[ http://issues.apache.org/jira/browse/DERBY-558?page=all ]
A B resolved DERBY-558:
-----------------------
Resolution: Fixed
I ran the repro attached to this issue as well as the new test case in
lang/subqueryFlattening.sql against the trunk (10.2) and the 10.1 branch to
verify that the changes have been committed and that things are working as they
should. It all looks good, so I'm resolving and closing this issue. Thanks
for committing, Satheesh.
> 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: d558.patch, 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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