Developing an efficient query system for encrypted XML documents

XQuery is a query and functional programming language that is designed for querying the data in XML documents. This paper addresses how to efficiently query encrypted XML documents using XQuery, with the key point being how to eliminate redundant decryption so as to accelerate the querying process. We propose a processing model that can automatically translate the XQuery statements for encrypted XML documents. The implementation and experimental results demonstrate the practicality of the proposed model.     

Query optimization in XML structured-document databases

While the information published in the form of XML-compliant documents keeps fast mounting up, efficient and effective query processing and optimization for XML have now become more important than ever. This article reports our recent advances in XML structured-document query optimization. In this article, we elaborate on a novel approach and the techniques developed for XML query optimization. Our approach performs heuristic-based algebraic transformations on XPath queries, represented as PAT algebraic expressions, to achieve query optimization. This article first presents a comprehensive set of general equivalences with regard to XML documents and XML queries. Based on these equivalences, we developed a large set of deterministic algebraic transformation rules for XML query optimization. Our approach is unique, in that it performs exclusively deterministic transformations on queries for fast optimization. The deterministic nature of the proposed approach straightforwardly renders high optimization efficiency and simplicity in implementation. Our approach is a logical-level one, which is independent of any particular storage model. Therefore, the optimizers developed based on our approach can be easily adapted to a broad range of XML data/information servers to achieve fast query optimization. Experimental study confirms the validity and effectiveness of the proposed approach.     

Indexing and querying XML using extended Dewey labeling scheme

Finding all the occurrences of a tree pattern in an XML database is a core operation for efficient evaluation of XML queries. The Dewey labeling scheme is commonly used to label an XML document to facilitate XML query processing by recording information on the path of an element. In order to improve the efficiency of XML tree pattern matching, we introduce a novel labeling scheme, called extended Dewey, which effectively extends the existing Dewey labeling scheme to combine the types and identifiers of elements in a label, and to avoid the scan of labels for internal query nodes to accelerate query processing (in I/O cost). Based on extended Dewey, we propose a series of holistic XML tree pattern matching algorithms. We first present TJFast to answer an XML twig pattern query. To efficiently answer a generalized XML tree pattern, we then propose GTJFast, an optimization that exploits the non-output nodes. In addition, we propose TJFastTL and GTJFastTL based on the tag + level data partition scheme to further reduce I/O costs by level pruning. Finally, we report our comprehensive experimental results to show that our set of XML tree pattern matching algorithms are superior to existing approaches in terms of the number of elements scanned, the size of intermediate results and query performance.     

Secure query processing against encrypted XML data using Query-Aware Decryption

Dissemination of XML data on the internet could breach the privacy of data providers unless access to the disseminated XML data is carefully controlled. Recently, the methods using encryption have been proposed for such access control. However, in these methods, the performance of processing queries has not been addressed. A query processor cannot identify the contents of encrypted XML data unless the data are decrypted. This limitation incurs overhead of decrypting the parts of the XML data that would not contribute to the query result. In this paper, we propose the notion of Query-Aware Decryption for efficient processing of queries against encrypted XML data. Query-Aware Decryption allows us to decrypt only those parts that would contribute to the query result. For this purpose, we disseminate an encrypted XML index along with the encrypted XML data. This index, when decrypted, informs us where the query results are located in the encrypted XML data, thus preventing unnecessary decryption for other parts of the data. Since the size of this index is much smaller than that of the encrypted XML data, the cost of decrypting this index is negligible compared with that for unnecessary decryption of the data itself. The experimental results show that our method improves the performance of query processing by up to six times compared with those of existing methods. Finally, we formally prove that dissemination of the encrypted XML index does not compromise security.     

