[GitHub] [arrow-datafusion] Dandandan commented on issue #790: Rework GroupByHash to support grouping by nulls

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

Dandandan commented on issue #790:
URL: https://github.com/apache/arrow-datafusion/issues/790#issuecomment-888516528


   Yes, this (great) explanation / overview matches my suggestion. Somehow the ASCII art doesn't render well here?
   
   A couple of points:
   * Great idea to use a `Vec<ScalarValue>` first. It seems it should be quite easy to swap that to a typed alternative later, and probably involves less changes now
   * To convert the group by values it can re-use the `create_hashes` function from the hash join. This way it will also benefit from improvements there and is calculated on (multiple) Arrays directly.
   
   * Before implementing this proposal it is I think be possible with minimal changes to also to include a null part in the hashmap key, so the key would become something like `(bool, Vec<u8>)` as hashmap key. A null key would have `(False, vec![])` as value, an actual value would contain a True and the bytes of the contents.
   That will be a bit slower and consume more memory, but at least allows to group on nulls too.


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