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https://issues.apache.org/jira/browse/CALCITE-6788?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17913455#comment-17913455
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Stamatis Zampetakis commented on CALCITE-6788:
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In addition users can specify their own metadata handlers so
{{mq.getCumulativeCost(rel)}} is already customizable. Why do we want to change
the customization and push the responsibility to the planner?
Worth keeping in mind that this is gonna be a noticeable breaking change for
various projects that specify their own metadata providers or override the
{{RelOptPlanner#getCost}} method (e.g.,
[HiveVolcanoPlanner|https://github.com/apache/hive/blob/88e2175b2c04403e35b404634306f60d599c3d83/ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/cost/HiveVolcanoPlanner.java#L98])
so if we decide to move forward we must ensure that it's worth it.
> LoptOptimizeJoinRule should delegate costs to optimization planner
> ------------------------------------------------------------------
>
> Key: CALCITE-6788
> URL: https://issues.apache.org/jira/browse/CALCITE-6788
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.38.0
> Reporter: Claude Brisson
> Priority: Major
> Labels: pull-request-available
>
> {{LoptOptimizeJoinRule}} uses costs comparisons to recursively decide whether
> to add a join at the top of the joins tree or to push it down.
> When doing so, instead of directly calling {{mq.getCumulativeCost(rel)}}, it
> should rely on {{call.getPlanner.getCost(rel, mq)}}, which will be used
> thereafter to choose the best joins tree.
> This way, it becomes possible to customize the costs computation during the
> heuristic joins ordering phase by just overriding the {{getCost()}} method of
> the optimization planner.
> Without this patch, it is only possible to alter joins costs at the final
> phase of the algorithm, which defeats the goal of customizing them, including
> during the to-top/push-down phase.
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