Database Reference
In-Depth Information
Design Objectives
Before we briefly describe the activities of the design phase, let us understand the
main objectives of this phase. We want to represent the information requirements
in a format that can describe how data structures are perceived, how the structures
are related, and how data are to be stored. Data are stored for access and usage.
The ability to navigate through the structures and retrieve data must also somehow
be portrayed in the design. The design must express the data content.
Here is list of major objectives for the design, both at the level of physical storage
and at the higher level indicating the data structures:
•
Show data content in detail.
•
Represent data relationships.
•
Indicate how data structures may be accessed easily.
•
Denote information sharing.
•
Introduce data independence.
•
Secure and safeguard data.
•
Establish data integrity and consistency.
•
Allow easy data management and administration.
Logical Versus Physical Design
As noted above, the design phase produces two levels of design. Design at the two
levels represents two levels of data abstraction. At one level, you design the data
structures and arrange them in a manner that expresses their relationships. If your
database consists of data only on customers and orders, then the data structures in
one level of abstraction will represent the logical view of the data. This is the logical
design. At the other level of the design, you will indicate the files, blocks, records,
fields, data types, field lengths, keys, indexes, and so on. This level of design that
relates to physical storage is the physical design.
The following summarizes the distinction:
Logical design.
This is a high-level view of data contained in the entire database.
This is a conceptual view of the whole database. It consists of data structures and
their relationships. Consider how each user group looks at the data in the database.
A particular user group may be interested in only a few data elements from customer
data and most of the data elements in the order data. Then, from outside, the view of
the database for the user group consists only of the required data elements from cus-
tomer and order data. This set of data elements is the external view for that user
group. Therefore, in the design process, we come up with sets of external views for
the various user groups. The combined external views constitute the logical design.
Physical design.
The logical design shows the data structures; the physical design
represents how data relating to these structures are stored on physical storage. The
physical design takes the design closer to hardware. While performing physical
design, you have to consider physical hardware features. The physical design step is
the transformation of the logical design into a design in terms of hardware.
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