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https://issues.apache.org/jira/browse/CALCITE-4129?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17161607#comment-17161607
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Julian Hyde commented on CALCITE-4129:
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Imagine that you have a {{VolcanoPlanner}} that has just completed, and you
have not yet called {{CheapestPlanReplacer}}. But you should be able to compare
the cheapest plan with a {{RelNode}} without building it. You can just compare
the traversals.
> Support equality check for whole rel plan tree
> ----------------------------------------------
>
> Key: CALCITE-4129
> URL: https://issues.apache.org/jira/browse/CALCITE-4129
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Haisheng Yuan
> Priority: Major
> Labels: pull-request-available
> Time Spent: 1h 40m
> Remaining Estimate: 0h
>
> Currently the only way to check rel node tree deep equality is transforming
> into String by {{RelOptUtil.toString(rel)}} with
> {{SqlExplainLevel.EXPPLAN_ATTRIBUTES}}, which is inefficient. One example is
> RexSubQuery. It has to do it this way, because the rel being reference by
> RexSubQuery is possibly not yet registered to VolcanoPlanner, and the digest
> {{equals}} checks the input RelNode by identity (not content). That is OK for
> RelSubset and HepRelVertex, if the RelNode is already registered in planner,
> but not for plain RelNode that is outside of planner. Due to this, we have to
> implement another set of deep equals logic in our system.
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