yjshen commented on issue #5325:
URL:
https://github.com/apache/arrow-datafusion/issues/5325#issuecomment-1444994728
We could rewrite an aggregate query with distinct aggregations into an
expanded double aggregation, as Spark did:
Before
```
Aggregate(
key = ['key]
functions = [COUNT(DISTINCT 'cat1),
COUNT(DISTINCT 'cat2),
sum('value)]
output = ['key, 'cat1_cnt, 'cat2_cnt, 'total])
LocalTableScan [...]
```
After rewrite:
```
Aggregate(
key = ['key]
functions = [count('cat1) FILTER (WHERE 'gid = 1),
count('cat2) FILTER (WHERE 'gid = 2),
first('total) ignore nulls FILTER (WHERE 'gid = 0)]
output = ['key, 'cat1_cnt, 'cat2_cnt, 'total])
Aggregate(
key = ['key, 'cat1, 'cat2, 'gid]
functions = [sum('value)]
output = ['key, 'cat1, 'cat2, 'gid, 'total])
Expand(
projections = [('key, null, null, 0, cast('value as bigint)),
('key, 'cat1, null, 1, null),
('key, null, 'cat2, 2, null)]
output = ['key, 'cat1, 'cat2, 'gid, 'value])
LocalTableScan [...]
```
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