Hi Luis,

If you want to load the whole database into memory, you have to set the page cache size and the –Xmx java setting:

java -Dderby.storage.pageCacheSize=524288 -Xmx1G

Derby is polite and will not take up more cache memory than the default size, even if you increase the heap.  So you have to tell Derby how much memory you want allocated.  This can be very handy in an embedded database application where you don’t want the DB to take up all available memory.

Also, please make sure to make the distinction between an embedded in-process cache versus something like memcached.  The serialization and IPC (inter-process communication) overhead makes out-of-process databases much slower.  This explains why an in-process Java memory cache out performs memcached:
http://gregluck.com/blog/archives/2007/05/comparing-memcached-and-ehcache-performance/

And if part of your study is to compare multiple Java in-memory embedded databases, please note whether the database is concurrent rather than merely “thread-safe”.  A concurrent database allows queries and updates in parallel from multiple threads, while “thread-safe” typically means only one thread at a time can access or modify the embedded database.   Concurrency makes a big difference for scalability because multiple threads share the same cache of in-process memory, so the overall cache scales across multiple threads.  In a “thread-safe” model each process must have it’s own cache, which duplicates cache entries and limits scalability of the solution.

I did a performance study of multiple Java embedded databases for use in the Hadoop framework, and found that Derby offers superior multi-threaded performance:
http://mpouttuclarke.wordpress.com/2010/12/10/java-embedded-db-for-ip2location-in-hadoop/

Hadoop has a multi-threaded option, so concurrency within embedded Derby was a major selection criteria.

Anyway, enjoy your study, and I hope this helps!

Cheers,
Matt


On 4/28/11 7:22 PM, "Luis Ribeiro" <luisribeiro1989@gmail.com> wrote:

Dears,
    My name is Luis Ribeiro, I`m brazilian and I study Computer Science at Universidade Presbiteriana Mackenzie (http://www.mackenzie.br/portal/principal.php). I and Henrique Teixeira (cc) are on the last semester and we`re doing our final project. To be honest, we`re finishing its. It`s about Comparison between HD databases and In-Memory databases.
    We`re trying to use Apache Derby to know how it works with the simple systems that we developed.
    We have two doubts and I would be greatfull if you could answer to us:
    - Does exist some function or method that you use similar to "GROUP_CONCAT" (from MySQL)? I want to know if I can do this on a simple way with Apache Derby?

Original table
ID | NAME
1  | Luis
2  | Henrique
3  | Luis

After do the query
ID | NAME
1, 3 | Luis
2  | Henrique

    - What we need to do to have a exceptional performance with In Memory mode? I`d increased jvm heap, but it didn`t work.
 
    I`m sorry if I was offensive with this e-mail.

    Thank you very much.

Best regards,
Luis Ribeiro

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