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From Ronald Rudy <ronchal...@gmail.com>
Subject OutOfMemoryErrors on group by select
Date Mon, 08 Feb 2010 14:11:58 GMT
I've gotten to the point where I seem to be able to frequently induce OutOfMemoryErrors when
executing a statement like this one:

SELECT groupCol, count(*) FROM myTable GROUP BY groupCol;

The total number of records doesn't seem terribly large (about 400,000 records) though while
I'm executing the above statement there are likely some inserts and deletes happening concurrently.
 I don't need anything transactional here, I'm really just monitoring a table from a separate
network connection.

In our app the database runs in its own JSVC daemon as a NetworkServer with 512MB allocated
to it.  Page cache size is set to 2000.    Page size should be Derby default (4096 I think).
 

I cannot be 100% sure that the above is specifically what is causing the error, but I can
say that I haven't seen memory errors until I started monitoring this table frequently and
even then I really only saw it when the table size started getting a bit bigger (not big by
any stretch, but > 300k rows).

The table itself is not really big - there are 7 varchar columns along with an ID column.

In this production-level situation stability is more important than performance.  I'd like
Derby configured so that no matter what is requested SQL wise the daemon itself is not impacted.
 

I'd also like to be able to build in some handling code perhaps that if/when an OutOfMemoryError
is encountered, it will automatically restart - for this I might need to add a script with
-XX:OnOutOfMemoryError as a JVM option unless there's some way to handle this internally?

-Ron
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