Classifying schematic and data heterogeneity in multidatabasesystems

A complete framework for enumerating and classifying the types of multidatabase system (MDBS) structural and representational discrepancies is developed. The framework is structured according to a relational database schema and is both practical and complete. It was used to build the UniSQL/M commercial multidatabase system. This MDBS was built over Structured-Query-Language-based relational database systems and a unified relational and object-oriented database system named UniSQL/X. However, the results are substantially applicable to heterogeneous database systems that use a nonrelational data model (for example, an object-oriented data model) as the common data model and allow the formulation of queries directly against the component database schemas     

从面向对象数据库模式到关系数据库模式的转换

本文提出了一种从面向对象数据库模式到关系数据库模式的映射及基于该映射的模式转换算法。以查询为例,说明了面向对象数据库中的特有语义仍能保留在转换后的关系模式中,而且从面向对象数据库到关系数据库的基于该模式转换中操纵运算的转换也是切实可行的。所得的模式转换结果可应用于面向对象数据库和关系数据库之间的互操作。     

Translating object-oriented database transactions into relational transactions

In this paper, we present methods of translating transactions from object-oriented database (OODB) to relational database (RDB). The process involves schema mapping in data definition language and transaction translation in data manipulation language. They include scheme definition, data query and transaction operation of insert, update, and deletion. We also discuss the object-oriented features in OODB operations that are not supported by RDB, such as class hierarchy, class composition hierarchy, and set attribute, and provide a general solution to realize those mechanisms by traditional relation operations. The result of the transaction translation can be applied into adding object-oriented interface into relational database management system and to the interoperability between OODB and RDB.     

Transforming RDB schema into well-structured OODB schema

When transforming relational database (RDB) schema into object-oriented database(OODB) schema, much effort was put on examining key and inclusion dependency (ID) constraints to identify class and establish inheritance and association between classes. However, in order to further remove the original data redundancy and update anomaly, multi-valued dependency (MVD) should also be examined. In this paper, we discuss class structures and define well-structured classes. Based on MVDs, a theorem is given transforming a relation schema into a well-structured class. To transform RDB schema into OODB schema, a composition process simplifying the input RDB schema and an algorithm transforming the simplified RDB schema into well-structured OODB classes are developed.     

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.     

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     

面向对象数据库的查询转换成关系数据库的查询

文章基于从面向对象数据库(OODB)到关系数据库(RDB)的模式转换,提出了一种从面向对象数据库查询到关系数据库查询的转换算法。该算法主要针对关系数据库中不支持的,而面向对象数据库查询中特有的面向对象特征,诸如,类分层结构、类复合分层结构和集合属性,以及考虑由传统的关系运算来实现它们的语义的一般方法。这项研究的结果可以用于将关系数据库管理系统(RDBMS)升级为面向对象数据库管理系统(OODBMS),也可以用于面向对象数据库和关系数据库之间的互操作。     

Identifying extended entity-relationship object structures inrelational schemas

Relational schemas consisting of relation-schemes, key dependencies and key-based inclusion dependencies (referential integrity constraints) are considered. Schemas of this form are said to be entity-relationship (EER)-convertible if they can be associated with an EER schema. A procedure that determines whether a relational schema is EER-convertible is developed. A normal form is proposed for relational schemas representing EER object structures. For EER-convertible relational schemas, the corresponding normalization procedure is presented. The procedures can be used for analyzing the semantics of existing relational databases and for converting relational database schemas into object-oriented database schemas     

Integrating relational and object-oriented database systems using a metaclass concept

This paper presents a specific approach of integrating a relational database system into a federated database system. The underlying database integration process consist of three steps: first, the external database systems have to be connected to the integrated database system environment and the external data models have to be mapped into a canonical data model. This step is often called syntactic transformation including structural enrichment and leads to component schemas for each external DBMS. Second, the resulting schemas from the first step are used to construct export schemas which are then integrated into global, individual schemas or views in the third step. In this paper we focus on the first step for relational databases, i.e., the connection of a relational database system and the mapping of the relational model into a canonical data model. We take POSTGRES as the relational database system and the object-oriented federated database system VODAK as the integration platform which provides the open, object-oriented data model as the canonical data model for the integration. We show different variations of mapping the relational model. By exploiting the metaclass concept provided by VML, the modelling language of VODAK, we show how to tailor VML such that the canonical data model meets the requirements of integrating POSTGRES into the global database system VODAK in an efficient way.     

