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Date: Fri, 1 Jun 2007 10:15:15 -0700 (PDT)
From: "Gokhan Ergul (JIRA)" <jira@apache.org>
To: open-jpa-dev@incubator.apache.org
Subject: [jira] Commented: (OPENJPA-235) SQL reordering to avoid
 non-nullable foreign key constraint violations
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    [ https://issues.apache.org/jira/browse/OPENJPA-235?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12500765 ] 

Gokhan Ergul commented on OPENJPA-235:
--------------------------------------

Marcus,

I'm not familiar with the specifics of Reece's patch (yet), but to answer your question from a theoretical point of view: As your example demonstrates, not-nullable FKs are not the only reason for such dependencies. The real issue with ordering statements is not about nullable/not-nullable constraints, but rather with whether the target database supports deferred constraints or not (constraints enforced at transaction commit time, as opposed constraints enforced after each statement). If the target database supports deferred constraints, we'd have no need to reorder statements --assuming that resulting set of rows (or lack of them) at transaction commit do not violate any constraints. Otherwise the transaction will fail anyway, irrespective of if/how you order the statements. 

On a related note, since you have access to TCK tests, is there any chance you can apply the patch I posted to see if it breaks anything? If we could take that detached testcase out of the picture I'll go ahead mark OPENJPA-231 resolved, since the issue with attached testcase is the same with this one. 

Thanks,

Gokhan.


> SQL reordering to avoid non-nullable foreign key constraint violations
> ----------------------------------------------------------------------
>
>                 Key: OPENJPA-235
>                 URL: https://issues.apache.org/jira/browse/OPENJPA-235
>             Project: OpenJPA
>          Issue Type: Improvement
>          Components: kernel
>            Reporter: Reece Garrett
>            Assignee: Patrick Linskey
>             Fix For: 0.9.8
>
>         Attachments: merge-detached.patch, merge-multigen-collection-testcase.zip, openjpa-235-test.jar, openjpa-235-test1.jar, sqlreorder.patch, sqlReorder2.patch
>
>
> OpenJPA does not do any SQL statement re-ordering in order to resolve foreign key constraints. Instead, objects are always inserted in the order in which the user persists the instances.  When you persist in an order that would violate foreign key constraints, OpenJPA attempts to insert null and then update the foreign key value in a separate statement. If you use non-nullable constraints, though, you must persist your objects in the correct order.
> This improvement re-orders SQL statements as follows:
> 1. First, all insert statements execute. Inserts which have foreign keys with non-nullable constraints execute AFTER the foreign keys which they depend on have been inserted since no deferred update is possible.
> 2. Next, all update statements execute. No reordering is necessary.
> 3.  Finally, all delete statements execute. Like inserts, deletes execute in an order which does not violate non-nullable foreign key constraints.
> If a circular foreign key reference is found during the re-ordering process then re-ordering halts and the remaining unordered statements are left as is. There is nothing that can be done about the circular reference (other than fixing the schema) and the resulting SQL statements will not succeed.
> The net effect is that users do not need to worry about the persistence order of their objects regardless of non-nullable foreign key constraints. The only class modified was org.apache.openjpa.jdbc.kernel.OperationOrderUpdateManager. I have included a patch which includes my modifications to OperationOrderUpdateManager and test cases. The test cases I have provided fail on the current trunk but pass with my modifications. I have also verified that I did not break anything by using maven to run all test cases with my modifications in place.

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