A Methodoloqy for View Inteqration in Loqical Database Desiqn Shamkant 6. Navathe Database Systems Research and Development Center 512 Weil Hall University of Florida, Gainesville, Florida 32611 Suresh G. Gadqil Metropolitan Life Corporate S.Ystem Planninq 1 Madison Avenue New York, New York 10010 Abstract It is assumed that user views would be explicit- ly represented using some data model. The View This paper is based on a conceptual frame- Representation Model of Navathe and Schknlnick work for the design of databases consisting of [ 111 is used as a representative model and a view modeling, view integration, schema analysis methodology for inteqratinq such views is and mapping and physical design and optimiza- discussed. tion. View modeling involves the modeling of the user views using a conceptual/semantic data model; the view integration phase merqes user 1.1 THE DATA BASE DESIGN PROCESS views into a global model which is then mapped to an existing database environment and subse- The conceptual framework for the desiqn of quently optimized. Here we use the data model data bases as described in some of Navathe's proposed by Navathe and Schkolnick to model user previous work [11,16] and as qenerally accepted views and develop a methodo1og.y for view inte- at the 1978 New Orleans data base desiqn work- gration. View integration issues are examined shop [;I] may be described in terms of four in detail; operations for merging views are steps. The input to these four steps stems from defined and an approach to automating the view a Requirements Analysis step which provides a integration process is described. The proposed specification of data and processing require- approach is being partially implemented at the ments. The four steps are: University of Florida. A. View modelinrl: in this first sten the user's view of the real world is abstracted and 1. INTRODUCTION represented. It can be said without any doubt that one 8. View Integration: the user views are of the major obstacles to the use of database combined into a qlobal model of the data and any management software is the initial preparatory conflicts in the process are presented for effort during logical database design. It is resolution. certainly a difficult task to design a "com- munity view" of a single database which truly C. Schema analysis and mapping: the global reflects the aggregation of views with different model is mapped to the logical structure of an expectations, backgrounds and technical exper- existing database management system. l tise. For realistic databases used in business, industry and government with thousands of D. Physical schema design and optimization: potential users, an individual user or user the logical structure is analyzed with respect group cannot be expected to be aware of the to physical design alternatives and an "optimal" needs of the rest of the user community and a physical schema is constructed. designer cannot be knowledgeable enouqh to comprehend the requirements of a spectrum of This conceotual framework is illustrated in users. Fiqure 1. A natural way to start, therefore is b.y collecting the views of user groups or applica- 1.2 OBJECTIVE AND SCOPE tion areas individually. The problem of coping with thousands of data elements/attributes or A major problem in database design is the hundreds of data objects is ver.y difficult to lack of a structured desiqn methodoloqv and lack deal with manually [12]. In this paper we of automatic aids for developing complex data- present an approach to alleviating this prol?lem. bases. Research has been conducted with the proceedings of the Eighth International Conference 142 on Very Large Data Bases Mexico City, September,‘W82 goal of structuring this process [12,15,17-J. Instances of an entity tvpe are called entities. Further, a few automated techniques which Entities refer to physical things, persons, address parts of the database design process and concepts. An entity type may either be self- others oriented towards particular database man- identified or externally identified. An asso- agement systems are available; examples are ciation type refers to a collection of associa- PSL/PSA [14] used primarily for requirements tions. Associations are n-ar.y relationships specification; and Database Design Aid (DBDA) defined over entity types and association types. [12] used for a specific database management They are subdivided into two subtypes: simple system. We had proposed in [16] the framework associations and identifier associations. The for a structured database design methodology subtypes of a simple association type are: consisting of the above 4 steps and surveyed cateqorization, subsettinq and partitioninq research done to date related to those steps. types. In [ll] we described how the first step of view modeling may be accomplished. Connecter types in the N-S model connect an association type to some object tvoe which par- This paper addresses the View Integration ticipates