Some interesting about oracle, did you know? (一)

1, By default DBUA removes all the hidden parameters during upgrade.If you want to keep the hidden parametersinvoke DBUA with ‘keepHiddenParams’ option.e.g./dbua -keepHiddenParams
2, If you cannot start the Oracle database,you do not have valid backups to restore,lost system tablespace data files,data files corrupted !Is there a way to recover your data !Yes,Still you have an option to extract your data using an Oracle tool Data Unloading (DUL).DUL is an offline operation.DUL is the processof extracting (unloading) data from Oracle data files directly.Its an offline tool and it bypasses the Oracle Kernel.There are similarother tools availble in the market e.g DUDE / jDUL,AnySQL UnLoader (AUL),Oracle Salvage, Oracle Recovery , WisdomForce FastReader for Oracle,MyDUL,CLOUT,K2Tor Pro,FlashUnload
3,  Have you ever surfaced in a situation where database was hung and you were not able to make connection or couldn’t login to the database due to ORA-00020 maximum number of processes exceeded.If yes,You can use “-prelim” option (10g onwards) to login as SYSDBA.Remember This doesn’t create a session.
sqlplus -prelim / as sysdba
4,  Starting 11g Oracle has introduced a fixed table X$DBGALERTEXT, we can query this table to get the alert.log contents from sqlplus.
SQL> select message_text from X$DBGALERTEXT where rownum <= 50;Using DBMS_SYSTEM.KSDWRT procedure , We can write custom messages directly into the Oracle alertlog file.
5, Starting with Oracle Database release 11.2.0.2, setting JOB_QUEUE_PROCESSES to '0'(zero) causes both DBMS_SCHEDULER and DBMS_JOB jobs to not run. Previously, setting JOB_QUEUE_PROCESSES to 0 caused DBMS_JOB jobs to not run, but DBMS_SCHEDULER jobs were unaffected and would still run.
6, Index rebuilding in parallel changes the degree permanently .
alter index ABC_IDX rebuild parallel 4;
select degree from dba_indexes where index_name=’ABC_IDX’;
DEGREE
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4
NOTE: This could cause plan change.
Reference  http://www.dbagalaxy.com/duk/