Optimizing updates of recursive XML views of relations

XML publishing has been an emerging technique for transforming (portions of) a relational database into an XML document, for example, to facilitate interoperability between heterogeneous applications. Such applications may update the XML document and the source relational database must be updated accordingly. In this paper, we consider such XML documents as (possibly) recursively defined XML views of relations. We propose new optimization techniques to efficiently support XML view updates specified via an XPATH expression with recursion and complex filters. The main novelties of our techniques are: (1) we propose a space-efficient relational encoding of recursive XML views; and (2) we push the bulk of update processing inside a relational database. Specifically, a compressed representation of the XML views is stored as extended shared-inlining relations. A space-efficient and updatable 2-hop index is used to optimize XPATH evaluation on XML views. Updates of the XML views are evaluated on these relations and index. View update translation is handled by a heuristic procedure inside a relational database, as opposed to previous middleware approaches. We present an experimental study to demonstrate the effectiveness of our proposed techniques.     

Updating XML views published over relational databases: Towards the existence of a correct update mapping

XML data management using relational database systems has been intensively studied in the last few years. However, in order for such systems to be viable, they must support not only queries, but also updates over virtual XML views that wrap the relational data. While view updating is a long-standing difficult issue in the relational context, the flexible XML data model and nested XML query language both pose additional challenges for view updating.

This paper addresses the question, if for a given update over an XML view, a correct relational update translation exists. First, we propose a clean extended-source theory as criteria for determining whether a given translation mapping is correct. To determine the existence of such a correct mapping, we classify a view update as either un-translatable, conditionally or unconditionally translatable under a given update translation policy. This classification depends on several features of the XML view and the update: (a) granularity of the update at the view side, (b) properties of the view construction, and (c) types of duplication appearing in the view. These features are represented in the Annotated Schema Graph. This is further utilized by our Schema-driven Translatability Reasoning algorithm (STAR) to classify a given update into one of the three above update categories. The correctness of the algorithm is proven using our clean extended-source theory. This technique represents a practical approach that can be applied by any existing view update system in industry and academia for analyzing the translatability of a given update statement before translation of it is attempted. To illustrate the working algorithm, we provide a concrete case study on the translatability of XML view updates.     

Related Axis: The Extension to XPath Towards Effective XML Search

We investigate the limitations of existing XML search methods and propose a new semantics, related relationship, to effectively capture meaningful relationships of data elements from XML data in the absence of structural constraints. Then we make an extension to XPath by introducing a new axis, related axis, to specify the related relationship between query nodes so as to enhance the flexibility of XPath. We propose to reduce the cost of computing the related relationship by a new schema summary that summarizes the related relationship from the original schema without any loss. Based on this schema summary, we introduce two indices to improve the performance of query processing. Our algorithm shows that the evaluation of most queries can be equivalently transformed into just a few selection and value join operations, thus avoids the costly structural join operations. The experimental results show that our method is effective and efficient in terms of comparing the effectiveness of the related relationship with existing keyword search semantics and comparing the efficiency of our evaluation methods with existing query engines.     

Numerical constraints on XML data

Boundaries occur naturally in everyday life. This paper introduces numerical constraints into the framework of XML to take advantage of the benefits that result from the explicit specification of such boundaries. Roughly speaking, numerical constraints restrict the number of elements in an XML data fragment based on the data values of selected subelements. Efficient reasoning about numerical constraints provides effective means for predicting the number of answers to XQuery and XPath queries, the number of updates when using the XQuery update facility, and the number of encryptions or decryptions when using XML encryption. Moreover, numerical constraints can help to optimise XQuery and XPath queries, to exclude certain choices of indices from the index selection problem, and to generate views for efficient processing of common queries and updates.We investigate decision problems associated with numerical constraints in order to capitalise on the range of applications in XML data processing. To begin with we demonstrate that the implication problem is strongly coNP-hard for several classes of numerical constraints. These sources of potential intractability direct our attention towards the class of numerical keys that permit the specification of positive upper bounds. Numerical keys are of interest as they are reminiscent of cardinality constraints that are widely used in conceptual data modelling. At the same time, they form a natural generalisation of XML keys that are popular in XML theory and practice. We show that numerical keys are finitely satisfiable and establish a finite axiomatisation for their implication problem. Finally, we propose an algorithm that decides numerical key implication in quadratic time using shortest path methods.     

