As several mature implementations of data warehousing systems are fully operational, a crucial role in preserving their up-to-dateness is played by the ability to manage the changes that the data warehouse (DW) schema undergoes over time in response to evolving business requirements. In this paper we propose an approach to schema versioning in DWs, where the designer may decide to undertake some actions on old data aimed at increasing the flexibility in formulating cross-version queries, i.e., queries spanning multiple schema versions. After introducing an algebra of DW schema operations, we define a history of versions for data warehouse schemata and discuss the relationship between the temporal horizon spanned by a query and the schema on which it can consistently be formulated.
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