment also includes a flexible compilation system that uses a language-, domain-, and platform independent program intermediate representation to separate high level application logic and physical organisation. We provide a detailed introduction of AL, establish our program intermediate representation as a generally useful abstraction, and give a detailed explanation of the translation of AL programs into workloads for our experimental shared-memory processing engine.
Idris, Muhammad, Ugarte, Martín, Vansummeren, Stijn, Voigt, Hannes, Lehner, Wolfgang
15 June 2022
Modern application domains such as Composite Event Recognition (CER) and real-time Analytics require the ability to dynamically refresh query results under high update rates. Traditional approaches to this problem are based either on the materialization of subresults (to avoid their recomputation) or on the recomputation of subresults (to avoid the space overhead of materialization). Both techniques have recently been shown suboptimal: instead of materializing results and subresults, one can maintain a data structure that supports efficient maintenance under updates and can quickly enumerate the full query output, as well as the changes produced under single updates. Unfortunately, these data structures have been developed only for aggregate-join queries composed of equi-joins, limiting their applicability in domains such as CER where temporal joins are commonplace. In this paper, we present a new approach for dynamically evaluating queries with multi-way θ-joins under updates that is effective in avoiding both materialization and recomputation of results, while supporting a wide range of applications. To do this we generalize Dynamic Yannakakis, an algorithm for dynamically processing acyclic equi-join queries. In tandem, and of independent interest, we generalize the notions of acyclicity and free-connexity to arbitrary θ-joins. We instantiate our framework to the case where θ-joins are only composed of equalities and inequalities (<, ≤, =, >, ≥) and experimentally compare this algorithm, called IEDyn, to state of the art CER systems as well as incremental view maintenance engines. IEDyn performs consistently better than the competitor systems with up to two orders of magnitude improvements in both time and memory consumption.
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