Jdbc mysql "select ... for update" and isolation level...

Dear, I am now writing an application, in which some worker threads will get a waiting job from database (mysql) and perform the processing.
To avoid workers getting the same job id, I have studied the topics about locks, isolation level, and something like "select ... for update".
The storage engine being used is innoDB. Actually the problem seems solved after using "select ... for update" to get the id, but I am just curious that do I also need to care about the isolation level. It is because when I do some experiments by setting the isolation level to serializable, but NOT using "SELECT ... FOR UPDATE" (just simple select), exception something like "deadlock found and try to restart transaction" occurs. Is there any relationship between these two things? Any comments are welcome!! Thanks!!!

ashipj wrote:
can u plz provide me with the code (connection string). Am new to Mysql.
And also the driver u are using.
Thank youDon't hijack other's topics. Kindly start your own. Don't do that before you read the [Sun JDBC tutorial|http://java.sun.com/docs/books/tutorial/jdbc/index.html] and the [MySQL JDBC documentation|http://dev.mysql.com/doc/refman/5.0/en/connector-j.html].

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Row Selection for second and subsequent level nodes in tree table

Hi All,
We have a .jsff page with tree bindings to display a three-level hierarchy tree table.Suppose the hierarchy is
-> Department
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->->-> Employee details.
I have a use case where-in when we add the employee node within a department,the department node must expand and the newly added employee node must be selected.
In both the cases(for new department or employee), we are following the approach mentioned below:
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Hi,
Thanks for the reply.
I looked at the code given in sample 61.
In the sample, a new node is not added dynamically (for ex.either new location or its child nodes.)
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Select...for update and close()

I am seeing unexpected behaviour when working with select for update, and I hope to get some clarification about it. I have RTFM and checked out any online information I could find, but didn't see too much about row level locks. See below for the problem I am encountering
We have an application with lots of threads on multiple nodes that uses a combination of row level locking and status flags to mark a row as being processed.
We are locking the rows, and then after completing the select ... for update statement we are closing the JDBC statement.
On executing the statement.close() it appears as though the row level lock is lost. Can anyone confirm this behaviour, or tell me that I am misreading the behaviour that I am seeing.
Thanks in advance,
Robert Dawson

I'd expect this behavior regardless of whether autocommit was on. If a statement is closed, any locks held by that statement must be released. If they weren't, no other statement would have permission to remove the locks, so the rows would remain locked indefinitely.
Justin

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
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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.
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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

JDBC SELECT FOR UPDATE

I'm trying to issue a SELECT ... FROM ... FOR UPDATE, and under specific verified conditions runs an UPDATE (where the current is positioned!).
The error code I get is:
ORA-01002: Fetch out of sequence.
Here you are my code:
/////////////////////START
import java.sql.*;
import oracle.jdbc.*;
import oracle.sql.*;
public class SelForUpdDin{
static{
     try{
          Class.forName("oracle.jdbc.driver.OracleDriver");
     catch(Exception e){e.printStackTrace();}
// Queries and cursor name
public static void main (java.lang.String[] args){
String cursorName = null;
int codice = 0;
String cognome = null;
String job = null;
int manager = 0;
java.sql.Date dataAss = null;
int salario = 0;
int commissioni = 0;
int reparto = 0;
String rowid = null;
String sqlSelect = "SELECT empno, ename, "+
"job, mgr, hiredate, sal, comm, deptno, ROWID "+
"FROM scott.emp FOR UPDATE";
String sqlUpdate = "UPDATE scott.emp SET comm = ? WHERE ROWID = ? ";
try {
Connection con = DriverManager.getConnection("jdbc:Oracle:oci8:@","system","manager");
// Esecuzione della SELECT e produzione del RESULT SET
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery(sqlSelect);
PreparedStatement ps = con.prepareStatement(sqlUpdate);
while (rs.next()) {
codice = rs.getInt(1);
cognome = rs.getString(2);
job = rs.getString(3);
manager = rs.getInt(4);
dataAss = rs.getDate(5);
salario = rs.getInt(6);
commissioni = rs.getInt(7);
reparto = rs.getInt(8);
rowid = rs.getString(9);
// Applicazione della business logic
if (reparto == 30)
{ System.out.println (cognome + " in dept= "+ reparto +
" with salary=" + salario + " has a commission= " +
commissioni);
int newcomm = 5555;
ps.setInt(1,newcomm);
ps.setString(2,rowid);
ps.executeUpdate();
else
{ System.out.println (cognome + " in dept= "+ reparto +
" with salary=" + salario + " has a commission= " + commissioni);
} // if - else
} // end while
rs.close();
ps.close();
stmt.close();
catch(Exception e)
e.printStackTrace();
} // end main()
} // end class
//PS.
//Many thanks to Bachar and Elangovan that ansewerd me to my previous posting and addressed me towards the right solution

