Conceptual, Logical, and Physical views of data
- From: "David Cressey" <david.cressey@xxxxxxxxxxxxx>
- Date: Tue, 30 Aug 2005 10:07:32 GMT
My introduction to databases, back in the 1980s, was, in part, by way of
Martin. I got this description of multiple views of data: the conceptual,
logical, and physical views.
The conceptual view of data: the data as seen by the subject matter expert.
The meaning of each of the values, in context, is part of the subject matter
expert's domain. Database structure is not the domain of the SME, in that
role.
The logical view of data: the data as seen by the programmers. The people
who store and retrieve data from a database (or who write software that does
the same thing) view the data in terms of its appearance at the interface.
Structural aspects of the data that are visible at the programming
interface are part of the logical view. Structural aspects that are
transparent at the interface are not part of the logical view. Structural
alterations at the logical level may require program maintenance, but
structural changes at the physical level will not.
The physical view of data: the data as seen internally in the DBMS. This
view of the data might be of concern to the DBA, the Data Base Designer, or
to the DBMS engineer.
Missing from the above is the data as seen by the user. Users are generally
represented by the SME, though not always.
My understanding of the conceptual data model, the logical data model, and
the physical data model are influenced by the above.
In particular, I don't buy the concept that the logical model is unrelated
to inplementation. In the design process, the logical model is going to,
for all practical purposes, determine the portion of the physical model
that will be visible at the programming interface once construction is
complete.
Once construction is complete, the logical model is just as "real" as the
physical model. The terminology changes with the context, but the design
carries over. Thus "relations" will be implemented as "tables", but they
are equally "logical".
If you use SQL to construct the database, the relations in the logical
model will appear as tables and indexes. Tables and indexes have logical
features and physical features as well. In general, the logical features of
tables and indexes appear inside the parentheses in the CREATE TABLE or
CREATE INDEX statement, while the physical features appear somewhere else.
The fact that indexes are visible at all in the logical view is simply a
reflection of the fact that, in the SQL world, the programmer needs to know
something about indexes for performance reasons.
Having said that, one goal of logical data modelling is that the model be
protable between two different DBMS products without changes to the model.
Thus a good logical model for a relational database should be invariant on
whether the implementation will be in DB2 or Oracle.In practice, this is not
always possible, because choices of datatypes will be influenced by the
choice of a target DBMS, and those choices will become part of the logical
model.
.
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