[GitHub] [druid] imply-cheddar commented on pull request #13764: Fixing a data correctness issue in unnest when first row of an MVD is null

["imply-cheddar (via GitHub)" <gi...@apache.org>]

imply-cheddar commented on PR #13764:
URL: https://github.com/apache/druid/pull/13764#issuecomment-1422101133

   Did I understand you correctly that the previous (bad?) behavior was
   
   ```
   select * from mytest1, unnest(mv_to_array(c2)) as unnested(c3)
   
   __time c1 c2 c3
   2022-01-01T00:00:00.000Z 1 null null
   2022-01-01T00:00:00.000Z 2 ["A","B","C"] A
   2022-01-01T00:00:00.000Z 2 ["A","B","C"] B
   2022-01-01T00:00:00.000Z 2 ["A","B","C"] C
   ```
   
   I.e. when it gets to the second row that has an array of 3 values, it unnests it into 3 rows.
   
   And you changed the code to do
   
   ```
   select * from mytest1, unnest(mv_to_array(c2)) as unnested(c3)
   
   __time c1 c2 c3
   2022-01-01T00:00:00.000Z 1 null null
   2022-01-01T00:00:00.000Z 2 ["A","B","C"] null
   ```
   
   I.e. if it sees a null, it will use the null going forward an not unnest anything?
   
   If that understanding is correct, can you explain why the previous behavior is not the correct behavior?  It is what I had expected at least...


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