Alter sequence from a trigger - sql

Question

I have an auto increment trigger that goes something like as follows
CREATE OR REPLACE TRIGGER test_auto_inc
BEFORE INSERT ON TESTTABLE
FOR EACH ROW
BEGIN
IF :NEW.TestCol IS NULL THEN
SELECT Auto_Increment.nextval INTO :new.TestCol FROM dual;
END IF;
END;
and I want to add in an if statement that will somehow lead to a decrement in the sequence I am using which is as follows:
CREATE SEQUENCE Auto_Increment START WITH 1
INCREMENT BY 1
CACHE 100;
I need to decrement the count somehow like this
ALTER SEQUENCE Auto_Increment INCREMENT BY -1;
SELECT Auto_Increment.NEXTVAL FROM dual;
ALTER SEQUENCE Auto_Increment INCREMENT BY 1;
What is the best way to do this?
Thank you

Answer 1

That's most probably not a good idea. What real problem are you trying to solve?
Anyway, as you asked, here's one option which does that.
This is what you currently have:
SQL> create table testtable (testcol number, datum date);
Table created.
SQL> create sequence auto_increment start with 1 increment by 1 cache 100;
Sequence created.
Trigger:
SQL> create or replace trigger test_auto_inc
2 before insert on testtable
3 for each row
4 declare
5 l_seq number;
6 begin
7 l_seq := auto_increment.nextval;
8 if l_seq >= 3 then
9 execute immediate 'alter sequence auto_increment increment by -1';
10 l_seq := auto_increment.nextval;
11 execute immediate 'alter sequence auto_increment increment by 1';
12 end if;
13
14 :new.testcol := nvl(:new.testcol, l_seq);
15 end;
16 /
Trigger created.
Testing:
SQL> insert into testtable (datum) values (sysdate);
1 row created.
SQL> insert into testtable (datum) values (sysdate);
1 row created.
SQL> select * from testtable order by datum;
TESTCOL DATUM
---------- -------------------
1 20.03.2022 08:22:17
2 20.03.2022 08:22:26
SQL>
If you want to modify (alter, eh?) the sequence within a PL/SQL procedure (that's your trigger), you'll have to use dynamic SQL as this is the only way to run a DDL from PL/SQL. OK, let's add it (I also slightly modified your code; no need for select while fetching sequence value):
SQL> create or replace trigger test_auto_inc
2 before insert on testtable
3 for each row
4 declare
5 l_seq number;
6 begin
7 l_seq := auto_increment.nextval;
8 if l_seq >= 3 then
9 execute immediate 'alter sequence auto_increment increment by -1';
10 l_seq := auto_increment.nextval;
11 execute immediate 'alter sequence auto_increment increment by 1';
12 end if;
13
14 :new.testcol := nvl(:new.testcol, l_seq);
15 end;
16 /
Trigger created.
Nice, it compiled! Why wouldn't it, Oracle has no idea what's written between single quotes. Could've been the 1st statement from Moby Dick. Let's try it:
SQL> insert into testtable (datum) values (sysdate);
insert into testtable (datum) values (sysdate)
*
ERROR at line 1:
ORA-04092: cannot COMMIT in a trigger
ORA-06512: at "SCOTT.TEST_AUTO_INC", line 6
ORA-04088: error during execution of trigger 'SCOTT.TEST_AUTO_INC'
SQL>
Whooops! Can't commit in a trigger. Bummer! Is there a way out of it? Certainly - declare the trigger to be an autonomous transaction:
SQL> create or replace trigger test_auto_inc
2 before insert on testtable
3 for each row
4 declare
5 pragma autonomous_transaction;
6 l_seq number;
7 begin
8 l_seq := auto_increment.nextval;
9 if l_seq >= 3 then
10 execute immediate 'alter sequence auto_increment increment by -1';
11 l_seq := auto_increment.nextval;
12 execute immediate 'alter sequence auto_increment increment by 1';
13 end if;
14
15 :new.testcol := nvl(:new.testcol, l_seq);
16 end;
17 /
Trigger created.
Does it work? Yes!
SQL> insert into testtable (datum) values (sysdate);
1 row created.
SQL> select * from testtable order by datum;
TESTCOL DATUM
---------- -------------------
1 20.03.2022 08:22:17
2 20.03.2022 08:22:26
3 20.03.2022 08:26:55
SQL>
Yet another insert:
SQL> insert into testtable (datum) values (sysdate);
1 row created.
SQL> select * from testtable order by datum;
TESTCOL DATUM
---------- -------------------
1 20.03.2022 08:22:17
2 20.03.2022 08:22:26
3 20.03.2022 08:26:55
3 20.03.2022 08:27:31 --> see? Yet another testcol = 3!
SQL>
So, yes ... it can be done. Note, though, that if you used autonomous transaction within a trigger for purposes different from logging, you're most probably doing it wrong.