Viewers: A Data-World Analogue of Procedure Calls

A viewer is a reference-valued datum with a special meaning: a value of the data pointed by the viewer becomes a virtual part of data where the viewer is placed; the value virtually substit.utes the viewer. Viewers are considered to be a dat,a-world analogue of procedure calls. They possess a large conceptual and pragmatic potential as a result of new data semantics on which we can base a variet,y of well-organized data structures. Various applications of viewers, related to II& PLs and object-oriented data modelling, are presented: importing common att,ributes, inheritance and multiinheritance, stored selections, projections and joins, viewing a single relational structure as several hierarchical structures, etc. Met,hodological and formal aspects of the concept are discussed and a method of incorpe rating viewers into a. query language is presented. and pragmatic differences between viewers and views; thus they should be considered as distinct notions. The basic differences are as follows: l Views are properties of a particular (query) language. Viewers are not connected to any language: they are properties of data structures. l Names of views occur explicitly in queries. Names of vkwers never occur in queries (they occur in met,a-st.at.~ment,s only). l Views are evaluated dynamically (wh they are needed) which has consequences for performance. Viewers are properties of st.a.t.ic da,ta. t,hus there is no performance problem. however, sometimes they share negative properties of materialized views.