Join to table pa_expenditure_items_all using orig_transaction_reference

I'm executing the following query which is taking hours to run.  My DBA tells me the join xpct.orig_transaction_referenc  = ei.orig_transaction_reference is the issue because there is no index on orig_transaction_reference. 

When I do an expalin on this query, I get a Merge Join - Cartesian Product however the cost is not high (17).   Does anyone have any suggestions.

SELECT SUM((ei.denom_raw_cost))
              ,SUM(abs(ei.denom_raw_cost)) 

    FROM xx_pa_cost_trx_in_stg xpct, 
               apps.pa_expenditures_all e, 
               apps.pa_expenditure_items_all ei 

  WHERE xpct.orig_transaction_referenc  = ei.orig_transaction_reference