Improve planner's handling of SetOp plans.

Remove the code for inserting flag columns in the inputs of a SetOp.
That was the only reason why there would be resjunk columns in a
set-operations plan tree, so we can get rid of some code that
supported that, too.

Get rid of choose_hashed_setop() in favor of building Paths for
the hashed and sorted alternatives, and letting them fight it out
within add_path().

Remove set_operation_ordered_results_useful(), which was giving wrong
answers due to examining the wrong ancestor node: we need to examine
the immediate SetOperationStmt parent not the topmost node.  Instead
make each caller of recurse_set_operations() pass down the relevant
parent node.  (This thinko seems to have led only to wasted planning
cycles and possibly-inferior plans, not wrong query answers.  Perhaps
we should back-patch it, but I'm not doing so right now.)

Teach generate_nonunion_paths() to consider pre-sorted inputs for
sorted SetOps, rather than always generating a Sort node.

Patch by me; thanks to Richard Guo and David Rowley for review.

Discussion: https://postgr.es/m/1850138.1731549611@sss.pgh.pa.us

Author: tglsfdc

src/backend/optimizer/plan/planner.c

@@ -616,7 +616,7 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
  * setops is used for set operation subqueries to provide the subquery with
  * the context in which it's being used so that Paths correctly sorted for the
  * set operation can be generated.  NULL when not planning a set operation
- * child.
+ * child, or when a child of a set op that isn't interested in sorted input.
  *
  * Basically, this routine does the stuff that should only be done once
  * per Query object.  It then calls grouping_planner.  At one time,
@@ -1350,7 +1350,7 @@ preprocess_phv_expression(PlannerInfo *root, Expr *expr)
  * setops is used for set operation subqueries to provide the subquery with
  * the context in which it's being used so that Paths correctly sorted for the
  * set operation can be generated.  NULL when not planning a set operation
- * child.
+ * child, or when a child of a set op that isn't interested in sorted input.
  *
  * Returns nothing; the useful output is in the Paths we attach to the
  * (UPPERREL_FINAL, NULL) upperrel in *root.  In addition,
@@ -3467,8 +3467,7 @@ standard_qp_callback(PlannerInfo *root, void *extra)
 									  tlist);
 
 	/* setting setop_pathkeys might be useful to the union planner */
-	if (qp_extra->setop != NULL &&
-		set_operation_ordered_results_useful(qp_extra->setop))
+	if (qp_extra->setop != NULL)
 	{
 		List	   *groupClauses;
 		bool		sortable;