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https://issues.apache.org/jira/browse/CALCITE-4514?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17292268#comment-17292268
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Julian Hyde commented on CALCITE-4514:
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I was only suggesting caching the two child sets in variables (for the duration
of the call to the {{merge}} method), not a longer-lived cache in fields.
Assigning to a variable seemed to make sense because it looked as if you were
calling {{set.getChildSets(this)}} more than once.
> Fine tune the merge order of two RelSets, cache RelSet's childSet computation
> -----------------------------------------------------------------------------
>
> Key: CALCITE-4514
> URL: https://issues.apache.org/jira/browse/CALCITE-4514
> Project: Calcite
> Issue Type: Improvement
> Reporter: Botong Huang
> Priority: Minor
> Labels: pull-request-available
> Time Spent: 0.5h
> Remaining Estimate: 0h
>
> When merging two relsets, we have two preferences:
> 1. Merge parent relset into child relset
> 2. Merge newer relset into older relset
> Currently, when the two relsets are parent set of each other, we randomly
> pick a merge order without checking the second condition above. For
> performance reasons, we should, to avoid unnecessary churn.
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