Classes and Extents
InterSystems IRIS uses an unconventional and powerful interpretation of the object-table mapping.
All the stored instances of a persistent class compose what is known as the extent of the class, and an instance belongs to the extent of each class of which it is an instance. Therefore:
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If the persistent class Person has the subclass Student, the Person extent includes all instances of Person and all instances of Student.
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For any given instance of class Student, that instance is included in the Person extent and in the Student extent.
Indexes automatically span the entire extent of the class in which they are defined. The indexes defined in Person contain both Person instances and Student instances. Indexes defined in the Student extent contain only Student instances.
The subclass can define additional properties not defined in its superclass. These are available in the extent of the subclass, but not in the extent of the superclass. For example, the Student extent might include the FacultyAdvisor field, which is not included in the Person extent.
The preceding points mean that it is comparatively easy in InterSystems IRIS to write a query that retrieves all records of the same type. For example, if you want to count people of all types, you can run a query against the Person table. If you want to count only students, run the same query against the Student table. In contrast, with other object databases, to count people of all types, it would be necessary to write a more complex query that combined the tables, and it would be necessary to update this query whenever another subclass was added.