An ontology based approach to the integration of entity–relationship schemas

In schema integration, schematic discrepancies occur when data in one database correspond to metadata in another. We explicitly declare the context that is the meta information relating to the source, classification, property etc. of entities, relationships or attribute values in entity–relationship (ER) schemas. We present algorithms to resolve schematic discrepancies by transforming metadata into the attribute values of entity types, keeping the information and constraints of original schemas. Although focusing on the resolution of schematic discrepancies, our technique works seamlessly with the existing techniques resolving other semantic heterogeneities in schema integration.     

The study of indexing techniques on object oriented databases

An object-oriented database (OODB) has been becoming more important in recent years. It can deal with a large amount of complex objects and relationships that relational database (RDB) systems cannot handle well. However, the retrieval and update performance of an OODB depends on indexing techniques. In this paper, we study the indexing techniques on OODBs, based on an inheritance hierarchy and an aggregation hierarchy. Given the access probability and the size of each class, we propose a cost function to evaluate the gain of building an index on an inheritance hierarchy. For an aggregation hierarchy, we use a path-catenation technique to evaluate how to build index files on classes. Through some experiments, we found our methods have better retrieval performance than most ones proposed before.     

Formal transformation from fuzzy object-oriented databases to fuzzy XML

XML has become the standard for publishing and exchanging data on the Web. Since most of the business data nowadays are stored in structured databases including relational and object-oriented databases (OODB), it is of significance to automate the transformation process and generate the XML data containing information from existing databases. At the same time, information imprecision and uncertainty exist in many practical applications, and for this reason, fuzzy data modeling has been extensively investigated in various data models. As such, there is an increasing need to effectively publish fuzzy structured data as fuzzy XML documents for Web-based applications. In this paper, we take a significant step in a fundamental consolidation of fuzzy XML. In particular, we are interested in finding an XML schema that best describes the existing fuzzy object-oriented schema. To accomplish this, we first offer mapping formalisms to capture the semantics of fuzzy XML Schema and fuzzy object-oriented schema. To allow for better and platform independent sharing of data stored in an object-oriented format, we investigate the formal transformation from fuzzy OODB to fuzzy XML and develop a set of rules to assist in the transformation process.     

构建面向对象时态数据模型的研究

本文通过对现存关系数据库和面向对象数据库的比较分析，总结其优点与不足，并在结合了二者优点的UniSQL/X模型基地上，对该模型进行时态上的扩展，使它能够记录和处理时态信息。在查询代数中，引入了一 些特殊的时态操作，通过聚集、概括和时间参考三种关联对对象进行访问，而且此代数同时反映了时态关系代数和对象代数的特点。     

面向对象数据库中查询处理技术的研究

在面向对象数据库（OODB）中，查询处理是其中重要的部分。在联系关系数据库（RDB）查询处理的基础上，综合考虑了数据模型，查询模型和查询处理等因素，提出了一种全新的面向对象数据库系统（OODBS）查询策略，运用关系数据库系统（RDBS）实现技术可以有效地解决OODB领域的许多问题，使得OODB无论从语义上还是效率上都有可能成为最有前途的数据库系统。     

Producing Interoperable Queries for Relational and Object-Oriented Databases

In this paper, we develop techniques to produce interoperable queries with object and relational databases. A user poses a local query in a local query language, against a local object or relational schema. We transparently produce appropriate queries with respect to a remote target object or relational schema, corresponding to some remote database which contains data relevant to the user's query. Mapping knowledge to resolve representational heterogeneities in local and remote schemas is expressed in a canonical representation, CRmapping, and is independent of the particular data model. A canonical representation CRquery is also used to resolve heterogeneities of query languages. A set of heterogeneous transformation algorithms define the appropriate transformations from the local queries to the remote queries. The use of canonical representations (CR) allows us to represent queries independent of the particular query language, and to resolve representational conflicts in a uniform manner, independent of models and query languages.     

