Function execute_dfplan_body 

pub fn execute_dfplan_body(
    body: &str,
    session: &SessionContext,
    reader: Option<Arc<dyn ShufflePartitionReader>>,
) -> SqlResult<(SchemaRef, SqlStream)>

Decode a dfplan body and execute its assigned partition (executor seam).

Keeps DataFusion types out of the executor crate: the result streams as the crate-level crate::SqlStream. session supplies the runtime environment (memory pool, object stores); the decoded plan needs no tables registered on it. Map-stage plans read upstream shuffle data through reader; passing None leaves any ShuffleReadExec leaves unexecutable (coordinator-side decode).