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4.1 Data Warehouse: Basic Concepts 133

Virtual warehouse: A virtual warehouse is a set of views over operational databases.
For efficient query processing, only some of the possible summary views may be
materialized. A virtual warehouse is easy to build but requires excess capacity on
operational database servers.
“What are the pros and cons of the top-down and bottom-up approaches to data ware-
house development?” The top-down development of an enterprise warehouse serves as a
systematic solution and minimizes integration problems. However, it is expensive, takes
a long time to develop, and lacks flexibility due to the difficulty in achieving consistency
and consensus for a common data model for the entire organization. The bottom-
up approach to the design, development, and deployment of independent data marts
provides flexibility, low cost, and rapid return of investment. It, however, can lead to
problems when integrating various disparate data marts into a consistent enterprise data
warehouse.
A recommended method for the development of data warehouse systems is to imple-
ment the warehouse in an incremental and evolutionary manner, as shown in Figure 4.2.
First, a high-level corporate data model is defined within a reasonably short period
(such as one or two months) that provides a corporate-wide, consistent, integrated
view of data among different subjects and potential usages. This high-level model,
although it will need to be refined in the further development of enterprise data ware-
houses and departmental data marts, will greatly reduce future integration problems.
Second, independent data marts can be implemented in parallel with the enterprise

Multitier
data
warehouse

Distributed
data marts

Enterprise
Data Data data
mart mart warehouse

Model refinement Model refinement

Define a high-level corporate data model

Figure 4.2 A recommended approach for data warehouse development.