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From "A B (JIRA)" <derby-...@db.apache.org>
Subject [jira] Created: (DERBY-1260) Investigate impact of infinite cost estimates on arithmetic operations/comparisons in the optimizer.
Date Thu, 27 Apr 2006 05:54:03 GMT
Investigate impact of infinite cost estimates on arithmetic operations/comparisons in the optimizer.
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         Key: DERBY-1260
         URL: http://issues.apache.org/jira/browse/DERBY-1260
     Project: Derby
        Type: Task

  Components: Performance  
    Versions: 10.2.0.0    
    Reporter: A B
    Priority: Minor


For large, deeply nested queries and/or queries with a high number of FROM tables/expressions
with high row count estimates, the the resultant cost estimates can be multiplied--sometimes
many times over--throughout the optimization process, which means that the overall query estimate
can climb to a very large number very quickly.  If the query is big enough, this can actually
cause the optimizer to reach an estimated cost of INFINITY.

That said, the current optimizer logic for costing and comparing access paths does not expect--and
therefore does not account for--infinite cost estimates.  As a result the optimizer does comparisons
of, and basic arithmetic with, cost estimates and row counts that, when applied to estimtes
of Infinity, can give unexpected results.  One specific example of this is DERBY-1259.

I'm filing this issue for the task of investigating the optimizer code more closely to see
what other issues might exist when infinite cost estimates come into play.

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