Physical database design
The physical design of your database optimizes performance while ensuring data integrity by avoiding unnecessary data redundancies. During physical design, you transform the entities into tables, the instances into rows, and the attributes into columns.
After completing the logical design of your database, you now move to the physical design. You and your colleagues need to make many decisions that affect the physical design, some of which are listed below.
- How to translate entities into physical tables
- What attributes to use for columns of the physical tables
- Which columns of the tables to define as keys
- What indexes to define on the tables
- What views to define on the tables
- How to denormalize the tables
- How to resolve many-to-many relationships
- What designs can take advantage of hash access
Physical design is the time when you abbreviate the names that you chose during logical design. For example, you can abbreviate the column name that identifies employees, EMPLOYEE_NUMBER, to EMPNO. In Db2 for z/OS®, you need to abbreviate column names and table names to fit the physical constraint of a 30-byte maximum for column names and a 128-byte maximum for table names. For more information about the conventions and rules for database object names, see Naming conventions and SQL identifiers.
The task of building the physical design is a job that truly never ends. You need to continually monitor the performance and data integrity characteristics of the database as time passes. Many factors necessitate periodic refinements to the physical design.
Db2 lets you change many of the key attributes of your design with ALTER SQL statements. For example, assume that you design a partitioned table so that it stores 36 months' worth of data. Later you discover that you need to extend that design to 84 months' worth of data. You can add or rotate partitions for the current 36 months to accommodate the new design.
The remainder of this information includes some valuable information that can help you as you build and refine the physical design of your database. However, this task generally requires more experience with Db2 than most readers of this introductory level information are likely to have.