To specify that Caché should enforce uniqueness for the properties on which this index is based, use the following syntax:
Index name On property_expression_list [ Unique ];
Otherwise, omit this keyword or place the word Not immediately before the keyword.
The Unique keyword, if present, indicates that this is a unique index.
The property (or properties) indexed by a unique index are constrained to have unique values (that is, no two instances can have the same collated value) within the extent (set of all objects) of the class (table) that defines the index.
A unique index cannot also be a bitmap index.
Index SSNIdx On SSN [ Unique ] ;
If you omit this keyword, Caché does not enforce uniqueness for the properties on which this index is based.
“Index Definitions” in this book
“Defining and Building Indices” in the Caché SQL Optimization Guide
“Introduction to Compiler Keywords” in Using Caché Objects