UPDATE TABLE - ORACLE performance

Question

Hi,
I have table with many columns, and i want to update it :
*1)- *
update table T
set col1=val1,
col2=val2
where c1;
*2)-*
update table T
set col1=val1
where c1;
update table T
set col1=val1
where c1;
Any one know : How is bettre for the oracle performance the first statement or the second?and WHY?
My table is a huge table with many column, i need know the best query in ordre to dont disturb the oracle performance
Thanks

Answer 1

Yes, Your guess is right. You can test it with a small test case in your test environment. I have tested it in my environment. see the result below. Also wait for other experts to comment.
SQL> create table test_emp (ename varchar2(50),empno number primary key, sal number);
Table created.
SQL> insert into test_emp select object_name,object_id,dbms_random.value(1000,100000) from all_objects;
42502 rows created.
SQL> commit;
Commit complete.
SQL> exec dbms_stats.gather_table_stats(USER,'TEST_EMP',cascade=>true);
PL/SQL procedure successfully completed.
SQL> explain plan for
  2  UPDATE test_emp
  3  SET ename=INITCAP(ename), sal=sal+20
  4  WHERE empno>1500;
Explained.
SQL> select * from xplan;
PLAN_TABLE_OUTPUT
Plan hash value: 1067865627
| Id  | Operation        | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | UPDATE STATEMENT   |           | 41551 |  2110K|    85     (3)| 00:00:02 |
|   1 |  UPDATE         | TEST_EMP |       |       |         |           |
|*  2 |   TABLE ACCESS FULL| TEST_EMP | 41551 |  2110K|    85     (3)| 00:00:02 |
Query Block Name / Object Alias (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - UPD$1
   2 - UPD$1 / TEST_EMP@UPD$1
Predicate Information (identified by operation id):
   2 - filter("EMPNO">1500)
Column Projection Information (identified by operation id):
PLAN_TABLE_OUTPUT
   2 - (upd=2,4; cmp=3) "TEST_EMP".ROWID[ROWID,10],
       "ENAME"[VARCHAR2,50], "EMPNO"[NUMBER,22], "SAL"[NUMBER,22]
26 rows selected.
SQL> explain plan for
  2  UPDATE test_emp
  3  SET ename=INITCAP(ename)
  4  WHERE empno>1500;
Explained.
SQL> select * from xplan;
PLAN_TABLE_OUTPUT
Plan hash value: 1067865627
| Id  | Operation        | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | UPDATE STATEMENT   |           | 41551 |  1257K|    85     (3)| 00:00:02 |
|   1 |  UPDATE         | TEST_EMP |       |       |         |           |
|*  2 |   TABLE ACCESS FULL| TEST_EMP | 41551 |  1257K|    85     (3)| 00:00:02 |
Query Block Name / Object Alias (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - UPD$1
   2 - UPD$1 / TEST_EMP@UPD$1
Predicate Information (identified by operation id):
   2 - filter("EMPNO">1500)
Column Projection Information (identified by operation id):
PLAN_TABLE_OUTPUT
   2 - (upd=2; cmp=3) "TEST_EMP".ROWID[ROWID,10], "ENAME"[VARCHAR2,50],
       "EMPNO"[NUMBER,22]
26 rows selected.
SQL> exlain plan for
SP2-0734: unknown command beginning "exlain pla..." - rest of line ignored.
SQL> explain plan for
  2  UPDATE test_emp
  3  SET sal=sal+20
  4  WHERE empno>1500;
Explained.
SQL> select * from xplan;
PLAN_TABLE_OUTPUT
Plan hash value: 1067865627
| Id  | Operation        | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | UPDATE STATEMENT   |           | 41551 |  1095K|    85     (3)| 00:00:02 |
|   1 |  UPDATE         | TEST_EMP |       |       |         |           |
|*  2 |   TABLE ACCESS FULL| TEST_EMP | 41551 |  1095K|    85     (3)| 00:00:02 |
Query Block Name / Object Alias (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - UPD$1
   2 - UPD$1 / TEST_EMP@UPD$1
Predicate Information (identified by operation id):
   2 - filter("EMPNO">1500)
Column Projection Information (identified by operation id):
PLAN_TABLE_OUTPUT
   2 - (upd=3; cmp=2) "TEST_EMP".ROWID[ROWID,10], "EMPNO"[NUMBER,22],
       "SAL"[NUMBER,22]
26 rows selected.
SQL>