Improve handling of CustomPath/CustomPlan(State) children.

Allow CustomPath to have a list of paths, CustomPlan a list of plans,
and CustomPlanState a list of planstates known to the core system, so
that custom path/plan providers can more reasonably use this
infrastructure for nodes with multiple children.

KaiGai Kohei, per a design suggestion from Tom Lane, with some
further kibitzing by me.

src/backend/commands/explain.c

@@ -115,6 +115,8 @@ static void ExplainMemberNodes(List *plans, PlanState **planstates,
 				   List *ancestors, ExplainState *es);
 static void ExplainSubPlans(List *plans, List *ancestors,
 				const char *relationship, ExplainState *es);
+static void ExplainCustomChildren(CustomScanState *css,
+								  List *ancestors, ExplainState *es);
 static void ExplainProperty(const char *qlabel, const char *value,
 				bool numeric, ExplainState *es);
 static void ExplainOpenGroup(const char *objtype, const char *labelname,
@@ -1624,6 +1626,8 @@ ExplainNode(PlanState *planstate, List *ancestors,
 		IsA(plan, BitmapAnd) ||
 		IsA(plan, BitmapOr) ||
 		IsA(plan, SubqueryScan) ||
+		(IsA(planstate, CustomScanState) &&
+		 ((CustomScanState *) planstate)->custom_ps != NIL) ||
 		planstate->subPlan;
 	if (haschildren)
 	{
@@ -1678,6 +1682,10 @@ ExplainNode(PlanState *planstate, List *ancestors,
 			ExplainNode(((SubqueryScanState *) planstate)->subplan, ancestors,
 						"Subquery", NULL, es);
 			break;
+		case T_CustomScan:
+			ExplainCustomChildren((CustomScanState *) planstate,
+								  ancestors, es);
+			break;
 		default:
 			break;
 	}
@@ -2647,6 +2655,20 @@ ExplainSubPlans(List *plans, List *ancestors,
 	}
 }
 
+/*
+ * Explain a list of children of a CustomScan.
+ */
+static void
+ExplainCustomChildren(CustomScanState *css, List *ancestors, ExplainState *es)
+{
+	ListCell   *cell;
+	const char *label =
+		(list_length(css->custom_ps) != 1 ? "children" : "child");
+
+	foreach (cell, css->custom_ps)
+		ExplainNode((PlanState *) lfirst(cell), ancestors, label, NULL, es);
+}
+
 /*
  * Explain a property, such as sort keys or targets, that takes the form of
  * a list of unlabeled items.  "data" is a list of C strings.

src/backend/optimizer/plan/createplan.c

@@ -2157,6 +2157,16 @@ create_customscan_plan(PlannerInfo *root, CustomPath *best_path,
 {
 	CustomScan *cplan;
 	RelOptInfo *rel = best_path->path.parent;
+	List	   *custom_plans = NIL;
+	ListCell   *lc;
+
+	/* Recursively transform child paths. */
+	foreach (lc, best_path->custom_paths)
+	{
+		Plan   *plan = create_plan_recurse(root, (Path *) lfirst(lc));
+
+		custom_plans = lappend(custom_plans, plan);
+	}
 
 	/*
 	 * Sort clauses into the best execution order, although custom-scan
@@ -2172,7 +2182,8 @@ create_customscan_plan(PlannerInfo *root, CustomPath *best_path,
 															  rel,
 															  best_path,
 															  tlist,
-															  scan_clauses);
+															  scan_clauses,
+															  custom_plans);
 	Assert(IsA(cplan, CustomScan));
 
 	/*

src/backend/optimizer/plan/setrefs.c

@@ -1151,6 +1151,8 @@ set_customscan_references(PlannerInfo *root,
 						  CustomScan *cscan,
 						  int rtoffset)
 {
+	ListCell   *lc;
+
 	/* Adjust scanrelid if it's valid */
 	if (cscan->scan.scanrelid > 0)
 		cscan->scan.scanrelid += rtoffset;
@@ -1194,6 +1196,12 @@ set_customscan_references(PlannerInfo *root,
 			fix_scan_list(root, cscan->custom_exprs, rtoffset);
 	}
 
+	/* Adjust child plan-nodes recursively, if needed */
+	foreach (lc, cscan->custom_plans)
+	{
+		lfirst(lc) = set_plan_refs(root, (Plan *) lfirst(lc), rtoffset);
+	}
+
 	/* Adjust custom_relids if needed */
 	if (rtoffset > 0)
 	{

src/backend/optimizer/plan/subselect.c

@@ -2373,10 +2373,27 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 			break;
 
 		case T_CustomScan:
-			finalize_primnode((Node *) ((CustomScan *) plan)->custom_exprs,
-							  &context);
-			/* We assume custom_scan_tlist cannot contain Params */
-			context.paramids = bms_add_members(context.paramids, scan_params);
+			{
+				CustomScan *cscan = (CustomScan *) plan;
+				ListCell   *lc;
+
+				finalize_primnode((Node *) cscan->custom_exprs,
+								  &context);
+				/* We assume custom_scan_tlist cannot contain Params */
+				context.paramids =
+					bms_add_members(context.paramids, scan_params);
+
+				/* child nodes if any */
+				foreach (lc, cscan->custom_plans)
+				{
+					context.paramids =
+						bms_add_members(context.paramids,
+										finalize_plan(root,
+													  (Plan *) lfirst(lc),
+													  valid_params,
+													  scan_params));
+				}
+			}
 			break;
 
 		case T_ModifyTable: