diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index 0ab4014..4d3a87a 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -56,8 +56,8 @@
 typedef struct
 {
 	PlannerInfo *root;
-	int			nappinfos;
-	AppendRelInfo **appinfos;
+	int			nappinfos;	/* # of entries in appinfos array. */
+	AppendRelInfo **appinfos;	/* AppendRelInfo detecting the adjustment. */
 } adjust_appendrel_attrs_context;
 
 static RelOptInfo *recurse_set_operations(Node *setOp, PlannerInfo *root,
@@ -2037,6 +2037,9 @@ translate_col_privs(const Bitmapset *parent_privs,
  *	  update rtindexes appearing outside Vars, such as resultRelation and
  *	  jointree relids.
  *
+ * appinfos is an array of nappinfos AppendRelInfos indicating parent-child
+ * pairs.
+ *
  * Note: this is only applied after conversion of sublinks to subplans,
  * so we don't need to cope with recursion into sub-queries.
  *
@@ -2613,6 +2616,9 @@ build_child_join_sjinfo(PlannerInfo *root, SpecialJoinInfo *parent_sjinfo,
  *
  * The AppendRelInfos are returned in an array, which can be pfree'd by the
  * caller. *nappinfos is set to the number of entries in the array.
+ *
+ * The returned array and *nappinfos is usually passed down to
+ * adjust_appendrel_attrs(). See prologue of that function for more details.
  */
 AppendRelInfo **
 find_appinfos_by_relids(PlannerInfo *root, Relids relids, int *nappinfos)
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index 3b28d19..0d17999 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -2096,6 +2096,13 @@ typedef struct SpecialJoinInfo
  * will not have many different append parents, it doesn't seem worthwhile
  * to complicate things.
  *
+ * We expand multi-level partitioning hierarchy into corresponding inheritance
+ * hierarchy level by level. An intermediate partitioned partition acts as
+ * both, a parent (of its partitions) and a child (to the partitioned table
+ * whose partition it is). Per expand_single_inheritance_child() the
+ * AppendRelInfo of an intermediate parent appears before that of any of its
+ * children.
+ *
  * Note: after completion of the planner prep phase, any given RTE is an
  * append parent having entries in append_rel_list if and only if its
  * "inh" flag is set.  We clear "inh" for plain tables that turn out not