goldmedal opened a new issue, #13621:
URL: https://github.com/apache/datafusion/issues/13621
### Is your feature request related to a problem or challenge?
Given a SQL like
```sql
WITH vals AS
(
SELECT 1 x, 'a' y UNION ALL
SELECT 1 x, 'b' y UNION ALL
SELECT 2 x, 'a' y UNION ALL
SELECT 2 x, 'c' y
)
SELECT x, y
FROM vals
```
The unoptimized plan is
```
Projection: vals.x, vals.y
SubqueryAlias: vals
Union
Union
Union
Projection: Int64(1) AS x, Utf8("a") AS y
EmptyRelation
Projection: Int64(1) AS x, Utf8("b") AS y
EmptyRelation
Projection: Int64(2) AS x, Utf8("a") AS y
EmptyRelation
Projection: Int64(2) AS x, Utf8("c") AS y
EmptyRelation
```
It's fine for the unparser. However, after `EliminateNestedUnion`
optimization rule, the nested union will be flatten to one union like
```
Projection: vals.x, vals.y
SubqueryAlias: vals
Union
Projection: Int64(1) AS x, Utf8("a") AS y
EmptyRelation
Projection: Int64(1) AS x, Utf8("b") AS y
EmptyRelation
Projection: Int64(2) AS x, Utf8("a") AS y
EmptyRelation
Projection: Int64(2) AS x, Utf8("c") AS y
EmptyRelation
```
If we tried to unparse this plan, we will get the error message:
```
Error: This feature is not implemented: UNION ALL expected 2 inputs, but
found 4
```
### Describe the solution you'd like
Currently, the unpaser only supports unparsing an union with 2 input
https://github.com/apache/datafusion/blob/172e5573270ae4a3d152d5bf15c5c0008595090e/datafusion/sql/src/unparser/plan.rs#L630-L635
We should support unparsing the union with more 2 inputs for the Unparser.
### Describe alternatives you've considered
_No response_
### Additional context
_No response_
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