Improve-comments-for-trivial_subqueryscan.patch

diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index d95fd89807..5108dbaf81 100644
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/src/backend/optimizer/plan/setrefs.c
@@ -1349,8 +1349,22 @@ set_subqueryscan_references(PlannerInfo *root,
  * We can delete it if it has no qual to check and the targetlist just
  * regurgitates the output of the child plan.
  *
- * This might be called repeatedly on a SubqueryScan node, so we cache the
- * result in the SubqueryScan node to avoid repeated computation.
+ * This can be called from mark_async_capable_plan(), a helper function for
+ * create_append_plan(), before set_subqueryscan_references(), to determine
+ * triviality of a SubqueryScan that is a child of an Append node.  So we
+ * cache the result in the SubqueryScan node to avoid repeated computation.
+ *
+ * Note: when called from mark_async_capable_plan(), we determine the result
+ * before running finalize_plan() on the SubqueryScan node (if needed) and
+ * set_plan_references() on the subplan tree, but this would be safe because
+ * 1) finalize_plan() doesn't modify the tlist or quals for the SubqueryScan
+ * node (or that for any plan node in the subplan tree), 2)
+ * set_plan_references() modifies the tlist for every plan node in the
+ * subplan tree, but keeps const/resjunk columns as const/resjunk ones and
+ * preserves the length and order of the tlist, and 3) set_plan_references()
+ * might delete the topmost plan node like an Append or MergeAppend from the
+ * subplan tree and pull up the child plan node, but in that case, the tlist
+ * for the child plan node exactly matches the parent.
  */
 bool
 trivial_subqueryscan(SubqueryScan *plan)
@@ -1359,7 +1373,7 @@ trivial_subqueryscan(SubqueryScan *plan)
 	ListCell   *lp,
 			   *lc;
 
-	/* We might have detected this already (see mark_async_capable_plan) */
+	/* We might have detected this already; in which case reuse the result */
 	if (plan->scanstatus == SUBQUERY_SCAN_TRIVIAL)
 		return true;
 	if (plan->scanstatus == SUBQUERY_SCAN_NONTRIVIAL)