You can create a database index for any class property except a system-owned property. These database indexes, also known as "single indexes" are stored within the object store database. For properties that your users frequently search, single indexes enable you to reduce the amount of time it takes to process queries on this property. To search database indexes for class properties, use SQL syntax rather than VQL.
NOTE When
a class property is set or unset for single indexing it is automatically
set or unset in all subclasses.
To create database indexes for class properties