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https://issues.apache.org/jira/browse/DERBY-3900?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12650437#action_12650437
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Dag H. Wanvik commented on DERBY-3900:
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You are right of course, that only using WITH RS could cause more deadlocks
(although it would solve
the basic integrity issue of another transaction "manipulating the data while
we are working with it"). So having
non-sharabled locks a priori could be useful for some apps. A global switch
would not add any non-standard
syntax but isn't very flexible. SQL doesn't deal with locks, only isolation
level of course.. How do you see the switch working?
> SELECT ... FOR UPDATE cannot be used in many queries
> ----------------------------------------------------
>
> Key: DERBY-3900
> URL: https://issues.apache.org/jira/browse/DERBY-3900
> Project: Derby
> Issue Type: Improvement
> Components: SQL
> Reporter: Marco
>
> The documentation in
> http://db.apache.org/derby/docs/10.4/ref/rrefsqlj41360.html#rrefsqlj41360__sqlj15384
> says that SELECT ... FOR UPDATE cannot be used in many situations (e.g. when
> ORDER BY is present or when JOINs are used). I can very well understand that
> the current implementation using updatable cursors is very hard to implement
> when multiple tables are used and therefore these restrictions are probably
> necessary.
> However, besides that functionality, "FOR UPDATE" is extremely useful for
> transactional integrity: For example, we - http://www.jfire.org - use
> transaction isolation level read committed, because it provides good
> transaction safety combined with good performance. When modifying records, we
> first select the appropriate table rows with a SELECT FOR UPDATE in order to
> guarantee that the data we just read cannot be manipulated by another
> transaction while we are working with it.
> I do not see any reason why this locking behaviour should not be possible for
> certain queries. Therefore, I recommend to introduce a configuration setting
> (maybe a system property? or an option passed to the JDBC-URL?) that disables
> updatable queries completely (we don't need them anyway and probably it
> improves performance when not using them). With this option set, the SELECT
> ... FOR UPDATE should solely affect locks on rows - and work with all SELECT
> expressions - no matter whether they use JOIN, UNION, ORDER BY etc..
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