oracle - updateステートメントを最適化するにはどうすればよいですか

Question

UPDATE UMSD_FILE_SNAPSHOT FS
         SET (data_msisdn, data_timestamp) = (SELECT msisdn, sacct_estab_dt
                                                FROM UMSD_FILE_SNAPSHOT_1D
                                               WHERE cust_acct_no =
                                                     FS.cust_acct_no
                                                 AND cust_sacct_no = '1D'
                                                 AND ROWNUM < 2),
             (fax_msisdn, fax_timestamp) = (SELECT msisdn, sacct_estab_dt
                                              FROM UMSD_FILE_SNAPSHOT_1D
                                             WHERE cust_acct_no =
                                                   FS.cust_acct_no
                                               AND cust_sacct_no = '1F'
                                               AND ROWNUM < 2),
             (video_msisdn, video_timestamp) = (SELECT msisdn, sacct_estab_dt
                                                  FROM UMSD_FILE_SNAPSHOT_1D
                                                 WHERE cust_acct_no =
                                                       FS.cust_acct_no
                                                   AND cust_sacct_no = '1V'
                                                   AND ROWNUM < 2)
       WHERE NVL(twc_status, ' ') <> 'S'
       AND cust_sacct_no = '1';