Github user yhuai commented on the pull request:
https://github.com/apache/spark/pull/10228#issuecomment-163365216
oh, just realized that the plan for a query like `SELECT COUNT(DISTINCT a)
FROM table` will be
```
AGG-2 (count distinct)
Shuffle to a single reducer
AGG-1 (grouping on a)
Shuffle by a
Partial-AGG-1 (grouping on a)
```
Ideally, we should still use four aggregate operators like the one shown
below but without the overhead of using Expand.
```
AGG-2 (count distinct)
Shuffle to a single reducer
Partial-AGG-2 (count distinct)
AGG-1 (grouping on a)
Shuffle by a
Partial-AGG-1 (grouping on a)
```
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