fix: gracefully skip SortExec when sort columns are not in the output schema

When a query like `SELECT name FROM t ORDER BY id` is federated, the
sort column (`id`) is not in the plan's output schema (`[name]`).
`create_physical_sort_expr` fails because it cannot resolve `id`
against the output schema.

Previously this returned an error, breaking such queries. Now we
gracefully skip adding the local SortExec and rely on the remote
database to handle the sorting via the generated SQL.

Author: lukekim

datafusion-federation/src/sql/mod.rs

@@ -195,7 +195,8 @@ impl FederationPlanner for SQLFederationPlanner {
                 .collect::<Result<Vec<_>>>()
             {
                 Ok(physical_sort_exprs) if physical_sort_exprs.is_empty() => {
-                    return Err(DataFusionError::Plan("Top-level Sort was detected, but no physical sort expressions could be created".to_string()));
+                    // No sort expressions could be created — skip the local
+                    // SortExec and rely on the remote database for ordering.
                 }
                 Ok(physical_sort_exprs) => {
                     if let Some(lex_ordering) = LexOrdering::new(physical_sort_exprs) {
@@ -204,11 +205,11 @@ impl FederationPlanner for SQLFederationPlanner {
                         ));
                     }
                 }
-                Err(e) => {
-                    return Err(DataFusionError::Context(
-                        "Failed to create `PhysicalSortExpr`".to_string(),
-                        Box::new(e),
-                    ))
+                Err(_) => {
+                    // Sort columns are not in the output schema (e.g.
+                    // `SELECT name FROM t ORDER BY id`). The remote database
+                    // will handle sorting via the generated SQL; skip the
+                    // local SortExec.
                 }
             };
         }