Knut Anders Hatlen commented on DERBY-4620:
> * Would it be possible/make sense to try to allocate the memory for the hash tables while running the optimiser, and if that fails then fall back to nested loops?
That might work, but I'm a little worried that it might cause OOME in other threads accessing the database while the optimizer is working. It might also overcommit memory if statements are compiled sequentially and executed in parallel.
Somehow adjusting the size of the hash table to the amount of available memory sounds like a good idea, though. One variant (briefly mentioned in an earlier comment) is to use a fixed percentage of the total heap size. This is what BackingStoreHashtable does by default (1% of total heap size), but hash joins always override that default. We could let hash joins fall back to using BackingStoreHashtable's default if maxMemoryPerTable hasn't been set explicitly. With 100MB heap, maxMemoryPerTable would default to the same value as it currently does. (Maybe we'd want to increase it to a higher percentage. 1% is probably less than ideal in most cases.)
> Query optimizer causes OOM error on a complex query
> Key: DERBY-4620
> URL: https://issues.apache.org/jira/browse/DERBY-4620 > Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.5.3.0
> Environment: java version "1.6.0_17"
> Java(TM) SE Runtime Environment (build 1.6.0_17-b04)
> Java HotSpot(TM) Client VM (build 14.3-b01, mixed mode, sharing)
> Linux rocio.xxx 2.6.24-27-generic #1 SMP Fri Mar 12 01:10:31 UTC 2010 i686 GNU/Linux
> Reporter: Chris Wilson
> Labels: derby_triage10_8
> Attachments: estimate-sizes.diff, query-plan.txt
> I have a query that generates about 2,000 rows in a summary table. It runs fast enough (~30 seconds) on MySQL. The same query on Derby runs for about 10 minutes and then fails with an OutOfMemoryError.
> I have created a test case to reproduce the problem. It's not minimal because it relies on a rather large dataset to reproduce it, and it's not trivial, but I don't mind doing a bit of work trimming it if someone can point me in the necessary direction.
> You can check out the test case from our Subversion server here:
> http://rita.wfplogistics.org/svn/trackrita/rita/doc/derby-oom-slow-query > which includes a pre-built Derby database in "testdb.derby". If this database is deleted, "test.sh" will recreate it, but that takes about 10-15 minutes.
> Just modify the script "test.sh" to point to your Derby libraries, and run it (or just execute the commands in "movement_complete.sql") to demonstrate the problem. You can view the source of that file online here:
> http://rita.wfplogistics.org/trac/browser/rita/conf/movement_complete.sql > The first "insert into movement_complete" (starting around line 4) takes about 15 seconds to complete and inserts 5890 rows. The second, starting around line 54, does not complete in reasonable time on Derby. On MySQL, it runs in 28 seconds and inserts 2038 rows. On Derby, after 10 minutes I get:
> JAVA ERROR: java.lang.OutOfMemoryError: Java heap space
> ij> ERROR X0Y67: Cannot issue rollback in a nested connection when there
> is a pending operation in the parent connection.
> (process exits)
> It does not output the query plan in this case.
> Following the suggestion of Bryan Pendleton, I tried increasing the JVM memory limit from the default to 1024m, and this allows the query to finish executing quite quickly. I guess that means that the optimiser is just taking a lot of memory to
> optimise the query, and it spends forever in GC before finally hitting OOM and giving up when using the default settings.