Extending a Spatial Database System by Graphs and Object Class Hierarchies
Two important open problems in spatial database systems are the modeling and implementation of spatially embedded networks and the handling of inhomogeneous collections of spatial objects. We sketch a data model that together with relations provides graphs as explicit structures. Graphs allow to model spatial networks; they consist of nodes, edges, and explicit paths. Inhomogeneous collections of objects can be represented in object type hierarchies which are in turn realized by tuple hierarchies. The resulting model contains the relational model; hence it is still possible to ask standard relational queries. At the implementation level, we describe a simple general purpose data structure for the representation of graphs on external storage. lt is shown how such a graph representation module can be embedded into the architecture of an extensible relational database system, the Gral system, and also how it can be implemented with relatively little effort in such an environment. Hence we obtain a strategy to migrate from a given relational system with spatial data types to a system with graphs and object class hierarchies.
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