SQL & PL/SQL

I try to tune SQL in another [url http://forums.oracle.com/forums/message.jspa?messageID=1762095#1762095]thread. 
And then I find out one of difference between 'merge' and 'update' 
that is number of execution. 
 
Update : Rows * Rows
Merge  : Rows
 
Can we tune my update statement up to near merge? 

Here is tkprof results. 
There are only 300 rows in mytable for test.
 
 /**** UPDATE ****/
 
Misses in library cache during parse: 1
Misses in library cache during execute: 1
Optimizer mode: ALL_ROWS
Parsing user id: 27  (USHI)   (recursive depth: 1)
 
Rows     Row Source Operation
-------  ---------------------------------------------------
      1  SORT AGGREGATE (cr=3 pr=0 pw=0 time=125 us)
    300   TABLE ACCESS FULL MYTABLE (cr=3 pr=0 pw=0 time=338 us)
 

Rows     Execution Plan
-------  ---------------------------------------------------
      0  SELECT STATEMENT   MODE: HINT: ALL_ROWS
      1   SORT (AGGREGATE)
    300    TABLE ACCESS (FULL) OF 'MYTABLE' (TABLE)

********************************************************************************

update mytable t1
 set flag =
 (select 1+rn-last_value(rn2 ignore nulls) over (order by id) new_flag
    from
      (select
        rowid row_id
       ,id
       ,row_number() over (order by id) rn
       ,case when col1 = lag(col1) over (order by id)
              and col2 = lag(col2) over (order by id)
              and col3 = lag(col3) over (order by id)
             then null
             else row_number() over (order by id)
        end rn2
      from mytable
      ) t2
   where t2.row_id = t1.rowid
   )
 
call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          2          0           0
Execute      1      0.50       0.50          0        903        606         300
Fetch        0      0.00       0.00          0          0          0           0
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        2      0.50       0.50          0        905        606         300
 
Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: 27  (USHI)
 
Rows     Row Source Operation
-------  ---------------------------------------------------
      0  UPDATE  MYTABLE (cr=903 pr=0 pw=0 time=502019 us)
    300   TABLE ACCESS FULL MYTABLE (cr=3 pr=0 pw=0 time=635 us)
    300   WINDOW BUFFER (cr=900 pr=0 pw=0 time=494110 us)
    300    VIEW  (cr=900 pr=0 pw=0 time=488138 us)
  90000     WINDOW SORT (cr=900 pr=0 pw=0 time=431450 us)
  90000      TABLE ACCESS FULL MYTABLE (cr=900 pr=0 pw=0 time=94585 us)
 

Rows     Execution Plan
-------  ---------------------------------------------------
      0  UPDATE STATEMENT   MODE: ALL_ROWS
      0   UPDATE OF 'MYTABLE'
    300    TABLE ACCESS (FULL) OF 'MYTABLE' (TABLE)
    300    WINDOW (BUFFER)
    300     VIEW
  90000      WINDOW (SORT)
  90000       TABLE ACCESS (FULL) OF 'MYTABLE' (TABLE)

********************************************************************************
 
 
 /**** MERGE ****/
  
Misses in library cache during parse: 1
Misses in library cache during execute: 1
Optimizer mode: ALL_ROWS
Parsing user id: 27  (USHI)   (recursive depth: 1)
 
Rows     Row Source Operation
-------  ---------------------------------------------------
      1  SORT AGGREGATE (cr=3 pr=0 pw=0 time=113 us)
    300   TABLE ACCESS FULL MYTABLE (cr=3 pr=0 pw=0 time=328 us)
 

Rows     Execution Plan
-------  ---------------------------------------------------
      0  SELECT STATEMENT   MODE: HINT: ALL_ROWS
      1   SORT (AGGREGATE)
    300    TABLE ACCESS (FULL) OF 'MYTABLE' (TABLE)
 
********************************************************************************

merge into mytable t1
using
 (select row_id, 1+rn-last_value(rn2 ignore nulls) over (order by id) new_flag
    from
      (select
        rowid row_id
       ,id
       ,row_number() over (order by id) rn
       ,case when col1 = lag(col1) over (order by id)
              and col2 = lag(col2) over (order by id)
              and col3 = lag(col3) over (order by id)
             then null
             else row_number() over (order by id)
        end rn2
      from mytable
      )
   ) t2
   on (t2.row_id = t1.rowid)
 when matched then
      update set flag = new_flag
 when not matched then
      insert values (null,null,null,null,null)
 
call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.03       0.02          0          4          0           0
Execute      1      0.00       0.00          0          6        308         300
Fetch        0      0.00       0.00          0          0          0           0
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        2      0.03       0.03          0         10        308         300
 
Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: 27  (USHI)
 
Rows     Row Source Operation
-------  ---------------------------------------------------
      2  MERGE  MYTABLE (cr=6 pr=0 pw=0 time=9505 us)
    300   VIEW  (cr=6 pr=0 pw=0 time=6530 us)
    300    HASH JOIN OUTER (cr=6 pr=0 pw=0 time=5930 us)
    300     VIEW  (cr=3 pr=0 pw=0 time=4164 us)
    300      WINDOW BUFFER (cr=3 pr=0 pw=0 time=3557 us)
    300       VIEW  (cr=3 pr=0 pw=0 time=3284 us)
    300        WINDOW SORT (cr=3 pr=0 pw=0 time=2080 us)
    300         TABLE ACCESS FULL MYTABLE (cr=3 pr=0 pw=0 time=346 us)
    300     TABLE ACCESS FULL MYTABLE (cr=3 pr=0 pw=0 time=352 us)

********************************************************************************