On the other hand, in LAV, the source database is modeled as a set of views over formula_2. In this case formula_4 associates to each element of formula_3 a query over formula_2. Here the exact associations between formula_2 and formula_3 are no longer well-defined. As is illustrated in the next section, the burden of determining how to retrieve elements from the sources is placed on the query processor. The benefit of an LAV modeling is that new sources can be added with far less work than in a GAV system, thus the LAV approach should be favored in cases where the mediated schema is less stable or likely to change.