drop proc test2 
go
create proc test2 @ProductId integer,
               @ReferenceOrderID integer,
               @Quantity         integer
as

select *
from [Production].[TransactionHistory]
where (@productid        is not null and @productid        = ProductID)
or    (@ReferenceOrderID is not null and @ReferenceOrderID = ReferenceOrderID)  
--or    (@quantity is not null and @quantity = Quantity)    Uncomment This to force a scan
go

exec test2 @productid =  790,@ReferenceOrderID = NULL,@Quantity=NULL 
go
exec test2 @productid =  NULL,@ReferenceOrderID =61197 ,@Quantity=NULL