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https://issues.apache.org/jira/browse/DERBY-5443?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13226409#comment-13226409
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Mike Matrigali commented on DERBY-5443:
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Do you know if the updates for sequences always are fixed length changes to the
catalog row?
> reduce number of times sequence updater does it work on user thread rather
> than nested user thread.
> ---------------------------------------------------------------------------------------------------
>
> Key: DERBY-5443
> URL: https://issues.apache.org/jira/browse/DERBY-5443
> Project: Derby
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 10.9.0.0
> Reporter: Mike Matrigali
> Priority: Minor
> Attachments: blockingDDL.sql
>
>
> Currently the Sequence updater tries to do the system catalog update as part
> of the user thread, but in a nested user transaction. When this works
> all is well as the nested user transaction is immediately committed and thus
> the throughput of all threads depending on allocating sequences is
> optimized.
> In order to be able to commit the nested writable transaction independently
> the lock manager must treat the parent and nested transactions as two
> independent transactions and locks held by the parent will thus block the
> child. And in effect any lock that is blocked by the parent is a deadlock,
> but the lock manager does not understand this relationship and thus only will
> timeout and not recognize the implicit deadlock.
> Only 2 cases come to mind of the parent blocking the child in this manner for
> sequences:
> 1) ddl like create done in transaction followed by inserts into the table
> requiring sequence update.
> 2) users doing jdbc data dictionary lookups in a multistatment transaction
> resulting in holding locks on the system catalog rows and subsequently
> doing inserts into the table requiring sequence updates.
> The sequence updater currently never waits for a lock in the nested
> transaction and assumes any blocked lock is this parent deadlock case. It
> then falls back on doing the update in tranaction and then the system catalog
> lock remains until the user transaction commits which could then
> hold hostage all other inserts into the table. This is ok in the above 2
> cases as there is not any other choice since the user transaction is already
> holding the system hostage.
> The problem is the case where it was not a deadlock but just another thread
> trying to do the sequence update. In this case the thread should
> not be getting locks on the user thread.
> I am not sure best way to address this project but here are some ideas:
> 1) enhance lock manager to recognize the deadlock and then change to code to
> somehow do an immediately deadlock check for internal
> nested transactions, no matter what the system default is. Then the code
> should go ahead and use the system wait timeout on this lock
> and only fall over to using user transaction for deadlock (or maybe even
> throw a new "self deadlock" error that would only be possible for
> internal transactions).
> 2) somehow execute the internal system catalog update as part of a whole
> different transaction in the system. Would need a separate context.
> Sort of like the background daemon threads. Then no self deadlock is
> possible and it could just go ahead and wait. The downside is that then
> the code to "wait" for a new sequence becomes more complicated as it has
> to wait for an event from another thread. But seems like it could
> designed with locks/synchonization blocks somehow.
> 3) maybe add another lock synchronization that would only involve threads
> updating the sequences. So first an updater would request the
> sequence updater lock (with a key specific to the table and a new type)
> and it could just wait on it. It should never be held by parent
> transaction. Then it would still need the catalog row lock to do the
> update. I think with proper ordering this would insure that blocking on
> the catalog row lock would only happen in the self deadlock case.
> Overall this problem is less important as the size of the chunk of sequence
> is tuned properly for the application, and ultimately best if derby
> autotuned the chunk. There is a separate jira for auto tuning: DERBY-5295
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