Spatial information systems are employed to record the schematics of large networks for utilities and telecommunication organisations. Concurrent users access the spatial information systems to insert new designs, update designs and to record the current status of the network. Concurrency must be managed such that the data is not corrupted. This paper introduces a model for long duration transactions based on the use of integrity constraints. The model, called COLT (COnstraint-based Long Transaction), is domain-independent but is especially suitable for spatial information systems and domains where its intent naturally manifests constraints that relate data items. User-defined constraints and database integrity constraints are used both to specify the correctness criteria and to manage long duration ad-hoc transactions. The model is a generalisation of the traditional ACID transaction model. The approach enables the specification of the correctness criteria to be declarative and the user needs no knowledge of the semantics of other concurrently executing transactions. The COLT model is formally specified using the external actions of the I/O automata method and correctness is defined with respect to that method. Correctness is defined independent of implementation.

In this paper we present a new technique to improve the performance of spatial access methods by minimizing redundancy: the oversize shelf. Oversize shelves are additional disk pages that are attached to the interior nodes of a treebased spatial access method (such as the R-tree or the cell tree). These pages are used to accommodate very large data objects in order to avoid their excessive fragmentation. Whenever inserting a new object into the tree, one now has to decide whether to store it on an oversize shelf or insert it into the corresponding subtrees. For this purpose, we developed an analytic model for the behaviour of dynamic spatial access methods under insertion and deletions. The model yields a threshold value for the size of an object, such that it is more favourable to put it on the oversize shelf if and only if its size is greater than the threshold value. Otherwise the insertion into the corresponding subtrees is preferable. Practical experiments indicate that this policy results in a faster computation of search queries, and that the cell tree with oversize shelves is one of the most efficient spatial access methods currently available.