address review: remove unnecessary stats computation in reverse path, improve docs

- Remove dead stats computation in reverse_file_groups branch
  (reverse path is always Inexact, so all_non_overlapping is unused)
- Add reverse prefix matching documentation to pushdown_sort module

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: zhuqi-lucas

datafusion/datasource/src/file_scan_config.rs

@@ -1405,24 +1405,11 @@ impl FileScanConfig {
                 false
             }
         } else {
-            // When reversing, check if reversed groups are non-overlapping
-            if let Some(sort_order) = LexOrdering::new(order.iter().cloned()) {
-                let projected_schema = new_config.projected_schema()?;
-                let projection_indices = new_config
-                    .file_source
-                    .projection()
-                    .as_ref()
-                    .and_then(|p| ordered_column_indices_from_projection(p));
-                let result = Self::sort_files_within_groups_by_statistics(
-                    &new_config.file_groups,
-                    &sort_order,
-                    &projected_schema,
-                    projection_indices.as_deref(),
-                );
-                result.all_non_overlapping
-            } else {
-                false
-            }
+            // When reversing, files are already reversed above. We skip
+            // statistics-based sorting here because it would undo the reversal.
+            // Note: reverse path is always Inexact, so all_non_overlapping
+            // is not used (is_exact is false).
+            false
         };
 
         if is_exact && all_non_overlapping {

datafusion/physical-optimizer/src/pushdown_sort.rs

@@ -47,7 +47,9 @@
 //! - **Non-overlapping detection**: when files have non-overlapping ranges and matching
 //!   within-file ordering, the combined scan is `Exact` (sort eliminated)
 //! - **Prefix matching**: if data has ordering [A DESC, B ASC] and query needs
-//!   [A DESC], the existing ordering satisfies the requirement
+//!   [A DESC], the existing ordering satisfies the requirement (`Exact`).
+//!   If the query needs [A ASC] (reverse of the prefix), a reverse scan is
+//!   used (`Inexact`, `SortExec` retained)
 //!
 //! Related issue: <https://github.com/apache/datafusion/issues/17348>