Consider cheap startup paths in add_paths_to_append_rel

6b94e7a6d did this for ordered append paths to allow fast startup
MergeAppends, however, nothing was done for the Append case.

Here we adjust add_paths_to_append_rel() to have it build an AppendPath
containing the cheapest startup paths from each of the child relations
when the append rel has "consider_startup" set.

Author: Andy Fan, David Rowley
Discussion: https://www.postgresql.org/message-id/CAKU4AWrXSkUV=Pt-gRxQT7EbfUeNssprGyNsB=5mJibFZ6S3ww@mail.gmail.com

src/backend/optimizer/path/allpaths.c

@@ -1307,6 +1307,8 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 {
 	List	   *subpaths = NIL;
 	bool		subpaths_valid = true;
+	List	   *startup_subpaths = NIL;
+	bool		startup_subpaths_valid = true;
 	List	   *partial_subpaths = NIL;
 	List	   *pa_partial_subpaths = NIL;
 	List	   *pa_nonpartial_subpaths = NIL;
@@ -1346,6 +1348,20 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 		else
 			subpaths_valid = false;
 
+		/*
+		 * When the planner is considering cheap startup plans, we'll also
+		 * collect all the cheapest_startup_paths and build an AppendPath
+		 * containing those as subpaths.
+		 */
+		if (rel->consider_startup && childrel->pathlist != NIL &&
+			childrel->cheapest_startup_path->param_info == NULL)
+			accumulate_append_subpath(childrel->cheapest_startup_path,
+									  &startup_subpaths,
+									  NULL);
+		else
+			startup_subpaths_valid = false;
+
+
 		/* Same idea, but for a partial plan. */
 		if (childrel->partial_pathlist != NIL)
 		{
@@ -1478,6 +1494,11 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 												  NIL, NULL, 0, false,
 												  -1));
 
+	/* build an AppendPath for the cheap startup paths, if valid */
+	if (startup_subpaths_valid)
+		add_path(rel, (Path *) create_append_path(root, rel, startup_subpaths,
+												  NIL, NIL, NULL, 0, false, -1));
+
 	/*
 	 * Consider an append of unordered, unparameterized partial paths.  Make
 	 * it parallel-aware if possible.

src/test/regress/expected/union.out

@@ -1432,3 +1432,24 @@ where (x = 0) or (q1 >= q2 and q1 <= q2);
  4567890123456789 |  4567890123456789 | 1
 (6 rows)
 
+--
+-- Test the planner's ability to produce cheap startup plans with Append nodes
+--
+-- Ensure we get a Nested Loop join between tenk1 and tenk2
+explain (costs off)
+select t1.unique1 from tenk1 t1
+inner join tenk2 t2 on t1.tenthous = t2.tenthous
+   union all
+(values(1)) limit 1;
+                       QUERY PLAN                       
+--------------------------------------------------------
+ Limit
+   ->  Append
+         ->  Nested Loop
+               Join Filter: (t1.tenthous = t2.tenthous)
+               ->  Seq Scan on tenk1 t1
+               ->  Materialize
+                     ->  Seq Scan on tenk2 t2
+         ->  Result
+(8 rows)
+

src/test/regress/sql/union.sql

@@ -540,3 +540,14 @@ select * from
    union all
    select *, 1 as x from int8_tbl b) ss
 where (x = 0) or (q1 >= q2 and q1 <= q2);
+
+--
+-- Test the planner's ability to produce cheap startup plans with Append nodes
+--
+
+-- Ensure we get a Nested Loop join between tenk1 and tenk2
+explain (costs off)
+select t1.unique1 from tenk1 t1
+inner join tenk2 t2 on t1.tenthous = t2.tenthous
+   union all
+(values(1)) limit 1;