c

crates/polars-plan/src/plans/optimizer/projection_pushdown/projection.rs

@@ -72,6 +72,7 @@ pub(super) fn process_projection(
             expr_arena.add(AExpr::Column(name.clone()))
         };
 
+        // Clear all accumulated projections since we only project a single column from this level.
         acc_projections.clear();
         projected_names.clear();
         add_expr_to_accumulated(expr, &mut acc_projections, &mut projected_names, expr_arena);
@@ -82,6 +83,8 @@ pub(super) fn process_projection(
         // names that are created on this level.
         let mut remove_names = PlHashSet::new();
 
+        // If there are non-scalar projections we must project at least one of them to maintain the
+        // output height.
         let mut opt_non_scalar = None;
         let mut projection_has_non_scalar = false;
 
@@ -120,6 +123,7 @@ pub(super) fn process_projection(
             })
             .collect();
 
+        // Remove aliased before adding new ones.
         if !remove_names.is_empty() {
             if !projected_names.is_empty() {
                 for name in remove_names.iter() {