[jira] Closed: (DERBY-1357) Short-circuit logic in optimizer appears to be incorrect...

["A B (JIRA)" <de...@db.apache.org>]

     [ http://issues.apache.org/jira/browse/DERBY-1357?page=all ]

A B closed DERBY-1357.
----------------------


Changes have been committed for almost 2 months, so closing the issue.

> 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: 10.0.2.0, 10.0.2.1, 10.1.1.0, 10.2.1.0, 10.1.2.0, 10.1.1.1, 10.1.1.2, 10.1.2.1, 10.1.2.2, 10.1.2.3, 10.1.2.4
>            Reporter: A B
>         Assigned To: A B
>            Priority: Minor
>             Fix For: 10.2.1.0
>
>         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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