Logical and Physical Data Models

Topics: Data modeling, SQL, Database model Pages: 13 (4439 words) Published: December 1, 2012
The Physical Data Model (PDM) describes how the information represented in the Logical Data Model is actually implemented, how the information-exchange requirements are implemented, and how the data entities and their relationships are maintained. There should be a mapping from a given Logical Data Model to the Physical Data Model if both models are used. The form of the Physical Data Model can vary greatly, as shown in Figure 31. For some purposes, an additional entity-relationship style diagram will be sufficient. The Data Definition Language (DDL) may also be used. References to message format standards (which identify message types and options to be used) may suffice for message-oriented implementations. (Getting information from the LDM in form of file) Descriptions of file formats may be used when file passing is the mode used to exchange information. Interoperating systems may use a variety of techniques to exchange data, and thus have several distinct partitions in their Physical Data Model with each partition using a different form. The figure illustrates some options for expressing the Physical Data Model and an other table (in the original document) provides a listing of the types of information to be captured.

A physical data model (or database design) is a representation of a data design which takes into account the facilities and constraints of a given database management system. In the lifecycle of a project it typically derives from a logical data model, though it may be reverse-engineered from a given database implementation. A complete physical data model will include all the database artifacts required to create relationships between tables or to achieve performance goals, such as indexes, constraint definitions, linking tables, partitioned tables or clusters. Analysts can usually use a physical data model to calculate storage estimates; it may include specific storage allocation details for a given database system. As of 2012 seven main databases dominate the commercial marketplace: Informix, Oracle, Postgres, SQL Server, Sybase, DB2 and MySQL. Other RDBMS systems tend either to be legacy databases or used within academia such as universities or further education colleges. Physical data models for each implementation would differ significantly, not least due to underlying operating-system requirements that may sit underneath them. For example: SQL Server runs only on Microsoft Windows operating-systems, while Oracle and MySQL can run on Solaris, Linux and other UNIX-based operating-systems as well as on Windows. This means that the disk requirements, security requirements and many other aspects of a physical data model will be influenced by the RDBMS that a database administrator (or an organization) chooses to use. Overview

Logical data models represent the abstract structure of a domain of information. They are often diagrammatic in nature and are most typically used in business processes that seek to capture things of importance to an organization and how they relate to one another. Once validated and approved, the logical data model can become the basis of a physical data model and inform the design of a database. Logical data models should be based on the structures identified in a preceding conceptual data model, since this describes the semantics of the information context, which the logical model should also reflect. Even so, since the logical data model anticipates implementation on a specific computing system, the content of the logical data model is adjusted to achieve certain efficiencies. The term 'Logical Data Model' is sometimes used as a synonym of 'Domain Model' or as an alternative to the domain model. While the two concepts are closely related, and have overlapping goals, a domain model is more focused on capturing the concepts in the problem domain rather than the structure of the data associated with that domain.


The ANSI/SPARC three level architecture, which...
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