[GitHub] [incubator-druid] gianm commented on issue #8013: GroupBy: Fix improper uses of StorageAdapter#getColumnCapabilities.

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

gianm commented on issue #8013: GroupBy: Fix improper uses of StorageAdapter#getColumnCapabilities.
URL: https://github.com/apache/incubator-druid/pull/8013#issuecomment-508576456
 
 
   It looks like the tests were failing because ExpressionVirtualColumn always set its capabilities to be singly-valued, which is a bug, since ever since #7588 they might be multi-valued. However, that bug was probably not detected since it was masked by _this_ bug (which prevents groupBy from using its all-singly-valued-dimension optimization if some of the columns involved are virtual columns).
   
   I pushed a fix for the ExpressionVirtualColumn issue and updated the top comment. In this fix I just set it to always be "true". This isn't ideal, since it means singly-valued optimizations won't work on top of it, but I didn't see an easy way for the ExpressionVirtualColumn to determine upfront if it will be singly-valued or not. I think this should be possible in the future as we add more upfront type info to the expression system, so I added a comment saying as much.
   
   /cc @clintropolis 

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