Decrement oracle sequence when an exception occurred - sql

Question

I try to insert a new record in my database using pl/sql, so first of all I generate a new sequence like so:
select my_seq.nextval into seqId from dual;
And then i try to insert a new record using the generated seqId like this :
insert into myTable (id) values seqId ;
But when an error occurred during the insertion I want to decrement my sequence in an exception block. Does anyone have an idea please?

Answer 1

As you were already told, you shouldn't be doing this at all. Anyway, here's how you might do it.
Sample table and a sequence:
SQL> create table mytable (id number primary key);
Table created.
SQL> create sequence seq;
Sequence created.
SQL> set serveroutput on
Procedure which inserts seq.nextval into mytable and decrements the sequence in a case of a failure. I'm doing it in a simplest way - dropping it and recreating with the start parameter set to the last fetched value minus 1. DBMS_OUTPUT calls are here just to show what's going on in the procedure.
SQL> create or replace procedure p_test as
2 seqid number;
3 begin
4 seqid := seq.nextval;
5 dbms_output.put_line('Sequence number to be inserted = ' || seqid);
6
7 insert into mytable(id) values (seqid);
8
9 exception
10 when others then
11 dbms_output.put_line(sqlerrm);
12 execute immediate 'drop sequence seq';
13 execute immediate 'create sequence seq start with ' || to_char(seqid - 1);
14 end;
15 /
Procedure created.
Let's test it: this should insert 1:
SQL> exec p_test;
Sequence number to be inserted = 1
PL/SQL procedure successfully completed.
SQL> select * from mytable;
ID
----------
1
So far so good. Now, I'll manually insert ID = 2 so that the next procedure call violates unique constraint:
SQL> insert into mytable values (2);
1 row created.
Calling the procedure again:
SQL> exec p_test;
Sequence number to be inserted = 2
ORA-00001: unique constraint (SCOTT.SYS_C007547) violated
PL/SQL procedure successfully completed.
OK; procedure silently completed. It didn't insert anything, but it decremented the sequence:
SQL> select * from mytable;
ID
----------
1 --> populated with the first P_TEST call
2 --> populated manually
SQL> select seq.nextval from dual;
NEXTVAL
----------
1 --> sequence is decremented from 2 to 1
If I delete the offending ID = 2 and try again:
SQL> delete from mytable where id = 2;
1 row deleted.
SQL> exec p_test;
Sequence number to be inserted = 2
PL/SQL procedure successfully completed.
SQL> select * from mytable;
ID
----------
1
2
SQL> select seq.nextval from dual;
NEXTVAL
----------
3
SQL>
Right; kind of works, but it's not worth the pain.
Besides, as you commented that there are 20 rows in a table. What will you do if someone deletes row #13. Will you decrement all values between #14 and 20? What if that's a primary key, referenced by some foreign keys?
Seriously, don't do it.

Answer 2

You can not decrement after the sequence is incremented (A sequence is either incremented or decremented not both) or reinitialize the sequence (start with ...) in Oracle. There is no way of doing that. Hope this helps.
However, if you want to continue this absurdity you can try this but you need to initialize your sequence first which is myseq.nextval;
then you can first try to insert currval and if succeeds then you can increment your sequence otherwise sequence will have its previous value.
Declare
currval pls_integer;
inc pls_integer;
Begin
select seq.currval into currval from dual;
insert into myTable (id) values (currval) ;
select seq.nextval into inc from dual;
Exception
when others then
do the exception handling;
end;