An Effective Semantic Cache for Exploiting XPath Query/View Answerability

Maintaining a semantic cache of materialized XPath views inside or outside the database is a novel,feasible and efficient approach to facilitating XML query processing.However,most of the existing approaches incur the following disadvantages:1) they cannot discover enough potential cached views sufficiently to effectively answer subsequent queries; or 2) they are inefficient for view selection due to the complexity of XPath expressions.In this paper,we propose SCEND, an effective Semantic Cache based on ...     

Incremental recomputation of active relational expressions

Database updates are small and incremental compared to database contents. It is therefore desirable that recomputations of active relational expressions-such as views, derived data, integrity constraints, active queries, and monitors-can also be performed incrementally. An efficient algorithm for the incremental recomputation of active relational expressions based on finite differencing techniques is presented. Database updates are modeled as incremental changes to database relations, and the algorithm derives, by update propagation, the minimal incremental relational expressions that need recomputation. The algorithm has applications in the maintenance of materialized views and derived data, the checking of integrity constraints, and the evaluation of active queries and monitors     

Query rewritings using views for XPath queries,framework, and methodologies

Query rewriting using views is a technique that allows a query to be answered efficiently by using pre-computed materialized views. It has many applications, such as data caching, query optimization, schema integration, etc. This issue has been studied extensively for relational databases and, as a result, the technology is maturing. For XML data, however, the work is inadequate. Recently, several frameworks have been proposed for query rewriting using views for XPath queries, with the requirement that a rewriting must be complete. In this paper, we study the problem of query rewriting using views for XPath queries without requiring that the rewriting be complete. This will increase its applicability since in many cases, complete rewritings using views do not exist. We give formal definitions for various concepts to formulate the problem, and then propose solutions. Our solutions are built under the framework for query containment. We look into the problem from both theoretic perspectives, and algorithmic approaches. Two methods to generate rewritings using views are proposed, with different characteristics in terms of generalities and efficiencies. The maximality properties of the rewritings generated by these methods are discussed.     

Improving the performance and functionality of Mondrian open‐source OLAP systems

For a long time, the design of relational databases has focused on the optimization of atomic transactions (insert, select, update or delete). Currently, relational databases store tactical information of data warehouses, mainly for select‐like operations. However, the database paradigm has evolved, and nowadays on‐line analytical processing (OLAP) systems handle strategic information for further analysis. These systems enable fast, interactive and consistent information analysis of data warehouses, including shared calculations and allocations. OLAP and data warehouses jointly allow multidimensional data views, turning raw data into knowledge. OLAP allows ‘slice and dice’ navigation and a top‐down perspective of data hierarchies. In this paper, we describe our experience in the migration from a large relational database management system to an OLAP system on top of a relational layer (the data warehouse), and the resulting contributions in open‐source ROLAP optimization. Existing open‐source ROLAP technologies rely on summarized tables with materialized aggregate views to improve system performance (in terms of response time). The design and maintenance of those tables are cumbersome. Instead, we intensively exploit cache memory, where key data reside, yielding low response times. A cold start process brings summarized data from the relational database to cache memory, subsequently reducing the response time. We ensure concurrent access to the summarized data, as well as consistency when the relational database updates data. We also improve the OLAP functionality, by providing new features for automating the creation of calculated members. This makes it possible to define new measures on the fly using virtual dimensions, without re‐designing the multidimensional cube. We have chosen the XML/A de facto standard for service provision. Copyright © 2008 John Wiley & Sons, Ltd.     

An efficient XML encoding and labeling method for query processing and updating on dynamic XML data

In this paper, we propose an efficient encoding and labeling scheme for XML, called EXEL, which is a variant of the region labeling scheme using ordinal and insert-friendly bit strings. We devise a binary encoding method to generate the ordinal bit strings, and an algorithm to make a new bit string inserted between bit strings without any influences on the order of preexisting bit strings. These binary encoding method and bit string insertion algorithm are the bases of the efficient query processing and the complete avoidance of re-labeling for updates. We present query processing and update processing methods based on EXEL. In addition, the Stack-Tree-Desc algorithm is used for an efficient structural join, and the String B-tree indexing is utilized to improve the join performance. Finally, the experimental results show that EXEL enables complete avoidance of re-labeling for updates while providing fairly reasonable query processing performance.     

