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https://issues.apache.org/jira/browse/TRAFODION-2645?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16189844#comment-16189844
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ASF GitHub Bot commented on TRAFODION-2645:
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Github user asfgit closed the pull request at:
https://github.com/apache/incubator-trafodion/pull/1246
> MDAM costing overestimates I/O cost
> -----------------------------------
>
> Key: TRAFODION-2645
> URL: https://issues.apache.org/jira/browse/TRAFODION-2645
> Project: Apache Trafodion
> Issue Type: Bug
> Components: sql-cmp
> Affects Versions: 2.2-incubating
> Environment: All
> Reporter: David Wayne Birdsall
> Assignee: David Wayne Birdsall
>
> The method MDAMCostWA::compute (optimizer/ScanOptimizer.cpp) has logic to
> calculate the total cost of an MDAM plan and compare it to the cost of a
> single subset plan. In the case of a Trafodion table, the cost logic for MDAM
> cost has an error. It is adding the total number of probes (totalRqsts) to
> the number of seeks (totalSeeks) in order to calculate I/O costs.
> TotalSeeks models direct access disk seeks. These happen at the block level
> and are an I/O cost.
> TotalRqsts models the number of probes, which is a CPU cost. A probe is a
> direct access within a block that has already been read into memory. This is
> a vastly smaller unit of cost than an I/O operation.
> Note that TotalRqsts is already modeled as a message cost since it is added
> to TotalRows.
> The effect of the bug is to vastly penalize MDAM plans having large numbers
> of probes. This will show up the most on the largest tables.
> The fix is simply not to add totalRqsts to totalSeeks. However, in making
> this fix, it is likely that other MDAM-related parameters will need tuning.
> So this change needs to be done carefully.
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