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https://issues.apache.org/jira/browse/CALCITE-4501?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Vladimir Ozerov resolved CALCITE-4501.
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Resolution: Won't Fix
Closing as "Won't fix" as there is no agreement on the correctness of the fix
in the community.
> Clear costs in VolcanoPlanner when setting the new root
> -------------------------------------------------------
>
> Key: CALCITE-4501
> URL: https://issues.apache.org/jira/browse/CALCITE-4501
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.26.0
> Reporter: Vladimir Ozerov
> Assignee: Vladimir Ozerov
> Priority: Minor
> Time Spent: 20m
> Remaining Estimate: 0h
>
> In production-grade systems, the optimization is typically performed in
> several phases. The same instance of {{VolcanoPlanner}} tends to be re-used
> between the phases.
> In some sophisticated scenarios, we may want to alter the cost function
> between phases.
> For example, to adjust the weights of some operators depending on some
> heuristics, or even switch to a completely different cost model.
> One way to do that is to re-instantiate the cluster and the planner with the
> new cost function.
> However, it requires copying of the current rel tree, because its nodes still
> point to the old cluster. This may negatively affect the performance.
> The alternative approach could be to change the state of the cost factory.
> However, it would not work for two reasons:
> # The cluster object has a reference to {{MetadataQuery}} with possibly
> cached costs. This may lead to a mix of different cost models.
> # {{VolcanoPlanner}} caches zero and infinite costs in a final variable.
> In this issue, I propose to introduce two slight modifications to the
> {{VolcanoPlanner}}:
> # Clear the metadata query from within {{setRoot}} method.
> # Make {{infCost}} and {{zeroCost}} non-final, and also re-initialize them in
> the {{setRoot}} method.
> The {{setRoot}} method is convenient for these purposes because it is invoked
> from the {{RuleSetProgram}}, so the desired cost cleanup will work with no
> modifications to the application's code.
> Please let me know if you think that this change is not valid.
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