create or replace function calc_sort_key return raw deterministic 
is 
    l_result raw(30) := hextoraw('41424344'); -- 'ABCD' 
begin 
    return l_result; 
end; 

select calc_sort_key from dual 

select utl_raw.cast_to_varchar2(calc_sort_key) from dual 

select utl_raw.length(calc_sort_key) from dual 

create table x ( 
    id number(38) primary key, 
    sort_key as (calc_sort_key()) 
) 

create unique index sort_key_ui on x (sort_key) 

drop table x 

create table x ( 
    id number(38) primary key, 
    sort_key raw(30) as (calc_sort_key()) 
) 

create table x ( 
    id number(38) primary key, 
    physical_column raw(30) 
) 

create unique index physical_column_ui on x (physical_column) 

create or replace function calc_sort_key return x.physical_column%type deterministic 
is 
    l_result raw(30) := hextoraw('41424344'); -- 'ABCD' 
begin 
    return l_result; 
end; 

alter table x add ( 
    sort_key as (calc_sort_key()) 
) 

create unique index sort_key_ui on x (sort_key)