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From "Dag H. Wanvik (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3900) SELECT ... FOR UPDATE cannot be used in many queries
Date Tue, 25 Nov 2008 01:36:44 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3900?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12650437#action_12650437

Dag H. Wanvik commented on DERBY-3900:

You are right of course, that only using WITH RS could cause more deadlocks (although it would
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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