基于X-RESTORE查询XML视图

基于转换 XML文档到关系数据库中进行存储与查询的策略 ,研究了 XML视图查询的有效计算问题 .提出了XML 视图查询的合成重写技术 ,它能够消除视图查询中所有在视图结构上的路径导航操作 ,并将视图查询中所有在原文档结构上的路径导航操作以及所有谓词操作下推到视图定义中去 ,与视图定义中的路径导航操作相结合 ,形成统一的在原文档结构上的路径导航操作 .视图合成重写不仅避免了对视图中不出现在最后结果中的中间 XML 片段的构造 ,而且允许将查询中的所有内存密集型或数据密集型操作尽量下推到关系引擎中去执行 ,从而提高 XQuery查询的执行性能     

UnQL: a query language and algebra for semistructured data based on structural recursion

Abstract. This paper presents structural recursion as the basis of the syntax and semantics of query languages for semistructured data and XML. We describe a simple and powerful query language based on pattern matching and show that it can be expressed using structural recursion, which is introduced as a top-down, recursive function, similar to the way XSL is defined on XML trees. On cyclic data, structural recursion can be defined in two equivalent ways: as a recursive function which evaluates the data top-down and remembers all its calls to avoid infinite loops, or as a bulk evaluation which processes the entire data in parallel using only traditional relational algebra operators. The latter makes it possible for optimization techniques in relational queries to be applied to structural recursion. We show that the composition of two structural recursion queries can be expressed as a single such query, and this is used as the basis of an optimization method for mediator systems. Several other formal properties are established: structural recursion can be expressed in first-order logic extended with transitive closure; its data complexity is PTIME; and over relational data it is a conservative extension of the relational calculus. The underlying data model is based on value equality, formally defined with bisimulation. Structural recursion is shown to be invariant with respect to value equality. Received: July 9, 1999 / Accepted: December 24, 1999     

Dynamically Updating XML Data: Numbering Scheme Revisited

Almost all existing approaches use certain numbering scheme to encode XML elements to facilitate query processing when XML data is stored in databases. For example, under the most popular region-based numbering scheme, the starting and ending positions of an element in a document are used as the code to identify the element so that the ancestor/descendant relationship between two elements can be determined by merely examining their codes. While such numbering scheme can greatly improve query performance, renumbering large amount of elements caused by updates becomes a performance bottleneck if XML documents are frequently updated. Unfortunately, no satisfactory work has been reported for efficient update of XML data. In this paper, we first formalize the XML data update problem by defining the basic operators to support most XML update queries. We then present a new numbering scheme that not only requires minimal code-length in comparison with existing numbering schema but also improves update performance when XML data is frequently updated at arbitrary positions. The fundamental difference between our new scheme and existing ones is that, instead of maintaining the explicit codes for elements, we only store the necessary information and generate the codes when they are needed in query processing. In addition to present the basic scheme, we also discuss some optimization techniques to further reduce the update cost. Results of a comprehensive performance study are provided to show the advantages of the new scheme.     

基于XML Schema的XML存储

文章介绍了一个在关系数据库中,基于XMLSchema的XML存储方案。描述了一个以树模型为模型,XMLSchema为模式的XML数据库的存储系统。首先给出了在关系数据库中存储XMLSchema的方法,在此基础上,又给出了存储XML文档的方法。与通常的XML分解存储方案不同之处在于,在该方案中,XMLSchema被保存到数据库中,未作模式映射,避免了模式映射通常会带来的数据丢失和数据要分散到多个关系表中的问题;对XML文档的存储和查询都是基于XMLSchema的;并且由于所有基于同一个模式的XML文档共享该模式的结构,不必对结构信息进行重复存储,减少了存储空间;最后,由于我们为每一个元素赋予了一个唯一ID值,在进行查询的时候,可以利用该ID值进行定位,具有和XPath表达式相同的作用,但是更为简便。     

