A survey of schema versioning issues for database systems
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paper_title: Versions and change notification in an object-oriented database system

paper_content:

The authors have built a prototype object-oriented database system called ORION to support applications from the CAD/CAM (computer-aided-design/computer-aided-manufacturing), AI (artificial-intelligence), and office-information-system domains. Advanced functions supported in ORION include versions, change notification, composite objects, dynamic schema evolution, and multimedia data. The versions and change notification features are based on a model that the authors developed earlier. They have integrated their model of versions and change notification into the ORION object-oriented data model, and also provide an insight into system overhead that versions and change notification incur. >
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paper_title: Issues in Software Maintenance

paper_content:

Abstract : Up to a few years ago the area of software maintenance was largely ignored. Interest has increased in the last few years due to several factors. First, the increased volume of enhancement and maintenance with more systems from that of ten years ago has restricted resources available for new development. Second, there has been a growing awareness that tools and aids which assist development of information systems may have little effect on operational systems. Third, the management of information systems has come under increasing scrutiny. In this report we highlight some of the major issues that surfaced during several extensive operational software studies. These sources have pointed to significant questions that must be addressed concerning the roles of the users in operations and maintenance, the management of maintenance, and the types of tools and techniques that are needed in maintenance. (Author)
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paper_title: Data model issues for object-oriented applications

paper_content:

Presented in this paper is the data model for ORION, a prototype database system that adds persistence and sharability to objects created and manipulated in object-oriented applications. The ORION data model consolidates and modifies a number of major concepts found in many object-oriented systems, such as objects, classes, class lattice, methods, and inheritance. These concepts are reviewed and three major enhancements to the conventional object-oriented data model, namely, schema evolution, composite objects, and versions, are elaborated upon. Schema evolution is the ability to dynamically make changes to the class definitions and the structure of the class lattice. Composite objects are recursive collections of exclusive components that are treated as units of storage, retrieval, and integrity enforcement. Versions are variations of the same object that are related by the history of their derivation. These enhancements are strongly motivated by the data management requirements of the ORION applications from the domains of artificial intelligence, computer-aided design and manufacturing, and office information systems with multimedia documents.
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paper_title: Semantic heterogeneity as a result of domain evolution

paper_content:

We describe examples of problems of semantic heterogeneity in databases due to “domain evolution”, as it occurs in both single- and multidatabase systems. These problems occur when the semantics of values of a particular domain change over time in ways that are not amenable to applying simple mappings between “old” and “new” values. The paper also proposes facilities and strategies for solving such problems.
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paper_title: Temporally oriented data definitions: managing schema evolution in temporally oriented databases

paper_content:

A simplifying — yet unrealistic — assumption widely held throughout the research of Temporally Oriented Data Models (TODM) is that the associated schema never changes. The implications of allowing data structures to evolve over time within a TODM and related databases are examined in this paper, and key issues and concepts are identified. Specifically, Temporally Oriented Data Definition (TODD) raises questions with respect to (1) the evolution of meanings in databases, (2) the nature of the temporal prevalence of database schema, and (3) the general principles that may guide the implementation of a TODM database with TODD.
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paper_title: Managing Schema Versions in a Time-Versioned Non-First-Normal-Form Relational Database

paper_content:

Support of time versions is a very advanced feature in a DBMS. However, full flexibility of history processing is achieved only if we can also change the database schema dynamically, without touching the history. A technique for achieving this goal is here presented, in the frames of the Non-First- Normal-Form (NF2) relational data model. The environment is a pilot DBMS supporting this model, developed by the Advanced Information Management (AIM) project at the IBM Heidelberg Scientific Center. The technical solution pursues to minimize the storage space and the number of data versions. One way to achieve this is to avoid the immediate update of all data instances in the context of a schema change. Transformations between versions enable the correct interpretation of data. The management of time-related queries becomes complicated, when schema changes are involved. The paper describes a technique of applying global views over different schema versions, when formulating the queries and their results.
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paper_title: Predictions and Challenges for Database Systems in the Year 2000

paper_content:

Pcrmirsion to copy without fee ail or part of ihir material ir granted provided that ibe copier are not made or diritibuted jot direct commercial advantage, tbe VLDB copyright notiee and tbe title oj the publication and ilr data appear, and noiice ir given tbot copying ir by permirrion of ihe Very Lar/c Data Baee Endowment. To copy oibcrwiee, or to npublirh, reqrirer a jet and/or rpccial permirrion jrom the Endowment.
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paper_title: SQL/SE: a query language extension for databases supporting schema evolution

paper_content:

The incorporation of a knowledge of time within database systems allows for temporally related information to be modelled more naturally and consistently. Adding this support to the metadatabase further enhances its semantic capability and allows elaborate interrogation of data. This paper presents SQL/SE, an SQL extension capable of handling schema evolution in relational database systems.
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paper_title: Schema evolution and the relational algebra

paper_content:

