You should start the X-Window manager only when you really need it

I tend to do a lot of routine tasks from the command line. Most commercial Linux distributions, however, default to starting an X-Window manager following system initialization. That's a lot of system resources to have running.

To change this behavior, edit the /etc/inittab file and locate the line that reads: id:5:initdefault and change it to id:3:initdefault.The system will start with a command line login, and when the need arises to run a window manager, it's easy to simply type startx

Best way to delete oracle flashback logs

The deletion of flashback logs is handled internally, which means you should not delete the flashback logs manually unless you disable the FLASHBACK DATABASE

According to the Oracle Documentation, flashback log files are deleted automatically when:

1. If the flash recovery area is full, then an archived redo log may be automatically deleted by the flash recovery area to make space for other files. In such a case, any flashback logs that would require the use of that redo log file for the use of FLASHBACK DATABASE are also deleted.
   
2. If the database needs to create a new flashback log and the flash recovery area is full or there is no disk space, then the oldest flashback log is reused instead.

Make sure there aren't any guaranteed restore points in place, causing flashback logs to be retained longer than they should. Use this query to check:

select * from v$restore_point;

When to use "SELECT FOR UPDATE NOWAIT"

• Verify the data has not changed since you queried it out (preventing lost updates). 
• Lock the row (preventing the UPDATE or DELETE from blocking).

Solution to a blocked SELECT FOR UPDATE

add the NOWAIT clause and it will no longer block. Instead, your application will report back to the end user that the row is already locked

How to relink oracle binaries

$ORACLE_HOME/bin/relink all

How to easily identify your trace file

It can be hard to identify individual trace files. One way to
make it easier is to specify a trace file identifier for your session. This can be done
by setting the TRACEFILE_IDENTIFIER parameter from within your session:

ALTER SESSION SET tracefile_identifier=GUY

Now when we look in the trace file directory, the tracefile can be identified
by the trailing “GUY”

Oracle Flashback monitoring scripts

PROMPT How Far Back Can We Flashback To (Time)?
PROMPT
select to_char(oldest_flashback_time,'dd-mon-yyyy hh24:mi:ss') "Oldest Flashback Time"
 from v$flashback_database_log;

PROMPT
PROMPT How Far Back Can We Flashback To (SCN)?
PROMPT
col oldest_flashback_scn format 99999999999999999999999999
select oldest_flashback_scn from v$flashback_database_log;

PROMPT
PROMPT Flashback Area Usage
SELECT * FROM   V$FLASH_RECOVERY_AREA_USAGE;

PROMPT
col ROUND(SPACE_LIMIT/1048576) heading "Space Allocated (MB)" format 999999
col round(space_used/1048576) heading "Space Used (MB)" format 99999
col name Heading "Flashback Location" format a40

select name, round(space_limit/1048576),round(space_used/1048576)
 from  v$RECOVERY_FILE_DEST;

Upgrading from 11gr1 to 11gr2 Using Data Pump Export/Import

• Export data from the current database using the Export utility shipped with the
  current database
• Install the new Oracle Database software
• If the new database has the same name as the current database, then shut down
  the current database before creating the new database
• Create the new database
• Start SQL*Plus in the new Oracle Database environment
• Connect to the database instance as a user with SYSDBA privileges
• Start an Oracle Database instance using STARTUP
• Optionally, you can change the storage parameters from the source database
• Use the Import utility of the new database to import the objects exported from the
  current database
• After the import, check the import log file for information about which imports of
  which objects completed successfully and, if there were failures, which failed
• If changes are made to the current database after the export, then make sure those
  changes are propagated to the new database before making it available to users

Script to Check for invalid objects

spool invalid_pre.lst
select substr(owner,1,12) owner,
substr(object_name,1,30) object,
substr(object_type,1,30) type, status from
dba_objects where status <>'VALID';
spool off

Automatic memory management

What is a log file sync ?

Log file sync is a client wait event.  It is the wait event your clients wait on when 
they say "commit".  It is the wait for LGWR to actually write their redo to disk and 
return back to them.  You can "tune" this by making lgwr faster (no raid 5 for example) 
and committing less frequently and generating less redo (BULK operations generate less 
redo than row by row do)

The other one is a background wait. LGWR is waiting for forgrounds to finish a current 
copy affecting the data LGWR is about to process.

HOWEVER, that said, tuning either of these will have no noticable affect on your systems 
performance whatsoever!  It certainly looks like "enqueue" is your wait and that is all 
about application design - those are heavyweight locks induced by the application logic 
itself.  You would be best served by looking at the *application* not at the "system" at 
this point. 

More infos here