pg_cost_append_v1.patch

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index ce32b8a4..c0bf602 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -1742,6 +1742,33 @@ cost_sort(Path *path, PlannerInfo *root,
 }
 
 /*
+ * cost_append
+ *	  Determines and returns the cost of an Append node.
+ *
+ * Though Append doesn't do any selection or projection, it's not free.  So we
+ * try to add cost per input tuple which is arbitrarily calculated as
+ * DEFAULT_APPEND_COST_FACTOR * cpu_tuple_cost.
+ *
+ * 'input_startup_cost' is the sum of the input streams' startup costs
+ * 'input_total_cost' is the sum of the input streams' total costs
+ * 'tuples' is the number of tuples in all the streams
+ */
+void
+cost_append(Path *path,
+			Cost input_startup_cost, Cost input_total_cost,
+			double tuples)
+{
+	Cost		startup_cost = 0;
+	Cost		run_cost = 0;
+
+	/* Add Append node overhead. */
+	run_cost += cpu_tuple_cost * DEFAULT_APPEND_COST_FACTOR * tuples;
+
+	path->startup_cost = startup_cost + input_startup_cost;
+	path->total_cost = startup_cost + run_cost + input_total_cost;
+}
+
+/*
  * cost_merge_append
  *	  Determines and returns the cost of a MergeAppend node.
  *
@@ -1800,6 +1827,9 @@ cost_merge_append(Path *path, PlannerInfo *root,
 	 */
 	run_cost += cpu_operator_cost * tuples;
 
+	/* Add MergeAppend node overhead like we do it for the Append node */
+	run_cost += cpu_tuple_cost * DEFAULT_APPEND_COST_FACTOR * tuples;
+
 	path->startup_cost = startup_cost + input_startup_cost;
 	path->total_cost = startup_cost + run_cost + input_total_cost;
 }
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 2d491eb..c8fdf1c 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1212,6 +1212,8 @@ create_append_path(RelOptInfo *rel, List *subpaths, Relids required_outer,
 				   int parallel_workers, List *partitioned_rels)
 {
 	AppendPath *pathnode = makeNode(AppendPath);
+	Cost		input_startup_cost;
+	Cost		input_total_cost;
 	ListCell   *l;
 
 	pathnode->path.pathtype = T_Append;
@@ -1227,32 +1229,31 @@ create_append_path(RelOptInfo *rel, List *subpaths, Relids required_outer,
 	pathnode->subpaths = subpaths;
 
 	/*
-	 * We don't bother with inventing a cost_append(), but just do it here.
-	 *
-	 * Compute rows and costs as sums of subplan rows and costs.  We charge
-	 * nothing extra for the Append itself, which perhaps is too optimistic,
-	 * but since it doesn't do any selection or projection, it is a pretty
-	 * cheap node.
+	 * Add up the sizes and costs of the input paths.
 	 */
 	pathnode->path.rows = 0;
-	pathnode->path.startup_cost = 0;
-	pathnode->path.total_cost = 0;
+	input_startup_cost = 0;
+	input_total_cost = 0;
 	foreach(l, subpaths)
 	{
 		Path	   *subpath = (Path *) lfirst(l);
 
 		pathnode->path.rows += subpath->rows;
-
-		if (l == list_head(subpaths))	/* first node? */
-			pathnode->path.startup_cost = subpath->startup_cost;
-		pathnode->path.total_cost += subpath->total_cost;
 		pathnode->path.parallel_safe = pathnode->path.parallel_safe &&
 			subpath->parallel_safe;
 
+		if (l == list_head(subpaths))	/* first node? */
+			input_startup_cost = subpath->startup_cost;
+		input_total_cost += subpath->total_cost;
+
 		/* All child paths must have same parameterization */
 		Assert(bms_equal(PATH_REQ_OUTER(subpath), required_outer));
 	}
 
+	/* Now we can compute total costs of the Append */
+	cost_append(&pathnode->path, input_startup_cost, input_total_cost,
+				pathnode->path.rows);
+
 	return pathnode;
 }
 
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h
index 306d923..cd42e5a 100644
--- a/src/include/optimizer/cost.h
+++ b/src/include/optimizer/cost.h
@@ -31,6 +31,12 @@
 
 #define DEFAULT_EFFECTIVE_CACHE_SIZE  524288	/* measured in pages */
 
+/*
+ * Arbitrarily use 50% of the cpu_tuple_cost to cost append node. Note that
+ * this value should be multiplied with cpu_tuple_cost wherever applicable.
+ */
+#define DEFAULT_APPEND_COST_FACTOR 0.5
+
 typedef enum
 {
 	CONSTRAINT_EXCLUSION_OFF,	/* do not use c_e */
@@ -106,6 +112,9 @@ extern void cost_sort(Path *path, PlannerInfo *root,
 		  List *pathkeys, Cost input_cost, double tuples, int width,
 		  Cost comparison_cost, int sort_mem,
 		  double limit_tuples);
+extern void cost_append(Path *path,
+			Cost input_startup_cost, Cost input_total_cost,
+			double tuples);
 extern void cost_merge_append(Path *path, PlannerInfo *root,
 				  List *pathkeys, int n_streams,
 				  Cost input_startup_cost, Cost input_total_cost,