Continue sequence

Hi,

Which is the correct way to obtain a continue sequence without fail
(hole/gap) for a composite-id ?

Example:

CREATE TABLE nf
(
number INTERGER NOT NULL,
serie VARCHAR(2) NOT NULL,
CONSTRAINT pky_nf PRIMARY KEY (number,serie)
)

number serie
1 A
2 A
1 B
3 A
4 A
2 B

I think that I will need to use LOCK TABLE or SELECT FOR UPDATE, but in
both cases if the client terminal shutdown suddenly the tuple/table
stays locked on the server for a indeterminate time.

I did a test with the following commands using pgadmin III:

BEGIN;
LOCK TABLE nf IN EXCLUSIVE MODE;

I restarted my terminal and checked in another terminal that the LOCK
condition was there. After 10 minutes, to unlock the table, I send a
KILL command for the process that was locking the table/tuple.

Best regards,

Rogério Augusto Bassete - BRAZIL/SP