Evaluating recursive queries in CAD using an extended projection function

Entities such as mechanical assemblies are recursive when they contain other assemblies at a lower level of detail. In this paper we show how recursive CAD entities can be manipulated using an extended projection function. The new function finds nested sub-entities within entities that are modeled as trees of aggregations and generalizations. It allows these sub-entities to be found using expressions that are independent of the complexity of an entity´s data structure. We describe the new function, its relationship to the transitive closure function, and its implementation in a CAD database system.