XML文档压缩技术比较研究

XML已经成为互联网上信息交换和信息表示的事实标准。然而XML文档中包含大量重复出现的标签和结构等冗余信息,导致XML文档在查询处理和数据交换时付出更高的代价,特别在带宽和资源受限的设备上显得更为突出。压缩技术是解决这一问题的重要途径。搜集了近几年提出的各种XML压缩方法,从压缩率、压缩与解压时间、内存消耗、查询性能等方面比较分析了六个具有代表性的XML压缩技术,最后简要归纳了各自的优点和存在的不足,并探讨未来努力的方向。     

XML压缩算法的分析与比较

XML为在Web上发布和交换异质数据提供了相当的灵活性。但由于这种语言自身就有冗余的特性所以XML文档在体积都大于有相同数据内容的其他类型的文档。随着XML在Web上应用的扩展其数据大小自然也会随之而增加,而这实质上增加了数据的存储量、处理量和交换量,因此XML文档的体积问题也阻碍了XML的应用,特别是阻碍了XML在具有带宽和内存容量限制的应用上如移动通讯应用。在这篇文章中,我们将大致的介绍一下最近提出的几种针对XML的压缩算法并分析它们在解决XML文档体积问题上的技术和效能。     

Comparative Analysis of XML Compression Technologies

XML provides flexibility in publishing and exchanging heterogeneous data on the Web. However, the language is by nature verbose and thus XML documents are usually larger in size than other specifications containing the same data content. It is natural to expect that the data size will continue to grow as XML data proliferates on the Web. The size problem of XML documents hinders the applications of XML, since it substantially increases the costs of storing, processing and exchanging the data. The hindrance is more apparent in bandwidth- and memory-limited settings such as those applications related to mobile communication. In this paper, we survey a range of recently proposed XML specific compression technologies and study their efforts and capabilities to overcome the size problem. First, by categorizing XML compression technologies into queriable and unqueriable compressors, we explain the efforts in the representative technologies that aim at utilizing the exposed structure information from the input XML documents. Second, we discuss the importance of queriable XML compressors and assess whether the compressed XML documents generated from these technologies are able to support direct querying on XML data. Finally, we present a comparative analysis of the state-of-the-art XML conscious compression technologies in terms of compression ratio, compression and decompression times, memory consumption, and query performance.     

XML压缩算法的分析与比较

XML为在Web上发布和交换异质数据提供了相当的灵活性。但由于这种语言自身就有冗余的特性所以XML文档在体积都大于有相同数据内容的其他类型的文档。随着XML在web上应用的扩展其数据大小自然也会随之而增加．而这实质上增加了数据的存储量、处理量和交换量，因此XML文档的体积问题也阻碍了XML的应用，特别是阻碍了XML在具有带宽和内存容量限制的应用上如移动通讯应用。在这篇文章中，我们将大致的介绍一下最近提出的几种针对XML的压缩算法并分析它们在解决XML文档体积问题上的技术和效能。     

利用自动机高效处理XML路径表达式查询

在XML查询处理中,应用于绝大多数XML查询语言中的路径表达式在定位和查询XML数据和数据的结构关系方面具有极强的表达能力,并且由于XML数据的半结构化性,使得XML路径表达式查询的查询处理技术的研究与传统的数据库查询处理技术相比有着全新的特点和挑战.一些目前已有的查询处理技术可以用来处理路径表达式,但是查询处理中产生的大量中间结果导致了这些方法应用在大规模XML文档和复杂的路径表达式查询中时查询效率急剧下降.文中利用自动机技术设计了一个处理XML路径表达式查询的高效方法--SAM.SAM的基本思想是将路径表达式查询转化成一个与之完全等价的自动机,然后将其与从XML文档中抽象出来的模式路径相匹配.文中同时也给出了基于SAM方法的针对路径表达式中"//"操作符计算的有效解决方案.实验证明:SAM是一种非常有效的查询方法,在计算大数据量复杂路径表达式查询时具有非常高的效率,是一种实用的XML路径表达式查询方法.     

