Optimize my Query

SELECT  
 DISTINCT 
 pdeep_code, pdeep_value, pdeep_uom, pdeep_status_code,
      pdeep_status_desc, pnum_value, permission_type_name, maker_product_name,
      adeep_value, adeep_uom, adeep_status_code, adeep_status_desc,
       char_num_value, interface_id, interface_name, preferred_flag,
       pdeep_lead_time, permission_lead_time, PEC_lead_time, PEC_lead_time_status,
       permission_lead_time_status, onnet_lead_time, onnet_lead_time_status,
        a.permission_deep_char_id, a.permission_maker_char_id,
       a.permission_product_type_id, a.permission_maker_name_id, a.pnum_value
    FROM  vw_pps_withpermission a,
	 psu_product p1, psu_cases a2, 
         psu_ref_subproduct sp
    WHERE a2.subproduct_cd = sp.subproduct_cd (+)
	 AND a2.product_cd = p1.product_cd
	 --ADDED TO TAKE CARE OF FULL SCAN , if you take off this join,cost increases significantly.
	 AND A.CASE_ID=A2.CASE_ID
	 --ADDITION ENDS
      AND p1.product_valid_cd = 1
      AND parent_product_id = p1.parent_product_cd
    
      AND a2.case_valid_cd = 1
	   AND a.case_id = :b2
      AND a.permission_maker_char_id = :b1
     
      AND NOT EXISTS (SELECT 'X'
                      FROM psu_parent_ship_permission_deeps
                      WHERE case_id = a2.case_id
                        AND ship_deep_char_id = a.ship_deep_char_id
                        AND permission_deep_char_id = a.permission_deep_char_id
                        AND permission_maker_char_id = a.permission_maker_char_id
                        AND permission_product_type_id = a.permission_product_type_id
                        AND permission_maker_name_id = a.permission_maker_name_id
                        AND valid_cd = 0)
     
    ORDER BY a.pnum_value ASC