Also, due to possible quality problems of the processed data,
the designer is obliged to define a data flow for logical exceptions, which is
responsible for the flow of the problematic data, that is, the rows that violate
integrity or business rules. Moreover, a very crucial topic is the semantics of
the ETL workflow. These semantics are generated by the combination of the
data flow and the execution sequence: the data flow defines what each process
does and the execution plan defines in which order and combination.
??? Mapping.conceptual.to.logical.models: Another issue that has to be solved
is the transition between the aforementioned phases (i.e., conceptual and
logical) of the data warehouse life cycle. On one hand, there exists a simple
model, sufficient for the early stages of the data warehouse design. On the
other hand, there exists a logical model that offers formal and semantically
founded concepts to capture the particularities of an ETL process.
The goal of this transition should be to facilitate the integration of the results
accumulated in the early phases of a data warehouse project into the logical
model, such as the collection of requirements from the part of the users, the
analysis of the structure and content of the existing data sources, along with
their intentional mapping to the common data warehouse model.
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