Structural Repairs of Multidimensional Databases

In a multidimensional (MD) database, dimensions may be subject to semantic conditions that are not enforced by MD DBMSs or data warehouse applications. Strictness and homogeneity are possibly two of them; and are crucial for the efficiency and correctness of answering MD aggregate queries and updating materialized aggregate views. Dimensions may become inconsistent, i.e. non-strict or heterogeneous, as the result of update operations. As a methodology to restore consistency, we propose and investigate changes to the dimension schema, as an alternative to changes on the dimension instance. We introduce the notion of minimal structural repair, and establish that under certain conditions, a structural repair reduces the cost wrt changing the dimension instance. We also show that it allows for a correct rewriting of queries posed to the original MD model into queries in terms of the new schema. Finally, we show how query-scoped calculated members in MDX can be used to create virtual repairs that simulate structural repairs.