How to improve performance of this select statement (between a large table and view)?

Hiya,

I have the following select that takes ages to complete:

select     a~head_no

               b~vality_to

appending table lt_itab up to 1 rows from crm_ace_refkey

as a inner join v_acedsoh as b on

b~ref_key = a~ref_key
where a~head_no = ls_kaufn-kaufn.

The crm_ace_refkey table has around 9 million rows and viewv_acedsoh has around 20 million rows.

I'm a bit stumped on how anything could be improved so any advice would be great!

Thanks in advance!