In this paper we discuss extensions to the conventional relational algebra to support both aspects of transaction time, evolution of a database’s contents and evolution of a database’s schema. We dene a relation’s schema to be the relation’s temporal signature, a function mapping the relation’s attribute names onto their value domains, and class, indicating the extent of support for time. We also introduce commands to change a relation, now dened as a triple consisting of a sequence of classes, a sequence of signatures, and a sequence of states. A semantic type system is required to identify semantically incorrect expressions and to enforce consistency constraints among a relation’s class, signature, and state following update. We show that these extensions are applicable, without change, to historical algebras that support valid time, yielding an algebraic language for the query and update of temporal databases. The additions preserve the useful properties of the conventional algebra. A database’s schema describes the structure of the database; the contents of the database must adhere to that structure [Date 1976, Ullman 1982]. Schema evolution refers to changes to the database’s schema over time. Conventional databases allow only one schema to be in force at a time, requiring restructuring (also termed logical reorganization [Sockut & Goldberg 1979]) when the schema is modied. With the advent of databases storing past states [McKenzie 1986], it becomes desirable to accommodate multiple schemas, each in eect for an interval in the past. Schema versioning refers to retention of past schemas resulting from schema evolution. In an earlier paper [McKenzie & Snodgrass 1987A] we proposed extensions to the conventional relational algebra [Codd 1970] that model the evolution of a database’s contents. We did not, however, consider the evolution of a database’s schema. In this paper, we provide further extensions to the conventional relational algebra that model the evolution of a database’s schema. The extensions that support evolution of a database’s contents are repeated here for completeness and because the extensions supporting schema evolution are best explained in concert with those earlier extensions.
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paper_title: Versions of Schema for Object-Oriented Databases

paper_content:

Version control is one of the important database requirements for design environments. Various models of versions have been proposed and implemented. However, research in versions has been focused exclusively on versioning single design objects. In a multi-user design environment where the schema (definition) of the design objects may undergo dynamic changes, it is important to be able to version the schema, as well as version the single design objects. In this paper, we propose a model of versions of schema by extending our model of versions of single objects. In particular, we present the semantics of our model of versions of schema for object-oriented databases, explore issues in implementing the model, and examine a few alternatives to our model of versions of schema.
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paper_title: The Historical Relational Data Model (HRDM) and Algebra Based on Lifespans

paper_content:

Critical to the design of an historical database model is the representation of the “existence” of objects across the temporal dimension — for example, the “birth,” “death,” or “rebirth” of an individual, or the establishment or dis-establishment of a relationship. The notion of the “lifespan” of a database object is proposed as a simple framework for expressing these concepts. An object's lifespan is simply those periods of time during which the database models the properties of that object. In this paper we propose the historical relational data model (HRDM) and algebra that is based upon lifespans and that views the values of all attributes as functions from time points to simple domains. The model that we obtain is a consistent extension of the relational data model, and provides a simple mechanism for providing both time-varying data and time-varying schemes.
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paper_title: Quantifying Schema Evolution

paper_content:

Abstract Achieving correct changes is the dominant activity in the application software industry. Modification of database schemata is one kind of change which may have severe consequences for database applications. The paper presents a method for measuring modifications to database schemata and their consequences by using a thesaurus tool. Measurements of the evolution of a large-scale database application currently running in several hospitals in the UK are presented and interpreted. The kind of measurements provided by this in-depth study is useful input to the design of change management tools.
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paper_title: Semantics and implementation of schema evolution in object-oriented databases

paper_content:

Object-oriented programming is well-suited to such data-intensive application domains as CAD/CAM, AI, and OIS (office information systems) with multimedia documents. At MCC we have built a prototype object-oriented database system, called ORION. It adds persistence and sharability to objects created and manipulated in applications implemented in an object-oriented programming environment. One of the important requirements of these applications is schema evolution, that is, the ability to dynamically make a wide variety of changes to the database schema. In this paper, following a brief review of the object-oriented data model that we support in ORION, we establish a framework for supporting schema evolution, define the semantics of schema evolution, and discuss its implementation.
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paper_title: Version Management in an Object-Oriented Database

paper_content:

We describe a database system that includes a built-in version control mechanism that can be used in the definition of any new object types. This database system is object-oriented in the sense that it supports data abstraction, object types, and inheritance.
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paper_title: Temporal semantics in information systems: a survey

paper_content:

Abstract If a computer system is to deal with temporal semantics, it must understand the nature of time and have the ability to accept and reason with time-related facts. This reasoning ranges from the knowledge and use of the chronological nature of a given calendar system, to the more complex nature of inductive reasoning between related events and time periods. This paper investigates the handling of time as it has been applied to the fields of data modelling and artificial intelligence. Systems using the techniques are investigated. Significant features and properties are then extracted and examined where they are pertinent to systems capable of modelling temporal data.
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paper_title: A consensus glossary of temporal database concepts

paper_content:

