Kunci Utama Komposit PostgreSQL dan Peningkatan Serial?

Tampilkan data seperti yang disarankan oleh @Juan

select
    row_number() over (
        partition by modulecode order by modulecode
    ) as sessionid, 
    modulecode
from schedule

Kemudian ketika pengguna meminta sessionid tertentu dari modul tertentu lakukan:

select *
from schedule
where sessionid = (
    select sessionid
    from (
        select
            sessionid,
            row_number() over (order by sessionid) as module_sessionid
        from schedule
        where modulecode = 'B'
    ) s
    where module_sessionid = 2
)