bvolpato opened a new issue, #20971:
URL: https://github.com/apache/datafusion/issues/20971

   ### Describe the bug
   
   The Substrait producer and consumer do not support DataFusion's 
`Expr::Placeholder` (bind parameters like `$1`, `$name`). Both directions 
return `not_impl_err!`:
   
   **Producer** 
([producer/expr/mod.rs:145](https://github.com/apache/datafusion/blob/main/datafusion/substrait/src/logical_plan/producer/expr/mod.rs#L145)):
   ```rust
   Expr::Placeholder(expr) => not_impl_err!("Cannot convert {expr:?} to 
Substrait"),
   ```
   
   **Consumer** 
([substrait_consumer.rs:367-373](https://github.com/apache/datafusion/blob/main/datafusion/substrait/src/logical_plan/consumer/substrait_consumer.rs#L367-L373)):
   ```rust
   async fn consume_dynamic_parameter(
       &self,
       _expr: &DynamicParameter,
       _input_schema: &DFSchema,
   ) -> datafusion::common::Result<Expr> {
       not_impl_err!("Dynamic Parameter expression not supported")
   }
   ```
   
   ### To Reproduce
   
   Any query involving bind parameters (e.g. prepared statements) cannot be 
serialized to or deserialized from Substrait:
   
   ```rust
   // Producer: fails
   let expr = Expr::Placeholder(Placeholder::new_with_field("$1".into(), 
Some(Arc::new(Field::new("$1", DataType::Int32, false)))));
   // Cannot convert to Substrait
   
   // Consumer: fails
   // A Substrait plan containing DynamicParameter cannot be consumed
   ```
   
   ### Expected behavior
   
   The mapping between DataFusion `Placeholder` and Substrait 
`DynamicParameter` should work in both directions:
   
   | DataFusion | Substrait |
   |---|---|
   | `Placeholder.id` (`"$1"`) | `DynamicParameter.parameter_reference` 
(extracted index) |
   | `Placeholder.field` (type info) | `DynamicParameter.type` |
   
   ### Additional context
   
   `DynamicParameter` was recently added to the Substrait spec and is now 
supported across the ecosystem:
   - **substrait-java**: [PR 
#752](https://github.com/substrait-io/substrait-java/pull/752) (merged)
   - **substrait-go**: [PR 
#215](https://github.com/substrait-io/substrait-go/pull/215) (open)
   - **substrait-rs**: [PR 
#481](https://github.com/substrait-io/substrait-rs/pull/481) (open, CI green)
   
   DataFusion is the last major consumer without support.


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