Unlinked versus Linked Indexes

An unlinked index is an index in which there are no index pointers in the child entities.

Considerations

Unlinked indexes can be added and removed without restructuring the database, provided the control length of the entity is not changed.
Building or rebuilding an unlinked index is faster because there are no index pointers to be maintained.
Additional CPU and I/Os are required to locate an index entry for the current entity occurrence. For example, changing the index key value or erasing an entity occurrence both require the retrieval of the index entry.
This additional overhead occurs because the DBMS must search the index to find the entry, whereas in a linked relationship there is a direct pointer to the SR8 occurrence containing the entry.
Linked indexes require additional storage space.