Questions on Locking-- URGENT

Question

Here are my questions on Locking.:
1)
How do I use v$lock to figure out lock details ? Details like
whos has locked,on which table,who else is waiting, and what type
of locking (shared or exlusive ?), since when it is locked.
2)
Why and when exclusive/shared locks are used ? Could you give me
example please ?
3)
Could you tell me significance of v$mystat and v$locked_object ?
4)
Do we have page level locking in Oracle ?
Thanks in advance,
DN

Answer 1

Details about locks : All locks acquired by statements within a transaction are held for the duration of the transaction.
Oracle releases all locks acquired by the statements within a transaction when an explict or implied commit or roll back is executed. Oracle also releases locks acquired after a savepoint when rolling back to the savepoint.
Note: Only transactions not waiting for the previously locked resources can acquire locks on now available resources. Waiting transactions continue to wait until after the original transaction commits or completely rolls back.
1. How do I use v$lock to figure out lock details? Details like
whos has locked,on which table,who else is waiting, and what type
of locking (shared or exlusive ?), since when it is locked.
Better way is to use Oracle OEM. The TYPE column in v$lock shows the type of wait events and we also have the LMODE column (lock mode) and REQUEST columns.
Here is a comprehensive v$lock query by Deepak Baranwal, listing the lock types:
set echo off
col sid form 9999
col id1 form 9999999999
col id2 form 999999999
col lmode head "Lock Held" form a14
col request1 head "Lock Request" form a16
col type head "Lock Type" form a15
col ctime head "Time|Held" form 999999
col block head "No Of |Sessions|Waiting|For This|Lock" form 99999
select sid,
DECODE(TYPE,
'BL','Buffer hash table',
'CF','Control File Transaction',
'CI','Cross Instance Call',
'CS','Control File Schema',
'CU','Bind Enqueue',
'DF','Data File',
'DL','Direct-loader index-creation',
'DM','Mount/startup db primary/secondary instance',
'DR','Distributed Recovery Process',
'DX','Distributed Transaction Entry',
'FI','SGA Open-File Information',
'FS','File Set',
'IN','Instance Number',
'IR','Instance Recovery Serialization',
'IS','Instance State',
'IV','Library Cache InValidation',
'JQ','Job Queue',
'KK','Redo Log "Kick"',
'LS','Log Start/Log Switch',
'MB','Master Buffer hash table',
'MM','Mount Definition',
'MR','Media Recovery',
'PF','Password File',
'PI','Parallel Slaves',
'PR','Process Startup',
'PS','Parallel Slaves Synchronization',
'RE','USE_ROW_ENQUEUE Enforcement',
'RT','Redo Thread',
'RW','Row Wait',
'SC','System Commit Number',
'SH','System Commit Number HWM',
'SM','SMON',
'SQ','Sequence Number',
'SR','Synchronized Replication',
'SS','Sort Segment',
'ST','Space Transaction',
'SV','Sequence Number Value',
'TA','Transaction Recovery',
'TD','DDL enqueue',
'TE','Extend-segment enqueue',
'TM','DML enqueue',
'TS','Temporary Segment',
'TT','Temporary Table',
'TX','Transaction',
'UL','User-defined Lock',
'UN','User Name',
'US','Undo Segment Serialization',
'WL','Being-written redo log instance',
'WS','Write-atomic-log-switch global enqueue',
'XA','Instance Attribute',
'XI','Instance Registration',
decode(substr(TYPE,1,1),
'L','Library Cache ('||substr(TYPE,2,1)||')',
'N','Library Cache Pin ('||substr(TYPE,2,1)||')',
'Q','Row Cache ('||substr(TYPE,2,1)||')',
'????')) TYPE,
id1,id2,
decode(lmode,
0,'None(0)',
1,'Null(1)',
2,'Row Share(2)',
3,'Row Exclu(3)',
4,'Share(4)',
5,'Share Row Ex(5)',
6,'Exclusive(6)') lmode,
decode(request,
0,'None(0)',
1,'Null(1)',
2,'Row Share(2)',
3,'Row Exclu(3)',
4,'Share(4)',
5,'Share Row Ex(5)',
6,'Exclusive(6)') request1,
ctime, block
from
v$lock
where sid>5
and type not in ('MR','RT')
order by decode(request,0,0,2),block,5
2 .Why and when exclusive/shared locks are used ? Could you give me
example please ?
Exclusive Lock Mode : Prevents the associates resource from being shared. This lock mode is obtained to modify data. The first transaction to lock a resource exclusively is the only transaction that can alter the resource until the exclusive lock is released.
Share Lock Mode : Allows the associated resource to be shared, depending on the operations involved. Multiple users reading data can share the data, holding share locks to prevent concurrent access by a writer (who needs an exclusive lock). Several transactions can acquire share locks on the same resource.
Oracle Lock Types
DML locks (data locks)
DDL locks (dictionary locks)
Oracle Internal Locks/Latches
Oracle Distributed Locks
Oracle Parallell Cache Management Locks
3. Could you tell me significance of v$mystat and v$locked_object ?
v$locked_object show the information about Who is locking and what object they locking. This view is similar to v$mystat except that it shows cumulated statistics for all sessions.
4. Do we have page level locking in Oracle ? No
Sachin