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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-532) Support deferrable constraints
Date Wed, 13 Nov 2013 23:27:22 GMT

    [ https://issues.apache.org/jira/browse/DERBY-532?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13821980#comment-13821980
] 

Mike Matrigali commented on DERBY-532:
--------------------------------------

As part of trying to understand deferred constraints I did a bit of searching on the Web.
 And one thing that kept popping up is a lot of experts recommending that database designers
not pick deferred constraints, if at all possible.  One of the main reasons for this is that
there was consensus that many if not most applications do not code for errors coming out of
a "commit".   I believe the standard has always allowed for commit to fail,
but previous to this I think in derby the only thing that would make commit fail would be
some sort of server
crashing even like a low level bug, or the transaction logging system running out resources
to write the 
commit log record.  

So I suggest that we highlight in the documentation for this project and for the individual
new ddl that using this feature will call for applications to have error handling for commit
to fail.  This may be new for many applications.  Probably would be good to show some sample
code on how to handle a deferred unique constraint failing at commit time.

> Support deferrable constraints
> ------------------------------
>
>                 Key: DERBY-532
>                 URL: https://issues.apache.org/jira/browse/DERBY-532
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>            Reporter: Jörg von Frantzius
>            Assignee: Dag H. Wanvik
>              Labels: derby_triage10_11
>         Attachments: deferredConstraints.html, deferredConstraints.html, deferredConstraints.html,
deferredConstraints.html, derby-532-import-1.diff, derby-532-import-1.status, derby-532-import-2.diff,
derby-532-import-3.diff, derby-532-import-3.status, derby-532-more-tests-1.diff, derby-532-more-tests-1.stat,
derby-532-serializable-scan-1.diff, derby-532-serializable-scan-2.diff, derby-532-serializable-scan-2.stat,
derby-532-syntax-binding-dict-1.diff, derby-532-syntax-binding-dict-1.status, derby-532-syntax-binding-dict-2.diff,
derby-532-syntax-binding-dict-2.status, derby-532-syntax-binding-dict-all-1.diff, derby-532-testAlterConstraintInvalidation.diff,
derby-532-testAlterConstraintInvalidation.status, derby-532-unique-pk-1.diff, derby-532-unique-pk-1.status,
derby-532-unique-pk-2.diff, derby-532-unique-pk-3.diff, derby-532-unique-pk-3.status, derby-532-xa-1.diff,
derby-532-xa-2.diff, derby-532-xa-3.diff, derby-532-xa-3.status
>
>
> In many situations it is desirable to have constraints checking taking place only at
transaction commit time, and not before. If e.g. there is a chain of foreign key constraints
between tables, insert statements have to be ordered to avoid constraint violations. If foreign
key references are circular, the DML has to be split into insert statements and subsequent
update statements by the user.
> In other words, with deferred constraints checking, life is much easier for the user.
Also it can create problems with softwares such as object-relational mapping tools that are
not prepared for statement ordering and thus depend on deferred constraints checking.



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