4,057 research outputs found
An LSH Index for Computing Kendall's Tau over Top-k Lists
We consider the problem of similarity search within a set of top-k lists
under the Kendall's Tau distance function. This distance describes how related
two rankings are in terms of concordantly and discordantly ordered items. As
top-k lists are usually very short compared to the global domain of possible
items to be ranked, creating an inverted index to look up overlapping lists is
possible but does not capture tight enough the similarity measure. In this
work, we investigate locality sensitive hashing schemes for the Kendall's Tau
distance and evaluate the proposed methods using two real-world datasets.Comment: 6 pages, 8 subfigures, presented in Seventeenth International
Workshop on the Web and Databases (WebDB 2014) co-located with ACM SIGMOD201
Histogram-Aware Sorting for Enhanced Word-Aligned Compression in Bitmap Indexes
Bitmap indexes must be compressed to reduce input/output costs and minimize
CPU usage. To accelerate logical operations (AND, OR, XOR) over bitmaps, we use
techniques based on run-length encoding (RLE), such as Word-Aligned Hybrid
(WAH) compression. These techniques are sensitive to the order of the rows: a
simple lexicographical sort can divide the index size by 9 and make indexes
several times faster. We investigate reordering heuristics based on computed
attribute-value histograms. Simply permuting the columns of the table based on
these histograms can increase the sorting efficiency by 40%.Comment: To appear in proceedings of DOLAP 200
A Nine Month Progress Report on an Investigation into Mechanisms for Improving Triple Store Performance
This report considers the requirement for fast, efficient, and scalable triple stores as part of the effort to produce the Semantic Web. It summarises relevant information in the major background field of Database Management Systems (DBMS), and provides an overview of the techniques currently in use amongst the triple store community. The report concludes that for individuals and organisations to be willing to provide large amounts of information as openly-accessible nodes on the Semantic Web, storage and querying of the data must be cheaper and faster than it is currently. Experiences from the DBMS field can be used to maximise triple store performance, and suggestions are provided for lines of investigation in areas of storage, indexing, and query optimisation. Finally, work packages are provided describing expected timetables for further study of these topics
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