SELECT FOR UPDATE with the SKIP LOCK clause

Hi,
I have a query regarding the SELECT FOR UPDATE with the SKIP LOCK clause.
Whether this will be really good for parallel processing.
Also if we are selecting a set of records in a cursor whether the lock will be done at the records level once we fetch the records?
Also do we have any known issues with this one?
We are trying to figure out whether this will fit for business requirement, we are trying to do a implement a threading kind of thing for our stored procedure invocation in background using shell script.
Any suggestion or feedback on this will be helpful for us.
Thanks a lot for the support given....
Regards,
Basil Abraham.

http://www.oracle.com/technology/oramag/oracle/08-mar/o28plsql.html
Please read the above thread for few information...Thanks!

Similar Messages

FOR UPDATE...SKIP LOCKED - Is it supported to use in 10.2?

Hi.
Is the "SKIP LOCKED" feature in SELECT..FOR UPDATE..SKIP LOCKED supported to use in 10.2.0.3?
It does exactly what we want to do. The patchsets for 10g also contains bug fixes for this feature.
I know that it's supported (and documented) for 11g, but we have not upgraded yet.
Also I'm fully aware of the locking mechanism using this feature (it does not lock when opening a cursor,
it locks when the rows are fetched).
Regards
Krister Lundmark

Also I'm fully aware of the locking mechanism using this feature (it does not lock when opening a cursor,
it locks when the rows are fetched).Usual FOR UPDATE statement locks data at the moment of cursor opening - according to the concepts it has to get
locked consistent snapshot before fetching. SKIP LOCKED differs from usual FOR UPDATE in that it
tries to lock at the moment of fetch and skips rows already locked by other transactions:
The first transaction:
SQL> select ename from emp where ename = 'KING' for update;
ENAME
KINGThe second: I wait at the moment of cursor opening, not fetching
SQL> declare
2 cursor x is select * from emp for update;
3 begin
4 open x;
5 end;
6 /Now I lock 'KING' and open FOR UPDATE SKIP LOCKED cursor
SQL> var rc refcursor
SQL> exec open :rc for select ename from emp where ename in ('KING','ALLEN') for update skip locked;
PL/SQL procedure successfully completed.I can lock 'ALLEN' in another transaction - cursor opening didn't lock it:
SQL> select ename from emp where ename = 'ALLEN' for update;
ENAME
ALLENAnd then I fetch cursor I find it's empty - I can't find unlocked rows at the fetch stage:
SQL> print rc
no rows selectedRgds.

How to SELECT FOR UPDATE with CMP (Oracle)

The most common database (Oracle) by default uses a scheme that does not fit into any of those isolation levels. A SELECT statement selects data at the start of the transactions, whereas a SELECT ... FOR UPDATE does something quite different. It is essential to do SELECT FOR UPDATEs before updating the row as SELECT does no lock. It's a hack that works well in practice.
1. Which isolation level is this?
2. More fundamentally, how an earth is it possible to use this scheme with CMP?! You would have to distinguish load() from loadForUpdate()! Is CMP inconsistent with Oracle?
This is a pretty big whole in the CMP spec!

No. thats no goes well.
Transaction serializable in Oracle uses a optimistic
concurrency system. And for update is a
pessimistic concurrency.
With optimistic: the system is faster but it can fail
With pessimistic: if doesnt fail (usually;)
You can solve the proble with many differents systems:
1. Edit the .xml descriptor files ans change the sql sentences.
And my prefer one.
2. Make a new jdbc driver that inherits from the original
oracledriver.
The new driver give u in "getConnection()" a new connection class that inherits from the original connection.
The executestatement and preparestatement adds the
string "for update" if the stattement was starting by select.
Configure your container to use the new driver.

HT1414 my daughter has iphone 3gs and it required an update , so i connected the iphone to itunes for update and the iphone locked up, I had to reset back to factory settings and now the iphone will not activate , what can i do

My daughter has an iphone 3gs and it required an update. So i connected the iphone to itunes for the update and the iphone locked up . I had to reset the iphone back to factory settings and now the iphone will not activate , Please advise what i can do to get this working for her ?

