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https://issues.apache.org/jira/browse/CALCITE-3085?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16846349#comment-16846349
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Danny Chan commented on CALCITE-3085:
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I don't think it's a good idea to remove the stack, this stack provides ability
for child nodes to have access to parent nodes, although this is not that
common but removing this will cause a regression. We can not make sure other
engines with Calcite do not use this stack.
> Unused stack field in RelShuttleImpl
> ------------------------------------
>
> Key: CALCITE-3085
> URL: https://issues.apache.org/jira/browse/CALCITE-3085
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Laurent Goujon
> Assignee: Laurent Goujon
> Priority: Minor
> Labels: pull-request-available
> Time Spent: 10m
> Remaining Estimate: 0h
>
> {{RelShuttleImpl}} class has a protected {{stack}} field which is being
> populated when going over children, but those content is actually never used.
> In Calcite code base, no subclasses are actually using the content of the
> stack ({{CorelMapBuilder}} is populating the stack but does not read the
> content back either).
> Searching code on github didn't show any usage of it either (but this is not
> foolprof).
> As maintaining this stack has a non-negligible impact on memory/performance,
> I would suggest to remove the field.
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