GAV systems model the global database as a set of views over formula_3. In this case formula_4 associates to each element of formula_2 a query over formula_3. Query processing becomes a straightforward operation due to the well-defined associations between formula_2 and formula_3. The burden of complexity falls on implementing mediator code instructing the data integration system exactly how to retrieve elements from the source databases. If any new sources join the system, considerable effort may be necessary to update the mediator, thus the GAV approach appears preferable when the sources seem unlikely to change.