multiple sequence number for one column

hi group,

we need help on one postgresql locking issue:

Originally we have a table like below;

id bigint not null nextval('xxx)',
customer_id int not null,
insert_record_date timestamp not null
...

so this id column is using a sequence number that applies to all
customers. And it's the primary key of the table

recently we made the change to use id and customer_id as the composite
primary key. And id will have a sequence number within each
customer_id. So now we can't use sequence number any more. To insert
the data into the new table, we have to calculate the id for that
particular customer_id first like

SELECT INTO v_ID COLESCSE(MAX("ID),0)+1
WHERE "Customer_ID"=P_A_Customer_ID

And then insert into this table with the id getting from the above
query. We also used:

PERFORM pg_advisory_lock('"Schema"."TABLE"'::regclass::integer,
P_A_Customer_ID

Then we found out this lock doesn't work. If two transactions for the
same customer comes in very closely, the second one will try to get
the max(id) from the table while the first one is still working on the
insert and then it will be a problem.

We thought of using SHARE UPDATE EXCLUSIVE but it will interfere with
regular maintenance like Vacuum and also it will block other updates
etc.

Is there a good way to solve this issue?

Thanks a lot