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.