How to check database link?

SQL> SELECT 1 FROM dual@mydblink

How to configure EM (Enterprose Manager) if has any problem?

On linux terminal;
set ORACLE_SID=<YOURSID>
emca -deconfig dbcontrol db -repos drop;
then;
emca -config dbcontrol db -repos create;
or
emca -config dbcontrol db -repos recreate;

How to delete archive log files until specific time in RMAN?

RMAN> delete archivelog until time ‘SYSDATE-10’;
i: it will delete them until 10 days ago

Also it has a no prompt option;
RMAN> delete noprompt archivelog until time ‘SYSDATE-10′;

How to check/open archive log in sqlplus?

Check in archive log open:
SQL> SELECT LOG_MODE FROM SYS.V$DATABASE;
or
SQL> archive log list;

Open achieve log mode:
SQL> startup mount;
SQL> alter database archivelog;
SQL> alter database open;

Enterprise Manager and OEM Agent commands in Oracle

Please login to database server with ssh as oracle user and run that commands.

Go to /bin path
su - grid12
cd agent/agent_inst/bin

How to start/stop Enterprise Manager
./emctl start dbconsole;
./emctl stop dbconsole;

for 12c grid EM
/home/grid12/oms/bin/emctl start oms
/home/grid12/oms/bin/emctl stop oms

How to start/stop Oem Agent
Be sure run root.sh as root user and run below commands as oracle user:
./emctl stop agent
./emctl start agent

How to check Oem Agent
./emctl status agent

How to restore Oem Agent
Run as grid12 user:
./emctl clearstate agent; and then
./emctl upload agent;

How to alter a profile in Oracle?

Please login to database server with ssh as oracle user and run that commands.

sqlplus / as sysdba

SQL> ALTER PROFILE engineer LIMIT 
SESSIONS_PER_USER 5
IDLE_TIME 60
CONNECT_TIME 1200;

SQL> exit

-- SESSIONS_PER_USER; Specify the number of concurrent sessions to which you want to limit the user.
-- IDLE_TIME; Specify the permitted periods of continuous inactive time during a session, expressed in minutes. Long-running queries and other operations are not subject to this limit.
-- CONNECT_TIME; Specify the total elapsed time limit for a session, expressed in minutes.

How to define new profile or change old profile in Oracle?

Please login to database server with ssh as oracle user and run that commands.

sqlplus / as sysdba

SQL> CREATE PROFILE engineer LIMIT 
SESSIONS_PER_USER 1
IDLE_TIME 30
CONNECT_TIME 600;

SQL> exit

-- SESSIONS_PER_USER; Specify the number of concurrent sessions to which you want to limit the user.
-- IDLE_TIME; Specify the permitted periods of continuous inactive time during a session, expressed in minutes. Long-running queries and other operations are not subject to this limit.
-- CONNECT_TIME; Specify the total elapsed time limit for a session, expressed in minutes.

other parameters:
CPU_PER_SESSION; Specify the CPU time limit for a session, expressed in hundredth of seconds.
CPU_PER_CALL; Specify the CPU time limit for a call (a parse, execute, or fetch), expressed in hundredths of seconds.
LOGICAL_READS_PER_SESSION; Specify the permitted number of data blocks read in a session, including blocks read from memory and disk.
LOGICAL_READS_PER_CALL; Specify the permitted number of data blocks read for a call to process a SQL statement (a parse, execute, or fetch).
PRIVATE_SGA; Specify the amount of private space a session can allocate in the shared pool of the system global area (SGA). Refer to size_clause for information on that clause.
COMPOSITE_LIMIT; Specify the total resource cost for a session, expressed in service units. Oracle Database calculates the total service units as a weighted sum of CPU_PER_SESSION, CONNECT_TIME, LOGICAL_READS_PER_SESSION, and PRIVATE_SGA.

Change current profile
ALTER PROFILE DEFAULT LIMIT IDLE_TIME 60;
ALTER PROFILE DEFAULT LIMIT CONNECT_TIME 6000;
ALTER PROFILE DEFAULT LIMIT SESSIONS_PER_USER 1;

Add this parameter into init.ora file permanently or apply this for already running instance temporary:
Option 1: 
*.resource_limit=TRUE 
Option 2:
alter system set resource_limit = true;

How to know system date in Oracle?

Please run that SQL in your editor's SQL window.

select sysdate from dual;

How to connect linux machine with XStart -interface?

If you have XManager tool, you can easily connect to linux machine like Windows machines.

Firstly, change runlevel in /etc/inittab as follow;
Id:3:initdefault;
Then, reboot the machine.

