Issueeditor 1 Rakeshagrawal
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JUNE1991,Volume 14, No. 2 a quarterlybulletin of the IEEEComputerSociety technicalcommittee on Data Engineering CONTENTS Letterfrom the IssueEditor 1 RakeshAgrawal TheTwo Facets of Object—Oriented DataModels 3 SergeAbiteboulandParisC.Kanellakis TheoreticalFoundations for OODB’s — a PersonalPerspective 8 CatrielBeen A First—OrderFormalization of Object—OrientedLanguages 13 MichaelKifer On DataRestructuring and Mergingwith ObjectIdentity 18 RichardHull, SurjatiniWidjojo,Dave Wile, andMasatoshiYoshikawa DataStructuresand DataTypes for Object—OrientedDatabases 23 Vat Breazu—Tannen,PeterBuneman, andAtsushiOhori Foundations of the 02 DatabaseSystem 28 C. Lecluseand P. Richard Updating the Schema of an Object—OrientedDatabase 33 AlbertaCoen—Porisini~LuigiLavazza, andRobertoZicari An Overview of IntegrityManagement in Object—OrientedDatabases 38 WonKim, Yoon—JoonLee, andJungyunSeo SupportingViews in Object—OrientedDatabases 43 Marc H. SchollandH.—J.Schek AlgebraicQueryProcessing in EXTRA/EXCESS 48 Scott L. VandenbergandDavid J. DeWitt ENCORE: An Object—OrientedApproach to DatabaseModelingand’Querying 53 Stanley B. ZdonikandGailMitchell QueryOptimization in Revelation, an Overview 58 ScottDaniels,GoetzGraefe,ThomasKeller,DavidMaier, Dun Schmidt, andBennetVance Calls for Papers 63 SPECIALISSUE ON FOUNDATIONS OF OBJECT—ORIENTEDDATABASESYSTEMS 1951-1991 THE EISTTTUIE ~ ELECTEJCAL — + ~ ~. IEEE June1991,Volume 14, No.2 Editor—In—Chief,DataEngineering Chairperson, TC Dr. Won Kim Prof.JohnCarlis UNISQL, Inc. Dept. of ComputerScience 9390ResearchBoulevard University of Minnesota Austin, TX 78759 Minneapolis, MN 55455 (512)343—7297 AssociateEditors PastChairperson, IC Dr. RakeshAgrawal ProfLarryKerschberg IBMAlmadonResearchCenter Dept. of InformationSystemsandSystemsEngineering 650 HarryRoad GeorgeMasonUniversity SanJose,Calif. 95120 4400UniversityDrive (408)927—1734 Fairfax, VA 22030 S (703)764—6192 Prof.AhmedElmagarmid Distribution Department of ComputerSciences PurdueUniversity IEEEComputerSociety WestLafayette,Indiana47907 1730MassachusettsAve. (317)494—1998 Washington,D.C. 20036—1903 (202)371—1012 Prof. YannisIoannidis Department of ComputerSciences University of Wisconsin Madison,Wisconsin 53706 (608)263—7764 Prof. Kyu—YoungWhang Department of ComputerScience KAIST P.O. Box 150 Chung—Ryang,Seoul,Korea DataEngineeringBulletin is a quarterlypublication of Opinionsexpressed in contributions are those of the the IEEEComputerSocietyTechnicalCommittee on individualauthorratherthan the officialposition of DataEngineering. Its scope of interestincludes:data the TC on DataEngineering, the IEEEComputer structuresand models,accessstrategies,access Society, or organizations with which the authormay controltechniques,databasearchitecture,database be affiliated. machines,intelligentfrontends,massstorage for verylargedatabases,distributeddatabasesystems and techniques,databasesoftwaredesign and im Membership in the DataEngineeringTechnical plementation,databaseutilities,databasesecurity Committee is open to individualswhodemonstrate and relatedareas. willingness to activelyparticipate in the variousacti vities of the TC. A member of the IEEEComputer Society mayjoin the TC as a full member. A non— Contribution to the Bulletin is herebysolicited.News member of the ComputerSociety mayjoin as a par items,letters,technicalpapers,bookreviews,meet ticipatingmember, with approvalfrom at leastone ing previews,summaries,casestudies, etc., should officer of the TC. Both full members and participat be sent to the Editor. All letters to the Editor will be ing members of the TC are entitled to receive the considered for publicationunlessaccompanied by a quarterlybulletin of the TC free of charge, untilfur request to the contrary.Technicalpapers are unre thernotice. formed. Letterfrom the IssueEditor TheoreticalFoundations of Object-OrientedDatabaseSystems Object-orientation has emerged as a major theme in current work on databasesystems, and several reseaithprototypes and commercialdatabaseproductsbased on object-orientedparadigm are in different stages of development. The enthusiasm and energy devoted to the development of object-oriented databasesystemsmatch, if not exceed, the development effort spent on relationalsystems in the late seventies and earlyeighties.Interestingly,however, the development of object-orienteddatabasesystems has taken a verydifferentevolutionarypath. While the relationalsystemsstarted with a strongtheoretical foundation, there is no consensus yet on a formal theory for object-oriented database systems. Experimentalsystemsandproductsseem to be driving this field at this stage. This issuepresents a sampling of some recentattempts to provide a theoreticalfoundation for object- orienteddatabasesystems. The