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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