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From "Bergquist, Brett" <BBergqu...@canoga.com>
Subject RE: Unexpected behavior for concurrent selection of an uncommitted record inserted in a different thread
Date Fri, 07 Feb 2014 19:20:00 GMT
I think this is the behavior as described in this document:

http://docs.oracle.com/javadb/10.5.3.0/devguide/rdevconcepts8424.html

See the table entry on read-committed and it looks like the insert statement has an exclusive
lock on inserted record and as such the select by T2 for that specific record will block on
the exclusive lock.

-----Original Message-----
From: Jacopo Cappellato [mailto:jacopo.cappellato@gmail.com] 
Sent: Friday, February 07, 2014 12:59 PM
To: derby-user@db.apache.org
Subject: Unexpected behavior for concurrent selection of an uncommitted record inserted in
a different thread

Hi all!

While I was writing some unit tests for the Apache OFBiz project (that by default runs on
Derby) I noticed a behavior of Derby that I didn't expect and I would love to get your opinion.
Here is my use case:

* Derby 10.10.1.1
* there are two concurrent transactions T1 and T2
* isolation level is "Read Committed"
* in transaction T1 a record with primary key 123 is inserted in a table; then other long
running tasks are executed (i.e. the transaction is not immediately committed)
* in the meantime T2 attempts to select from the same table the record with primary key 123

Behavior: T2 blocks on the select statement waiting for transaction T1 to release the write
lock; this can cause a lock wait timeout Expected behavior: since T1 is not committed, T2
should not be able to select the record; I was expecting that the select statement in T2 would
return an empty result set rather than blocking waiting for the lock held by T1 to be released;
in fact this is what we get with MySQL and Postgres.

What do you think?

Thanks,

Jacopo Cappellato

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