clintropolis commented on pull request #11955:
URL: https://github.com/apache/druid/pull/11955#issuecomment-974574311


   >@clintropolis with regard to whether virtual column implementations would 
handle this properly, I want to stress that virtual columns referencing other 
virtual columns is already possible for cursors backed by regular segments or 
by IncrementalIndexes. They do their own virtual column handling and they both 
already work this way. The VirtualizedColumnSelectorFactory is updated in this 
patch to match that behavior. It's used by RowBasedSegments and by 
PostJoinCursor.
   
   Hmm, in that case there might be a couple of other things worth fixing so 
stuff works good.
   
   these should probably use `this` instead of the inspector of the underlying 
index:
   
https://github.com/apache/druid/blob/master/processing/src/main/java/org/apache/druid/segment/QueryableIndexColumnSelectorFactory.java#L196
   
https://github.com/apache/druid/blob/master/processing/src/main/java/org/apache/druid/segment/vector/QueryableIndexVectorColumnSelectorFactory.java#L270
   
https://github.com/apache/druid/blob/master/processing/src/main/java/org/apache/druid/segment/virtual/VirtualizedColumnInspector.java#L55
   
   I've had it in my head that this is broken for some reason, but admit the 
last time i really dug into it was a couple of years ago, so trying to remember 
things.


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