in the association type. They are step. It assumes the use of the specific data divided into two subtypes: directed and undi- model namely, the Navathe and Schkolnick model rected. A directed connector type implies [ll], for representing views and analyzes the certain rules of insertion and deletion in the process of view integration. The aaper dis- context of an association type. In qeneral, cusses the different problems related to the connector types are used to show three types of view integration process and proposes an dependencies among object types: ownership, approach to the development of a software system deletion and null dependency. They are illus- for automating the construction of integrated trated in Fiqure 3. views. We are presently implementing these ideas in the database design aid in conjunction The N-S model representation conventions with a data dictionary system being developed at allow view diagrams to be drawn. These diaqrams the University of Florida. Efforts are also are supplemented with assertions to state com- under way to consolidate the research on view plicated interdependencies among instances. An integration by collaborating with the University assertion lanquage may be defined to state these of Rome where similar work is being pursued on assertions. We are presently investigating the the extended E-R model [2]. To our knowledqe, adequacy of a language based on the first order very little work has been done on the view predicate calculus. The language must be capa- integration problem, barring a few exceptions ble of expressing the following kind of asser- c51. tions: For the view diagram in Fig. 18 - "Procedures performed by the SERVICE 2. MODELING USER VIEWS instance called Hospital-trust are always performed free" is an assertion in natural For ease of discussion and because of some language. The same in an assertion of its desirable features we have chosen the language could be stated as - model proposed by Navathe and Schkolnick [ll] (henceforth abbreviated as the N-S model) as a s.Service-Name = Hospital-trust & vehicle for modeling user views. A brief <s,p> e PROCEDURE-IDENTIFIER & description of the model and its advantages <p,c,r> E PERFORMED ==> follows. <p,c,r> E PERFORMED-FREE. where: s,p,c,r are respectively instances of 2.1 SUMMARY OF THE NAVATHE-SCHKOLNICK MODEL entity types SERVICE, PROCEDURE, SCHEDULE and PERSONNEL. The N-S model includes the two as ects of user requirements identified by Kahn [6 !i : "the Assertions pertaining to a single view are Information Structure perspective" describing termed intra-view assertions whereas those data by means of entity types and association pertaining to multiple views are termed inter- types, and "the usage perspective" giving a view assertions. description of processing and insertion/deletion of instances of these types. The discussion of For a further clarification of the N-S the view integration process itself is mostly model the reader is referred to the Appendix in invariant to the model selected and will be done the paper and to Clll. in a general way. The N-S model allows the modeling of user views in explicit terms. The Note: The words 'type' and 'instance' will be model uses two type constructs: objects and used in the subsequent discussion only connectors. The object type is divided into two when they seem necessary. subtypes: entity type and association type. Proceedings of the Eighth International Conference ., 143 on Very Large Data Baser MexitiCity, September,1969 2.2 ADVANTAGESOF THE N-S MODEL Equivalent Views are defined as views which have the same information content but different a) It allows association types to be structures. The term information content refers defined over entity types, over association to the functional and non-functional associa- types, or a combination of both unlike the E-R tions among attributes. Information content has model [23. been defined formally for relational databases in terms of functional dependence and direct b) Models the existence-dependencies among table look-up associations [l]. In this paper entities separately (in terms of identifier we will assume that an acceptable measure of associations) from the relationships (or associ- information content has been defined for the ations) among entities (unlike Smith/Smith user c0mmunit.v in question. C131) l It thus defines identification paths clearly. (e.g. PROCEDUREin Fig. 19 is exis- A tarqet of integration is an object type tence-dependent on SERVICE b.y virtue of identi- or a c-o?;-type which is compared among fier association PROCEDURE-IDENTIFIER), different views for possible integration. C Describes a view of terms of object A view integration operation is a process We 1which are either entities or associa- that is applied to the targets of integration tions). The internal hierarchy of descriptor giving rise to new object types, connector elements (attribute types) within an object type types, attribute types etc.