create table qz_tab as select level as n from dual connect by level <= 170;
select count(distinct dbms_rowid.rowid_block_number(rowid)) from qz_tab;
set timing on serverout on
declare
  i int := 0;
  procedure Lck
  is
    pragma autonomous_transaction;
    m int;
    t timestamp := systimestamp;
  begin
    i := i + 1;
    select n into m from qz_tab where n = i for update nowait;
    if systimestamp - t > interval '0.5' second then
      dbms_output.put_line(i||') '||(systimestamp - t));
    end if;
    Lck;
  end Lck;
begin
  Lck;
exception
  when others then
    dbms_output.put_line('i='||i);
    raise;
end;
/
/
/
drop table qz_tab;

COUNT(DISTINCTDBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID))
---------------------------------------------------
                                                  1
i=170
declare
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired
ORA-06512: at line 21


Elapsed: 00:00:00.07
i=170
declare
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired
ORA-06512: at line 21


Elapsed: 00:00:00.03
i=170
declare
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired
ORA-06512: at line 21


Elapsed: 00:00:00.03