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From Oyvind.Bakk...@Sun.COM
Subject Re: [jira] Commented: (DERBY-231) "FOR UPDATE" required for updatable result set to work
Date Fri, 28 Oct 2005 13:44:44 GMT
Dag H. Wanvik wrote:
> Hi,
> 
> Andreas> >   2) Applications use the FOR UPDATE clause to control locking for
> Andreas> > future updates with read only ResultSets.
> Andreas> >
> Andreas> 
> Andreas> Note currently it throws an exception if the statement is not updatable 
> Andreas> i.e contains a join or order by.
> 
> I guess what you mean here is that the FOR UPDATE is not in general
> available as a means for locking for future updates.
> 
> To Dan's point, my tests indicate that the current Derby
> implementation for forward-only updatable result sets only sets a row
> update lock while on the current row.
> 
> In contrast, Oracle's "FOR UPDATE" places locks on the entire result
> set for the duration of the transaction (see below). The usefulness as
> a way to control locking would be more useful if the Derby locking was
> closer to what Oracle does, at the expense of concurrency.
> 
> Dag

Just a little pick at the wording... What's "useful" behaviour depends 
on the application and its needs. If you don't need update locks on the 
entire result set, the "usefulness" of such a behaviour is negative, 
since it only reduces concurrency and, as such, overall performance.

-- 
Oyvind Bakksjo
Sun Microsystems, Database Technology Group
Trondheim, Norway
http://weblogs.java.net/blog/bakksjo/

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