Enable multiple index feature

The multiple key option enables the multiple index feature for a collection, which allows defining any number of indexes for a reference collection without creating a new schema version. "multiple key" is enabled for a collection automatically, as soon as at least two sort (order) keys are defined for the collection. The option should be set in advance for single indexed collection in order to allow creating additional indexes for the collection later on without database reorganization.

The multiple key feature creates an additional access level between the instance and the index of the referenced collection. Hence, this option cannot be changes for an existing database without upgrading the database explicitly (create new schema version).