Effects of Deterministic Property on Function-Based Indexes

Unlike computed columns, function-based index keys must be deterministic. A computed column is still conceptually a column, which, once evaluated and stored, does not require reevaluation. A function or expression, however, must be reevaluated upon each appearance in a query.

You cannot use preevaluated data, such as index data, unless the function always evaluates to the same results with the same input set .

Related concepts
Function-Based Indexes