[GitHub] [arrow] alamb commented on pull request #8222: ARROW-10043: [Rust][DataFusion] Implement COUNT(DISTINCT col)

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alamb commented on pull request #8222:
URL: https://github.com/apache/arrow/pull/8222#issuecomment-696842287


   @drusso  I think you are correct that we would need a separate group by operator for each count distinct and then combine them together:
   
   so `SELECT c1, COUNT(DISTINCT c2), COUNT(DISTINCT c3) FROM t1 GROUP BY c1` might look like
   
   ```
   HashAggregateExec: // this second phase then counts
     group_expr:
       Column(c1)
     aggr_expr:
       CountReduce(Column(c2))
     input:
       HashAggregateExec: // this first agg expr finds all distinct values of (c1,c2)
         group_expr:
           Column(c1), Column(c2)
           input:
             CsvExec:
   
   JOIN ON (c1):
   
   HashAggregateExec: // this second phase then counts
     group_expr:
       Column(c1)
     aggr_expr:
       CountReduce(Column(c3))
     input:
       HashAggregateExec: // this first agg expr finds all distinct values of (c1,c2)
         group_expr:
           Column(c1), Column(c3)
           input:
             CsvExec:
   ```
   
   Or something. I like you suggestion to get an implementation in (this one) and then iterate as needed


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