[ http://issues.apache.org/jira/browse/DERBY-558?page=all ]
A B updated DERBY-558:
----------------------
Attachment: repro.sql
Attaching a simplified reproduction of the hang. Note that this particular
reproduction is completely contrived and nonsensical, but it nonetheless
demonstrates the problem. In order to reproduce, start ij with the
"derby.optimizer.noTimeout" property set to true, connect to a database, and
then run the attached sql script:
> java -Dderby.optimizer.noTimeout=true org.apache.derby.tools.ij
ij version 10.2
ij> connect 'jdbc:derby:testdb;create=true';
ij> run 'repro.sql';
Note that the hang won't reproduce if "noTimeout" is false (which is the
default) because eventually the optimizer will decide that it's taking too long
and will quit. That's nice because it means most people won't ever see this
problem :) However, when noTimeout is set to true the query _should_ still
finish (even if it takes longer), so I _do_ think this is a bug.
> Optimizer hangs with query that uses more than 6 tables and does subquery
> flattening.
> -------------------------------------------------------------------------------------
>
> Key: DERBY-558
> URL: http://issues.apache.org/jira/browse/DERBY-558
> Project: Derby
> Type: Bug
> Versions: 10.1.1.0, 10.0.2.1, 10.0.2.0, 10.1.2.0, 10.2.0.0, 10.0.2.2
> Environment: Running query in "ij" with derby.optimizer.noTimeout=true
> Reporter: A B
> Assignee: A B
> Fix For: 10.1.2.0, 10.2.0.0
> Attachments: repro.sql
>
> I was running a query that has a large number (hundreds) of tables in it and
> I set the derby property "derby.optimizer.noTimeout" to true to see what plan
> Derby would choose as the _best_ plan for the query. When doing so, I ran
> into a situation where the optimizer hung forever--which is wrong. I expect
> that setting "noTimeout" to true might cause the query to run more slowly
> (since it has to evaluate ALL possible join orders for all of the tables in
> question), but it should _not_ cause the optimizer to hang forever.
> I noticed that "subquery flattening" is peformed on the query, which
> introduces dependencies between the various tables and thus restricts the
> possible join orders that the optimizer can choose (see
> http://db.apache.org/derby/docs/10.1/tuning/ctuntransform25868.html). I was
> eventually able to track the problem down to code in OptimizerImpl where, for
> queries with more than 6 tables, a certain "jumping" algorithm is used to try
> to allow the optimizer to find a better plan more quickly.
> Long story short, there is logic in the "jumping" mechanism that tries to put
> the tables into a legal join order, but in certain (rare) cases where
> multiple join order dependencies have to be enforced, the jump logic can end
> up looping indefinitely, causing the "hang" in the optimizer.
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