Datawarehouse Design
There are two
major approaches to data warehouse design.
1.
Bottom-up approach
• This approach
is recommended by Kimball.
• In the
bottom-up approach data marts are first created to provide reporting and
analytical capabilities for specific business processes.
• Data marts
contain, primarily, dimensions and facts. Facts can contain either atomic data
and, if necessary, summarized data. The single data mart often models a
specific business area such as Sales or Production.
• These data
marts can eventually be integrated to create a comprehensive data warehouse.
• The integration
of the data marts in the data warehouse is centered on the conformed
dimensions.
• The actual
integration of two or more data marts is then done by a process known as
"Drill across". A drill-across works by grouping (summarizing) the
data along the keys of the (shared) conformed dimensions of each fact
participating in the "drill across" followed by a join on the keys of
these grouped (summarized) facts.
• Some consider
it an advantage of the Kimball method, that the data warehouse ends up being
"segmented" into a number of logically self contained and consistent
data marts, rather than a big and often complex centralized model.
• Business value
can be returned as quickly as the first data mart is built.
2.
Top-down approach
• This approach
is recommended by Bill Inmon.
• Inmon is one of
the leading proponents of the top-down approach to data warehouse design, in
which the data warehouse is designed using a normalized enterprise data model.
• In the Inmon
vision the data warehouse is at the center of the "Corporate Information
Factory" (CIF), which provides a logical framework for delivering business
intelligence (BI) and business management capabilities.
• The top-down
design methodology generates highly consistent dimensional views of data across
data marts since all data marts are loaded from the centralized repository.
• Generating new
dimensional data marts against the data stored in the data warehouse is a relatively
simple task.
• The main
disadvantage to the top-down methodology is that it represents a very large
project with a very broad scope, cost and time.
• In addition,
the top-down methodology can be inflexible and unresponsive to changing
departmental needs during the implementation phases.
