Fix missing outfuncs.c support for IncrementalSortPath.

For debugging purposes, Path nodes are supposed to have outfuncs
support, but this was overlooked in the original incremental sort patch.

While at it, clean up a couple other minor oversights, as well as
bizarre choice of return type for create_incremental_sort_path().
(All the existing callers just cast it to "Path *" immediately, so
they don't care, but some future caller might care.)

outfuncs.c fix by Zhijie Hou, the rest by me

Discussion: https://postgr.es/m/324c4d81d8134117972a5b1f6cdf9560@G08CNEXMBPEKD05.g08.fujitsu.local

src/backend/nodes/outfuncs.c

@@ -1954,14 +1954,30 @@ _outProjectSetPath(StringInfo str, const ProjectSetPath *node)
 	WRITE_NODE_FIELD(subpath);
 }
 
+static void
+_outSortPathInfo(StringInfo str, const SortPath *node)
+{
+	_outPathInfo(str, (const Path *) node);
+
+	WRITE_NODE_FIELD(subpath);
+}
+
 static void
 _outSortPath(StringInfo str, const SortPath *node)
 {
 	WRITE_NODE_TYPE("SORTPATH");
 
-	_outPathInfo(str, (const Path *) node);
+	_outSortPathInfo(str, node);
+}
 
-	WRITE_NODE_FIELD(subpath);
+static void
+_outIncrementalSortPath(StringInfo str, const IncrementalSortPath *node)
+{
+	WRITE_NODE_TYPE("INCREMENTALSORTPATH");
+
+	_outSortPathInfo(str, (const SortPath *) node);
+
+	WRITE_INT_FIELD(nPresortedCols);
 }
 
 static void
@@ -4055,6 +4071,9 @@ outNode(StringInfo str, const void *obj)
 			case T_SortPath:
 				_outSortPath(str, obj);
 				break;
+			case T_IncrementalSortPath:
+				_outIncrementalSortPath(str, obj);
+				break;
 			case T_GroupPath:
 				_outGroupPath(str, obj);
 				break;

src/backend/optimizer/README

@@ -387,6 +387,7 @@ RelOptInfo      - a relation or joined relations
   ProjectionPath - a Result plan node with child (used for projection)
   ProjectSetPath - a ProjectSet plan node applied to some sub-path
   SortPath      - a Sort plan node applied to some sub-path
+  IncrementalSortPath - an IncrementalSort plan node applied to some sub-path
   GroupPath     - a Group plan node applied to some sub-path
   UpperUniquePath - a Unique plan node applied to some sub-path
   AggPath       - an Agg plan node applied to some sub-path

src/backend/optimizer/util/pathnode.c

@@ -2798,7 +2798,7 @@ create_set_projection_path(PlannerInfo *root,
  * 'limit_tuples' is the estimated bound on the number of output tuples,
  *		or -1 if no LIMIT or couldn't estimate
  */
-SortPath *
+IncrementalSortPath *
 create_incremental_sort_path(PlannerInfo *root,
 							 RelOptInfo *rel,
 							 Path *subpath,
@@ -2834,7 +2834,7 @@ create_incremental_sort_path(PlannerInfo *root,
 
 	sort->nPresortedCols = presorted_keys;
 
-	return pathnode;
+	return sort;
 }
 
 /*

src/include/nodes/pathnodes.h

@@ -1655,7 +1655,10 @@ typedef struct SortPath
 } SortPath;
 
 /*
- * IncrementalSortPath
+ * IncrementalSortPath represents an incremental sort step
+ *
+ * This is like a regular sort, except some leading key columns are assumed
+ * to be ordered already.
  */
 typedef struct IncrementalSortPath
 {

src/include/optimizer/pathnode.h

@@ -184,16 +184,16 @@ extern ProjectSetPath *create_set_projection_path(PlannerInfo *root,
 												  RelOptInfo *rel,
 												  Path *subpath,
 												  PathTarget *target);
-extern SortPath *create_incremental_sort_path(PlannerInfo *root,
+extern SortPath *create_sort_path(PlannerInfo *root,
 								  RelOptInfo *rel,
 								  Path *subpath,
 								  List *pathkeys,
-											  int presorted_keys,
 								  double limit_tuples);
-extern SortPath *create_sort_path(PlannerInfo *root,
+extern IncrementalSortPath *create_incremental_sort_path(PlannerInfo *root,
 														 RelOptInfo *rel,
 														 Path *subpath,
 														 List *pathkeys,
+														 int presorted_keys,
 														 double limit_tuples);
 extern GroupPath *create_group_path(PlannerInfo *root,
 									RelOptInfo *rel,