Скачать или смотреть How to avoid Oracle Sequence gap?

-- Create table
create table rep.t1(s1 number,a1 varchar2(100));
-- Create sequence
create sequence REP.seq1
minvalue 1
maxvalue 999999999
start with 1
increment by 1
nocache
order;
Open 3 sessions:
1-- Insert rec 1 in session 1
insert into rep.t1(s1,a1) values(REP.seq1.NEXTVAL,'Rec1');
2--- Insert rec 2 in session 2
insert into rep.t1(s1,a1) values(REP.seq1.NEXTVAL,'Rec2');
3-- Insert rec 3 in session 3
insert into rep.t1(s1,a1) values(REP.seq1.NEXTVAL,'Rec3');
4-- Commit rec 1 in session 1
commit;
5-- rollback rec 2 in session 2
rollback;
6-- Commit rec 3 in session 3
commit;
7-- select al records, you will find gap
select * from rep.t1;
S1 A1
1 Rec1
3 Rec3