COSS (as COSP) are "Totals" table, so why would you join COEP (detail ) and COSS (total) If you have COEP record then COSP record don't carry much information. Actually only records of COSS that do not relate to COEP may be of interest. (check value type available in COEP vs COSP which has more type available) Would you not prefer join COBK header of documents to COEP for missing informations ?
Regards,
Raymond