Optimize generate_orderedappend_paths

david-rowley · david-rowley · commit 94cad7a3e604 · 2023-02-20T22:48:58.000+13:00
In generate_orderedappend_paths(), when match_partition_order_desc was true, we would lcons() items to various lists in a loop over each live partition. When the number of live partitions was large, the lcons() could show up in profiles due to it having to perform memmove() to make way for the new list item. Here we adjust things so that we just perform the loop over the live partitions backwards when match_partition_order_desc is true. This allows us to simplify the logic in the loop. Now, as far as the guts of the loop knows, there's no difference between match_partition_order and match_partition_order_desc. We can just set match_partition_order to true so that we build the correct list of paths for the asc and desc case. Per idea from Andres Freund. Discussion: https://postgr.es/m/20230217002351.nyt4y5tdzg6hugdt@awork3.anarazel.de
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
@@ -1700,9 +1700,11 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
 		List	   *total_subpaths = NIL;
 		List	   *fractional_subpaths = NIL;
 		bool		startup_neq_total = false;
-		ListCell   *lcr;
 		bool		match_partition_order;
 		bool		match_partition_order_desc;
+		int			end_index;
+		int			first_index;
+		int			direction;
 
 		/*
 		 * Determine if this sort ordering matches any partition pathkeys we
@@ -1723,10 +1725,38 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
 			 (!partition_pathkeys_desc_partial &&
 			  pathkeys_contained_in(partition_pathkeys_desc, pathkeys)));
 
+		/*
+		 * When the required pathkeys match the reverse of the partition
+		 * order, we must build the list of paths in reverse starting with the
+		 * last matching partition first.  We can get away without making any
+		 * special cases for this in the loop below by just looping backward
+		 * over the child relations in this case.
+		 */
+		if (match_partition_order_desc)
+		{
+			/* loop backward */
+			first_index = list_length(live_childrels) - 1;
+			end_index = -1;
+			direction = -1;
+
+			/*
+			 * Set this to true to save us having to check for
+			 * match_partition_order_desc in the loop below.
+			 */
+			match_partition_order = true;
+		}
+		else
+		{
+			/* for all other case, loop forward */
+			first_index = 0;
+			end_index = list_length(live_childrels);
+			direction = 1;
+		}
+
 		/* Select the child paths for this ordering... */
-		foreach(lcr, live_childrels)
+		for (int i = first_index; i != end_index; i += direction)
 		{
-			RelOptInfo *childrel = (RelOptInfo *) lfirst(lcr);
+			RelOptInfo *childrel = list_nth_node(RelOptInfo, live_childrels, i);
 			Path	   *cheapest_startup,
 					   *cheapest_total,
 					   *cheapest_fractional = NULL;
@@ -1822,25 +1852,6 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
 					fractional_subpaths = lappend(fractional_subpaths, cheapest_fractional);
 				}
 			}
-			else if (match_partition_order_desc)
-			{
-				/*
-				 * As above, but we need to reverse the order of the children,
-				 * because nodeAppend.c doesn't know anything about reverse
-				 * ordering and will scan the children in the order presented.
-				 */
-				cheapest_startup = get_singleton_append_subpath(cheapest_startup);
-				cheapest_total = get_singleton_append_subpath(cheapest_total);
-
-				startup_subpaths = lcons(cheapest_startup, startup_subpaths);
-				total_subpaths = lcons(cheapest_total, total_subpaths);
-
-				if (cheapest_fractional)
-				{
-					cheapest_fractional = get_singleton_append_subpath(cheapest_fractional);
-					fractional_subpaths = lcons(cheapest_fractional, fractional_subpaths);
-				}
-			}
 			else
 			{
 				/*
@@ -1859,7 +1870,7 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
 		}
 
 		/* ... and build the Append or MergeAppend paths */
-		if (match_partition_order || match_partition_order_desc)
+		if (match_partition_order)
 		{
 			/* We only need Append */
 			add_path(rel, (Path *) create_append_path(root,
