Database Reference
In-Depth Information
PRESENTATION
LAYER
Desktop
Client
Analytical
Server
APPLICATION
LAYER
Data
Warehouse
RDBMS
Server
DATA
LAYER
Figure 20-17
The ROLAP model.
The ROLAP Model
In the relational OLAP (ROLAP) model, data are stored as
rows and columns in the relational format. This model presents data to the users in
the form of business dimensions. To hide the storage structure to the user and
present data multidimensionally, a semantic layer of metadata is created. The meta-
data layer supports the mapping of dimensions to the relational tables. Additional
metadata support summarizations and aggregations. You may store the metadata in
relational databases.
Now take a look at Figure 20-17. This figure shows the architecture of the
ROLAP model.
What you see is a three-tier architecture. The analytical server in the middle tier
application layer creates multidimensional views on the fly. The multidimensional
system at the presentation layer provides a multidimensional view of the data to
the users. When the users issue complex queries based on this multidimensional
view, the queries are transformed into complex SQL directed to the relational data-
base. Unlike the MOLAP model, static multidimensional structures are not created
and stored.
True ROLAP has three distinct characteristics:
•
Supports all the basic OLAP features and functions discussed above.
•
Stores data in a relational form.
•
Supports some form of aggregation.
Local storing of multidimensional cubes is a variation of ROLAP provided by
vendors. This is how it works:
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