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https://issues.apache.org/jira/browse/CALCITE-5715?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17726831#comment-17726831
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Julian Hyde commented on CALCITE-5715:
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I've been confused by the summaries of a few of your recent cases. (This, and
CALCITE-5506 and CALCITE-5717.) When you use the word 'prune' I tend of think
of pruning fields or RexNodes. Can we find a clearer terminology? Summaries are
important, because that is what ends up in the release notes and becomes the
documentation.
Also, I personally refuse to review a PR until I understand its goal. The
specification needs to be good enough that people don't need to read the
implementation. Otherwise we are not doing software engineering.
In the case of CALCITE-5717 I found it clearer to reference the before and
after RelNode classes: "Replace Project on Aggregate with Values".
Pruning a tree with N branches results in a tree with N-1 branches. So in this
case, which removes a trivial Project, I wouldn't refer to the process as
'pruning'.
> Prune old nodes after projection merging in ProjectMergeRule
> ------------------------------------------------------------
>
> Key: CALCITE-5715
> URL: https://issues.apache.org/jira/browse/CALCITE-5715
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.34.0
> Reporter: Benchao Li
> Priority: Major
> Fix For: 1.35.0
>
>
> We already have many similar usages (prune old nodes when the new one is
> obviously better, e.g. in
> [CalcMergeRule|https://github.com/apache/calcite/blob/c56d5564628de6b3265f960764a6f6fc43935a75/core/src/main/java/org/apache/calcite/rel/rules/CalcMergeRule.java#L85-L90]
> to reduce the search scope, I propose to also add this for
> {{ProjectMergeRule}}. Do you have any concerns about this?
> In {{ProjectMergeRule}}, there is a case that after merging the two
> {{Project}}, we'll find the new Project is trivial and we'll transform to the
> bottomProject's
> input(https://github.com/apache/calcite/blob/c56d5564628de6b3265f960764a6f6fc43935a75/core/src/main/java/org/apache/calcite/rel/rules/ProjectMergeRule.java#L132-L139).
> In this case, both topProject and bottomProject could be pruned?
> The reason I propose to do this optimization for {{ProjectMergeRule}} is that
> I met a problem that two sets will point to each other after projection
> merging, and the planner will run into dead loop. (It's hard to show the
> details, I haven't got a simple reproducible demo yet)
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