Joining two flat files

Hi Gurus,

I'm a newbie to SAP BODS. We have a scenario, where we are joining two flat files to get single column. Both Primary and Reference file records were suppose to be in millions. We don't have any staging DB to push down to sql operations.

In this case, is it better to go for JOIN or LOOK UP?  Is there is any other way to improve performance other than staging DB?

Thanks