Considerable effort has been devoted to minimizing XPath queries under the assumption that the minimal query is faster than the original query. However, little attention has been paid to the validity of the assumption. In this paper, we provide a detailed analysis on the effectiveness of XPath query minimization and present an extensive experimental evaluation on the effectiveness using six publicly available XQuery engines. To the best of our knowledge, this is the first work done towards this objective. Experiments on real and synthetic data sets show that although the assumption is valid for some cases, the performance of the minimal query is often lower than or almost equal to that of the original query.

This letter introduces a new reference frame to improve the performance of motion estimation and compensation in video coding, based on a video stabilization technique. The proposed method synthesizes the new reference frame from the previous frame in a way that the new reference and current frames have the same camera orientations. The overhead data for each frame to transmit from an encoder to a decoder is only three rotational angles along the x, y, and z axes. Since the new reference and current frames have the same camera orientations, the proposed method significantly improves the performance of motion estimation and compensation for video sequences having dynamic camera motion by up to 0.98 dB with negligible overhead data.

As data volumes explode, data storage costs become a large fraction of total IT costs. We can reduce the costs substantially by using compression. However, it is generally known that database compression is not suitable for write-intensive workloads. In this paper, we provide a comprehensive solution to improve the performance of compressed databases for write-intensive OLTP workloads. We find that storing data too densely in compressed pages incurs many future page splits, which require exclusive locks. In order to avoid lock contention, we reduce page splits by sacrificing a couple of percent of space savings. We reserve enough space in each compressed page for future updates of records and prevent page merges that are prone to incur page splits in the near future. The experimental results using TPC-C benchmark and MySQL/InnoDB show that our method gives 1.5 times higher throughput with 33% space savings compared with the uncompressed counterpart and 1.8 times higher throughput with only 1% more space compared with the state-of-the-art compression method developed by Facebook.

XQuery has become the standard for querying XML. Just like SQL, XQuery allows nested expressions. To optimize XQuery processing, a lot of research has been done on normalization, i.e., transforming nested expressions to equivalent unnested ones. Previous normalization rules are classified into two categories—source-level/ and algebra-level/—depending on whether a construct is specified in the XQuery syntax or as equivalent algebraic expressions. From an implementation point of view, the former is preferable to the latter since it can be implemented in a variety of XQuery engines with different algebras. However, existing source-level rules have several problems: They do not handle quantified expressions, incur duplicated query results, and use many temporary files. In this paper, we propose new source-level normalization rules that solve these problems. Through analysis and experiments, we show that our normalization rules can reduce query execution time from hours to a few seconds and can be adapted to a variety of XQuery engines.