Database Reference
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relational data model, do you represent and perceive data as being contained in
two-dimensional tables or relations.
From the relational data model we proceed to the physical design to represent
the information requirements in physical hardware storage. Semantic and conven-
tional data models are logical data models; these are conceptual data models. They
are conceptual representations, not representations of how data are actually stored.
Actual data are going to reside on physical storage, and physical design stipulates
how, where, and which data are stored in physical storage. This chapter covers the
physical design of relational database systems. The emphasis is exclusively on rela-
tional databases. When we discuss any aspect of physical design, assume that the
discussion refers to relational databases.
INTRODUCTION TO PHYSICAL DESIGN
For a moment, think of the logical design process in terms of its output. Take, for
instance, the logical design of a relational database system. At the end of the process,
you arrive at a set of two-dimensional tables or relations. In the standard notation,
you represent the set of relations, indicating the columns, primary keys, and foreign
keys. This is the output from the logical design. The output is still at a conceptual
level, not at the practical and physical level for storing actual data.
To store and manage data on physical storage, the representation of the logical
design must be transformed into a representation for actual data storage. Why is
this necessary? Computer systems do not store data in the form of tables with
columns and rows; they typically store data in a different way. Computer storage
systems store data as files and records. Physical design is the process of imple-
menting the database on physical storage. Therefore, in the physical design process,
you are concerned with the features of the storage system on which your database
will reside. Furthermore, you are also concerned with the functions, features, and
facilities of the DBMS selected to manage your database.
Figure 12-1 illustrates the transition to physical design in the DDLC. Note the
place of physical design in the overall design process.
Logical to Physical Design
Logical design produces a data model in one of the standard conventional models.
For the purpose of our discussions here, we will focus on the relational data model
as the chosen conventional model. Although logical design is independent of any
particular DBMS and details of physical hardware and systems software, physical
design must necessarily take these into account. Physical design has to be tailored
to a target DBMS. The features of the host computer system must be used to shape
the output of the physical design process. The physical designer must know how the
target DBMS works and must be very familiar with the capabilities and working of
the host computer system.
So in physical design, the last stage of the database design process, your objec-
tive is to proceed from the logical design to implement the database system as a set
of records, files, and other data structures on physical storage. When doing this, you
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