多数据库系统中基于XIDM的模式映射方法研究

建立多数据库系统中多种模式间的映射是多数据库解决模式异构性的关键，而选用一种适用于这些映射的集成数据模型又是模式映射的基础。为了集成数据库、文件系统、Web文档等多种异构系统中的数据，提出了一种基于XML的集成数据模型（XIDM)作为多数据库系统的公共数据模型，基于XIDM模型，给出了全局模式与输出模式、输出模式与局部模式之间的模式映射方法，并对全局映射和局部映射在多数据库原型系统Panorama中的实现进行了示例，最后从数据模型的集成能力和模式映射的可操作性方面对基于XIDM的模式映射方法进行了性能比较和分析。     

一种适合多数据库系统的查询表示方法

1 引言随着分布计算和网络技术的不断发展,传统的数据库技术已越来越不能满足数据共享和互操作的需要。同时,已有的数据库系统又不可能全部丢弃.因而研制能同时访问和处理来自多个数据库中数据的多数据库系统已成为必然趋势。多数据库系统是解决已存的、异构的、分布的多个局部数据库系统之间数据共享和集成的问题。由于多数据库系统具有异构性、分布性和局部自治性的特点,使得多数据库查询处理与传统数据库查询处理有很大的不同。多数据库系统呈现给用户的是全局模式,用户使用全局查询语言提交对多数据库的查询,而所需的数据又必须从各局部数据库获得,所以必须将全局查询转换成与局部数据库对应的局部查询。在全局查询转换为局部查询的过程中,需要     

多数据库系统中查询分解算法的研究

多数据库系统允许用户使用一个集成模式和简单的全局查询语言同时访问多个异构的、自治的数据库系统。全局查询分解处理是多数据库系统中的一个很重要的问题。本文给出了一种多数据库环境中的模式信息管理方法,基于这些模式信息,我们提出一种易于实现的查询分解算法。由于多数据库查询分解处理与模式集成的实现紧密相关,所以本文对多数据库系统的模式集成作了一些描述。     

基于本体的信息集成框架中包装器的设计

将本体应用在信息集成框架中能够在语义层次上消除底层数据源的异构,但是本体只相当于一个知识库,在定义用户接口时,需要赋予其一个语法结构,这个语法结构可作为与用户交互的全局模式,从本体到全局模式的转换可以用包装器来实现。而此全局模式和各个数据源之间的局部模式也需要映射,这些映射也可以用包装器来实现。该文提出了基于本体的信息集成框架中一种包装器的设计,通过将本体转换为XMLSchema作为全局模式,并利用XSLT实现全局模式和局部模式的映射,从而屏蔽了数据源的异构性。     

多数据库系统中的查询处理

在多数据库系统中,为实现局部系统的互操作,通常需要建立局部系统的集成视图u多数据库系统的用户针对集成视图表达全局查询请求后,多数据库系统的查询处理器经多级转换和处理后将全局查请求等价地转换为优化的针对局部模式的一组局邮子查询,主局部系统取得的部分查询结果经组装后取得最终的查询结果。     

基于元数据的分布异构数据集成研究

将“虚拟法”和“数据仓库法”结合,通过统一的数据交换中心解决数字气田分布异构数据集成问题。建立包含全局模式和全局-局部映射的元数据字典,屏蔽异构数据细节,由此解决异构数据的各种冲突。提出一种由分布数据源的局部模式归并建立全局模式和映射表的算法,以提高模式集成的自动能力。     

基于SVM的人物实体全局模式动态构建

针对结构化网页中人物实体全局模式构建问题,提出了基于SVM的动态构建方法。构建分为两个阶段,第一个阶段是来自同一数据源的人物实体结构化实例到人物实体局部模式的转化,第二个阶段是利用SVM分类器完成人物实体局部模式到人物实体全局模式的映射。本方法能适应数据源的不断变化,保证了全局模式的完整性。通过实验,验证了构建算法的有效性和可行性,并对随着结构化网页不断增多时全局模式的稳定性进行了考察。     

