Data Volume and its Usage:
In physical database design, the first important issue is the estimation of
volume of current and future data and the methods and patterns of its usage.
- Estimates of data size (database size) are
used to select physical storage devices and estimate the costs of storage.
- Estimates of usage paths or patterns are
used to select the file organization and access methods, to plan for the use
of indexes and to plan a strategy for data distribution.
For example, we have explained the estimation of
data volume in the following figure below.
The number written within the entity types
indicates the total possible instances of that entity types and number written
with the relationship line indicates instances attached with other entity type.
For example there are 500 students in a university and there are ten classes
daily. In each class, there are averagely 50 students. Following data estimation
can be finalized;
Total records of students = 500
Number of classes per month = 10 x 25 = 250
Number of papers taken by all students = 500 x 5 = 2500
Student attendance records per month = 10 x 25 = 250 etc.
Keeping in view these statistics, the database designer should estimate the
total storage required for the database. Moreover future expansion of data
should also be kept in mind.
Data usage means type and frequency of reports and forms that will be
generated or used in conjunction with the database. If data is used most frequently then
fast storage devices are required. Similarly, if selective or random data is
needed, then you need effective indexes and data should be kept on magnetic
disks rather than magnetic tapes.