Modify query to use indexspace

hi all,

i have a cursor with the where clause as

where

(trn_isin = :smx-isin

or

trn_isin = :ws-und-isin

or

trn_cusip = : smx-cusip )

here trn_isin is index ..

now the selection criteria is either of the 3 values should be present in the table

if im using the above where clause the query is going for a tablespace scan
maybe because of OR ... can anyone modify this query to do a indexspace scan ?

thanks,
ashimer

if need arise i can make trn_cusip as another index ..
IN predicate should work or else joins ..