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’
