create table dropme_t as select rownum num, 0 status from dual connect by level <= 10e3 order by dbms_random.value;
Table created

declare cursor c is select * from dropme_t where status = 0 for update skip locked;
begin 
  for i in c loop dbms_output.put_line('Закреплено: '|| i.num); 
                      update dropme_t set status =1 where current of c; 
                      exit; 
  end loop; 
commit; 
end;
/
Закреплено: 4790
PL/SQL procedure successfully completed

SQL> /
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SQL> 

declare l_num number; try integer := 0;
  try_limit constant integer := 100e3;
begin
  loop
    try := try + 1;
    update dropme_t set status = 1 where status = 0 and num = round(dbms_random.value(1,10e3))
    returning num into l_num;
  exit when sql%rowcount = 1;
  exit when try >= try_limit;
  end loop;
  if try < try_limit then
    dbms_output.put_line('Закреплено: '|| l_num);
  else
    dbms_output.put_line('Нишмогла я...');
  end if;
end;
/
Закреплено: 4457
PL/SQL procedure successfully completed

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SQL> 

SQL> update dropme_t set num = num*1000;
10000 rows updated

SQL> create index dropme_t$num on dropme_t(num);
Index created

SQL> 
declare l_num number; try integer := 0;
  try_limit constant integer := 100e5;
begin
  loop
    try := try + 1;
    update dropme_t set status = 1 where status = 0 and num = round(dbms_random.value(1,10e6))
    returning num into l_num;
  exit when sql%rowcount = 1;
  exit when try >= try_limit;
  end loop;
  if try < try_limit then
    dbms_output.put_line('Закреплено: '|| l_num);
  else
    dbms_output.put_line('Нишмогла я...');
  end if;
end;
/
Закреплено: 2803000
PL/SQL procedure successfully completed

SQL> /
Закреплено: 9299000
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SQL> /
Закреплено: 6073000
PL/SQL procedure successfully completed

select v, count(*)
     , stdDev(count(*)) over() 
     , median(count(*)) over() 
     , avg(count(*)) over() 
  from t group by v