variable tmp number;
begin
  select max(id)+1 into :tmp from table1;
  execute immediate 'create sequence table1_seq start with ' || :tmp;
end;
/

alter sequence table1_seq cache 2000
/

create or replace trigger table1_tr
before insert on table1 for each row
begin
  if :new.id is null then
    select table1_seq.nextval into :new.id from dual;
    end if;
end;
/

create unique index table1_id_uq on table1(id)
  tablespace t_idx
/

alter table table1 add constraint table1_id_pk
  primary key(id) using index table1_id_uq
/