Chapter 3 Building Physical Diagrams

Defining indexes in a physical diagram

An index is a data structure associated with a table that is logically ordered by the values of a key. It improves database performance and access speed.

You normally create indexes for columns that you access regularly, and where response time is important. Indexes are most effective when they are used on columns that contain mostly unique values.

Example

In a table called Author, you create indexes for the primary key Author ID and the column Author name, but not for the column City. The values for city, are not likely to be unique, nor searched regularly, and do not help reduce query time.