Adjust reltarget assignment for UPPERREL_PARTIAL_DISTINCT rel

A comment in grouping_planner() claimed that the PlannerInfo
upper_targets array was not used in core code.  However, the code that
generated the paths for the UPPERREL_PARTIAL_DISTINCT rel made that
comment untrue.

Here we adjust the create_distinct_paths() function signature to pass
down the PathTarget the same as is done for create_grouping_paths(),
thus making the aforementioned comment true again.

In passing adjust the order of the upper_targets[] assignments.  These
seem to be following the reverse enum order apart from
UPPERREL_PARTIAL_DISTINCT.

Also, update the header comment for generate_gather_paths() to mention
the function is also used to create gather paths for partial distinct
paths.

Author: Richard Guo, David Rowley
Discussion: https://postgr.es/m/CAMbWs48u9VoVOouJsys1qOaC9WVGVmBa+wT1dx8KvxF5GPzezA@mail.gmail.com

src/backend/optimizer/path/allpaths.c

@@ -3053,10 +3053,10 @@ set_worktable_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
  *
  * If we're generating paths for a scan or join relation, override_rows will
  * be false, and we'll just use the relation's size estimate.  When we're
- * being called for a partially-grouped path, though, we need to override
+ * being called for a partially-grouped or partially-distinct path, though, we
- * the rowcount estimate.  (It's not clear that the particular value we're
+ * need to override the rowcount estimate.  (It's not clear that the
- * using here is actually best, but the underlying rel has no estimate so
+ * particular value we're using here is actually best, but the underlying rel
- * we must do something.)
+ * has no estimate so we must do something.)
  */
 void
 generate_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_rows)

src/backend/optimizer/plan/planner.c

@@ -190,10 +190,12 @@ static void create_one_window_path(PlannerInfo *root,
 								   WindowFuncLists *wflists,
 								   List *activeWindows);
 static RelOptInfo *create_distinct_paths(PlannerInfo *root,
-										 RelOptInfo *input_rel);
+										 RelOptInfo *input_rel,
+										 PathTarget *target);
 static void create_partial_distinct_paths(PlannerInfo *root,
 										  RelOptInfo *input_rel,
-										  RelOptInfo *final_distinct_rel);
+										  RelOptInfo *final_distinct_rel,
+										  PathTarget *target);
 static RelOptInfo *create_final_distinct_paths(PlannerInfo *root,
 											   RelOptInfo *input_rel,
 											   RelOptInfo *distinct_rel);
@@ -1644,8 +1646,8 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 		 */
 		root->upper_targets[UPPERREL_FINAL] = final_target;
 		root->upper_targets[UPPERREL_ORDERED] = final_target;
-		root->upper_targets[UPPERREL_PARTIAL_DISTINCT] = sort_input_target;
 		root->upper_targets[UPPERREL_DISTINCT] = sort_input_target;
+		root->upper_targets[UPPERREL_PARTIAL_DISTINCT] = sort_input_target;
 		root->upper_targets[UPPERREL_WINDOW] = sort_input_target;
 		root->upper_targets[UPPERREL_GROUP_AGG] = grouping_target;
 
@@ -1695,7 +1697,8 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 		if (parse->distinctClause)
 		{
 			current_rel = create_distinct_paths(root,
-												current_rel);
+												current_rel,
+												sort_input_target);
 		}
 	}							/* end of if (setOperations) */
 
@@ -4568,12 +4571,14 @@ create_one_window_path(PlannerInfo *root,
  * Build a new upperrel containing Paths for SELECT DISTINCT evaluation.
  *
  * input_rel: contains the source-data Paths
+ * target: the pathtarget for the result Paths to compute
  *
  * Note: input paths should already compute the desired pathtarget, since
  * Sort/Unique won't project anything.
  */
 static RelOptInfo *
-create_distinct_paths(PlannerInfo *root, RelOptInfo *input_rel)
+create_distinct_paths(PlannerInfo *root, RelOptInfo *input_rel,
+					  PathTarget *target)
 {
 	RelOptInfo *distinct_rel;
 
@@ -4601,7 +4606,7 @@ create_distinct_paths(PlannerInfo *root, RelOptInfo *input_rel)
 	create_final_distinct_paths(root, input_rel, distinct_rel);
 
 	/* now build distinct paths based on input_rel's partial_pathlist */
-	create_partial_distinct_paths(root, input_rel, distinct_rel);
+	create_partial_distinct_paths(root, input_rel, distinct_rel, target);
 
 	/* Give a helpful error if we failed to create any paths */
 	if (distinct_rel->pathlist == NIL)
@@ -4643,7 +4648,8 @@ create_distinct_paths(PlannerInfo *root, RelOptInfo *input_rel)
  */
 static void
 create_partial_distinct_paths(PlannerInfo *root, RelOptInfo *input_rel,
-							  RelOptInfo *final_distinct_rel)
+							  RelOptInfo *final_distinct_rel,
+							  PathTarget *target)
 {
 	RelOptInfo *partial_distinct_rel;
 	Query	   *parse;
@@ -4664,7 +4670,7 @@ create_partial_distinct_paths(PlannerInfo *root, RelOptInfo *input_rel,
 
 	partial_distinct_rel = fetch_upper_rel(root, UPPERREL_PARTIAL_DISTINCT,
 										   NULL);
-	partial_distinct_rel->reltarget = root->upper_targets[UPPERREL_PARTIAL_DISTINCT];
+	partial_distinct_rel->reltarget = target;
 	partial_distinct_rel->consider_parallel = input_rel->consider_parallel;
 
 	/*