Web-based learning programs enable learners to browse Web pages individually through a predefined path or a personalized routine. However, they provide less social context and cooperative context for cooperation. The main idea presented is the concept of "informal cooperative learning program". It is defined as a set of structured learning steps. Learners can go through all steps by following the structure at different paces, but they are supported to perform cooperative learning when they happen to be at the same step. This idea has been implemented in our prototype system COMPILE (cooperative open multimedia-based process-centered integrated learning environment).

The BR/sup 2/-directory representation, a directory structure for grid files, and a join algorithm for the evaluation of general n-ary joins on grid files are presented. It is shown that the CPU cost of the join algorithm is successfully reduced by introducing an inner join. A comparison with the hash join algorithm and a join algorithm on k-d trees for equijoins is based on a cost model developed for query processing with grid files. The join algorithm outperforms the hash join, a specialized join method for equijoins, and the join algorithm on k-d trees for equijoins.>

Gral is an extensible database system, based on the formal concept of a many-sorted relational algebra. Many-sorted algebra is used to define any application's query language, its query execution language, and its optimiztion rules. In this paper we describe Gral's optimization component. It provides (1) a sophisticated rule language—rules are transformations of abstract algebra expressions, (2) a general optimization framework under which more specific optimization algorithms can be implemented, and (3) several control mechanisms for the application of rules. An optimization algorithm can be specified as a series of steps. Each step is defined by its own collection of rules together with a selected control strategy. The general facilities are illustrated by the complete design of an example optimizer—in the form of a rule file—for a small nonstandard query language and an associated execution language. The query language includes selection, join, ordering, embedding derived values, aggregate functions, and several geometric operations. The example shows in particular how the special processing techniques of a geometric database systems, such as spatial join methods and geometric index structures, can be integrated into query processing and optimization of a relational database system. A similar, though larger, optimizer is fully functional within the geometric database system implemented as a Gral prototype.