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From "Kristian Waagan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3900) SELECT ... FOR UPDATE cannot be used in many queries
Date Mon, 24 Nov 2008 08:03:44 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3900?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12650132#action_12650132
] 

Kristian Waagan commented on DERBY-3900:
----------------------------------------

I agree with Knut Anders, there are scenarios where using the SELECT ... FOR UPDATE with more
complex queries would be beneficial.
The benchmark SPECjAppServer2004  is one such example, and it seems the reporter also has
a concrete example.

We should probably read up on the standards, and try to introduce the extended functionality
in a way that doesn't break standards compliance (having a switch has already been suggested).

> 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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