This document contains definitions of a wide range of concepts specific to and widely used within temporal databases. In addition to providing definitions, the document also includes separate explanations of many of the defined concepts. Two sets of criteria are included. First, all included concepts were required to satisfy four relevance criteria, and, second, the naming of the concepts was resolved using a set of evaluation criteria. The concepts are grouped into three categories: concepts of general database interest, of temporal database interest, and of specialized interest. This document is a digest of a full version of the glossary1. In addition to the material included here, the full version includes substantial discussions of the naming of the concepts.The consensus effort that lead to this glossary was initiated in Early 1992. Earlier status documents appeared in March 1993 and December 1992 and included terms proposed after an initial glossary appeared in SIGMOD Record in September 1992. The present glossary subsumes all the previous documents. It was most recently discussed at the "ARPA/NSF International Workshop on an Infrastructure for Temporal Databases," in Arlington, TX, June 1993, and is recommended by a significant part of the temporal database community. The glossary meets a need for creating a higher degree of consensus on the definition and naming of temporal database concepts.
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paper_title: Temporally oriented data definitions: managing schema evolution in temporally oriented databases

paper_content:

A simplifying — yet unrealistic — assumption widely held throughout the research of Temporally Oriented Data Models (TODM) is that the associated schema never changes. The implications of allowing data structures to evolve over time within a TODM and related databases are examined in this paper, and key issues and concepts are identified. Specifically, Temporally Oriented Data Definition (TODD) raises questions with respect to (1) the evolution of meanings in databases, (2) the nature of the temporal prevalence of database schema, and (3) the general principles that may guide the implementation of a TODM database with TODD.
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paper_title: Management Of Schema Evolution In Databases

paper_content:

This paper presents a version model which handles database schema changes and which takes evolution into account. Its originality is in allowing the development of partial schema versions, or views of a schema. These versions are created in the same database from a common schema. We define the set of authorised modifications on a schema and the rules which guarantee its coherence after transformation. Mechanisms allowing data to be associated with each version are also integrated in the model.
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paper_title: An incremental mechanism for schema evolution in engineering domains

paper_content:

The authors focus on one class of schema revisions necessitated by a very basic phenomenon: a given individual object evolves into a family of objects which are similar to it in many ways. This is commonly called the version problem. In theoretical terms, one can handle the above schema change in the standard, object-oriented database models by the interposition of suitable abstractions into the existing type lattice. There are practical and engineering difficulties with such schema changes. The authors propose an incremental mechanism called instance inheritance which is well suited to handling the schema changes without the attendant practical costs. The authors formally characterize this augmentation to the standard database models, and show examples of its applications. >
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paper_title: The Use of Information Capacity in Schema Integration and Translation

paper_content:

In this paper, we carefully explore the assumptions behind using information capacity equivalence as a measure of correctness for judging transformed schemas in schema integration and translation methodologies. We present a classification of common integration and translation tasks based on their operational goals and derive from them the relative information capacity requirements of the original and transformed schemas. We show that for many tasks, information capacity equivalence of the schemas is not strictly required. Based on this, we present a new definition of correctness that reflects each undertaken task. We then examine existing methodologies and show how anomalies can arise when using those that do not meet the proposed correctness criteria.
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paper_title: Structural schema integration with full and partial correspondence using the dual model

paper_content:

Abstract The integration of views and schemas is an important part of database design and evolution and permits the sharing of data across complex applications. The view and schema integration methodologies used to date are driven purely by semantic considerations, and allow integration of objects only if that is valid from both semantic and structural view points. We discuss a new integration method called structural integration that has the advantage of being able to integrate objects that have structural similarities, even if they differ semantically. This is possible by using the object-oriented Dual Model which allows separate representation of structure and semantics. Structural integration has several advantages, including the identification of shared common structures that is important for sharing of data and methods.
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paper_title: A theory of attributed equivalence in databases with application to schema integration

paper_content:

The authors present a common foundation for integrating pairs of entity sets, pairs of relationship sets, and an entity set with a relationship set. This common foundation is based on the basic principle of integrating attributes. Any pair of objects whose identifying attributes can be integrated can themselves be integrated. Several definitions of attribute equivalence are presented. These definitions can be used to specify the exact nature of the relationship between a pair of attributes. Based on these definitions, several strategies for attribute integration are presented and evaluated. >
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paper_title: The Use of Information Capacity in Schema Integration and Translation

paper_content:

In this paper, we carefully explore the assumptions behind using information capacity equivalence as a measure of correctness for judging transformed schemas in schema integration and translation methodologies. We present a classification of common integration and translation tasks based on their operational goals and derive from them the relative information capacity requirements of the original and transformed schemas. We show that for many tasks, information capacity equivalence of the schemas is not strictly required. Based on this, we present a new definition of correctness that reflects each undertaken task. We then examine existing methodologies and show how anomalies can arise when using those that do not meet the proposed correctness criteria.
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paper_title: Temporal semantics in information systems: a survey

paper_content:

