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From "Dag H. Wanvik (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-1357) Short-circuit logic in optimizer appears to be incorrect...
Date Mon, 29 Jun 2009 14:35:47 GMT

     [ https://issues.apache.org/jira/browse/DERBY-1357?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Dag H. Wanvik updated DERBY-1357:

    Derby Categories: [Performance]

> Short-circuit logic in optimizer appears to be incorrect...
> -----------------------------------------------------------
>                 Key: DERBY-1357
>                 URL: https://issues.apache.org/jira/browse/DERBY-1357
>             Project: Derby
>          Issue Type: Bug
>    Affects Versions:,,,,,
>            Reporter: A B
>            Assignee: 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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