Hi
The documentation gives following explanation for the error :
ORA-01002 fetch out of sequence
Cause: In a host language program, a FETCH call was issued out of sequence. A successful parse-and-execute call must be issued before a fetch. This can occur if an attempt was made to FETCH from an active set after all records have been fetched. This may be caused by fetching from a SELECT FOR UPDATE cursor after a commit. A PL/SQL cursor loop implicitly does fetches and may also cause this error.
Action: Parse and execute a SQL statement before attempting to fetch the data.
In your program you should set auto commit to false as follows :
con.setAutoCommit(false);
Do this before executing the SELECT FOR UPDATE sql query.
At the end of program you can commit to save the updations as follows:
con.commit();
This should solve the problem.
Chandar

Difference in select for update of - in Oracle Database 10g and 11g

Hi, I found out that Oracle Database 10g and 11g treat the following PL/SQL block differently (I am using scott schema for convenience):
DECLARE
  v_ename bonus.ename%TYPE;
BEGIN
  SELECT b.ename
    INTO v_ename
    FROM bonus b
    JOIN emp e ON b.ename = e.ename
    JOIN dept d ON d.deptno = e.deptno
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     FOR UPDATE OF b.ename;
END;
/While in 10g (10.2) this code ends successfully (well NO_DATA_FOUND exception is raised but that is expected), in 11g (11.2) it raises exception "column ambiguously defined". And that is definitely not expected. It seems like it does not take into account table alias because I found out that when I change the column in FOR UPDATE OF e.empno (also does not work) to e.mgr (which is unique) it starts working. So is this some error in 11g? Any thoughts?
Edited by: Libor Nenadál on 29.4.2010 21:46
It seems that my question was answered here - http://stackoverflow.com/questions/2736426/difference-in-select-for-update-of-in-oracle-database-10g-and-11g

The behaviour seems like it really is a bug and can be avoided using non-ANSI syntax. (It makes me wonder why Oracle maintains two query languages while dumb me thinks that this is just a preprocessor matter and query engine could be the same).

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.

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[Forms6]Select for update before update and commit

Hi,
In the KEY-COMMIT forms trigger
My code finds the primary keys of the rows to be updated,
then there is "select ...where "these keys".. for update nowait"
then there is a commit
(There also is a pragma exception init -54 "Resource Busy")
At runtime I get an error:
"Cannot insert rows here".
Maybe I should lock these rows before triggering the Key-commit?
--> message edited
Oops, I just reproduced the code in Forms 10,
I select for update from emp where empno=7839 then update King's sal then commit_form;
and everything works fine.
So I have to locate where this error message comes from.
--< End of edit
Many thanks for your help.
Edited by: JeanParis on Dec 23, 2009 10:31 PM

Hi,
The issue is solved on my Developer 2000 10 installed on Windows XP.
At work on Forms 6 I have hidden the warning with a :system.message_level.
What I'm searching now is a script to check the locks in the situation where
I don't have access to the catblock.sql script because I'm not DBA on the database. (but I can see DBA_LOCKS).
The problem starts because on the master block there is an item that can ben changed
from 'CREAT' to 'VALID', and in the When Validate Item trigger on that item I cannot issue a go_block( 'xxx' ).
I have to go to the 'xxx' block in order to start a first record - next loop to gather values and update a table accordingly.
Many thanks for your care.
JBM.

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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

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.