issuecuntains 12 papers. Thesepaperscovervariousaspects of object- orienteddatabasesystems,includingmodeling,schemaevolution,integrityconstraints,views, and queries. Abiteboul and Kanellakis point out the two facets of object-oriented data models — structural and behavioral — thatreflect the twoorigins ofobject-orienteddatabasesystems — relationaldatabasetheory andobject-orientedprogramming. Theyformalize and analyzethese twofacets andgiveexamples of their integration. Been argues for the extension of the existinglogic-basedapproaches to databases and programming languages for modelingobject-orienteddatabasesystems. He presents an initial modelapproach that unifies the theories of relationaldatabases,deductiveprogramming, and abstract data types. He also argues for functions as first class values and a flexiblefunctiondefinitionfacility to modelbehavioral aspects of object-orienteddatabasesystems. Kiferpresents the salientfeatures of F-logic that make it possible to provide a full logicalformalization of object-orientedlanguages. F-logicbreaks the distinctionbetweenclasses,objects, and attributeswhich allowsqueries that may return sets of attributes,classes, or any other aggregation that involves these higher-orderentities. It is alsopossible to defineparametricclasses in F-logic. Object identity is a central concept in object-orienteddatabasesystems. Hull, Widjojo, Wile, and Yoshikawadifferentiatebetweenobjectidentities and values,describe a formalmodelwhichencompasses objectidentities and values, and examine the impact of objectidentity in the contexts of data structuring and merging. Breazu-Tannen,Buneman, and Ohori argue that the object-oriented database systems can be best understood in the framework of typed languages. They address the demandsplaced on programming languages by the addition of operations on records and “bulk” datatypessuch as sets. Lecluse and Richard describe the foundations of the 02 database system. The 02 data model differentiatesbetweenvalues and objects, and between types and classes. It also supports the notions of the consistency of a classhierarchy and databaseschema. Coen-Ponsini,Lavazza, and Zicariaddress the problem of schemaevaluation in object-orienteddatabase systems. Theydifferentiatebetweenstructural and behavioralconsistency of a schema and outline their solutions for maintainingschemaconsistency in the presence of schemamodifications. Object-oriented data models give rise to additionalconstraints beyond those meaningful under the relational model. Kim, Lee, and Seo present a framework for classifyingintegrityconstraints in the context of an object-oriented datamodel on the basis of theirperformanceimpact. Scholl and Schek describe how views may be supported in object-orienteddatabasesystems. They how introduceobject-preserving querysemantics of a genericobject-oriented querylanguage and show viewsdefined by suchqueryexpressions may be updated.Dynamicreclassificationallowsobjects to gain and loosetypes. 1 Vandenberg and DeWiudescribe the algebraicfundamentalsunderlying the processing and optimization of EXCESSqueries in the EXTRA/EXCESSDBMS. They describe the algebraicstructures and their operators, the algebra’sexpressivepower, and the algebraicqueryoptimization. Zdonikand Mitchellpresent the ENCOREdatamodel and its queryalgebra,calledEQUAL. Thisalgebra generalizesrelationaloperations by providing the capability to access and produceencapsulated,logically complexobjects. Finally,Daniels,(Iraefe,Keller,Maier,Schmidt, and Vancediscussqueryoptimization in object-oriented databases in the context of Revelationproject. Theydescnbemodelingfeatures that supportuser-defined data types,consider their impact on queryoptimization, and discuss the Revelationapproach to these problems. Beforeclosing, I would like to thank the authors for providingexcellentpapers at a shortnotice. It is my fondhopethat this issuewill provideimpetus for furtherreseaith in this importantarea. RakeshAgrawal IBMAlmadenResearchCenter SanJose,California95120 THE JUHF~H~IFhIN~ ~U9T~~IFIL~EElF1E~H (.i.s ~urwui:r~i~ui~:sr FEIISF~ W1TF~ U~LITE~ A NationalSearch forcomputerbased applications to help persons with physical or learningdisabilities is be ing conducted by The JohnsHopkins University with grants from the Na tional ScienceFoundation and MCI CommunicationsCorporation. A grandprizeof$10,000andmorethan 100 otherprizes will be awarded for the best ideas, systems,devices and computerprograms developed by Professionals,Amateurs,andStudents. Entrydeadline is August 23, 1991. Formore informationwrite to Computing to AssistPersons withDisabilities P.O. Box 1200 Laurel,MD20723 2 The TwoFacets of Object-OrientedDataModels SERGEABITEBOULS PARIS C. KANELLAKISt 0. Introduction:Object-orienteddatabasesystems(OODBs) are new softwaresystemsbuiltusingtechniquesfrom