Oracle nls length semantics

Questo breve script plsql converte tutte le colonne VARCHAR2 e CHAR da BYTE a CHAR:

begin
 for c1 in (select owner,table_name,column_name,data_type,data_length 
            from dba_tab_columns where data_type in ('VARCHAR2','CHAR') 
            and char_used='B' and owner in ('USER1','USER2'))
 loop
    begin
    execute immediate 'alter table ' || c1.owner || '.' || c1.table_name || ' modify ' ||
                       c1.column_name || ' ' || c1.data_type || '( ' || c1.data_length || ' char)';
    exception
      when others
          then
              null;
    end;
 end loop;
end;
/