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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3900) SELECT ... FOR UPDATE cannot be used in many queries
Date Mon, 02 Feb 2009 10:06:00 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3900?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12669544#action_12669544
] 

Knut Anders Hatlen commented on DERBY-3900:
-------------------------------------------

I found this comment in CursorNode.determineUpdateMode():

		// The FOR UPDATE clause has two uses:
		//
		// for positioned cursor updates
		//
		// to change locking behaviour of the select
		// to reduce deadlocks on subsequent updates
		// in the same transaction.
		//
		// We now support this latter case, without requiring
		// that the source of the rows be able to implement
		// a positioned update.

So it seems there is (or has been) support in the code for getting this locking behaviour
in statements that don't allow positioned updates.

> 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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