HCH for checking containment of XPath fragment

XPath is ubiquitous in XML applications for navigating XML trees and selecting a set of element nodes.In XPath query processing,one of the most important issues is how to effciently check containment relationship between two XPath expressions.To get out of the intricacy and complexity caused by numerous XPath features,we investigate this issue on a frequently used fragment of XPath expressions that consists of node tests,the child axis(/),the descendant axis(//),branches([])and label wildcards(*).Prior work has shown that homomorphism technology can be used for containment checking.However,homomorphism is the sufficient but not necessary condition for containment.For special classes of this fragment,the homomorphism algorithm returns false negatives.To address this problem,this paper proposes two containment techniques,conditioned homomorphism and hidden conditioned homomorphism,and then presents sound algorithms for checking containment.Experimental results confirm the practicability and efficiency of the proposed algorithms.     

Understanding the implications of view update policies

Database views are traditionally described as unmaterialized queries, which may be coincidentally updatable according to some fixed criteria. One of the problems in updating through views lies in determining whether a given view modification can be correctly translated by the system. To define an updatable view, a view designer must be aware of how an update request in the view will be mapped into updates of the underlying relations. Furthermore, because of side effects, the view designer must also be made aware of the effects of isolated updates back into the view. To address this problem, we present a general algorithm that predicts the effects of arbitrary mapping policies. Given an update policy, this algorithm indicates whether a desired update will, in fact, occur in the view and describes all possible side effects it may have, documenting the conditions under which they occur. The algorithm subsumes the results obtained by other view design tools, and generalizes their use to encompass a larger class of views.     

Maintaining materialized views without accessing base data

Access to a database through a user view can be serviced quickly when the view is materialized, i.e. the transformed data is explicitly stored. In the presence of database updates, however, the materialized view can become costly to maintain; often it must be completely rederived from the base data using the view definition. Under some conditions the view can be updated directly given only the view definition, the current contents of the materialized view, and the update operation (still expressed against the base data), without accessing the base data itself. In this paper, we consider relational views defined by projection, selection, and join. We present necessary and sufficient conditions on the view definition, contents, and update operations for insertions and deletions to be reflected in the view without reference to base data. Because the possibility of such view-based updating is dependent on the current contents of view, we call the update conditionally autonomously computable.     

基于XPath的XML查询优化

随着XML作为Internet上数据表示和交换的标准,如何高效地进行XML数据的查询己经变得越来越重要,许多XML查询语言也随之出现。这些查询语言虽然种类繁多,但都有个共同特征:使用基于XPath数据模型下规则路径表示来查询XML数据。研究表明,当前的关系数据库技术在处理规则路径表示的查询时通常效率不高。文章在介绍了传统的基于遍历树的方法的基础上重点讨论了基于路径分解的查询处理算法,并对选择连接顺序算法提出了基于动态规划思想的改进。     

Schema-aware XPath filtering on XML document streams

The XML stream filtering is gaining widespread attention from the research community in recent years. There have been many efforts to improve the performance of the XML filtering system by utilizing XML schema information. In this paper, we design and implement an XML stream filtering system, SFilter, which uses DTD or XML schema information for improving the performance. We propose the simplification and two kinds of optimization, one is static and the other is dynamic optimization. The Simplification and static optimization transform the XPath queries to make automata as an index structure for the filtering. The dynamic optimization are done in runtime at the filtering time. We developed five kinds of static optimization and two kinds of dynamic optimization. We present the novel filtering algorithm for the resulting transformed XPath queries and runtime optimizing. The experimental result shows that our system filters the XML streams efficiently.     

从XML模式到关系模式的影射

XML在关系数据库中的存储问题是XML研究领域中的一个重要问题。该文在总结多种影射方法的基础之上,提出了一个利用DTD文档,在关系数据库中通过多阶段XML文档信息的提取与变换,最终实现XML模式到关系模式的影射。该文对于影射中的一些难点问题,如集合值的影射、递归元素的影射、关系表的分散等方面提出了解决办法,实现了一个较为完整、高效的影射方案。