Managing Schema Changes in Object-Relationship Databases

Changing a database schema may aaect the preexistent data instances and programs. Two major strategies has been followed to deal with schema changes side eeects on data and programs. The rst, called data restructuring consists in converting data to render them conform to the modiied schema. However it doesn't concern the programs. The second approach allows the coexistence of several schema versions into the same database. Such an approach uses the emulation mechanism to achieve programs conformity to the new schema. However, it introduces data redundancy and both restructuring and emulation only consider atomic changes. We introduce an approach combining both databases restructuring and emulation mechanisms. We consider the data instances as a set of multiple facets, each one representing according to a particular schema version. To avoid data redundancy and to achieve programs adaptation to schema change, only one facet is physically stored. In fact we deene mapping functions allowing every program deened for a particular facet to be performed even if this facet is not eeectively stored. Our approach is applicable for both atomic and complex schema changes.