Generally this is a sign that the iPhone had previously been
hacked/modified/jailbroken and the update relocked it to the
original wireless carrier. If this is the case, only that wireless
carrier can unlock your iPhone. You must contact them to see
if they offer unlocking and if you qualify.
Where did you acquire this iPhone?
What wireless carrier did you use before this problem?
Does the app Cydia appear on your iPhone?
What does it say when you look at Settings=>General=>About=>Carrier?
Unauthorized modification of iOS
http://support.apple.com/kb/HT3743

Lock Cascade With Select for UPDATE

If I had a employee table and a phone table with a parent/child relationship and a primary key constraint on the employee table-will issuing a select for update on the employee also lock the corresponding child rows on the phone table ?
If not how can I bring this about ?

You only need two sessions:
First session: Issue the 'select for update'
statements for the row(s) in both tables, don't
rollback or commit
Second session: try to update a row that you tried to
lock in the first session (with NOWAIT).
Thanks. I can try this definitely. A basic question.
You are asking me to do a join on both the tables right ?
Not two individual SQL statements ?
Updating the primary key is known as a Bad Idea (tm).
The key should never be touched because it should be
meaningless. When you have a column that holds 'real'
information it is no candidate for a primary key.
Rgds,
GuidoYes I am aware of that. I was just wondering what is the meaning behind this statement from this link ?
http://www.akadia.com/services/ora_locks_survival_guide.html
And the exact phrase from that link under the section Referential Integrity Locks (RI Locks)
"RI constraints are validated by the database via a simple SELECT from the dependent (parent) table in question-very simple, very straightforward. If a row is deleted or a primary key is modified within the parent table, all associated child tables need to be scanned to make sure no orphaned records will result. "
Thanks again.

How to avoid 'select for update'

Hi,
we are using the bc4j framework of jdev 3.2.3. We have a View which collects data from several tables in different database schemes, so we naturally have to use the 'union clause'. But if we try to make an update on a row (via 'setAttribute'), we got a DMLException ORA-02014 telling us that we should not use 'select for update' on views whith unions.
Since i cannot avoid using a union clause, is there a way to avoid the 'select for update' in the bc4j framework?
Please help, its urgent!
Thanx,
Dietmar

SELECT FOR UPDATE is used for our implementation or row-level locking.
If you are using pessimistic locking mode, this will occur the first time any attribute is modified.
If you are using optimistic locking mode, it will be deferred until post/commit time.
If you are using "none" locking mode, it will not happen in your application may hang indefinitely if another session has locked the row.
Are you asking how to avoid locking?
Do you mean to be updating this view with a union over multiple databases?

Inconsistent Locking with Select for Update

Hi,
I seem to be having some issues in using SELECT FOR UPDATE and was hoping to get some insight from the Oralce Guru's out there.
I have a J2EE application, running in WebLogic 8.1.4 using Oralce 9.2.0.1.0.
The application contains code that requires locking to be done on a specific table with multiple transactions (tx) requesting the same lock. Eg:
Tx 1: Select * from Zone where Zoneid = 'Zone1' for update (Obtains lock)
Tx 2: Select * from Zone where Zoneid = 'Zone1' for update (waits)
Tx 100: Select * from Zone where Zoneid = 'Zone1' for update
Tx1 commits.
It appears that the following transactions, i.e. Tx2 - Tx100 do not seem to execute in the order the lock was requested. That is Tx 100 always appears to be the second last transaction to execute, after which some arbitrary transaction between Tx2 - Tx99 will execute after Tx100 has committed.
This seems to tell me that the lock is not being handed in a FIFO manner and is causing us great pain as our data is not longer consistent.
Does anyone know how i might be able to trace which transaction is being awarded the lock? Also if anyone has any suggestion on how to troubleshoot/solve this issue, greatly appreciated.
TIA
Prem

Oracle does not have a lock queue/manager at all. The locked status of a record is essentially an attribute of the record itself. It is stored on the datablock header. When a transaction requests a lock and can't get it, and is willing to wait (SELECT FOR UPDATE without NOWAIT), it first spins while waiting for the lock (four times as I recall), then sleeps waiting for the lock. The the more times it sleeps before getting the lock, the longer it will sleep before trying again.
What is likely happening here is that transaction 100 is still spinning when transaction 1 commits, so checks back more frequently and gets the lock first. The rest get the lock whenever they wake up and noone else has taken the lock.
If you need the transaction to occur in order, then I do not think you can use Oracle's native locking mechanism. Depending on what exactly you are trying to do, you may want to look at Advanced Queueing, or possibly the built-in package DBMS_LOCK.
HTH
John

