alamb opened a new issue, #7786:
URL: https://github.com/apache/arrow-datafusion/issues/7786
### Is your feature request related to a problem or challenge?
The current the plan for a nested `UNION` is still nested resulting in
unecessary GROUPing
```
❯ explain SELECT 1 UNION (SELECT 1 UNION SELECT 1);
+---------------+-----------------------------------------------------------------------------------------+
| plan_type | plan
|
+---------------+-----------------------------------------------------------------------------------------+
| logical_plan | Aggregate: groupBy=[[Int64(1)]], aggr=[[]]
|
| | Union
|
| | Projection: Int64(1)
|
| | EmptyRelation
|
| | Aggregate: groupBy=[[Int64(1)]], aggr=[[]]
|
| | Union
|
| | Projection: Int64(1)
|
| | EmptyRelation
|
| | Projection: Int64(1)
|
| | EmptyRelation
|
```
### Describe the solution you'd like
As suggested by @jackwener:
https://github.com/apache/arrow-datafusion/pull/7695#discussion_r1349639782,
ideally the plan should look like the following (the unions should be unnested
and run through a single group by):
```
❯ explain SELECT 1 UNION (SELECT 1 UNION SELECT 1);
+---------------+-----------------------------------------------------------------------------------------+
| plan_type | plan
|
+---------------+-----------------------------------------------------------------------------------------+
| logical_plan | Aggregate: groupBy=[[Int64(1)]], aggr=[[]]
|
| | Union
|
| | Projection: Int64(1)
|
| | EmptyRelation
|
| | Projection: Int64(1)
|
| | EmptyRelation
|
| | Projection: Int64(1)
|
| | EmptyRelation
|
```
### Describe alternatives you've considered
_No response_
### Additional context
@maruschin added a nice optimization (as well as very nice tests) to unnest
`UNION ALL` in https://github.com/apache/arrow-datafusion/pull/7695
```sql
❯ explain SELECT 1 UNION ALL (SELECT 1 UNION ALL SELECT 1);
+---------------+----------------------------------------+
| plan_type | plan |
+---------------+----------------------------------------+
| logical_plan | Union |
| | Projection: Int64(1) AS Int64(1) |
| | EmptyRelation |
| | Projection: Int64(1) AS Int64(1) |
| | EmptyRelation |
| | Projection: Int64(1) AS Int64(1) |
| | EmptyRelation |
```
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