An entry in an index can refer to several records that have the same values for the index fields. A unique index does not permit these multiple entries. The index fields of a unique index thus have key function, that is they already uniquely identify each record of the table.

The primary index of a table is always a unique index since the index fields form the key of the table, uniquely identifying each data record.

You can define a secondary index as a unique index when you create it. This ensures that there are no double records in the table fields contained in the index. An attempt to maintain an entry violating this condition in the table results in termination due to a database error.

The accessing speed does not depend on whether or not an index is defined as a unique index. A unique index is simply a means of defining that certain field combinations of data records in a table are unique.