How to get posting key BSCHL from invoice no

Hi,

I need Posing key BSCHL from BSEG table based on invoice no (VBRK-VBELN) and billing type (VBRK-FKART) and invoice date (FKDAT).

If I directly fetching from BSEG table to get posting key, system is taking much time and also my program is taking much time as BSEG is cluster table.

If any Fucnion module or BAPI is available to get posting key, anybody can please provide it.

Thanks