Update Clause (can These Queries be more tunned)

Question

Can anyone suggest me on following Queries which we can write in some other fashion (in line view, etc..) or can it be more optimised or multiple select can be minimised. I anted to reduce time of these updates. Its taking more time to update. I have tried For Loop, cursors, but still taking more time.. I have also created indexes. But it reduced hardly any time.
I have also attached execution plan for the same..
No. : 1
          UPDATE cbr_fw_acct_mast A
          SET A.bal_eod = (SELECT b.bal_principal
                              FROM ol_td_aux_table@fwdat B
WHERE b.cod_acct_no = a.cod_acct_no
                                   AND b.dat_post = var_l_dt_process
AND b.dat_txn = (SELECT MAX(c.dat_txn)
                                                       FROM ol_td_aux_table@fwdat c
                                                       WHERE b.cod_acct_no = c.cod_acct_no
                                                       AND c.dat_post = b.dat_post)
AND b.ctr_batch_no = (SELECT MAX(c.ctr_batch_no)
                                                                 FROM ol_td_aux_table@fwdat c
                                                            WHERE b.cod_acct_no = c.cod_acct_no
                                                            AND c.dat_post = b.dat_post
                                                                 AND b.dat_txn = c.dat_txn)
AND b.ref_sys_tr_aud_no = (SELECT MAX(c.ref_sys_tr_aud_no)
                                                                      FROM ol_td_aux_table@fwdat c
                                                                      WHERE b.cod_acct_no = c.cod_acct_no
                                                                      AND c.dat_post = b.dat_post
                                                                      AND b.dat_txn = c.dat_txn
                                                                      AND b.ctr_batch_no = c.ctr_batch_no)
AND b.ctr_updat_srlno = (SELECT MAX(c.ctr_updat_srlno)
                                                                 FROM ol_td_aux_table@fwdat c
                                                                 WHERE b.cod_acct_no = c.cod_acct_no
                                                                 AND c.dat_post = b.dat_post
                                                                 AND b.dat_txn = c.dat_txn
                                                                 AND b.ctr_batch_no = c.ctr_batch_no
                                                                 AND b.ref_sys_tr_aud_no= c.ref_sys_tr_aud_no) )
          WHERE A.cod_prod_category = 'TD'
          AND EXISTS (SELECT DISTINCT cod_acct_no
                              FROM ol_td_aux_table@fwdat
                         WHERE dat_post = var_l_dt_process
                         AND A.cod_acct_no = cod_acct_no)
          AND A.cod_acct_stat = 5;
Query Plan
UPDATE STATEMENT Cost =
UPDATE CBR_FW_ACCT_MAST
FILTER
TABLE ACCESS BY INDEX ROWID CBR_FW_ACCT_MAST 1
INDEX RANGE SCAN IN_CBR_FW_ACCT_MAST_2 NON-UNIQUE
REMOTE
FILTER
REMOTE
SORT AGGREGATE
REMOTE
SORT AGGREGATE
Query Plan
REMOTE
SORT AGGREGATE
REMOTE
SORT AGGREGATE
REMOTE
No. : 2
          UPDATE cbr_fw_acct_txns A
          SET A.cod_cc_atm_brn = (SELECT RTRIM(B.ca_name)
                                        FROM xf_ol_st_cotxn_mmdd@fwdat B
                                        WHERE B.cod_acct_no = A.cod_acct_no
                                        AND B.dat_post_stl = var_l_dat_process
AND B.dat_txn_stl = A.dat_txn
                              AND B.ctr_batch_no = A.ctr_batch_no
AND B.ref_sys_tr_aud_no = A.ref_sys_tr_aud_no
                                        AND B.ref_subseq_no = A.ref_sub_seq_no
AND B.cod_txn_mnemonic = A.cod_txn_mnemonic
AND B.cod_msg_typ = A.cod_msg_typ
AND B.cod_reply = 0
AND B.fwd_inst_id = '00000009900')
WHERE ( A.cod_acct_no, A.dat_post, A.ctr_batch_no
               , A.ref_sys_tr_aud_no, A.cod_txn_mnemonic, A.cod_msg_typ ) IN
( SELECT cod_acct_no, var_l_dat_process, ctr_batch_no,
                         ref_sys_tr_aud_no, cod_txn_mnemonic, cod_msg_typ
FROM     xf_ol_st_cotxn_mmdd@fwdat
               WHERE dat_post_stl = var_l_dat_process
AND cod_reply = 0
AND fwd_inst_id = '00000009900' ) ;
Query Plan
UPDATE STATEMENT Cost =
UPDATE CBR_FW_ACCT_TXNS
NESTED LOOPS
VIEW VW_NSO_1
SORT UNIQUE
REMOTE
TABLE ACCESS BY INDEX ROWID CBR_FW_ACCT_TXNS 1
INDEX RANGE SCAN IN_CBR_FW_ACCT_TXNS_1 NON-UNIQUE
REMOTE
No. : 3
          UPDATE     cbr_fw_acct_mast A
          SET          A.dat_acct_close = ( SELECT MAX(dat_value)
                                             FROM td_audit_trail@fwdat B
                                        WHERE B.cod_acct_no = A.cod_acct_no)
          WHERE     A.cod_prod_category = 'TD'
          AND     EXISTS ( SELECT DISTINCT c.cod_acct_no
                              FROM td_audit_trail@fwdat c
                              WHERE A.cod_acct_no = c.cod_acct_no)
          AND A.cod_acct_stat in (1);
Query Plan
UPDATE STATEMENT Cost =
UPDATE CBR_FW_ACCT_MAST
FILTER
TABLE ACCESS BY INDEX ROWID CBR_FW_ACCT_MAST 1
INDEX RANGE SCAN IN_CBR_FW_ACCT_MAST_2 NON-UNIQUE
REMOTE
SORT AGGREGATE
REMOTE

Answer 1

Point 1. Querying from remote databases is very difficult to tune. It is also notoriously bad for performance. Running correlated sub-queries against tables in a remote database is bound to be agonisingly slow.
How often do you run these queries? If these are one-offs you should just take the pain but if you are going to be running these queries often it would be worth replacing the remote calls with materalized views on the local database.
Point 2. You don't need DISTINCT with EXISTS. You're introducing an unnecessary sort there. But I don't think that's what's killing you.
Cheers, APC