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Date: Fri, 14 Jul 2006 11:07:14 -0700 (PDT)
From: "Bryan Pendleton (JIRA)" <derby-dev@db.apache.org>
To: derby-dev@db.apache.org
Subject: [jira] Commented: (DERBY-1357) Short-circuit logic in optimizer
 appears to be incorrect...
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    [ http://issues.apache.org/jira/browse/DERBY-1357?page=comments#action_12421183 ] 
            
Bryan Pendleton commented on DERBY-1357:
----------------------------------------

Hi Army,

Your change looks very good to me. I like the way you named the boolean temp variable; it makes the code substantially easier to read. And thanks for adding the good comments in the code.

One question, though: what is the actual symptom of this bug? If I am understanding it correctly, the symptom is that the optimizer may pointlessly continue to investigate a possible query plan which it should already be able to reject as being too expensive.

Does that mean that the only symptom here is that the optimizer may waste some resources?

I guess the thing I'm still struggling with is the bit of the change which adds "reloadBestPlan = true" at about line 530. Does that part of the patch have any actual user-visible impact on which query plan would be chosen, such that we need to have a release note here? Or is that just another bit of resource wastage that we're improving?


> 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, 10.2.0.0, 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: 10.2.0.0
>
>         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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