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https://issues.apache.org/jira/browse/CALCITE-2363?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16514707#comment-16514707
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Julian Hyde commented on CALCITE-2363:
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I am a little skeptical that you can make it fully extensible without making it
significantly harder to use. I kind of like how it handles the common 80% of
cases fairly easily, and you have to build a custom visitor for the other 20%.
That was actually a design goal when I wrote it.
That said, refactoring may be possible - I’d like to see what you can come up
with.
> Refactor RelShuttle not to depend on structures of RelNodes
> -----------------------------------------------------------
>
> Key: CALCITE-2363
> URL: https://issues.apache.org/jira/browse/CALCITE-2363
> Project: Calcite
> Issue Type: Improvement
> Reporter: Teruyoshi Zenmyo
> Assignee: Julian Hyde
> Priority: Major
>
> Currently, RelShuttle implementations depends on the structures of RelNodes.
> For instance, RelShuttleImp.visit(LogicalAggregate aggregate) knows the first
> element of getInput() is the next element to be visited.
> [https://github.com/apache/calcite/blob/master/core/src/main/java/org/apache/calcite/rel/RelShuttleImpl.java#L75]
>
> By this refactoring, implementations of RelShuttle will become free from
> invoking super.visit methods.
> https://github.com/apache/calcite/blob/master/core/src/main/java/org/apache/calcite/sql2rel/CorrelationReferenceFinder.java#L61
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