Advances in Databases and Information Systems

Abstract

Abstract A minimal extension of relational data model for logical modeling of the category of time in databases isoffered. Thetimeisconsideredasatotallyorderedset. Intheframeofsuggesteddatamodelthehomogeneousmodel of representation of time in relations is adopted. Allowing the homogeneity, the modeling of thetemporal aspect of databases is achieved by means of two temporal attributes. The temporal attribute isconsidered as abstract data type. A formal definition of temporal relation which is based on abstraction ofgeneralization is given. The semantics of operators update and delete is changed. A temporal algebra andcalculus are developed. Also new aggregate functions are suggested considering the time aspect of the objectbehavior in the given enterprise. In order to give temporal integrity constrains closed formulas are used. Theproblems of partial support of evolution of the database schemes are considered. 1 Introduction A great deal of research both in the area of DM and, in particular, in relational models has been doneafter the publications of early works of E. F. Codd. Results of this research are conceptual DM in whichadvanced declarative and/or procedural apparatus for data modeling is maintained. But the problems ofeffective organization and manipulation of data do notexhaust all the specter of problems in the theory ofDB. Problems of logical modeling of category of time in databases(CTDB) accept a special importance. Theroots of such research are in that within the frame of traditional DM it is possible to present onestate of themodeling object only. This brings to the loss of information which is necessary to take certain decision. Thusthe aim of CTDB is to present information on evolution of real world objects in DB. It should be mentionedthat problems of modeling of time are nota prerogative of information processing systems only. In scientificpublications known are works in the field of artificial intelligence, logic, linguistics, etc.The explicit presentation of the temporal context and its representation in constructions of informationsystems assumes development of special language tools because the temporal dimension, temporal attributesof relations of DB have a special semantics. In particular, the creation of temporal DM assumes the necessityof preservation in DB of those values of properties and relations of objects which are no longer true. Besides,the traditional interpretation of operations of DB updating changes: each actualization of DB is considered asa insertion of new data in it (the information of its previous state preserved).In general, DM is based upon a conception of consistency of DB. This means that it is impossible topresent in DB a fact and its contradiction simultaneously. Thisconception (with CTDB)should be extended bypossibilityto represent the stateof DBindifferent discrete moments of time (DMT).The traditional invariantsof DB take place in each of these states. Additional means are necessary for presentation of DB invariants,that take into account the category of time. An effective realization of CTDB requires to store information onobjects in those DMT only, that correspond to an certain event of the life of the object. The missing explicitinformation on the object in the interval between events should be inferable.Nowadays, CTDB supporting research in DM develops intensively. Such research is necessary for furtherdevelopment of conceptual DM for solution practical problems. On the other hand the appearance of cheaperenvironments of storage (optical disks, for instance) makes it possible to realize the temporal DB practically.Advanced in Databases and Information Systems, 1997 1