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From "A B (JIRA)" <derby-...@db.apache.org>
Subject [jira] Created: (DERBY-1357) Short-circuit logic in optimizer appears to be incorrect...
Date Tue, 30 May 2006 20:52:31 GMT
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: 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
    Priority: Minor


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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