db-derby-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "A B (JIRA)" <derby-...@db.apache.org>
Subject [jira] Assigned: (DERBY-1357) Short-circuit logic in optimizer appears to be incorrect...
Date Fri, 07 Jul 2006 17:49:30 GMT
     [ http://issues.apache.org/jira/browse/DERBY-1357?page=all ]

A B reassigned DERBY-1357:
--------------------------

    Assign To: A B

> 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
>     Assignee: 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.

-- 
This message is automatically generated by JIRA.
-
If you think it was sent incorrectly contact one of the administrators:
   http://issues.apache.org/jira/secure/Administrators.jspa
-
For more information on JIRA, see:
   http://www.atlassian.com/software/jira


Mime
View raw message