goldmedal opened a new issue, #13601:
URL: https://github.com/apache/datafusion/issues/13601
### Is your feature request related to a problem or challenge?
Given a SQL:
```SQL
select * from unnest([1,2,3]) as t(c1)
```
DataFusion plans the unnest to `Projection/Unnest/Projection` pattern like
```
Projection: *
SubqueryAlias: t
Projection: UNNEST(make_array(Int64(1),Int64(2),Int64(3))) AS c1
Projection:
unnest_placeholder(make_array(Int64(1),Int64(2),Int64(3)),depth=1) AS
UNNEST(make_array(Int64(1),Int64(2),Int64(3)))
Unnest:
lists[unnest_placeholder(make_array(Int64(1),Int64(2),Int64(3)))|depth=1]
structs[]
Projection: make_array(Int64(1), Int64(2), Int64(3)) AS
unnest_placeholder(make_array(Int64(1),Int64(2),Int64(3)))
EmptyRelation
```
If we try to unparse the plan back to the SQL, the result is
```
SELECT * FROM (SELECT UNNEST([1, 2, 3]) AS
"UNNEST(make_array(Int64(1),Int64(2),Int64(3)))") AS t (c1)
```
However, some databases (e.g.
[BigQuery](https://cloud.google.com/bigquery/docs/arrays#querying_nested_arrays)),
only support using `UNNEST` as a table function.
### Describe the solution you'd like
We can introduce a new option for the unparser `Dialect`
```rust
pub trait Dialect: Send + Sync {
...
/// Allow to unparse the unnest plan as a table function.
fn unnest_as_table_function(&self) -> bool {
false
}
...
}
```
If we found the pattern of plan like `Projection/Unnest/Projection`, we can
try to unparse it to an `ast::query::TableFactor::UNNEST`.
### Describe alternatives you've considered
_No response_
### Additional context
We may need to ignore other instances of `UNNEST` usage, such as:
```sql
SELECT UNNEST(column1), UNNEST(column2) FROM (SELECT column1, column2 FROM
xxx);
```
This issue aims to enable a roundtrip for the `UNNEST` table function.
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