When search clauses are not optimizable

The following set of clauses on the authors table includes the substring function on the au_fname column:

    where substring(au_fname,1,4) = "Fred" 
         and city = "Miami"

Due to the use of the substring function on a column name, the set of optimizable clauses does not include the where clause on the au_fname column:

Selecting best index for the SEARCH CLAUSE:
    authors.city = ’Miami’