synonym - after dba privileges user is not able to access the option. only schema owner can access

create or replace trigger logon_trg
   after logon on database
 begin
    if user in ('A','B') then
        execute immediate 'alter session set current_schema=X';
    end if;
 end;
/
----------------------

create or replace trigger logon_trg
   after logon on database
declare
usr varchar2(1000);
 begin
select wm_concat(''''|| username || '''') into usr from dba_users;
    if user in (usr) then
        execute immediate 'alter session set current_schema=LDBO';
    end if;
 end;
/

SYS@NBS11G> select wm_concat(''''|| username || '''') from dba_users;
select wm_concat(''''|| username || '''') from dba_users
*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character string buffer too small
ORA-06512: at "WMSYS.WM_CONCAT_IMPL", line 30