Table lock

What is Table Lock??
What is Latch???

What is LOCK TABLE ???

Use the LOCK TABLE statement to lock one or more tables, table partitions, or table sub partitions in a specified mode. This lock manually overrides automatic locking and permits or denies access to a table or view by other users for the duration of your operation.
Some forms of locks can be placed on the same table at the same time. Other locks allow only one lock for a table.
A locked table remains locked until you either commit your transaction or roll it back, either entirely or to a save point before you locked the table.
A lock never prevents other users from querying the table. A query never places a lock on a table. Readers never block writers and writers never block readers.

What is Latch??

A latch is an internal Oracle mechanism used to protect data structures in the SGA from simultaneous access. Atomic hardware instructions like TEST-AND-SET is used to implement latches. Latches are more restrictive than locks in that they are always exclusive. Latches are never queued, but will spin or sleep until they obtain a resource, or time out.


What is the difference between locks, latches, enqueues and semaphores? for DBA??
 
A latch is an internal Oracle mechanism used to protect data structures in the SGA from simultaneous access. Atomic hardware instructions like TEST-AND-SET is used to implement latches. Latches are more restrictive than locks in that they are always exclusive. Latches are never queued, but will spin or sleep until they obtain a resource, or time out.
Enqueues and locks are different names for the same thing. Both support queuing and concurrency. They are queued and serviced in a first-in-first-out (FIFO) order.
Semaphores are an operating system facility used to control waiting. Semaphores are controlled by the following Unix parameters: semmni, semmns and semmsl. Typical settings are:
semmns = sum of the "processes" parameter for each instance
(see init<instance>.ora for each instance)
semmni = number of instances running simultaneously;
semmsl = semmns

Steps for releasing lock on a table:

Finding Locks


select session_id "sid",SERIAL# "Serial",
substr(object_name,1,20) "Object",
substr(os_user_name,1,10) "Terminal",
substr(oracle_username,1,10) "Locker",
nvl(lockwait,'active') "Wait",
decode(locked_mode,
2, 'row share',
3, 'row exclusive',
4, 'share',
5, 'share row exclusive',
6, 'exclusive', 'unknown') "Lockmode",
OBJECT_TYPE "Type"
FROM
SYS.V_$LOCKED_OBJECT A,
SYS.ALL_OBJECTS B,
SYS.V_$SESSION c
WHERE
A.OBJECT_ID = B.OBJECT_ID AND
C.SID = A.SESSION_ID
ORDER BY 1 ASC, 5 Desc

Finding Blocking sessions :

select l1.sid, ' IS BLOCKING ', l2.sid
from v$lock l1, v$lock l2 where l1.block =1 and l2.request > 0
and l1.id1=l2.id1 and l1.id2=l2.id2

select s1.username '@' s1.machine ' ( SID=' s1.sid ' ) is blocking '
s2.username '@' s2.machine ' ( SID=' s2.sid ' ) ' AS blocking_status
from v$lock l1, v$session s1, v$lock l2, v$session s2 where s1.sid=l1.sid and s2.sid=l2.sid
and l1.BLOCK=1 and l2.request > 0 and l1.id1 = l2.id1 and l2.id2 = l2.id2 ;

Sessions with highest CPU consumption :
SELECT s.sid, s.serial#, p.spid as "OS PID",s.username, s.module, st.value/100 as "CPU sec"
FROM v$sesstat st, v$statname sn, v$session s, v$process p
WHERE sn.name = 'CPU used by this session' -- CPU
AND st.statistic# = sn.statistic#
AND st.sid = s.sid
AND s.paddr = p.addr
AND s.last_call_et <> (SYSDATE - 240/1440) -- sessions logged on within 4 hours
ORDER BY st.value;

Sessions with the highest time for a certain wait  :

SELECT s.sid, s.serial#, p.spid as "OS PID", s.username, s.module, se.time_waited
FROM v$session_event se, v$session s, v$process p
WHERE se.event = '&event_name'
AND s.last_call_et <> (SYSDATE - 240/1440) -- sessions logged on within 4 hours
AND se.sid = s.sid
AND s.paddr = p.addr
ORDER BY se.time_waited;

Sessions with highest DB Time usage :

SELECT s.sid, s.serial#, p.spid as "OS PID", s.username, s.module, st.value/100 as "DB Time (sec)"
, stcpu.value/100 as "CPU Time (sec)", round(stcpu.value / st.value * 100,2) as "% CPU"
FROM v$sesstat st, v$statname sn, v$session s, v$sesstat stcpu, v$statname sncpu, v$process p
WHERE sn.name = 'DB time' -- CPU
AND st.statistic# = sn.statistic#
AND st.sid = s.sid
AND sncpu.name = 'CPU used by this session' -- CPU
AND stcpu.statistic# = sncpu.statistic#
AND stcpu.sid = st.sid
AND s.paddr = p.addr
AND s.last_call_et <> (SYSDATE - 240/1440) -- sessions logged on within 4 hours
AND st.value > 0; 
 


Step1:To verify the lock object Here is the import query:
---------------------------------------------------------------

SELECT o.owner, o.object_name, o.object_type, o.last_ddl_time, o.status, l.session_id, l.oracle_username, l.locked_mode
FROM dba_objects o, gv$locked_object l
WHERE o.object_id = l.object_id;

Step 2: Find the serial# for the sessions holding the lock:

SQL> select SERIAL# from v$session where SID=667;

SERIAL#
----------
21091

SQL> alter system kill session '667,21091';

System altered.