There are some network database constructs that have no direct parallel in a relational database environment. Optional sets are one of these. To make an optional set (indexed or chained) 'visible' to the optimizer, you must do these steps:
1- Create a network schema that defines the set as MA (Mandatory Automatic).
2- Create an SQL schema that points to this new network schema.
3- Reference this SQL schema in your SQL DML statements.
4- If the optimizer chooses an optional set as the access route, the statement will access only those record occurrences that are members of the set. This is true regardless of whether the statement is doing retrieval or update.