SQL> create table dropme_t1 (id number, dt timestamp);

Table created.

SQL> create unique index ix_dropme_t1 on dropme_t1 (dt);

Index created.

SQL> insert into dropme_t1 select level, localtimestamp from dual connect by level<=2;
insert into dropme_t1 select level, localtimestamp from dual connect by level<=2
*
ERROR at line 1:
ORA-00001: unique constraint (IX_DROPME_T1) violated

SQL>  create or replace trigger trg_dropme_t1_pseudoseq before insert on dropme_t1 for each row
  2  begin
  3      :new.dt := current_timestamp;
  4 end;
  5/

Trigger created.

SQL> begin
  2      for v in 1..1e6 loop
  3          insert into dropme_t1(id) values(v);
  4      end loop;
  5  end;
  6  /

PL/SQL procedure successfully completed.

SQL> create table dropme_t2 (n number(1));

Table created.

SQL> insert into dropme_t2 values(0.99999999999999999999999999999999);

1 row created.

SQL> select * from dropme_t2;

         N
----------
         1