Doubt on retrieving of objectid from CDHDR Table?

Hi,

Doubt on retrieving of objectid from CDHDR Table?i am having hardcoded Objctclas and Date time based on these 2 i have to retrieve the changed Sales document no from CDHDR.But also it is taking more time,performance is a big problem.is ther any alternative for retriving the changed sales document Number.

Thanks & Regards,

Gopi.