Abstract If a computer system is to deal with temporal semantics, it must understand the nature of time and have the ability to accept and reason with time-related facts. This reasoning ranges from the knowledge and use of the chronological nature of a given calendar system, to the more complex nature of inductive reasoning between related events and time periods. This paper investigates the handling of time as it has been applied to the fields of data modelling and artificial intelligence. Systems using the techniques are investigated. Significant features and properties are then extracted and examined where they are pertinent to systems capable of modelling temporal data.
---
paper_title: Semantic heterogeneity as a result of domain evolution

paper_content:

We describe examples of problems of semantic heterogeneity in databases due to “domain evolution”, as it occurs in both single- and multidatabase systems. These problems occur when the semantics of values of a particular domain change over time in ways that are not amenable to applying simple mappings between “old” and “new” values. The paper also proposes facilities and strategies for solving such problems.
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paper_title: Schema evolution in database systems: an annotated bibliography

paper_content:

Schema Evolution is the ability of a database system to respond to changes in the real world by allowing the schema to evolve. In many systems this property also implies a retaining of past states of the schema. This latter property is necessary if data recorded during the lifetime of one version of the schema is not to be made obsolete as the schema changes. This annotated bibliography investigates current published research with respect to the handling of changing schemas in database systems.
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paper_title: A Taxonomy for Schema Versioning Based on the Relational and Entity Relationship Models

paper_content:

Recently there has been increasing interest in both the problems and the potential of accommodating evolving schema in databases, especially in systems which necessitate a high volume of structural changes or where structural change is difficult. This paper presents a taxonomy of changes applicable to the Entity-Relationship Model together with their effects on the underlying relational model expressed in terms of a second taxonomy relevant to the relational model.
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paper_title: An architecture for automatic relational database sytem conversion

paper_content:

Changes in requirements for database systems necessitate schema restructuring, database translation, and application or query program conversion. An alternative to the lengthy manual revision process is proposed by offering a set of 15 transformations keyed to the relational model of data and the relational algebra. Motivations, examples, and detailed descriptions are provided.
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paper_title: Extending the relational algebra to support transaction time

paper_content:

In this paper we discuss extensions to the conventional relational algebra to support transaction time. We show that these extensions are applicable to historical algebras that support valid time, yielding a temporal algebraic language. Since transaction time concerns the storage of information in the database, the notion of state is central. The extensions are formalized using denotational semantics. The additions preserve the useful properties of the conventional relational algebra.
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paper_title: Algebra and query language for a historical data model

paper_content:

We propose a «state» oriented view of historical databases. We propose an algebra for historical relations which contains classical as well as some new operators. The operators are simple to comprehend, unlike in other research proposals. Were are also able to formulate a completeness criteria for the proposed model. Finally, we extend the popular SQL query language for use with historical databases. Again, the extensions are consistent with the simple basis of standard SQL
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paper_title: An algebraic language for query and update of temporal databases

paper_content:

Although time is a property of events and objects in the real world, conventional relational database management systems (RDBM's) can't model the evolution of either the objects being modeled or the database itself. Relational databases can be viewed as snapshot databases in that they record only the current database state, which represents the state of the enterprise being modeled at some particular time. We extend the relational algebra to support two orthogonal aspects of time: valid time, which concerns the modeling of time-varying reality, and transaction time, which concerns the recording of information in databases. In so doing, we define an algebraic language for query and update of temporal databases. ::: The relational algebra is first extended to support valid time. Historical versions of nine relational operators (i.e., union, difference, cartesian product, selection, projection, intersection, $\Theta$-join, natural join, and quotient) are defined and three new operators (i.e., historical derivation, non-unique aggregation, and unique aggregation) are introduced. Both the relational algebra and this new historical algebra are then encapsulated within a language of commands to support transaction time. The language's semantics is formalized using denotational semantics. Rollback operators are added to the algebras to allow relations to be rolled back in time. The language accommodates scheme and contents evolution, handles single-command and multiple-command transactions, and supports queries on valid time. The language is shown to have the expressive power of the temporal query language TQuel. ::: The language supports both unmaterialized and materialized views and accommodates a spectrum of view maintenance strategies, including incremental, recomputed, and immediate view materialization. Incremental versions of the snapshot and historical operators are defined to support incremental view materialization. A prototype query processor was built for TQuel to study incremental view materialization in temporal databases. Problems that arise when materialized views are maintained incrementally are discussed, and solutions to those problems are proposed. ::: Criteria for evaluating temporal algebras are presented. Incompatibilities among the criteria are identified and a maximal set of compatible evaluation criteria is proposed. Our language and other previously proposed temporal extensions of the relational algebra are evaluated against these criteria.
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paper_title: A Temporal Relational Algebra as Basis for Temporal Relational Completeness

paper_content:

We define a temporal algebra that is applicable to anytemporal relational data model supporting discrete linearbounded time. This algebra has the five basicrelational algebra operators extended to the temporaldomain and an operator of linear recursion. Weshow that this algebra has the expressive power of asafe temporal calculus based on the predicate temporallogic with the until and since temporal operators.In [CrC189], a historical calculus was proposed as abasis for historical relational completeness. We proposethe temporal algebra defined in this paper andthe equivalent temporal calculus as an alternative basisfor temporal relational completeness.
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paper_title: Extending the database relational model to capture more meaning

paper_content:

During the last three or four years several investigators have been exploring “semantic models” for formatted databases. The intent is to capture (in a more or less formal way) more of the meaning of the data so that database design can become more systematic and the database system itself can behave more intelligently. Two major thrusts are clear. (1) the search for meaningful units that are as small as possible— atomic semantics ; (2) the search for meaningful units that are larger than the usual n -ary relation— molecular semantics . In this paper we propose extensions to the relational model to support certain atomic and molecular semantics. These extensions represent a synthesis of many ideas from the published work in semantic modeling plus the introduction of new rules for insertion, update, and deletion, as well as new algebraic operators.
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paper_title: Adding time dimension to relational model and extending relational algebra

paper_content:

Abstract A methodology for adding the time dimension to the relational model is proposed and relational algebra is extended for this purpose. We propose time-stamping attributes instead of adding time to tuples. Each attribute value is stored along with a time interval over which it is valid. Non-first normal form realations are used. A relation can have atomic, set-valued, triplet-valued, or set triplet-valued attributes. The last two types of attributes preserve the time (history). Furthermore, new algebraic operations are defined to extract information from historical relations. These operations convert one attribute type to another and do selection over the time dimension. Algebraic rules and identities for the new operations are also included.
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paper_title: A homogeneous relational model and query languages for temporal databases

paper_content:

In a temporal database, time values are associated with data item to indicate their periods of validity. We propose a model for temporal databases within the framework of the classical database theory. Our model is realized as a temporal parameterization of static relations. We do not impose any restrictions upon the schemes of temporal relations. The classical concepts of normal forms and dependencies are easily extended to our model, allowing a suitable design for a database scheme. We present a relational algebra and a tuple calculus for our model and prove their equivalence. Our data model is homogeneous in the sense that the periods of validity of all the attributes in a given tuple of a temporal relation are identical. We discuss how to relax the homogeneity requirement to extend the application domain of our approach.
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paper_title: The Historical Relational Data Model (HRDM) and Algebra Based on Lifespans

paper_content:

Critical to the design of an historical database model is the representation of the “existence” of objects across the temporal dimension — for example, the “birth,” “death,” or “rebirth” of an individual, or the establishment or dis-establishment of a relationship. The notion of the “lifespan” of a database object is proposed as a simple framework for expressing these concepts. An object's lifespan is simply those periods of time during which the database models the properties of that object. In this paper we propose the historical relational data model (HRDM) and algebra that is based upon lifespans and that views the values of all attributes as functions from time points to simple domains. The model that we obtain is a consistent extension of the relational data model, and provides a simple mechanism for providing both time-varying data and time-varying schemes.
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paper_title: Evaluation of relational algebras incorporating the time dimension in databases

paper_content:

The relational algebra is a procedural query language for relational databases. In this paper we survey extensions of the relational algebra that can query databases recording time-varying data. Such an algebra is a critical part of a temporal DBMS. We identify 26 criteria that provide an objective basis for evaluating temporal algebras, Seven of the criteria are shown to be mutually unsatisfiable, implying there can be no perfect temporal algebra, Choices made as to which of the incompatible criteria are satisfied characterize existing algebras Twelve time-oriented algebras are summarized and then evaluated against the criteria. We demonstrate that the design space has in some sense been explored in that all combinations of basic design decisions have at least one representative algebra. Coverage of the remaining criteria provides one measure of the quality of each algebra We argue that all of the criteria are independent and that the criteria identified as compatible are indeed so, Finally, we list plausible properties proposed by others that are either subsumed by other criteria, are not well defined, or have no objective basis for being evaluated. The algebras realize many different approaches to what appears initially to be a straightforward design task.
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paper_title: Relational completeness of data base sublanguages

paper_content:

In the near future, we can expect a great variety of languages to be proposed for interrogating and updating data bases. This paper attempts to provide a theoretical basis which may be used to determine how complete a selection capability is provided in a proposed data sublanguage independently of any host language in which the sublanguage may be embedded. A relational algebra and a relational calculus are defined. Then, an algorithm is presented for reducing an arbitrary relation-defining expression (based on the calculus) into a semantically equivalent expression of the relational algebra. Finally, some opinions are stated regarding the relative merits of calculusoriented versus algebra-oriented data sublanguages from the standpoint of optimal search and highly discriminating authorization schemes. RJ 987 Hl7041) March 6, 1972 Computer Sciences
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paper_title: Architecture of the ORION next-generation database system

paper_content:

Various architectural components of ORION-1 and ORION-1SX are described and a review of the current implementation is provided. The message handler receives all messages sent to the ORION system. The object subsystem provides high-level data management functions, including query optimization, schema management, long data management (including text search) and support for versionable objects, composite objects, and multimedia objects. The transaction management subsystem coordinates concurrent object accesses and provides recovery capabilities. The storage subsystem manages persistent storage of objects and controls the flow of objects between the secondary storage device and main memory buffers. In ORION-1, all subsystems reside in one computer. The ORION-1SX architecture is significantly different from ORION-1 in the management of shared data structures and distribution of these subsystems and their components. >
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paper_title: Management Of Schema Evolution In Databases

paper_content:

This paper presents a version model which handles database schema changes and which takes evolution into account. Its originality is in allowing the development of partial schema versions, or views of a schema. These versions are created in the same database from a common schema. We define the set of authorised modifications on a schema and the rules which guarantee its coherence after transformation. Mechanisms allowing data to be associated with each version are also integrated in the model.
---
paper_title: Versions of Schema for Object-Oriented Databases

paper_content:

Version control is one of the important database requirements for design environments. Various models of versions have been proposed and implemented. However, research in versions has been focused exclusively on versioning single design objects. In a multi-user design environment where the schema (definition) of the design objects may undergo dynamic changes, it is important to be able to version the schema, as well as version the single design objects. In this paper, we propose a model of versions of schema by extending our model of versions of single objects. In particular, we present the semantics of our model of versions of schema for object-oriented databases, explore issues in implementing the model, and examine a few alternatives to our model of versions of schema.
---
paper_title: Version Management in an Object-Oriented Database

paper_content:

We describe a database system that includes a built-in version control mechanism that can be used in the definition of any new object types. This database system is object-oriented in the sense that it supports data abstraction, object types, and inheritance.
---
paper_title: Versions of Schema for Object-Oriented Databases

paper_content:

Version control is one of the important database requirements for design environments. Various models of versions have been proposed and implemented. However, research in versions has been focused exclusively on versioning single design objects. In a multi-user design environment where the schema (definition) of the design objects may undergo dynamic changes, it is important to be able to version the schema, as well as version the single design objects. In this paper, we propose a model of versions of schema by extending our model of versions of single objects. In particular, we present the semantics of our model of versions of schema for object-oriented databases, explore issues in implementing the model, and examine a few alternatives to our model of versions of schema.
---
paper_title: Semantics and implementation of schema evolution in object-oriented databases

paper_content:

Object-oriented programming is well-suited to such data-intensive application domains as CAD/CAM, AI, and OIS (office information systems) with multimedia documents. At MCC we have built a prototype object-oriented database system, called ORION. It adds persistence and sharability to objects created and manipulated in applications implemented in an object-oriented programming environment. One of the important requirements of these applications is schema evolution, that is, the ability to dynamically make a wide variety of changes to the database schema. In this paper, following a brief review of the object-oriented data model that we support in ORION, we establish a framework for supporting schema evolution, define the semantics of schema evolution, and discuss its implementation.
---
paper_title: Version Management in an Object-Oriented Database

paper_content:

We describe a database system that includes a built-in version control mechanism that can be used in the definition of any new object types. This database system is object-oriented in the sense that it supports data abstraction, object types, and inheritance.
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paper_title: Meta Operations for Type Management in Object-Oriented Databases: — A Lazy Mechanism for Schema Evolution

paper_content:

In object-oriented database systems, type definitions are used as the basis of object manipulation. They may change causing the systems' schemata evolve dynamically. In this paper, we first clarify the concept of schema evolution in databases and discuss its existing solutions. Next we propose a lazy evaluation method of schema evolution which minimize the amount of object manipulation. It is realized in a system which incorporates the concept of persistent meta-object, while meta-object interprets meta-message and principally maintains type validness while schema evolves. Better balance between the availability of object and the speed of accessing object is obtained by our method.
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paper_title: The time relational model

paper_content:

Existing data base management systems (DBMSs) allow users to operate upon the latest (committed) data base state. However, many real world applications require storing and accessing historical information. Furthermore, there are no provisions in current DBMSs for the distinction between the physical time a given data item is entered, and the time period to which it pertains. Many data base updates in real applications are either "retroactive", i.e., their effectiveness takes place sometime in the past, or "proactive", i.e., their effectiveness will take place sometime in the future. ::: The Time-Relational Model is an architecture which integrates comprehensive time processing capabilities into the Relational Model of data bases for managing changes of data values, data manipulation rules, and data structure. However, those who are not concerned about Time in their particular applications may perceive this model as a regular relational model. ::: A fundamental concept is the time-view of data and transaction programs. Its major objective is to automatically and dynamically provide multiple and complete data base views reflecting different points in time, including all system components that may change over time. The time-view theorem provides necessary and sufficient conditions for achieving any desirable, dynamic time-view. The time-consistency theorem states the conditions for which a given computation can be reproduced again and again in the face of data base changes over time. The basic relational algebra operators are extended to include the time-view concept, resulting in the time relational algebra. An implementation architecture is developed which demonstrates the feasibility of such model. The central idea is to gain more functions and solutions based on the presence of time related information and, thus, spread the overall operational and implementation cost across multiple sources. Recovery Manager and Concurrency Control Procedures are developed. The recovery manager uses only the data itself for its own operation rather, than a logging facility; there is no need for a locking mechanism to provide each user with an appropriate isolation level from other concurrent users. ::: A real-life case study is described based on the Informatics Inc. Manufacturing Planning System - PRODUCTION-IV.
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paper_title: The temporal query language TQuel

paper_content:

Recently, attention has been focused on temporal databases , representing an enterprise over time. We have developed a new language, Tquel , to query a temporal database. TQuel was designed to be a minimal extension, both syntactically and semantically, of Quel, the query language in the Ingres relational database management system. This paper discusses the language informally, then provides a tuple relational calculus semantics for the TQuel statements that differ from their Quel counterparts, including the modification statements. The three additional temporal constructs defined in Tquel are shown to be direct semantic analogues of Quel's where clause and target list. We also discuss reducibility of the semantics to Quel's semantics when applied to a static database. TQuel is compared with ten other query languages supporting time.
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paper_title: Formal semantics for time in databases

paper_content:

The concept of a historical database is introduced as a tool for modeling the dynamic nature of some part of the real world. Just as first-order logic has been shown to be a useful formalism for expressing and understanding the underlying semantics of the relational database model, intensional logic is presented as an analogous formalism for expressing and understanding the temporal semantics involved in a historical database. The various components of the relational model, as extended to include historical relations, are discussed in terms of the model theory for the logic IL s , a variation of the logic IL formulated by Richard Montague. The modal concepts of intensional and extensional data constraints and queries are introduced and contrasted. Finally, the potential application of these ideas to the problem of natural language database querying is discussed.
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paper_title: Quantifying Schema Evolution

paper_content:

Abstract Achieving correct changes is the dominant activity in the application software industry. Modification of database schemata is one kind of change which may have severe consequences for database applications. The paper presents a method for measuring modifications to database schemata and their consequences by using a thesaurus tool. Measurements of the evolution of a large-scale database application currently running in several hospitals in the UK are presented and interpreted. The kind of measurements provided by this in-depth study is useful input to the design of change management tools.
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paper_title: Formal semantics for time in databases

paper_content:

The concept of a historical database is introduced as a tool for modeling the dynamic nature of some part of the real world. Just as first-order logic has been shown to be a useful formalism for expressing and understanding the underlying semantics of the relational database model, intensional logic is presented as an analogous formalism for expressing and understanding the temporal semantics involved in a historical database. The various components of the relational model, as extended to include historical relations, are discussed in terms of the model theory for the logic IL s , a variation of the logic IL formulated by Richard Montague. The modal concepts of intensional and extensional data constraints and queries are introduced and contrasted. Finally, the potential application of these ideas to the problem of natural language database querying is discussed.
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paper_title: Database relations with null values

paper_content:

Abstract A new formal approach is proposed for modeling incomplete database information by means of null values. The basis of our approach is an interpretation of nulls which obviates the need for more than one type of null. The conceptual soundness of this approach is demonstrated by generalizing the formal framework of the relational data model to include null values. In particular, the set-theoretical properties of relations with nulls are studied and the definitions of set inclusion, set union, and set difference are generalized. A simple and efficient strategy for evaluating queries in the presence of nulls is provided. The operators of relational algebra are then generalized accordingly. Finally, the deep-rooted logical and computational problems of previous approaches are reviewed to emphasize the superior practicability of the solution.
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paper_title: Null values in nested relational databases

paper_content:

The desire to extend the applicability of the relational model beyond traditional data-processing applications has stimulated interest in nested or non-first normal form relations in which the attributes of a relation can take on values which are sets or even relations themselves. In this paper, we study the role of null values in the nested relational model using an open world assumption. We extend the traditional theory and study the properties of extended operators for nested relations containing nulls. The no-information, unknown, and non-existent interpretation of nulls are discussed and the meaning of “empty set” is clarified. Finally, contrary to several previous results, we determine that the traditional axiomatization of functional and multivalued dependencies is valid in the presence of nulls.
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paper_title: Schema evolution and the relational algebra

paper_content:

In this paper we discuss extensions to the conventional relational algebra to support both aspects of transaction time, evolution of a database’s contents and evolution of a database’s schema. We dene a relation’s schema to be the relation’s temporal signature, a function mapping the relation’s attribute names onto their value domains, and class, indicating the extent of support for time. We also introduce commands to change a relation, now dened as a triple consisting of a sequence of classes, a sequence of signatures, and a sequence of states. A semantic type system is required to identify semantically incorrect expressions and to enforce consistency constraints among a relation’s class, signature, and state following update. We show that these extensions are applicable, without change, to historical algebras that support valid time, yielding an algebraic language for the query and update of temporal databases. The additions preserve the useful properties of the conventional algebra. A database’s schema describes the structure of the database; the contents of the database must adhere to that structure [Date 1976, Ullman 1982]. Schema evolution refers to changes to the database’s schema over time. Conventional databases allow only one schema to be in force at a time, requiring restructuring (also termed logical reorganization [Sockut & Goldberg 1979]) when the schema is modied. With the advent of databases storing past states [McKenzie 1986], it becomes desirable to accommodate multiple schemas, each in eect for an interval in the past. Schema versioning refers to retention of past schemas resulting from schema evolution. In an earlier paper [McKenzie & Snodgrass 1987A] we proposed extensions to the conventional relational algebra [Codd 1970] that model the evolution of a database’s contents. We did not, however, consider the evolution of a database’s schema. In this paper, we provide further extensions to the conventional relational algebra that model the evolution of a database’s schema. The extensions that support evolution of a database’s contents are repeated here for completeness and because the extensions supporting schema evolution are best explained in concert with those earlier extensions.
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paper_title: Quantifying Schema Evolution

paper_content:

Abstract Achieving correct changes is the dominant activity in the application software industry. Modification of database schemata is one kind of change which may have severe consequences for database applications. The paper presents a method for measuring modifications to database schemata and their consequences by using a thesaurus tool. Measurements of the evolution of a large-scale database application currently running in several hospitals in the UK are presented and interpreted. The kind of measurements provided by this in-depth study is useful input to the design of change management tools.
---
paper_title: Handling discovered structure in database systems

paper_content:

Most database systems research assumes that the database schema is determined by a database administrator. With the recent increase in interest in knowledge discovery from databases and the predicted increase in the volume of data expected to be stored it is appropriate to reexamine this assumption and investigate how derived or induced, rather than database administrator supplied, structure can be accommodated and used by database systems. The paper investigates some of the characteristics of inductive learning and knowledge discovery as they pertain to database systems and the constraints that would be imposed on appropriate inductive learning algorithms is discussed. A formal method of defining induced dependencies (both static and temporal) is proposed as the inductive analogue to functional dependencies. The Boswell database system exemplifying some of these characteristics is also briefly discussed.
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Title: A Survey of Schema Versioning Issues for Database Systems

Section 1: Background
Description 1: Provide an introduction to the problems of schema modifications and their implications for database management and administration.

Section 2: Pragmatic considerations
Description 2: Discuss the practical constraints and considerations for proposed solutions to schema versioning and evolution.

Section 3: Outline of this paper
Description 3: Summarize the structure and main sections of the paper.

Section 4: Handling heterogeneous schemata
Description 4: Define terms related to heterogeneous schemata and discuss their relationship with schema evolution.

Section 5: Schema modification, evolution and versioning
Description 5: Provide definitions and distinctions between schema modification, schema evolution, and schema versioning.

Section 6: Data and view integration
Description 6: Discuss associated research areas like data and view integration and their relevance to schema evolution.

Section 7: Temporal database systems
Description 7: Offer a brief overview of temporal database systems and their concepts related to schema versioning.

Section 8: Domain/type evolution
Description 8: Explore the issues and solutions related to the evolution of domains in data models.

Section 9: Relation/class evolution
Description 9: Examine the challenges and methodologies for evolving relation and class structures.

Section 10: Algebras supporting schema evolution
Description 10: Review different algebras proposed for supporting schema evolution in database systems.

Section 11: Schema conversion mechanisms
Description 11: Outline various proposed mechanisms for converting schemas at the physical level.

Section 12: Data conversion mechanisms
Description 12: Discuss approaches for converting data to align with new schema versions.

Section 13: Access right considerations
Description 13: Address the potential violations of access rights due to schema evolution changes.

Section 14: Concurrency considerations and concurrent schemata
Description 14: Discuss the concurrency issues that arise with schema modifications in a multi-user environment.

Section 15: Issues in query language support
Description 15: Explore the challenges and potential solutions for supporting schema evolution in database query languages.

Section 16: Levels of support for schema evolution in query languages
Description 16: Propose different approaches for handling schema changes in query languages.

Section 17: Completed schemata
Description 17: Discuss the concept of completed schemata and its application for data retrieval and backup purposes.

Section 18: Problems presented by null values
Description 18: Examine how null values can present issues in the context of an evolving schema.

Section 19: Schema valid-time support
Description 19: Discuss the potential merits and implementation considerations for schema valid-time support.

Section 20: Schema-time projection
Description 20: Define schema-time projection and methods for specifying and constructing effective schemas.

Section 21: Schema-time selection
Description 21: Address how schema-time selection helps access data based on schema formats and considerations related to data update.

Section 22: Version naming
Description 22: Present methods for naming schema versions to track and manage schema changes effectively.

Section 23: Casting of output attribute domains
Description 23: Discuss the relevance of casting or converting attribute domains for stability in applications using evolving schemas.

Section 24: Other related research issues
Description 24: Identify other research areas related to schema versioning and evolution, including pragmatic limitations and automated schema evolution.

Section 25: Further Research
Description 25: Summarize the current research landscape and outline directions for future research in schema versioning and evolution.