Java.sql.SQLSyntaxErrorException: ORA-02014: cannot select FOR UPDATE from

Experts,
I am getting "java.sql.SQLSyntaxErrorException: ORA-02014: cannot select FOR UPDATE from view with DISTINCT, GROUP BY, etc" exception and it leads to one or more setter methods of the VO. I checked the respective attribute in the VO to see if it refers to any LOV with sql having Distinct or group by clause, but it is a normal attribute.
Any idea how do i resolve this error ?
Error Stack::
java.sql.SQLSyntaxErrorException: ORA-02014: cannot select FOR UPDATE from view with DISTINCT, GROUP BY, etc.
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     at org.apache.myfaces.trinidad.component.UIXComponentBase.invokeOnComponent(UIXComponentBase.java:1424)
     at org.apache.myfaces.trinidad.component.UIXComponentBase.invokeOnChildrenComponents(UIXComponentBase.java:1330)
     at org.apache.myfaces.trinidad.component.UIXComponentBase.invokeOnComponent(UIXComponentBase.java:1424)
     at org.apache.myfaces.trinidad.component.UIXComponentBase.invokeOnChildrenComponents(UIXComponentBase.java:1330)
     at oracle.adf.view.rich.component.fragment.UIXInclude.invokeOnComponent(UIXInclude.java:147)
     at org.apache.myfaces.trinidad.component.UIXComponentBase.invokeOnChildrenComponents(UIXComponentBase.java:1330)
     at org.apache.myfaces.trinidad.component.UIXComponentBase.invokeOnComponent(UIXComponentBase.java:1424)
     at org.apache.myfaces.trinidad.component.UIXComponentBase.invokeOnChildrenComponents(UIXComponentBase.java:1330)
     at org.apache.myfaces.trinidad.component.UIXComponentBase.invokeOnComponent(UIXComponentBase.java:1424)
     at oracle.adf.view.rich.component.rich.RichDocument.invokeOnComponent(RichDocument.java:168)
     at javax.faces.component.UIComponent.invokeOnComponent(UIComponent.java:720)
     at javax.faces.component.UIComponentBase.invokeOnComponent(UIComponentBase.java:678)
     at oracle.adfinternal.view.faces.lifecycle.LifecycleImpl._executePhase(LifecycleImpl.java:334)
     at oracle.adfinternal.view.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:186)
     at javax.faces.webapp.FacesServlet.service(FacesServlet.java:265)
     at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:227)
     at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:125)
     at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:300)
     at weblogic.servlet.internal.TailFilter.doFilter(TailFilter.java:26)
     at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
     at oracle.adf.model.servlet.ADFBindingFilter.doFilter(ADFBindingFilter.java:205)
     at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
     at oracle.adfinternal.view.faces.webapp.rich.RegistrationFilter.doFilter(RegistrationFilter.java:106)
     at org.apache.myfaces.trinidadinternal.webapp.TrinidadFilterImpl$FilterListChain.doFilter(TrinidadFilterImpl.java:446)
     at oracle.adfinternal.view.faces.activedata.AdsFilter.doFilter(AdsFilter.java:60)
     at org.apache.myfaces.trinidadinternal.webapp.TrinidadFilterImpl$FilterListChain.doFilter(TrinidadFilterImpl.java:446)
     at org.apache.myfaces.trinidadinternal.webapp.TrinidadFilterImpl._doFilterImpl(TrinidadFilterImpl.java:271)
     at org.apache.myfaces.trinidadinternal.webapp.TrinidadFilterImpl.doFilter(TrinidadFilterImpl.java:177)
     at org.apache.myfaces.trinidad.webapp.TrinidadFilter.doFilter(TrinidadFilter.java:92)
     at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
     at com.awRostamani.leadWeb.ui.filter.SecurityCheckFilter.doFilter(SecurityCheckFilter.java:80)
     at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
     at oracle.adf.library.webapp.LibraryFilter.doFilter(LibraryFilter.java:175)
     at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
     at oracle.security.jps.ee.http.JpsAbsFilter$1.run(JpsAbsFilter.java:111)
     at oracle.security.jps.util.JpsSubject.doAsPrivileged(JpsSubject.java:313)
     at oracle.security.jps.ee.util.JpsPlatformUtil.runJaasMode(JpsPlatformUtil.java:413)
     at oracle.security.jps.ee.http.JpsAbsFilter.runJaasMode(JpsAbsFilter.java:94)
     at oracle.security.jps.ee.http.JpsAbsFilter.doFilter(JpsAbsFilter.java:161)
     at oracle.security.jps.ee.http.JpsFilter.doFilter(JpsFilter.java:71)
     at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
     at oracle.dms.servlet.DMSServletFilter.doFilter(DMSServletFilter.java:136)
     at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
     at weblogic.servlet.internal.RequestEventsFilter.doFilter(RequestEventsFilter.java:27)
     at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
     at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.wrapRun(WebAppServletContext.java:3715)
     at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.run(WebAppServletContext.java:3681)
     at weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.java:321)
     at weblogic.security.service.SecurityManager.runAs(SecurityManager.java:120)
     at weblogic.servlet.internal.WebAppServletContext.securedExecute(WebAppServletContext.java:2277)
     at weblogic.servlet.internal.WebAppServletContext.execute(WebAppServletContext.java:2183)
     at weblogic.servlet.internal.ServletRequestImpl.run(ServletRequestImpl.java:1454)
     at weblogic.work.ExecuteThread.execute(ExecuteThread.java:209)
     at weblogic.work.ExecuteThread.run(ExecuteThread.java:178)
thnks

