You can create indexes on the values of one or more SQL built-in functions.

create index idx_namelen on Persons(length(firstname));

The statement above allows the query optimizer to instantly find records based on name length (for example, WHERE length(firstname) > 10) without having to calculate the length for every row in the table during the search.

create index idx_modtime on Persons(modification_time());

The statement above allows the database to quickly locate records based on their last change date without scanning the entire table.