Using Caché Objects - InterSystems Documentation

9
Indices
Indexes provide a mechanism for optimizing searches across the instances of a persistent
class; they define a specific sorted subset of commonly requested data associated with a class.
They are very helpful in reducing overhead for performance-critical searches.
Indexes automatically span the entire extent of the class in which they are defined. If a Person
class has a subclass Student, all indexes defined in Person contain both Person objects and
Student objects. Indexes defined in the Student class contain only Student objects.
Indexes can be sorted on one or more properties belonging to their class. This allows you a
great deal of specific control of the order in which results are returned. (See the Properties
keyword, below.)
In addition, indexes can store additional data that is frequently requested by queries based
on the sorted properties. By including additional data as part of an index, you can greatly
enhance the performance of the query that uses the index; when the query uses the index to
generate its result set, it can do so without accessing the main data storage facility. (See the
Data keyword below.)
For additional information on indices, refer to the Index chapter within Using Caché SQL.
9.1 Index Keywords
You can modify an index definition through the use of one or more index keywords. Each
keyword is optional and has a default value if not explicitly specified.
The available index keywords include:
Using Caché Objects 63