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From "Dag H. Wanvik (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-1908) Investigate: What's the "unit" for optimizer cost estimates?
Date Mon, 29 Jun 2009 14:29:47 GMT

     [ https://issues.apache.org/jira/browse/DERBY-1908?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Dag H. Wanvik updated DERBY-1908:

    Derby Categories: [Performance]

> Investigate: What's the "unit" for optimizer cost estimates?
> ------------------------------------------------------------
>                 Key: DERBY-1908
>                 URL: https://issues.apache.org/jira/browse/DERBY-1908
>             Project: Derby
>          Issue Type: Task
>          Components: SQL
>            Reporter: A B
> Derby optimizer decisions are necessarily based on cost estimates.  But what are "units"
for these cost estimates?  There is logic in OptimizerImpl.getNextPermutation() that treats
cost estimates as if their unit is milliseconds--but is that really the case?
> The answer to that question may in fact be "Yes, the units are milliseconds"--and maybe
the unexpected cost estimates that are sometimes seen are really caused by something else
(ex. DERBY-1905).  But if that's the case, it would be great to look at the optimizer costing
code (see esp. FromBaseTable.estimateCost()) to verify that all of the "magic" of costing
really makes sense given that the underlying unit is supposed to be milliseconds.
> Also, if the stats/cost estimate calculations are truly meant to be in terms of milliseconds,
I can't help but wonder on what machine/criteria the determination of milliseconds is based.
 Is it time to update the stats for "modern" machines, or perhaps (shooting for the sky) to
dynamically adjust the millisecond stats based on the machine that's running Derby and use
the adjusted values somehow?  I have no answers to these questions, but I think it would be
great if someone out there was inclined to discuss/investigate these kinds of questions a
bit more...

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