Implement a sequence that increments by value of a function - oracle

Question

I am looking to create a simple sequence:
CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 3
INCREMENT BY 4
CACHE 20000;
However, I want to semi-randomize the incrementing values with a function determined by some factors implemented on the java side.
I thought the first step for testing this might look something like this:
CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 3
INCREMENT BY myfunction
CACHE 20000;
I tried this:
CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 3
INCREMENT BY (declare
rtrnt number;
begin
rtrnt :=semiRandomize();
end;
)
CACHE 20000;
which I realize is ridiculous.. but there must be some way to do something like this. Any pointers?

Answer 1

How about this? Not really a sequence, but - might suit your needs. It is a function that selects a random number and stores it into a table with a primary key. If the number has already been used, it is skipped. The function checks whether all numbers have been used; if so, it raises an error.
In this example, I'm creating a 5-numbers "sequence" (so that it fails soon enough).
Table & function:
SQL> create table t_seq (supseq number constraint pk_tseq primary key);
Table created.
SQL> create or replace function f_supseq
2 return number
3 as
4 l_range number := 5; -- number of values in a "sequence"
5 l_seq number; -- a new "sequence" number
6 l_cnt number; -- number of used numbers
7 pragma autonomous_transaction;
8 begin
9 select count(*) into l_cnt from t_seq;
10 if l_cnt < l_range then
11 -- there are still some available numbers so - let's get them
12
13 -- don't let anyone mess with the table
14 lock table t_seq in exclusive mode;
15 while l_seq is null loop
16 begin
17 insert into t_seq (supseq) values
18 (round(dbms_random.value(1, l_range)))
19 returning supseq into l_seq;
20 exception
21 when dup_val_on_index then
22 -- that number has already been used; skip it
23 null;
24 end;
25 end loop;
26 commit;
27 else
28 raise_application_error(-20001, 'No more available numbers');
29 end if;
30
31 return l_seq;
32 end;
33 /
Function created.
Fetching random "sequence" values:
SQL> select f_supseq from dual;
F_SUPSEQ
----------
2
SQL> select f_supseq from dual;
F_SUPSEQ
----------
4
SQL> select f_supseq from dual;
F_SUPSEQ
----------
1
SQL> select f_supseq from dual;
F_SUPSEQ
----------
3
SQL> select f_supseq from dual;
F_SUPSEQ
----------
5
SQL> select f_supseq from dual;
select f_supseq from dual
*
ERROR at line 1:
ORA-20001: No more available numbers
ORA-06512: at "SCOTT.F_SUPSEQ", line 28
Table contents:
SQL> select * From t_seq;
SUPSEQ
----------
1
2
3
4
5
SQL>