Data Warehouse Concepts - Data Warehouse Systems: Design and Implementation - page 78

several data marts that were independently created need to be integrated into
a data warehouse for the entire enterprise.
In some other situations, an OLAP server does not exist and/or the client
tools directly access the data warehouse. This is indicated by the arrow
connecting the data warehouse tier to the front-end tier. This situation
is illustrated in Chap. 6 , where the same queries for the Northwind case
study are expressed both in MDX (targeting the OLAP server) and in
SQL (targeting the data warehouse). An extreme situation is where there is
neither a data warehouse nor an OLAP server. This is called a virtual data
warehouse , which defines a set of views over operational databases that are
materialized for ecient access. The arrow connecting the data sources to
the front-end tier depicts this situation. Although a virtual data warehouse
is easy to build, it does not provide a real data warehouse solution, since it
does not contain historical data, does not contain centralized metadata, and
does not have the ability to clean and transform the data. Furthermore, a
virtual data warehouse can severely impact the performance of operational
databases.
Finally, a data staging area may not be needed when the data in the source
systems conforms very closely to the data in the warehouse. This situation
typically arises when there is one data source (or only a few) having high-
quality data. However, this is rarely the case in real-world situations.

3.5 Data Warehouse Design

Like in operational databases (studied in Sect. 2.1 ), there are two major
methods for the design of data warehouses and data marts. In the top-
down approach , the requirements of users at different organizational levels
are merged before the design process starts, and one schema for the entire
data warehouse is built, from which data marts can be obtained. In the
bottom-up approach , a schema is built for each data mart, according to
the requirements of the users of each business area. The data mart schemas
produced are then merged in a global warehouse schema. The choice between
the top-down and the bottom-up approach depends on many factors that will
be studied in Chap. 10 in this topic.

Requirements
specification

Conceptual
design

Logical design Physical design

Fig. 3.6 Phases in data warehouse design