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https://issues.apache.org/jira/browse/OPENJPA-235?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12502561
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Craig Russell commented on OPENJPA-235:
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> Would it be sufficient to just always run the deletes first, then the
> updates, and finally the inserts, when there are unique constraints involved?
> Assuming that we optimize away any SQL operations for
> insert-then-delete-before-flush, it would seem that that algorithm would be
> guaranteed to work when no constraint violations were generated in the
> business code, which I think is all that we can really strive for in the case
> of unique constraints.
This algorithm isn't quite enough. Consider deleting an instance that has a
reference from an instance that is being updated to nullify that reference.
Then you want to do the update to nullify the reference before you do the
delete.
I think the general issue is that due to uniqueness and referential integrity
constraints you need to create a dependency graph and then order the operations
to preserve the dependencies. But you also want to preserve the order the user
made the changes to avoid deadlocks due to locking dependencies. And you also
want to reorder operations to take advantage of statement batching.
> 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, openjpa-235-test2.zip, sqlreorder.patch,
> sqlReorder2.patch, sqlReorderTests.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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