When trying to commit from business tester, then also the following error is coming
Jdev 11.1.1.5 - locking mode is optimistic.
OracleSQLBuilderImpl.doEntitySelect failed...
java.sql.SQLSyntaxErrorException: ORA-02014: cannot select FOR UPDATE from view with DISTINCT, GROUP BY, etc.
     at oracle.jdbc.driver.T4CTTIoer.processError(T4CTTIoer.java:457)
     at oracle.jdbc.driver.T4CTTIoer.processError(T4CTTIoer.java:405)
     at oracle.jdbc.driver.T4C8Oall.processError(T4C8Oall.java:889)
     at oracle.jdbc.driver.T4CTTIfun.receive(T4CTTIfun.java:476)
     at oracle.jdbc.driver.T4CTTIfun.doRPC(T4CTTIfun.java:204)
     at oracle.jdbc.driver.T4C8Oall.doOALL(T4C8Oall.java:540)
     at oracle.jdbc.driver.T4CPreparedStatement.doOall8(T4CPreparedStatement.java:217)
     at oracle.jdbc.driver.T4CPreparedStatement.executeForDescribe(T4CPreparedStatement.java:924)
     at oracle.jdbc.driver.OracleStatement.executeMaybeDescribe(OracleStatement.java:1261)
     at oracle.jdbc.driver.OracleStatement.doExecuteWithTimeout(OracleStatement.java:1419)
     at oracle.jdbc.driver.OraclePreparedStatement.executeInternal(OraclePreparedStatement.java:3752)
     at oracle.jdbc.driver.OraclePreparedStatement.executeQuery(OraclePreparedStatement.java:3806)
     at oracle.jdbc.driver.OraclePreparedStatementWrapper.executeQuery(OraclePreparedStatementWrapper.java:1667)
     at oracle.jbo.server.OracleSQLBuilderImpl.doEntitySelectForAltKey(OracleSQLBuilderImpl.java:869)
     at oracle.jbo.server.BaseSQLBuilderImpl.doEntitySelect(BaseSQLBuilderImpl.java:553)
     at oracle.jbo.server.EntityImpl.doSelect(EntityImpl.java:8259)
     at oracle.jbo.server.EntityImpl.lock(EntityImpl.java:5964)
     at oracle.jbo.server.EntityImpl.beforePost(EntityImpl.java:6484)
     at oracle.jbo.server.EntityImpl.postChanges(EntityImpl.java:6665)
     at oracle.jbo.server.DBTransactionImpl.doPostTransactionListeners(DBTransactionImpl.java:3286)
     at oracle.jbo.server.DBTransactionImpl.postChanges(DBTransactionImpl.java:3089)
     at oracle.jbo.server.DBTransactionImpl.commitInternal(DBTransactionImpl.java:2093)
     at oracle.jbo.server.DBTransactionImpl.commit(DBTransactionImpl.java:2374)
     at oracle.adf.model.bc4j.DCJboDataControl.commitTransaction(DCJboDataControl.java:1608)
     at oracle.adf.model.binding.DCDataControl.callCommitTransaction(DCDataControl.java:1416)
     at oracle.jbo.uicli.binding.JUCtrlActionBinding.doIt(JUCtrlActionBinding.java:1437)
     at oracle.adf.model.binding.DCDataControl.invokeOperation(DCDataControl.java:2149)
     at oracle.jbo.uicli.binding.JUCtrlActionBinding.invoke(JUCtrlActionBinding.java:740)
     at oracle.jbo.uicli.jui.JUActionBinding.actionPerformed(JUActionBinding.java:193)
     at oracle.jbo.uicli.controls.JUNavigationBar.doAction(JUNavigationBar.java:412)
     at oracle.jbo.jbotester.NavigationBar.doAction(NavigationBar.java:111)
     at oracle.jbo.uicli.controls.JUNavigationBar$NavButton.actionPerformed(JUNavigationBar.java:118)
     at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
     at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
     at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
     at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
     at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
     at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:272)
     at java.awt.Component.processMouseEvent(Component.java:6289)
     at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
     at java.awt.Component.processEvent(Component.java:6054)
     at java.awt.Container.processEvent(Container.java:2041)
     at java.awt.Component.dispatchEventImpl(Component.java:4652)
     at java.awt.Container.dispatchEventImpl(Container.java:2099)
     at java.awt.Component.dispatchEvent(Component.java:4482)
     at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
     at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
     at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
     at java.awt.Container.dispatchEventImpl(Container.java:2085)
     at java.awt.Window.dispatchEventImpl(Window.java:2478)
     at java.awt.Component.dispatchEvent(Component.java:4482)
     at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:644)
     at java.awt.EventQueue.access$000(EventQueue.java:85)
     at java.awt.EventQueue$1.run(EventQueue.java:603)
     at java.awt.EventQueue$1.run(EventQueue.java:601)
     at java.security.AccessController.doPrivileged(Native Method)
     at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
     at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
     at java.awt.EventQueue$2.run(EventQueue.java:617)
     at java.awt.EventQueue$2.run(EventQueue.java:615)
     at java.security.AccessController.doPrivileged(Native Method)
     at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
     at java.awt.EventQueue.dispatchEvent(EventQueue.java:614)
     at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
     at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
     at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
     at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
     at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
     at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
[348] JUErrorHandlerDlg.reportException(oracle.jbo.DMLException)
[349] DBG: afterActionPerformed :javax_swing_JToolBara1_175
Edited by: in the line of fire on Jul 12, 2011 2:54 PM