XCluster:基于聚类支持查询的XML多文档压缩方法

XML已成为各种网络应用中数据存储和数据交换的标准.XML数据管理面临的最大困难在于结构与数据混合存储导致大量数据冗余,这极大地增加了XML数据存储、交换和处理的代价.对XML文档进行压缩可以在一定程度上解决这个问题.但现有XML压缩方法大都仅压缩单文档中的冗余信息.利用XML文档间的相似性,提出一种支持查询的多XML文档压缩存储方法XCluster.XCluster先利用XML有根有序标签树上改进的pq-gram近似距离对XML文档集进行层次聚类;然后合并每个聚类结果子集中文档的结构得到结构代表并进行字典编码压缩;同时合并不同文档内同一标签下的值内容,并根据其数据类型进行相应编码压缩.实验结果表明,在真实和生成的XML多文档数据集上,XCluster比XGrind和XQilla具有更好的压缩效果和查询效率.     

Accomplishing Deterministic XML Query Optimization

As the popularity of XML (extensible Markup Language) keeps growing rapidly, the management of XML compliant structured-document databases has become a very interesting and compelling research area. Query optimization for XML structured-documents stands out as one of the most challenging research issues in this area because of the much enlarged optimization (search) space, which is a consequence of the intrinsic complexity of the underlying data model of XML data. We therefore propose to apply deterministic transformations on query expressions to most aggressively prune the search space and fast achieve a sufficiently improved alternative (if not the optimal) for each incoming query expression. This idea is not just exciting but practically attainable. This paper first provides an overview of our optimization strategy, and then focuses on the key implementation issues of our rule-based transformation system for XML query optimization in a database environment. The performance results we obtained from experimentation show that our approach is a valid and effective one.     

XML压缩方法的比较分析

搜集近几年提出的各种XML压缩方法,比较、分析具有代表性的XML压缩技术。实验表明,XMill拥有出色的压缩率、较少的压缩与解压时间、较低的内存消耗,以及最优的平均压缩性能,但不支持对压缩文档的直接查询。XGRIND与XPRESS的压缩性能不如XMill,却能较好地支持对压缩文档的直接查询。总结XML压缩方法各自的优点和不足,并探讨未来努力的方向。     

A syntactic approach to twig-query matching on XML streams

Query matching on XML streams is challenging work for querying efficiency when the amount of queried stream data is huge and the data can be streamed in continuously. In this paper, the method Syntactic Twig-Query Matching (STQM) is proposed to process queries on an XML stream and return the query results continuously and immediately. STQM matches twig queries on the XML stream in a syntactic manner by using a lexical analyzer and a parser, both of which are built from our lexical-rules and grammar-rules generators according to the user's queries and document schema, respectively. For query matching, the lexical analyzer scans the incoming XML stream and the parser recognizes XML structures for retrieving every twig-query result from the XML stream. Moreover, STQM obtains query results without a post-phase for excluding false positives, which are common in many streaming query methods. Through the experimental results, we found that STQM matches the twig query efficiently and also has good scalability both in the queried data size and the branch degree of the twig query. The proposed method takes less execution time than that of a sequence-based approach, which is widely accepted as a proper solution to the XML stream query.     

XML数据的路径表达式查询优化技术

路径表达式作为XML数据查询语言的核心部分,关于它的计算方法的研究成果已有很多,然而针对路径表达式本身进行优化的研究却相对较少.提出了两种针对路径表达式的优化策略:路径缩短策略和补路径策略,从而提高了XML路径查询效率.路径缩短策略根据XML文档模式信息,将路径表达式查询长度缩短,从而简化查询本身以降低需要的查询代价;而补路径策略则试图使用代价更小的等价路径表达式来替换原始查询.经过对实验数据的分析,这两种优化策略对于绝大多数路径表达式查询可以应用,并可大幅度地改进路径表达式的查询性能.     