从关系数据库到面向对象语义数据库的语义捕捉和模式转换*

本文提出了一个从关系数据库到面向对象语义数据库的两段模式转换方法。该方法是以语义确认为基础的．包含在局部关系模式中的语义可分为2类：一类是关系内语义，它是包含在一个局部关系模式内的语义;另一类是关系间语义，它是包含在局部关系模式之间的语义．第1阶段主要是将关系内语义捕捉到全局集成模式中来；而第2阶段则将关系间语义通过模式重构操作添加到全局集成模式中来．本文定义了一套用于建立转换机制的映射规则和转换操作．设计并实现了用于产生G关系的算法和一系列转换公式．     

异构信息源集成系统的模式集成研究

随着社会的发展,越来越多的企业开始将工作重心向集成体系结构转移。因此,对异构信息源集成的要求是非常迫切并会长期存在下去。本文提出了一种基于Java的关系数据模型(JIDM)作为集成系统的公共数据模型。在该模型的基础上,介绍了全局模式、输出模式以及局部模式之间的映射关系,解决了JIDM模型与关系模型、XML文件以及面向对象模型之间的映射问题。     

多数据库系统的数据模式集成与查询处理

在分析了多数据库系统数据模式体系结构的基础上 ,讨论了多数据库查询处理问题 :查询分解、查询转换和查询优化。给出了全局查询分解算法和全局查询优化算法     

Object-oriented query language access to relational databases: A semantic framework for query translation

This research investigates and approach to query processing in a multidatabase system that uses an objectoriented model to capture the semantics of other data models. The object-oriented model is used to construct a global schema, defining an integrated view of the different schemas in the environment. The model is also used as a self-describing model to build a meta-database for storing information about the global schema. A unique aspect of this work is that the object-oriented model is used to describe the different data models of the multidatabase environment, thereby extending the meta database with semantic information about the local schemas. With the global and local schemas all represented in an object-oriented form, structural mappings between the global schema and each local schema are then easily supported. An object algebra then provides a query language for expressing global queries, using the structural mappings to translate object algebra queries into SQL queries over local relational schema. The advantage of using an object algebra is that the object-oriented database can be viewed as a blackboard for temporary storage of local data and for establishing relationships between different databases. The object algebra can be used to directly retrieve temporarily-stored data from the object-oriented database or to transparently retrieve data from local sources using the translation process described in this paper.     

基于多库操作语言的异构数据库集成框架研究

为解决当前异构数据库集成框架中全局模式维护复杂的问题,提出了基于多库操作语言的异构数据库集成框架HDIFBML.主要内容包含关键模块设计、全局模式及模式映射概念,并定义了一种多库操作语言SMSQL.SMSQL定义了模式构建语言集和模式映射语言集来构建,维护全局模式,并通过字段转化解决不同模式集成时的结构/语义冲突.实践应用表明,HDIFBML屏蔽了底层异构数据库执行细节,可以灵活地维护全局模式,具有良好的可操作性、可维护性和可扩展性.     

关系数据库模式和本体间映射的研究综述

关系数据库模式和本体问映射是语义网研究中的一个重要问题.首先,给出关系数据库模式和本体间映射的形式化定义,并从建模思想和应用场景两个方面分析问题的难点.根据3个不同角度,即模型转换的途径、映射策略的适用范围以及映射结果的表达形式,调研当前存在的多种解决途径.在此基础上,进一步介绍并比较6个具有代表性的关系数据库模式和本体间映射的工具.最后,讨论存在的挑战,并指出未来可能的研究方向.     

基于源模式分裂的模式匹配算法

模式匹配就是在作为输入的模式中有对应语义关系的元素间产生一个映射.为了提高模式匹配的效率,提出了一种新型的模式匹配方法--源模式分裂模式匹配算法.它可以解决标准模式匹配难以解决的问题:1)源模式的某一个属性和多个目标模式的多个属性之间建立匹配关系;2)表格中的不同元组对应其他表格同一元组的不同属性值的匹配.在匹配过程中,该方法先搜索种类型属性,然后根据种类型属性建立选择条件,最后把源模式进行分裂形成视图,再重新生成候选匹配集合,从而提高模式匹配的质量.     

