Release 10.1A: OpenEdge Getting Started:
Database Essentials

Indexes

An index in a database operates like the index tab on a file folder. It points out one identifying column, such as a customer’s name, that makes it easier and faster to find the information you want.

When you use index tabs in a file folder, you use those pieces of information to organize your files. If you index by customer name, you organize your files alphabetically. If you index by customer number, you organize them numerically. Indexes in the database serve the same purpose.

You can use a single column to define a simple index, or a combination of columns to define a compound index. To decide which columns to use, you determine how the data in the table will be accessed. If users frequently look up customers by last name, then the last name is a candidate for an index. It is typical to base indexes on primary keys (columns that contain unique information).

An index has these advantages:

Faster row search and retrieval. It is more efficient to locate a row by searching a sorted index table than by searching an unsorted table.
In an application written with the Progress® 4GL, records are ordered automatically to support your particular data access patterns. No matter how you change the table, when you browse or print it, the rows appear in indexed order instead of their stored physical order on disk.
When you define an index as unique, each row is unique. This ensures that duplicate rows do not occur. A unique index can contain nulls. However, a primary key, although unique, cannot contain nulls.
A combination of columns can be indexed together to allow you to sort a table in several different ways at once (for example, sort the Projects table by a combined employee and date column).
Efficient access to data in multiple related tables.
When you design an index as unique, each key value must be unique. The database engine will prevent you from entering records with duplicate key values.