General databases and organization of the knowledges

The access to databases shows new difficulties of the on-line inquiries.These difficulties are linked to the organization of the knowledges about the consulted field. This organization takes place in different ways, not inevitably « logical », which emerge in the choice, by the user, of the decriptors indexing his inquiry and lead not to use very much the hierarchical relation. These difficulties pose the problem of the cognitive assistances to give to the user.