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From "A B (JIRA)" <derby-...@db.apache.org>
Subject [jira] Created: (DERBY-1907) Improve optimizer to prune query plans more aggressively.
Date Fri, 29 Sep 2006 21:25:19 GMT
Improve optimizer to prune query plans more aggressively.
---------------------------------------------------------

                 Key: DERBY-1907
                 URL: http://issues.apache.org/jira/browse/DERBY-1907
             Project: Derby
          Issue Type: Improvement
          Components: Performance, SQL
    Affects Versions: 10.1.3.2, 10.2.1.5, 10.3.0.0
            Reporter: A B


For large queries with numerous and deeply-nested subqueries like those in DERBY-1777, the
number of different index/joinOrder/joinStrategy combinations that the optimizer can process
is extremely high.  In the absence of an efficient timeout mechanism (see DERBY-1906) this
can lead to situations where the optimizer spends far too much time trying out different access
paths before picking a plan.  In the case of the DERBY-1777 queries, the optimizer can literally
spend hours trying out all of the possible combinations before deciding on the best one.

Even if/when the timeout mechanism is improved, it would still be great if the Derby optimizer
could somehow figure out how to aggressively prune out access paths that are known (or suspected)
to be sub-optimal.  This way the optimizer would spend less time optimizing "bad" access paths
and could therefore try out more "good" plans before timeout occurs.

I have no idea how this pruning would occur, but I do think it would be extremely beneficial
to the Derby optimization process when dealing with deeply nested queries with large FROM
lists.

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