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  1. Apache Drill
  2. DRILL-3130

Project can be pushed below union all / union to improve performance

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      A query such as

      Select a from 
      (select a, b, c, ..., union all select a, b, c, ...)
      

      will perform Union-All over all the specified columns on the two sides, despite the fact that only one column is asked for at the end. Ideally, we should perform ProjectPushDown rule for Union & Union-All to avoid them to generate results which will be discarded at the end.

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            vitalii Vitalii Diravka
            seanhychu Sean Hsuan-Yi Chu
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