Recovery: corrupt oracle block for test

C:\oracle\product\10.2.0\db_1\database>vim test.null
insert : 0
C:\oracle\product\10.2.0\db_1\database>dd if=test.null of=C:\PROJECT\TEST10G\DATAFILE\O1_MF_TEST_UND_6MBTLQRW_.DBF bs=8192 seek=3 count=1
0+1 records in
0+1 records out

Recovery: clear log file - not existing

http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:346817516082

08:00:42 38 4294967295 SYS@orcl> alter database clear unarchived logfile group 2;
alter database clear unarchived logfile group 2
*
ERROR at line 1:
ORA-00313: open failed for members of log group 2 of thread 1
ORA-00312: online log 2 thread 1: 'C:\ORACLE\PRODUCT\10.2.0\ORADATA\ORCL\REDO02.LOG'
ORA-27041: unable to open file
OSD-04002: unable to open file
O/S-Error: (OS 2) ═х єфрхЄё эрщЄш єърчрээ√щ Їрщы.


Elapsed: 00:00:00.10
08:00:50 38 4294967295 SYS@orcl> alter database open resetlogs;
alter database open resetlogs
*
ERROR at line 1:
ORA-01139: RESETLOGS option only valid after an incomplete database recovery


Elapsed: 00:00:00.37
08:12:44 38 4294967295 SYS@orcl> recover database until cancel;
Media recovery complete.
08:13:01 38 4294967295 SYS@orcl> alter database open resetlogs;

Database altered.

Performance: Block contention

select class, count, time/100 s from v$waitstat order by s desc;

General: Estimate index size

variable a number
variable u number
exec dbms_space.create_index_cost('create index t1_idx on t_1(object_name, object_type, owner)', :u, :a);

PL/SQL procedure successfully completed.

A
------------------------------------
7,340,032.0

U
------------------------------------
5,282,424.0