Tcrnporal Managcment of DataBase   

　　There are two components to temporal datamanagement: history data management and versionmanagement. Both have been the subjects of researchfor over a decade. The troublesome aspect of tempo-ral data management is that the boundary betweenapplications and database systems has not beenclearly drawn．Specifically, it is not clear how muchof the typical semantics and facilities of temporaldata management can and should be directly incorpo-rated in a database systqm，and how much should belcft to applications and users．In this section, we willprovide a list of short-term research issues thatshould be examined to shed light on this fundamentalquestion.   

　　The focus of research into history data manage-ment has been on defining the semantics of time andtime interval, and issues related to understanding thesemantics of queries and updates against histo叮datastored in an attribute of a record. Typically, in thecontext of relational databases，a temporal attribute isdefined to hold a sequence of history data for theattribute. A history data consists of a data item and atime interval for which the data item is valid. A quenmav- then be issued to retrieve historv data for aspecified time intei-val for the temporal attribute. Themechanism for supporting temporal attributes is simi-lar to that for supporting set-valued attributes in adatabase system, such as UnISQL   

　　In the absence of a support for temporal attrib-utes, application developers who need to model andmanage history data have simply simulated temporalattributes by creating attributc(s) for the time inter-val, along with the" temporal" attribute. This ofcourse may result in duplication of records in a table,and more complicated search predicates in queries．The one necessary topic of research in history data.nanagement is to quantitatively establish the per-.ormance (and even productivity) differences betweenasing a database system that directly supports  attributes and using a conventional DataBascs,system that does not support either the set-valuedattributes or temporal attributes．If it can be convinc ngly established that the benefits of a DataBasc, in that supports temporal attributes arc substantial,DataBase vendors will be strongly motivated to aug-ment their systems with history data management.