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From "Patrick Linskey (JIRA)" <j...@apache.org>
Subject [jira] Commented: (OPENJPA-182) db2 update lock syntax WITH <isolation> USE AND KEEP UPDATE LOCKS
Date Thu, 05 Apr 2007 19:55:32 GMT

    [ https://issues.apache.org/jira/browse/OPENJPA-182?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12487086
] 

Patrick Linskey commented on OPENJPA-182:
-----------------------------------------

> > I know that Oracle allows you to add a FOR UPDATE clause 
> > to a query, and this affects the results of that query. In Sun 
> > appserver CMP we use this to set exclusive locks on rows 
> > where we want pessimistic locking behavior just for certain tables.
> 
> Again, this is exactly what our existing lock levels and APIs do.

The current patches in this issue encompass two separate bits of functionality:

1. an update-override setting. (In the context of my patch, I'm not convinced that this is
necessary, since I suspect that the code that calls toSelect() might set the forUpdate boolean
based on the values in the JDBCFetchConfiguration anyways.)

2. an isolation-override setting.

Currently, this patch implements all of this only in the DB2Dictionary. As of right now, the
first feature is something that is useful for all sorts of databases, via syntax like Oracle's
SELECT ... FOR UPDATE. However, we only know how to implement the second feature for DB2,
and not for any other database. In Oracle, "ALTER SESSION" can be used to change the isolation
level of a given session, but I'm not sure of the semantics of this operation. I believe that
Abe's question is: Do other databases (Sybase? Derby?) also have semantics for changing the
isolation level of a particular query?

> db2 update lock syntax  WITH <isolation> USE AND KEEP UPDATE LOCKS
> ------------------------------------------------------------------
>
>                 Key: OPENJPA-182
>                 URL: https://issues.apache.org/jira/browse/OPENJPA-182
>             Project: OpenJPA
>          Issue Type: New Feature
>          Components: jdbc
>         Environment: db2 database driver for zOS, AS400, Unix, Windows, Linux
>            Reporter: David Wisneski
>         Assigned To: David Wisneski
>         Attachments: OPENJPA-182.patch, openJPA182.patch
>
>
> A while back we changed the syntax of update locking from FOR UPDATE OF  to  WITH RS
USE AND KEEP UPDATE LOCKS.   Additional changes are required because 
> 1.  if isolation=serializable is configured, then the syntax should be  WITH RR USE AND
KEEP UDPATE LOCKS
> 2.  when using DB2/400 on iSeries machines, the syntax is WITH RS USE AND KEEP EXCLUSIVE
LOCKS  or WITH RR USE AND KEEP EXCLUSIVE LOCKS because DB2/400 only supports read or exclusive
locks. 
> 3.  DB2 supports both a FETCH FIRST  ROWS and update LOCKS clauses.
> So we change supportsLockingWithSelectRange = true in the AbstractDB2Dictionary class
and change the DB2Dictionary to append the correct LOCKS syntax depending on vendor, release
and isolation level.

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