When does select for update release locks

Hello all,
Does anyone know when Oracle realeases the row locks when a
select for update is issued?
Does Oracle realase the row lock at the time when an actual update statement is
issued for the locked row, or does it wait until a commit statment is executed?
So for example, can I lock several rows with a select for update clause, and then
issue update statements as many times as I want on each locked row without
having to worry about the lock being released until I issue a commit statement.
Thanks,
David

yes.
The lock is released only when your transaction ends. A transaction can end because of:
1). Commit.
2). Rollback.
3). client disconnects.
etc. etc...

Select for update returns no rows even though there is no locking thread

I'm using Ibatis library over oracle sql for my query. The select for update statement returns no rows. This happens intermittently. When this was happening last time, I executed the select statement on sqldeveloper (but without the 'for update') and got rows. This situation is not easily reproducible so I've not yet been able to ascertain whether rows are returned on sqldeveloper with the 'for update' clause. But I know for sure that there was no other thread locking the rows. How could this be happening?

The select for update statement returns no rowsWhy do you think that a select for update will always return rows?
the for update clause if there not to garantee the presence of rows but to lock the row when it is present
sql> select * from t;
         A          B C
         1          1 step1
         2          2 step2
         3          3 step3Then session 1 issues the following select
SELECT     *
      FROM t
     WHERE a = 1
FOR UPDATE NOWAIT;If session 2 issues the same select before session 1 commits or rolls back
SELECT     *
      FROM t
     WHERE a = 1
FOR UPDATE NOWAIT;It will get the following error
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specifiedBut if session 2 issue such a kind of select
sql> SELECT     *
2        FROM t
3       WHERE a = 99
4 FOR UPDATE NOWAIT;
no rows selectedYou see then that a select for update can return no rows
Best Regards
Mohamed Houri

Pros and cons of select for update clause

hi,
Can anybody explain what are the
pros and cons of select for update clause
11.2.0.1

As commented, there are no pros versus cons in this case.
What is important is to understand conceptually what this do and why it would be use.
Conceptually, a select for update reads and locks row(s). It is known as pessimistic locking.
Why would you want to do that? Well, you have a fat client (Delphi for example) and multiple users. When userA updates an invoice, you want that invoice row(s) locked and prevent others from making updates at the same time. Without locking, multiple users updating the same invoice will result in existing updated data being overwritten by old data that also has been updated. A situation called lost updates.
For web based clients that are stateless, pessimistic locking does not work - as the clients do not have state and pessimistic locking requires state. Which means an alternative method to select for update needs to be used to prevent lost updates. This method is called optimistic locking.
So it is not about pros versus cons. It is about understanding how the feature/technique/approach works and when to use it.. and when it is not suited to use it. All problems are not nails. All solutions are not the large hammer for driving in nails.

Oracle select for update: not releasing lock

My JDBC code uses "select for update" to modify record in Oracle database. I tried to simulate network connection down situation.
After my JDBC code locked on a record using "select for update", I unplugged the network cable. I then tried to run the same code on another computer, but it could not accquire the lock, because the previous lock was not released. I tried sqlplus to lock the record, but failed also. The lock has been there for at least an hour now. I guess it may finally be released, but is there a way for oracle to release the lock as soon as the connection is down? Not know if it is a JDBC setting or oracle setting.

Dear Friend,
What you are trying to do is not correct way of checking the concurrency and transaction.
The reason is as listed below.
01.Always remember http is a stateless protocol and removing the connection or just closing the browser will never be informed to the database or to the application server thus the transaction monitor (TM)or processor will never release the lock as it will deem that the actor is manipulating the data.
02.As per locking goes every database is having a �TM� and the default isolation level setting is different like oracle uses serializable DB2udb 7.0 or db2/As400 uses repeatable read. You can change this setting by editing the default setting in the database but be very sure before touching it.
03. You can also transpose this with your Application server setting for that piece of code or Globally but again be very sure about it as it will change the entire gamete.
04. For releasing lock you have to manually do it or you can change the settings of App server or the Database to release the connection after some wait time.
Regards,
Ruchir

