In this lecture many applications process high volumes of streaming data, among them Internet traffic analysis, financial tickers, and transaction log mining. In general, a data stream is an unbounded data set that is produced incrementally over time, rather than being available in full before its processing begins. In this lecture, we give an overview of recent research in stream processing, ranging from answering simple queries on high-speed streams to loading real-time data feeds into a streaming warehouse for off-line analysis.We will discuss two types of systems for end-to-end stream processing: Data Stream Management Systems (DSMSs) and Streaming Data Warehouses (SDWs). A traditional database management system typically processes a stream of ad-hoc queries over relatively static data. In contrast, a DSMS evaluates static (long-running) queries on streaming data, making a single pass over the data and using limited working memory. In the first part of this lecture, we will discuss research problems in DSMSs, such as continuous query languages, non-blocking query operators that continually react to new data, and continuous query optimization. The second part covers SDWs, which combine the real-time response of a DSMS by loading new data as soon as they arrive with a data warehouse's ability to manage Terabytes of historical data on secondary storage.
This paper addresses the problem of explaining missing answers in queries that include selection, projection, join, union, aggregation and grouping (SPJUA). Explaining missing answers of queries is useful in various scenarios, including query understanding and debugging. We present a general framework for the generation of these explanations based on source data. We describe the algorithms used to generate a correct, finite, and, when possible, minimal set of explanations. These algorithms are part of Artemis, a system that assists query developers in analyzing queries by, for instance, allowing them to ask why certain tuples are not in the query results. Experimental results demonstrate that Artemis generates explanations of missing tuples at a pace that allows developers to effectively use them for query analysis.
Answering Why-Not questions consists in explaining to developers of complex data transformations or manipulations why their data transformation did not produce some specific results, although they expected them to do so. Different types of explanations that serve as Why-Not answers have been proposed in the past and are either based on the available data, the query tree, or both. Solutions (partially) based on the query tree are generally more efficient and easier to interpret by developers than solutions solely based on data. However, algorithms producing such query-based explanations so far may return different results for reordered conjunctive query trees, and even worse, these results may be incomplete. Clearly, this represents a significant usability problem, as the explanations developers get may be partial and developers have to worry about the query tree representation of their query, losing the advantage of using a declarative query language. As remedy to this problem, we propose the Ted algorithm that produces the same complete querybased explanations for reordered conjunctive query trees.
Abstract. Fuzzy duplicate detection aims at identifying multiple representations of real-world objects in a data source, and is a task of critical relevance in data cleaning, data mining, and data integration tasks. It has a long history for relational data, stored in a single table or in multiple tables with an equal schema. However, algorithms for fuzzy duplicate detection in more complex structures, such as hierarchies of a data warehouse, XML data, or graph data have only recently emerged. These algorithms use similarity measures that consider the duplicate status of their direct neighbors to improve duplicate detection effectiveness. In this chapter, we study different approaches that have been proposed for XML fuzzy duplicate detection. Our study includes a description and analysis of the different approaches, as well as a comparative experimental evaluation performed on both artificial and real-world data. The two main dimensions used for comparison are the methods effectiveness and efficiency. Our comparison shows that the DogmatiX system [44] is the most effective overall, as it yields the highest recall and precision values for various kinds of differences between duplicates. Another system, called XMLDup [27] has a similar performance, being most effective especially at low recall values. Finally, the SXNM system [36] is the most efficient, as it avoids executing too many pairwise comparisons, but its effectiveness is greatly affected by errors in the data.
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