关系数据库与面向对象数据库的集成

本文从ＲＤＢ（ｒｅｌａｔｉｏｎａｌｄａｔａｂａｓｅ）的缺欠、ＯＯＤＢ（ｏｂｊｅｃｔｏｒｉｅｎｔｅｄｄａｔａｂａｓｅ）的不足和新的数据库应用需求３个方面指出了统一关系范型和ＯＯ范型、集成ＲＤＢ与ＯＯＤＢ的趋势，讨论了集成ＲＤＢ与ＯＯＤＢ的３种途径，最后介绍了一个集成关系与对象模型的多媒体数据库系统ＴＨＣＩＭＨＹＰＥＲ的总体设计.     

An empirical investigation of factors influencing object-oriented database querying

Object-oriented databases (OODBs) provide an effective means for capturing complex data and semantic relationships underlying many real-world database applications. Because users' interactions with databases have increased significantly in today's era of client–server computing, it is important to examine users' ability to interact with such databases. We investigated a number of factors that potentially affect performance in writing queries on an OODB. First, we evaluated the utility of graphical and textual schemas associated with emerging OODBs from the perspective of database querying. Second, we examined the use of two different strategies (navigation and join) that could be used in writing OODB queries. Third, we examined a number of factors that potentially contribute to the complexity of an OODB query.Our exploratory study examined the performance of 20 graduate students in an experiment in which each participant wrote queries for two problems, one using a graphical OODB schema and the other a textual OODB schema. The participants had no prior exposure to the object-oriented data model. We found that there was no difference in query writing performance (either accuracy or time) using the graphical and textual schemas. Examination of query strategy revealed that a significant number of participants used a join strategy, rather than the navigation strategy that matches the database structure. Use of the join strategy resulted in significantly less accurate and slower query writing than did the navigation strategy. From the viewpoint of complexity, the number of objects referenced in a query, the number of starting points in the from clause, and the presence of special operators influenced both the accuracy and time of query writing.     

Model independent assertions for integration of heterogeneous schemas

Due to the proliferation of database applications, the integration of existing databases into a distributed or federated system is one of the major challenges in responding to enterprises' information requirements. Some proposed integration techniques aim at providing database administrators (DBAs) with a view definition language they can use to build the desired integrated schema. These techniques leave to the DBA the responsibility of appropriately restructuring schema elements from existing local schemas and of solving inter-schema conflicts. This paper investigates theassertion-based approach, in which the DBA's action is limited to pointing out corresponding elements in the schemas and to defining the nature of the correspondence in between. This methodology is capable of: ensuring better integration by taking into account additional semantic information (assertions about links); automatically solving structural conflicts; building the integrated schema without requiring conforming of initial schemas; applying integration rules to a variety of data models; and performing view as well as database integration. This paper presents the basic ideas underlying our approach and focuses on resolution of structural conflicts.     

OWL本体到关系数据库模式的映射

通过对现有本体存储模式的分析,以及对OWl本体和关系数据库模式之间的概念对应关系的分析,定义了从OWL本体到关系数据库模式的转换规则,给出了OWL本体存储模式的设计原则,并基于该原则提出了一种新的本体存储模式.针对本体描述语言OWL的构词特点,该模式提出将本体中的语义信息存放在不同的表中,以达到方便理解、结构稳定和提高效率的目的,适应于多数中小型本体的存储.     

多数据库系统中的模式映射方法

多数据库系统一般具有四级模式结构，全局用户只能访问全局模式，而最终的数据必须从各局部数据库系统中获得，因此必须建立多数据库系统的模式映射，它表示了局部模式通过输出模式集成为全局模式的相应转换。本文给出了一种多数据库系统中的模式映射方法，并使用 模式映射树来存储和表达这种模式映射。