How to find out if a sequence was initialized in this session?

Question

I need to read the current value of a sequence in a function. However, for the first time in each session I try to use currval(), I get following error:

currval of sequence "foo_seq" is not yet defined in this session

Hint for those who might find this question by googling for this error: you need to initialize the sequence for each session, either by nextval() or setval().

I could use something like lastval() or even setval('your_table_id_seq', (SELECT MAX(id) FROM your_table)); instead, but this seems seems either prone to gaps or slower than simple currval(). My aim is to avoid gaps and inconsistencies (I know some of the values will be added manually), so using nextval() before logic handling them is not ideal for my purpose. I would need this to initialize the sequence for the session anyway, but I would prefer to do something like this:

--start of the function here
IF is_not_initialized THEN
  SELECT setval('foo_seq', (SELECT MAX(id) FROM bar_table)) INTO _current;
ELSE
  SELECT currval('foo_seq') INTO _current;
END IF; 
--some magic with the _current variable and nextvalue() on the right position

The point is that I have no idea how might "is_not_initialized" look like and whether is it possible at all. Is there any function or other trick to do it?

EDIT: Actually, my plan is to let each group of customers choose between proper sequence, no sequence at all, and the strange "something like a sequence" I'm asking for now. Even if the customer wanted such a strange sequence, it would be used only for the columns where it is needed - usually because there are some analog data and we need to store their keys (usually almost gapless sequence) into the DB for backward compatibility.

Anyway, you are right that this is hardly proper solution and that no sequence might be better than such a messy workaround in those situations, so I'll think (and discuss with customers) again whether it is really needed.

Answer 1

Craig, a_horse and pozs have provided information which can help you understand principles of using sequences. Apart from the question how are you going to use it, here is a function which returns current value of a sequence if it has been initialized or null otherwise.

If a sequence seq has not been initialized yet, currval(seq) raises exception with sqlstate 55000.

create or replace function current_seq_value(seq regclass)
returns integer language plpgsql 
as $$
begin
    begin
        return (select currval(seq));
    exception
        when sqlstate '55000' then return null;
    end;
end $$;

select current_seq_value('my_table_id_seq')