SCHEMA VERSIONING FOR MULTITEMPORAL RELATIONAL DATABASES

In order to follow the evolution of application needs, a database mana gement system is easily expected to undergo changes involving database structure after implementation. Schema evolution concerns the ability of maintaining extant data in response to changes in database structu re. Schema versioning enables the use of extensional data through mult iple schema interfaces as created by a history of schema changes. Howe ver, schema versioning has been considered only to a limited extent in current literature. Also in the field of temporal databases, whereas a great deal of work has been done concerning temporal versioning of e xtensional data, a thorough investigation of schema versioning potenti alities has not yet been made. In this paper we consider schema versio ning in a broader perspective and introduce new design options whose d istinct semantic properties and functionalities will be discussed. Fir st of all, we consider solutions for schema versioning along transacti on time but also along valid time. Moreover, the support of schema ver sioning implies operations both at intensional and extensional level. Two distinct design solutions (namely single-and multi-pool) are prese nted for the management of extensional data in a system supporting sch ema versioning. Finally, a further distinction is introduced to define synchronous and asynchronous management of versioned data and schemat a. The proposed solutions differ in their semantics and in the possibl e operations they support. The mechanisms for the selection of data th rough a schema version are in many cases strictly related to the parti cular schema versioning solution adopted, that also affects the data d efinition and manipulation language at user-interface level. In partic ular, we show how the temporal language TSQL2, originally designed to support basic functionalities of transaction-time schema versioning, c an accordingly be extended. (C) 1997 Elsevier Science Ltd.