ASA SQL User's Guide
Working with Database Objects
Working with indexes
An index provides an ordering on the rows in a column or columns of a table. An index is like a telephone book that initially sorts people by surname, and then sorts identical surnames by first names. This ordering speeds up searches for phone numbers for a particular surname, but it does not provide help in finding the phone number at a particular address. In the same way, a database index is useful only for searches on a specific column or columns.
Indexes get more useful as the size of the table increases. The average time to find a phone number at a given address increases with the size of the phone book, while it does not take much longer to find the phone number of, say, K. Kaminski, in a large phone book than in a small phone book.
The database server query optimizer automatically uses an index when a suitable index exists and when using one will improve performance.