Data integration with uncertainty

This paper reports our first set of results on managing uncertainty in data integration. We posit that data-integration systems need to handle uncertainty at three levels and do so in a principled fashion. First, the semantic mappings between the data sources and the mediated schema may be approximate because there may be too many of them to be created and maintained or because in some domains (e.g., bioinformatics) it is not clear what the mappings should be. Second, the data from the sources may be extracted using information extraction techniques and so may yield erroneous data. Third, queries to the system may be posed with keywords rather than in a structured form. As a first step to building such a system, we introduce the concept of probabilistic schema mappings and analyze their formal foundations. We show that there are two possible semantics for such mappings: by-table semantics assumes that there exists a correct mapping but we do not know what it is; by-tuple semantics assumes that the correct mapping may depend on the particular tuple in the source data. We present the query complexity and algorithms for answering queries in the presence of probabilistic schema mappings, and we describe an algorithm for efficiently computing the top-k answers to queries in such a setting. Finally, we consider using probabilistic mappings in the scenario of data exchange.     

Query optimization in multidatabase systems considering schemaconflicts

In a multidatabase system, the participating databases are autonomous. The schemas of these databases may be different in various ways, while the same information is represented. A global query issued against the global database needs to be translated to a proper form before it can be executed in a local database. Since data requested by a query (or a part of a query) is sometimes available in multiple sites, the site (database) that processes the query with the least cost is the desired query processing site. The authors study the effect of differences in schemas on the cost of query processing in a multidatabase environment. They first classify schema conflicts to different types. For each type of conflict, they show how much more or less complex a translated query can become in comparison with the originally user-issued global query. Based on this observation, they propose an analytical method that considers the conflicts between local databases and finds the database(s) that renders the least execution cost in processing a global query. This research introduces a new level of query optimization (termed the schema-level optimization) in multidatabase environments. The results provide a new dimension of enhancement for the capability of a query optimizer in multidatabase systems     

基于OWL本体论映射的数据库网格语义模式集成研究

本文提出了一种在数据库网格中OWL本体论映射机制如何用于基于语义的数据库模式集成。方法是首先把关系模式转化为RDF/OWL语义描述以完成局部映射,再通过把局部数据语义与全局共享本体建立联系来完成全局映射。本质是把异构数据库模式的语义通过本体显性地表达出来,并在语义Web层完成模式的集成。特点是实现了在统一的语义层次上进行共享与查询,同时采用了局部映射与全局映射松耦合的构架,其特有的分层结构使得在跨库/单库环境中进行语义查询变得更加灵活。     

Preserving mapping consistency under schema changes

In dynamic environments like the Web, data sources may change not only their data but also their schemas, their semantics, and their query capabilities. When a mapping is left inconsistent by a schema change, it has to be detected and updated. We present a novel framework and a tool (ToMAS) for automatically adapting (rewriting) mappings as schemas evolve. Our approach considers not only local changes to a schema but also changes that may affect and transform many components of a schema. Our algorithm detects mappings affected by structural or constraint changes and generates all the rewritings that are consistent with the semantics of the changed schemas. Our approach explicitly models mapping choices made by a user and maintains these choices, whenever possible, as the schemas and mappings evolve. When there is more than one candidate rewriting, the algorithm may rank them based on how close they are to the semantics of the existing mappings.Received: 13 January 2004, Accepted: 26 March 2004, Published online: 12 August 2004Edited by: M. Carey     

On resolving schematic heterogeneity in multidatabase systems

The objective of a multidatabase system is to provide a single uniform interface to accessing multiple independent databases being managed by multiple independent, and possibly heterogeneous, database systems. One crucial element in the design of a multidatabase system is the design of a data definition language for specifying a schema that represents the integration of the schemas of multiple independent databases. The design of such a language in turn requires a comprehensive classification of the conflicts (i.e., discrepancies) among the schemas of the independent databases and development of techniques for resolving (i.e., homogenizing) all of the conflicts in the classification. An earlier paper provided a comprehensive classification of schematic conflicts that may arise when integrating multiple independent relational database (RDB) schemas into a single multidatabase (MDB) schema. In this paper, we provide a comprehensive classification of techniques for resolving the schematic conflicts that may arise when integrating multiple RDB schemas, or RDB schemas and object-oriented database (OODB) schemas, or multiple OODB schemas. The classification of conflict resolution techniques includes not only those necessary for resolving schematic conflicts identified in the earlier paper, but also additional conflicts that arise when OODBs become part of the databases to be integrated. Most of the conflict resolution techniques discussed in the paper have already been incorporated into SQL/M, a multidatabase language implemented in UniSQL/M, a commercially available multidatabase system from UniSQL, Inc. which integrated SQL-based relational database systems and the UniSQL/X unified relational and object-oriented database system.