"SELECT FOR UPDATE" genrating large redo

Hi,
Is there any obvious reason why a "SELECT FOR UPDATE" would cause massive amount of redo to be generated?
Derek

1. "SELECT FOR UPDATE" itself should modify block to record row level lock information. This can cause redo data. See following:
-- Create test table
SQL> create table for_update_test(id int);
Table created.
SQL> insert into for_update_test values(1);
1 row created.
SQL> commit;
Commit complete.
-- Get DBA
SQL> select dbms_rowid.rowid_relative_fno(rowid) as fno,
2 dbms_rowid.rowid_block_number(rowid) as blk_no
3 from for_update_test;
FNO BLK_NO
4 41980
-- Execute "Select for Update..."
SQL> connect user/password;
SQL> select * from for_update_test for update;
ID
1
-- Get Transaction ID
SQL> select xidusn, xidslot, xidsqn from v$transaction
where ses_addr = (select saddr from v$session where sid =
(select sid from v$mystat where rownum = 1));
SQL> /
XIDUSN XIDSLOT XIDSQN
2 24 24009
-- Dump corresponding block to identify which has been changed
SQL> alter system dump datafile 4 block 41980;
Block header dump: 0x0100a3fc
Object id on Block? Y
seg/obj: 0x1013b csc: 0x7df.1847891a itc: 2 flg: E typ: 1 - DATA
brn: 0 bdba: 0x100a3f9 ver: 0x01 opc: 0
inc: 0 exflg: 0
Itl Xid Uba Flag Lck Scn/Fsc
0x01 0x0004.023.00004719 0x00800b41.1359.02 C--- 0 scn 0x07df.184788fe
0x02 0x0002.018.00005dc9 0x00800c54.0eef.33 ---- 1 fsc 0x0000.00000000
data_block_dump,data header at 0x6682464
===============
tsiz: 0x1f98
hsiz: 0x14
pbl: 0x06682464
bdba: 0x0100a3fc
76543210
flag=--------
ntab=1
nrow=1
frre=-1
fsbo=0x14
fseo=0x1f92
avsp=0x1f7b
tosp=0x1f7b
0xe:pti[0] nrow=1 offs=0
0x12:pri[0] offs=0x1f92
block_row_dump:
tab 0, row 0, @0x1f92
tl: 6 fb: H-FL lb: 0x2 cc: 1
col 0: [ 2] c1 02
end_of_block_dump
End dump data blocks tsn: 4 file#: 4 minblk 41980 maxblk 41980
Here you can see that the row's lock byte is "2" and the corresponding ITL entry shows that the lock owner is the session that executed "Select for update...".
So, "Select for update" does really update block and generate redo for that change.
2. Note that even "Select " statement can generate redo data. This is caused by "delayed block cleanout". Google it for more information.

