alamb edited a comment 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 your suggestion to get an implementation in (this one)
and then iterate as needed
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