RRSi: indexing XML data for proximity twig queries

Twig query pattern matching is a core operation in XML query processing. Indexing XML documents for twig query processing is of fundamental importance to supporting effective information retrieval. In practice, many XML documents on the web are heterogeneous and have their own formats; documents describing relevant information can possess different structures. Therefore some “user-interesting” documents having similar but non-exact structures against a user query are often missed out. In this paper, we propose the RRSi, a novel structural index designed for structure-based query lookup on heterogeneous sources of XML documents supporting proximate query answers. The index avoids the unnecessary processing of structurally irrelevant candidates that might show good content relevance. An optimized version of the index, oRRSi, is also developed to further reduce both space requirements and computational complexity. To our knowledge, these structural indexes are the first to support proximity twig queries on XML documents. The results of our preliminary experiments show that RRSi and oRRSi based query processing significantly outperform previously proposed techniques in XML repositories with structural heterogeneity.

Coding-based Join Algorithms for Structural Queries on Graph-Structured XML Document

In many applications, XML documents need to be modelled as graphs. The query processing of graph-structured XML documents brings new challenges. In this paper, we design a method based on labelling scheme for structural queries processing on graph-structured XML documents. We give each node some labels, the reachability labelling scheme. By extending an interval-based reachability labelling scheme for DAG by Rakesh et al., we design labelling schemes to support the judgements of reachability relationships for general graphs. Based on the labelling schemes, we design graph structural join algorithms to answer the structural queries with only ancestor-descendant relationship efficiently. For the processing of subgraph query, we design a subgraph join algorithm. With efficient data structure, the subgraph join algorithm can process subgraph queries with various structures efficiently. Experimental results show that our algorithms have good performance and scalability. Support by the Key Program of the National Natural Science Foundation of China under Grant No.60533110; the National Grand Fundamental Research 973 Program of China under Grant No. 2006CB303000; the National Natural Science Foundation of China under Grant No. 60773068 and No. 60773063.     

PS+Pre/Post: A novel structure and access mechanism for wireless XML stream supporting twig pattern queries

XML data broadcast is an efficient way to disseminate XML data to a large number of mobile clients in mobile wireless networks. Recently, several indexing methods have been proposed to improve the performance of XML query processing in terms of access time and tuning time over XML streams. However, existing indexing methods cannot process twig pattern XML queries. In this paper, we propose a novel structure for streaming XML data called PS+Pre/Post by integrating the path summary technique and the pre/post labeling scheme. Our proposed XML stream structure exploits the benefits of the path summary technique and the pre/post labeling scheme to efficiently process different types of XML queries over the broadcast stream. Experimental results show that our proposed XML stream structure improves the performance of access time and tuning time in processing different types of XML queries.     

XML数据查询的并行算法

目前已经提出了多种查询XML数据的方法,然而这些传统的方法不能充分利用多处理器和多核心处理器的优势。本文提出了一种XML查询的并行算法,大幅提高了共享存储器多处理器、多核心处理器系统中XML数据的查询效率。     

Native XML数据库的文档编码机制研究

Native XML数据库快速查询的实现,可以采用基于XML文档编码的结构连接算法。而结构连接算法的实现需要对XML文档进行编码,以便于快速判断XML文档树结点之间的祖先后裔关系。在对现有编码机制进行综述的前提下,提出一种新的XML文档编码机制——前缀整除编码（PDIV）机制。该机制编码形式简单,只需要一个正整数即可充分表示结点在XML文档树中的位置信息;可以实现祖先后裔关系的快速查询;支持XML文档的更新操作;编码长度较短,编码长度约为o（ln（n））。     

RPE query processing and optimization techniques for XML databases

An extent join to compute path expressions containing parent-children and ancestor-descendent operations and two path expression optimization rules, path-shortening and path-complementing, are presented in this paper. Path-shortening reduces the number of joins by shortening the path while path-complementing optimizes the path execution by using an equivalent complementary path expression to compute the original one. Experimental results show that the algorithms proposed are more efficient than traditional algorithms.