DBMS_LOCK vs Select for update locking

Hi,
In one of our database packages, we are using the dbms_lock (that package is used to generate unique numbers) instead of select for update locking.
I want to ask what is more suitable here. Our application is oracle forms and oracle db on oracle app server.
Are there any limitations to select for update locking?
What are the conditions favourable to use DBMS_LOCK?
Regards,

DBMS_LOCK is extremely efficient. Try it. Put a call to obtain/free a lock in a loop and see how long it takes to loop a few thousand times (don't SLEEP in the loop, just obtain and free the lock). If you are having performance issues with DBMS_LOCK or your select it is probably because whatever it is that gets the lock (whichever way you get it) is taking a certain amount of time to finish.
I too, am curious as to what "issues" you had. In either case, a lock for consistent read will be different than a lock for update (as it should be) in terms of how it impacts other users.

Updatable View issues: cannot select FOR UPDATE from view with DI

Hi All,
I have a simple view XY and an instead of trigger on that to insert data into one table which is used in the my view. When I do insert statement on view XY it is working fine from sql but when i used same thing with page process of " Process Row of XY Automatic Row Processing (DML) " i am getting following error. I am using APEX 3.0 . Please help me.
ORA-20001: Error in DML: p_rowid=xxxx, p_alt_rowid=abc, p_rowid2=, p_alt_rowid2=. ORA-02014: cannot select FOR UPDATE from view with DISTINCT, GROUP BY, etc.
Thanks

James,
are you already on 3.0.1.00.07 or 3.0.1.00.08? Have a look at the release notes, it says something about a bug fix for some occurrences of ORA-02014.
Also have a look at the new substitution value/item FSP_DML_LOCK_ROW which turns locking off if you set it to FALSE. See http://www.oracle.com/technology/products/database/application_express/html/3.0.1_readme.html#CHDIDIAF and also http://download-west.oracle.com/docs/cd/B32472_01/doc/appdev.300/b32471/advnc.htm#BCGFDAIJ
Patrick
My APEX Blog: http://inside-apex.blogspot.com
The ApexLib Framework: http://apexlib.sourceforge.net
The APEX Builder Plugin: http://apexplugin.sourceforge.net/ New!

Whether two session with for update at the same time will wait for another?

Will this scenario happen?
One session executes: select ... for update/select ... for update nowait
The other session also executes: select ... for update/select ... for update nowait
Two sessions happens at the same time unluckily. Whether they will all stand by because of "for update" or quit immediately because of "nowait"?
Thx a lot.

HiddenName wrote:
This situation is called 'a deadlock'. Oracle will automaticaly detect a deadlock. One of the statements will fail with ORA-00060 deadlock detected;
No, it's not.
A deadlock is when sesson-A has a lock on something that session-B needs in order to continue, and at the same time, session-B has a lock on something that Session-A needs in order to continue. They BOTH have something the other needs.
In the OP's case, at least as he described it, it is simple serialization. The one with the lower scn will go, then the next.
PS.
Why don't you try? You can setup a small table with some records, open 2 sessions to the same database and for example issue UPDATE statements:
session 1: update where id=1
session 2: update where id=2
session 1: update where id=2 - this will hang waiting for session 2 to commit and free lock
session 2: update where id=1 - ORA-00060
You may also read about deadlocks in chapter on "Data Concurency and Consistency" of "Oracle Concepts'.

"All of the Playlists Selected for Updating No Longer Exist"

My family has 2 ipod shuffles. After purchasing a Nano recently, I inserted the cd that came with it. Apparently the existing iTunes software was removed and reinstalled. At any rate, the library still shows all the original songs, but when the Nano is plugged in, iTunes immediately gives a prompt stating "Songs on the iPod *** cannot be updated because all of the playlists selected for updating no longer exist." The library is there, the playlists that were there are there, and I can play the songs on my cpu. Is there anything I can do besides completely uninstalling everything and starting over?
iPod Nano Windows XP Pro

This user tip should help you sort out your missing playlist problem: Hudgie - iPod cannot sync because one or more playlists are missing

SELECT FOR UPDATE with the SKIP LOCK clause

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