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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 Fri, 07 Jul 2006 17:51:31 GMT
     [ http://issues.apache.org/jira/browse/DERBY-1357?page=all ]

A B updated DERBY-1357:

    Attachment: d1357_v1.patch

Attaching a patch, d1357_v1.patch, to address the incorrect logic as noted in the description.

I also had to update the lang/predicatePushdown.out master file because with the d1357_v1.patch
the order of a couple of qualifiers has changed.  Note that the query plans themselves are
exactly the same--the only thing that's changed is the the qualifier ordering for one query.
 This change of order occurs because as part of the costing code in FromBaseTable.estimateCost()
the optimizer transfers predicates from one predicate list to another, gets an estimated cost,
then puts the predicates back into the original list.  The methods to do this transferring
are in NestedLoopJoinStrategy.java and HashJoinStrategy.java.  In the former, the predicates
are transferred away in front-to-back order and then transferred back in back-to-front order,
which leads to a reversal of the relevant predicate ordering.  Ex. If we have a list L1 of
preds A,B,C and we transfer them to L2 in front-to-back order, L2 will end up with A,B,C.
 Then, when we transfer the predicates back to L1, if we process L2 in back-to-front order,
L1 will end up with C,B,A.  That said, with d1357_v1 applied the short-circuit logic prevents
the optimizer from trying to optimize a join order that is known to be bad.  This means that
we skip an unnecessary round of optimization and therefore skip one round of order reversal,
which means the order of the predicate qualifiers in the final plan is now different.

I ran derbyall on Red Hat with sane jars and ibm142, and saw no new failures.

Review/commit would be appreciated.

> Short-circuit logic in optimizer appears to be incorrect...
> -----------------------------------------------------------
>          Key: DERBY-1357
>          URL: http://issues.apache.org/jira/browse/DERBY-1357
>      Project: Derby
>         Type: Bug

>   Components: Performance
>     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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