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From "A B (JIRA)" <derby-...@db.apache.org>
Subject [jira] Updated: (DERBY-1357) Short-circuit logic in optimizer appears to be incorrect...
Date Tue, 18 Jul 2006 20:26:17 GMT
     [ http://issues.apache.org/jira/browse/DERBY-1357?page=all ]

A B updated DERBY-1357:

    Derby Info: [Patch Available, Release Note Needed]  (was: [Patch Available])

Possible RELEASE NOTE for this fix is as follows, based on suggestions from Bryan in the following



DERBY-1357: Short-circuit logic in optimizer appears to be incorrect.

Changes have been made to prevent the optimizer from spending time optimizing/evaluating join
orders that it already knows are bad.


The optimizer will now abandon sub-optimal join orders as soon as it realizes that they cost
more than the best join order so far.

This fix also ensures that, in the case of short-circuited join orders, Derby will still generate
(and execute) an overall plan that matches the "best path" decisions made by the optimizer--which
was not always the case prior to these changes.


Execution performance of large queries (esp. those with nested subqueries and/or with large
FROM clauses) may change.  The expectation is that the new (10.2) query plans will show improved
performance over the old ones. 


Since the optimizer is now spending less time evaluating sub-optimal join orders, it is possible
that it will be able to try out more join orders before optimizer "timeout" occurs.  As a
result the optimizer can sometimes find better plans than it did in earlier versions of Derby.


This was an intentional change to fix behavior that was not working correctly in earlier versions
of Derby. The expectation is that the new behavior--and the subsequent query plans--will lead
to improved performance over the old ones, so no further solution is required.


There is no way to disable/workaround this new behavior since the symptom as described above
is a good one for Derby.

That said, any user who notices a negative performance change after moving to Derby 10.2,
and who believes that the difference in performance is related to this optimizer change, is
encouraged to visit the following "performance diagnosis" page and to follow up with his/her
findings on the Derby mailing lists:



> Short-circuit logic in optimizer appears to be incorrect...
> -----------------------------------------------------------
>                 Key: DERBY-1357
>                 URL: http://issues.apache.org/jira/browse/DERBY-1357
>             Project: Derby
>          Issue Type: Bug
>          Components: Performance
>    Affects Versions:,,,,,,,,,,
>            Reporter: A B
>         Assigned To: A B
>            Priority: Minor
>             Fix For:
>         Attachments: d1357_v1.patch, d1357_v1.stat
> When considering different join orders for the FROM tables in a query, the optimizer
will decide to give up on a join order midway through if the cost of that (partial) join order
is already higher than the cost of some other *complete* join order that the optimizer previously
found.  This "short-circuiting" of a join order can save compilation time.
> That said, the logic to perform this "short-circuit" of a join order is currently as
follows (from OptimizerImpl.java):
>   /*
>   ** Pick the next table in the join order, if there is an unused position
>   ** in the join order, and the current plan is less expensive than
>   ** the best plan so far, and the amount of time spent optimizing is
>   ** still less than the cost of the best plan so far, and a best
>   ** cost has been found in the current join position.  Otherwise,
>   ** just pick the next table in the current position.
>   */
>   boolean joinPosAdvanced = false;
>   if ((joinPosition < (numOptimizables - 1)) &&
>     ((currentCost.compare(bestCost) < 0) ||
>     (currentSortAvoidanceCost.compare(bestCost) < 0)) &&
>     ( ! timeExceeded )
>     )
>   {
>     ...
>   }
> There are two "current costs" in this statement: one for the cost if the optimizer is
calculating a "sort avoidance" plan (which it does if there is a required row ordering on
the results) and one if it is calculating a plan for which row order is not important.
> I admit that I'm not all that familiar with what goes on with the costing of a sort-avoidance
plan, but inspection of the code shows that, when there is no required row ordering--i.e.
when we aren't looking for a sort-avoidance plan--the cost field of currentSortAvoidanceCost
will always be 0.0d. That in turn means that in the above "if" statement, the check for
>   ((currentCost.compare(bestCost) < 0) ||
>     (currentSortAvoidanceCost.compare(bestCost) < 0))
> will always return true (because bestCost should--in theory--always be greater than 0.0d).
 Thus, in the case where we don't have a required row ordering, the short-circuit logic will
fail even if currentCost is actually greater than bestCost.

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