A Method for Data Warehouse Design - Data Warehouse Systems: Design and Implementation - page 403

Define initial
conceptual
schema

Analysis-driven
approach

Define final
conceptual
schema and
mappings

Match the two
conceptual
schemas

Define initial
conceptual
schema

Source-driven
approach

Fig. 10.9 Steps for conceptual design in the analysis/source-driven approach

An ideal situation arises when both schemas cover the same analysis
aspects, that is, the users' needs are covered by the data in the operational
systems and no other data are needed to expand the analysis. In this
case, the schema is accepted, and mappings between elements of the source
systems and the data warehouse are specified. Additionally, documentation is
developed following the guidelines studied for the analysis-driven and source-
driven approaches. This documentation contains metadata about the data
warehouse, the source systems, and the ETL process. Nevertheless, in real-
world applications, it is seldom the case that both schemas will cover the
same aspects of analysis. Two situations may occur:
1. The users require less information than what the operational databases
can provide. In this case, it is necessary to determine whether users may
consider new aspects of analysis or whether to eliminate from the schema
those facts that are not of interest to users. Therefore, another iteration of
the analysis- and source-driven approaches is required, where either new
users will be involved or a new initial schema will be developed.
2. The users require more information than what the operational databases
can provide. In this case, the users may reconsider their needs and limit
them to those proposed by the analysis-driven solution. Alternatively, the
users may require the inclusion of external sources or legacy systems that
were not considered in the previous iteration but contain the necessary
data. Thus, new iterations of the analysis- and source-driven approaches
may again be needed.

10.5 Logical Design

As illustrated in Fig. 10.10 , two steps must be considered during the logical
design phase: first, the transformation of the conceptual multidimensional
schema into a logical schema; and second, the specification of the ETL
processes, considering the mappings and transformations indicated in the
previous phase. We shall refer next to these two steps.