[GitHub] [druid] clintropolis commented on pull request #11955: VirtualizedColumnSelectorFactory: Allow virtual columns to reference each other.

[GitBox <gi...@apache.org>]

clintropolis commented on pull request #11955:
URL: https://github.com/apache/druid/pull/11955#issuecomment-974495671


   This seems like it should work for `ExpressionVirtualColumns` without issue, but we might need to add some additional guards to some other virtual columns which expect to be built on top of a physical column to do things with dictionaries and indexes, such as `ListFilteredVirtualColumn`, looking into some code to see if this should be done now or can be done later.
   
   I'm also wondering if there is work SQL side to be done here to allow this, e.g. with `VirtualColumnRegistry` and how some of the expressions are being constructed, to allow SQL to plan into things like this. I think we would want some smarts to it so that it only breaks up an expression virtual column into multiple parts if there is some sub-part shared by all callers, but could also potentially be nice in the cases that have "native" optimized virtual column implementations, such as allowing expressions to call `ListFilteredVirtualColumn`.


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