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From "Reece Garrett (JIRA)" <j...@apache.org>
Subject [jira] Commented: (OPENJPA-235) SQL reordering to avoid non-nullable foreign key constraint violations
Date Thu, 07 Jun 2007 21:32:25 GMT

    [ https://issues.apache.org/jira/browse/OPENJPA-235?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12502535
] 

Reece Garrett commented on OPENJPA-235:
---------------------------------------

Hello Patrick,

Your patch works great for foreign keys but I wrote my patch in anticipation of adding uniqueness
constraint dependencies. Markus is correct, when you consider uniqueness constraints an insert
can depend on an update or a delete and an update can depend on another update or a delete.

Say you have an insert which depends on a delete because of a uniqueness constraint. The delete
should get ordered first. However, the delete was originally generated because there is an
update either nulling or replacing  a foreign key reference to it . If the delete runs before
the update a foreign key violation will occur because the row being deleted has not been dereferenced
by the update yet. Upon seeing that the delete precedes the update which it depends on the
code must update the offending foreign key to null before the delete executes (this assumes
the foreign key is nullable, if not then you're out of luck).

The code you committed does not know about the dependency between the delete and the update
and thus can not handle the above situation. In fact, without having my list of dereferenced
states for each state manager I do not see how the dependency can be calculated. 

Other than that, the code looks great. My test cases pass and I will attach them as a patch
("sqlReorderTests.patch"). 

Any input you have on the uniqueness constraint issue would be much appreciated because that's
the last piece to this puzzle. Secretly I'm hoping you guys will tackle this one because it
doesn't look too hard and will give you guys (another) advantage over the competition. I know
I can do it but it will require going a bit deeper into the code than I feel totally comfortable
with. For all updates and deletes I'd have to store the original values (as they stood at
the beginning of the transaction) for columns involved in uniqueness constraints. Then, when
calculating dependencies for inserts and updates where uniqueness constraints are involved
I'd check all updates and deletes going to the same table and see if they need to be run first
to prevent a unique key violation. Like I said, it shouldn't be rocket science as all the
data needed to do this is pulled from the database; just not stored for later access. Updates
have some of these original values stored for rollback but deletes don't store any of the
original values except the primary key and version. 

Basically, if there was a method "RowImpl.getOriginal(Column col)" that works just like the
existing method "RowImpl.getSet(Column col)"  but returns the original value for the specified
column then it would be smooth sailing from there.

-Reece

> 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
>
>
> 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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