How to Reset Oracle Sequence Safely? - oracle

Question

I want to reset my Oracle Sequence to 0 everyday my code like this:
create or replace procedure reset_seq(p_seq_name in varchar2) is
l_val number;
begin
execute immediate 'select ' || p_seq_name || '.nextval from dual'
INTO l_val; --1
execute immediate 'alter sequence ' || p_seq_name || ' increment by -' ||
l_val || ' minvalue 0'; --2
execute immediate 'select ' || p_seq_name || '.nextval from dual'
INTO l_val; --3
execute immediate 'alter sequence ' || p_seq_name ||
' increment by 1 minvalue 0'; --4
end;
but after run about 2 years later , suddenly I get an error, the increment is an negative int like -16 and the start value also is -16. So anyone can help me to explain this issue.
What I think is that.
the procedure run to step 3 ,after that the current increment is -16 and nextval is 0.
other client request the sequence ,but the current increment is -16 so the next value is -16
the exception occured when the procedure run to step 4.
but I’m not sure about this, can anyone explain it to me. Thanks.

Answer 1

The issue is that both your job-submitted procedure and your external program could both call nextval between steps 2 and 4; and could call in either order; and potentially the external program could make multiple calls.
You could try to mitigate that by changing both the procedure and the external call.
At the moment if step 3 errors the procedure terminates and step 4 is never reached, so all future calls to nextval will continue to error. So you could trap and ignore the ORA-08004 error, on the basis that you'll only get that if there has been an external call between steps 2 and 3, the sequence has therefore been incremented back to zero by that call and thus step 3 is effectively redundant in that scenario:
create or replace procedure reset_seq(p_seq_name in varchar2) is
l_val number;
e_8004 exception;
pragma exception_init(e_8004, -8004);
begin
execute immediate 'select ' || p_seq_name || '.nextval from dual'
INTO l_val; --1
execute immediate 'alter sequence ' || p_seq_name || ' increment by -' ||
l_val || ' minvalue 0'; --2
begin
execute immediate 'select ' || p_seq_name || '.nextval from dual'
INTO l_val; --3
exception
when e_8004 then
-- nextval has already been called by someone else
null;
end;
execute immediate 'alter sequence ' || p_seq_name ||
' increment by 1 minvalue 0'; --4
end;
Now if an external program calls nextval between steps 2 and 3 then you'll get the error at step 3 but ignore it, and step 4 will still happen.
Your external program will get a nextval value of zero if it makes its call between steps 2 and 3; and will get the ORA-08004 if it calls between steps 3 and 4 - or if it makes multiple calls in the window between steps 2 and 4. So, assuming you don't expect zero to be a valid result (which seems reasonable as you wouldn't normally ever get that), you can make repeated calls until you get a non-zero answer and no error. In pseudocode something like:
loop
val = seq.nextval;
if error == -8004 then continue;
if val == 0 then continue;
break;
end loop
You could consider putting that logic into a PL/SQL function and have your program call the function instead of accessing the sequence directly, both to hide that complexity and to avoid having to repeat it if you have more than one program potentially affected.