update and undo

Goal
----
Research how to skip the update on some columns.
Logically, Update table set column = column is an option.
Need to check the resource usage.

Result
------
Update column without change the value, still generate same amount of undo and redo.
The undo is proportion to target value size.

Tested on Oracle 11.1.0.7

Setup
-----

drop table t;
drop table t1;

create table t nologging
as
select rownum id, lpad('x',500,'x') c1
from dual
connect by level <= 50000;

create table t1 nologging
as
select rownum id, lpad('x',500,'x') c1
from dual
connect by level <= 50000;

set serveroutput on

Benchmark
---------
exec runstats_pkg.rs_start;
update t set id = id, c1 = c1;
commit;
exec runstats_pkg.rs_middle;
update t1 set id = id;
commit;
exec runstats_pkg.rs_stop;


Run1 ran in 414 hsecs
Run2 ran in 119 hsecs
run 1 ran in 347.9% of the time

Name                                  Run1        Run2        Diff
LATCH.shared pool                      117         120           3
STAT...redo synch time                  23           7         -16
STAT...redo buffer allocation           16           0         -16
LATCH.DML lock allocation               22           5         -17
STAT...physical reads                1,786       1,808          22
LATCH.redo writing                      62          22         -40
LATCH.redo allocation                   78          23         -55
STAT...CPU used by this sessio          73          16         -57
LATCH.enqueues                         152          84         -68
STAT...commit cleanouts                979       1,120         141
STAT...rollback changes - undo         142           0        -142
LATCH.cache buffers lru chain        4,909       5,184         275
STAT...Elapsed Time                    414         119        -295
STAT...DB time                         418         119        -299
STAT...IMU Redo allocation siz       9,412      15,396       5,984
STAT...consistent changes           48,013          30     -47,983
STAT...consistent gets              51,861       1,878     -49,983
STAT...redo entries                 54,046       3,579     -50,467
STAT...db block gets                56,883       2,111     -54,772
STAT...IMU undo allocation siz      63,992       1,080     -62,912
STAT...db block changes            110,023       7,330    -102,693
STAT...session logical reads       108,744       3,989    -104,755
STAT...physical read bytes      29,261,824  29,622,272     360,448
LATCH.cache buffers chains         487,509      26,861    -460,648
STAT...undo change vector size  32,362,024   2,399,528 -29,962,496
STAT...redo size                69,525,648   5,092,152 -64,433,496

Run1 latches total versus runs -- difference and pct
Run1        Run2        Diff       Pct
521,401      45,711    -475,690  1,140.65%