After that please follow the steps:

Open XStart-> Click New and input a session name 
-> Input IP into Host, select SSH protocol and username/password 
-> Then select execution command like "/usr/bin/gnome-session --display $DISPLAY"
-> Click Run button -> Lastly, you should see the linux window

or

Open XShell-> Connect and login to machine with ssh 
-> Input IP into Host, select SSH protocol and username/password 
-> Then run this command "/usr/bin/gnome-session --display $DISPLAY"
-> Lastly, you should see the linux window

How to check ports in Linux machines?

Please run the command as linux system user.

For example, you want to check .80 port
# netstat -anp | grep :80
or
# netstat -tulpn | grep :80

How to see last updated (DDL) table object in Oracle?

Please run below sql script as desired database user;

SQL> select * from all_objects order by last_ddl_time desc;

How to use Java binary for all linux system users?

To execute java binary (bin) for all users, please run these commands below;

sudo ln -s -v /opt/jdk1.7/bin/java /usr/bin/java
sudo ln -s -v /opt/jdk/bin/javac /usr/bin/javac
              <target_name>      <sembolic-file_name>

<target_name> ; java installation directory
<sembolic-file_name> ; binary directory of all users

How to see PGA size per oracle session?

Please execute following SQL lines as "sys";

select
    1048576+a.value+b.value   pga_size
from
   v$parameter a,
   v$parameter b
where
   a.name = 'sort_area_size'
and
   b.name = 'hash_area_size';

How to set SHMMAX value and what is that?

Setting the kernel.shmmax parameter does not reserve or allocate any shared memory. SHMMAX is a safeguard parameter that sets the upper limit for a process can allocate how much shared memory when requested.

Please execute following commands as “root”;
sysctl -w kernel.shmmax=2147483648
(2147483648 -> 2 GB)

or
modify /etc/sysctl.conf file

sysctl -p
(to apply it)

How to see process that used memory or CPU at most?

Please run the command as linux system user.

# top
at manual -> shift + f and then n (mem) or k (cpu)

to highlight the column: press b

to sort by time: press shift + t

to see all CPUs: press 1

to get the different graphical output: press t

Note: Type and press enter bold ones to run

What should you do if electricty down or reboot exists in lvm system?

For this case,
You should mount the logic volume (lv) to related path. 

For example, you are using ftp server;
Run this command after system is up:

mount /dev/my_vg_ora/vol01 /srv/ftp/myhome/​
(mount - logic volume path - mounted path)

How to see FRA (Flash Recovery Area) configuration?

Please execute the commands as "sys";

Check archive log configuration
SQL> archive log list;

Check archive log destination
SQL> show parameter db_recovery_file_dest;

Check archive log limit and left space
SQL> SELECT * FROM V$RECOVERY_FILE_DEST;

Check archive log files 
RMAN> list archivelog all;

How to add datafile as logic volume (LV) on the tablespace in Linux?

On linux terminal;
chown -R oracle:dba /dev/my_vg_ora/example01
chown -R oracle:dba /dev/mapper/my_vg_ora-example01
chmod 777 /dev/my_vg_ora
chmod 660 /dev/my_vg_ora/*

$ sqlplus
Enter user-name: sys as sysdba
@sys
to add database file please execute under query as system database role

SQL> ALTER TABLESPACE MYTABLESPACE ADD DATAFILE '/dev/my_vg_ora/example01' size 8000m reuse

To execute the command, make sure the instance is open and the tablespace is online and also check database datafiles;

SELECT * FROM dba_data_files;

How to kill session when system could not drop database user?

Please execute following commands as "sys user":

select inst_id, sid, serial# from gv$session where username = 'TEST_ALPER';
(inst_id -> instance id, sid -> service id, serial# -> serial number)

then use;

alter system kill session '44,61808,@1';
(44 -> inst_id, 61808 -> service id, @1 -> serial number)

finally drop user;

drop user TEST_ALPER cascade;

How to select top 10 records in Oracle 11g and 12c?

In 11g:

As you now that TOP keyword does not exist in 11g, but you can use rownum keyword to query top records.

For example, you want to see top 10 salary records based on employees:

Run that SQL in your editor's SQL window.

Wrong one;
SELECT *
FROM (
 SELECT employee_id, first_name, last_name, salary
 FROM employees
 )
WHERE rownum < 11
ORDER BY salary DESC;

* ORDER BY runs at last so that it should be in sub query.

Correct one;
SELECT *
FROM (
 SELECT employee_id, first_name, last_name, salary
 FROM employees
 ORDER BY salary DESC
 )
WHERE rownum < 11;

In 12c:
With 12c, you may use special syntax to see top records that is "FETCH" syntax.

SELECT employee_id, first_name, last_name, salary
FROM employees
ORDER BY salary DESC
FETCH FIRST 10 ROWS ONLY;

Also, you may disregard first 5 records inside 10 records.

SELECT employee_id, first_name, last_name, salary
FROM employees
ORDER BY salary DESC
OFFSET 5 ROWS 
FETCH NEXT 5 ROWS ONLY;

That's all. Bye :)