perf(cube): logical plan optimization: combine Limit(fetch)>Sort into Sort(fetch) in one pass

Author: srh

datafusion/optimizer/src/push_down_limit.rs

@@ -136,8 +136,13 @@ impl OptimizerRule for PushDownLimit {
                     }
                 } else {
                     sort.fetch = new_fetch;
-                    limit.input = Arc::new(LogicalPlan::Sort(sort));
-                    Ok(Transformed::yes(LogicalPlan::Limit(limit)))
+                    let new_plan = if skip > 0 {
+                        limit.input = Arc::new(LogicalPlan::Sort(sort));
+                        LogicalPlan::Limit(limit)
+                    } else {
+                        LogicalPlan::Sort(sort)
+                    };
+                    Ok(Transformed::yes(new_plan))
                 }
             }
             LogicalPlan::Projection(mut proj) => {