Indexes speed up queries dramatically if the table is much larger than the number of rows retrieved.
By creating indexes on columns by which you often search a table, you can reduce the number of rows that Derby has to scan, thus improving performance. Depending on the size of the table and the number of rows returned, the improvement can be dramatic.
Indexes work best when the number of rows returned from the query is a fraction of the number of rows in the table.
There are some trade-offs in using indexes: indexes speed up searches but slow down inserts and updates. As a general rule, every table should have at least a primary key constraint.
See Always create indexes for more information.