goldmedal opened a new issue, #13753:
URL: https://github.com/apache/datafusion/issues/13753
### Is your feature request related to a problem or challenge?
`LogicalPlan::Extension` allows the user to implement their custom logical
plan. I think it makes sense to allow to define custom unparsing behavior for
it.
### Describe the solution you'd like
I would like to introduce a new method for `UserDefinedLogicalNode`
```rust
/// User-defined nodes can override this method to provide a custom
/// implementation for the unparser.
fn unparse(
&self,
unparser: &Unparser,
query: &mut Option<QueryBuilder>,
select: &mut Option<SelectBuilder>,
relation: &mut Option<RelationBuilder>,
table_with_joins: &mut Option<TableWithJoinsBuilder>,
) -> Result<()> {
not_impl_err!("custom unparsing not implemented")
}
```
Then, we can handle `LogicalPlan::Extension` in the unparser like
```rust
LogicalPlan::Extension(extension) => {
extension.node.unparse(self, plan, query, Some(select),
Some(relation), None)
},
```
However, the required builders haven't been made public yet.
`UserDefinedLogicalNode` can't access the builders.
As per the comment in `ast.rs`, we planned to move builders to sqlparser-rs.
https://github.com/apache/datafusion/blob/6ac199919f323f1121f1662349ed93de3446c72a/datafusion/sql/src/unparser/ast.rs#L18-L22
I'm contemplating whether this move is required. When implementing a new
unparsing feature, we may need to add some additional helper functions to the
builder (e.g., `SelectBuilder::already_projection`). If we move it to the
upstream crate, it would be difficult to add helper functions when required.
I prefer to move builders to `datafusion-common` and make them public for
the user and `datafusion-expr`, where the `UserDefinedLogicalNode` is located.
### Describe alternatives you've considered
_No response_
### Additional context
_No response_
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