Can a sequence guarantee the given order? - oracle

Question

Using this Oracle sequence definition:
CREATE SEQUENCE MY_SPECIAL_COUNTER
START WITH 100
INCREMENT BY -1
MAXVALUE 100
MINVALUE 0
NOCYCLE
NOCACHE
ORDER
;
Is it guaranteed, that this sequence will ALWAYS return
each number once from 100 to 0
in the given order from 100 to 0 and
no single number will be obmitted and
independent of multiple concurrent session in the very moment of the request
the correct number?

Answer 1

Yes, the sequence will return each number once, starting with 100, progressively down to 0.
Yes, it will return them in order from 100 down to 0.
Correct, it will not skip any numbers.
Multiple sessions? That depends.
Consider the following:
create table junk ( id number );
CREATE SEQUENCE MY_SPECIAL_COUNTER
START WITH 100
INCREMENT BY -1
MAXVALUE 100
MINVALUE 0
NOCYCLE
NOCACHE
ORDER
;
-- in session 1 do the following:
insert into junk
select my_special_counter.nextval from dual;
insert into junk
select my_special_counter.nextval from dual;
SQL> select * from junk;
ID
----------
100
99
SQL>
-- in session 2 do the following:
insert into junk
select my_special_counter.nextval from dual;
SQL> select * from junk;
ID
----------
98
SQL> commit;
-- in session 1 do the following:
rollback;
-- in session 3 do the following:
SQL> select * from junk;
ID
----------
98
SQL>
As you can see once you introduce multiple sessions, all kinds of fun things can happen. Numbers can be "lost" or "skipped" .. note that it is NOT the sequence doing it, but rather the session or whoever pulled the sequence value then subsequently drops it (ie job abends, and rollsback, logic error, etc.)
Also, if session 1 pulls sequence first, but session 2 commits first, others will "think" session 2 inserted a number "out of order" .. so to understand what you're after, we really need to understand your requirements in detail to advise further.
However, that should help you understand sequences properly. :)
Good luck!