Should i use SELECT for update NOWAIT ?

Hi:
Do I need to use, in my pl/sql triggers and procedures, the SELECT FOR UPDATE NOWAIT sentence, to avoid locks before using update table sentences ? Is it common to use it on stored procedures and triggers?
Thanks
Joao Oliveira

First, what, exactly do you mean by "avoid locks"? I was interpreting that to mean "I want to avoid creating locks in my session that might block someone else", not "I want to avoid having my SELECT wait for locks to be released-- I want it to fail immediately". If you meant the latter, then SELECT ... FOR UPDATE NOWAIT would be what you want. If you meant the former, then pessimistic locking is not what you want.
Second, what sort of Oracle Forms architecture do you have? Are you still using old-school client-server applications? Or are you using a three-tiered approach? As Tom discusses in that thread, pessimistic locking is only an option when your client application is able to maintain database state across calls (i.e. client/server systems) not when you have stateless connections (which is the norm in the three-tier model). The old client-server versions of Forms would automatically and transparently do pessimistic locking. Since you didn't mention anything about your architecture, most of us probably assumed the more common stateless client architecture (note how Tom's answers progress over the 5 years in that thread as client/server architecture became less and less common).
Third, while your question is appropriate for either the Database - General forum or the SQL and PL/SQL forum, that generally means that you are free to post it either forum, not that it should be posted in both. The vast majority of the folks that hang out in one forum hang out in the other. It's also rather frustrating to answer a post in one forum only to discover that there is another post in a different forum where someone else had already covered the same points half an hour earlier or to discover that there was additional information in another thread that might have changed your answer.
Fourth, if you are going to do pessimistic locking, that requires that you are able to maintain state across various database calls, that you are locking on the lowest possible level of granularity, and that you are able to time out sessions relatively aggressively to ensure that someone doesn't open a record, thereby locking it, go to lunch (or have their system die) and then block everyone else from working. Assuming that is the case, and that you have some reasonable way to handle the error that gets generated other than simply retrying the operation, adding NOWAIT is certainly an option. Most applications, particularly those getting written today, cannot guarantee all these things, so pessimistic locking is generally not appropriate there.
Looking at your other thread (where there is new information that would be useful in this discussion, one of the reasons that multiple threads are generally a bad idea), it seems that you have an ERP application and you are concerned about the performance of entering orders. Obviously, there shouldn't be any locking issues on the ORDER or ORDER_DETAILS tables, assuming that multiple users aren't going to be inserting the same order at the same time. The contention would almost certainly come when multiple orders are trying to update the STOCK and INVENTORY tables, since multiple orders presumably rely on the same rows in those tables. In that case, I'm not sure what adding a NOWAIT would buy you-- unless you were going to roll back the entire order because someone is updating the STOCK row for #2 pencils and your order has an item of #2 pencils, you'd have to keep retrying the operation until you were able to modify the STOCK row, which would be less efficient than just letting that update block until the row was free.
Now, you could certainly redesign the application to minimize that contention by not trying to update what I assume are aggregate tables like STOCK and INVENTORY directly as part of your OLTP processing or, at least, by minimizing the time that you're locking a row. You could, for example, make STOCK and INVENTORY materialized views rather than tables that refresh ON COMMIT, which should decrease the time that your locks are held. You could also have those tables refreshed asynchronously, which would be even more efficient but may require that you reasses your holdback requirements.
Justin

Jdbc mysql "select ... for update" and isolation level...

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