diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index d8aa35d..8167583 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -404,6 +404,15 @@ subquery_planner(PlannerGlobal *glob, Query *parse,
 	expand_inherited_tables(root);
 
 	/*
+	 * Collapse multilevel inheritances into fewer levels.
+	 *
+	 * UNION ALL containing subselects on inherited tables leaves multilevel
+	 * inheritance after calling expand_inherited_tables().  Fold them in
+	 * order to make MergeAppend plan available for such queries.
+	 */
+	collapse_appendrels(root);
+
+	/*
 	 * Set hasHavingQual to remember if HAVING clause is present.  Needed
 	 * because preprocess_expression will reduce a constant-true condition to
 	 * an empty qual list ... but "HAVING TRUE" is not a semantic no-op.
diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index e249628..c7933b5 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -57,6 +57,11 @@ typedef struct
 	AppendRelInfo *appinfo;
 } adjust_appendrel_attrs_context;
 
+typedef struct {
+	AppendRelInfo	*child_appinfo;
+	Index	 		 target_rti;
+} transvars_merge_context;
+
 static Plan *recurse_set_operations(Node *setOp, PlannerInfo *root,
 					   double tuple_fraction,
 					   List *colTypes, List *colCollations,
@@ -98,6 +103,8 @@ static List *generate_append_tlist(List *colTypes, List *colCollations,
 					  List *input_plans,
 					  List *refnames_tlist);
 static List *generate_setop_grouplist(SetOperationStmt *op, List *targetlist);
+static Node *transvars_merge_mutator(Node *node,
+									 transvars_merge_context *ctx);
 static void expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte,
 						 Index rti);
 static void make_inh_translation_list(Relation oldrelation,
@@ -1211,6 +1218,131 @@ expand_inherited_tables(PlannerInfo *root)
 }
 
 /*
+ * collapse_appendrels
+ *      Pulling up the descendents by recursively combining successive
+ *      translations.
+ *
+ *      Although the same thing could be done in adjust_appendrel_attrs(),
+ *      doing it here all through at once is more efficient than individually
+ *      (and maybe repetitively) done there.
+ */
+void
+collapse_appendrels(PlannerInfo *root)
+{
+	Index	  		 last_parent_relid = 1;
+	AppendRelInfo	*last_parent = NULL;
+	ListCell 		*lcchild;
+	ListCell 		*lcparent;
+	bool			 full_search = false;
+
+	foreach(lcchild, root->append_rel_list)
+	{
+		AppendRelInfo	*ch_appinfo		 = (AppendRelInfo *)lfirst(lcchild);
+		Index			 ch_parent_relid = ch_appinfo->parent_relid;
+		
+		if (last_parent_relid != ch_parent_relid)
+		{
+			/*
+			 * Find the parent for the current child appinfo if parent relid
+			 * is different from that of preveous child.
+			 */
+			do
+			{
+				/*
+				 * For most cases, the relations are referred to as the parent
+				 * in their apperarence order in rtable from
+				 * append_rel_list. So start searching for the parent appinfos
+				 * from just after the last parent. If the incremental search
+				 * was failed, retry searching the entire list and exit on
+				 * failure.
+				 */
+				if (!last_parent)
+				{
+					/*
+					 * If this is the first time or the preveous search was
+					 * failed, set up for full search.
+					 */
+					lcparent = list_head(root->append_rel_list);
+					last_parent_relid = 1;
+					full_search = true;
+				}
+
+				last_parent = NULL;
+				for_each_cell(lcparent, lcparent)
+				{
+					/*
+					 * Children's and parents' apprelinfos are bonded via
+					 * rtable relations. So two apprelinfos are in
+					 * parent-child relationship when the child_relid of the
+					 * parent is equal to the parent_relid of the child.
+					 */
+					AppendRelInfo *papp = (AppendRelInfo*)lfirst(lcparent);
+					if (papp->child_relid == ch_parent_relid)
+					{
+						last_parent = papp;
+						last_parent_relid = ch_parent_relid;
+						
+						/* Search from the next cell next time. */
+						lcparent = lnext(lcparent);
+						full_search = false;
+						break;		/* Parent found */
+					}
+				}
+				/* Retry only when incremental search was failed. */
+			} while (!full_search && !last_parent);
+		}
+
+		/*
+		 * Replace child translated_vars so as to be a direct children of the
+		 * topmost relation.
+		 */
+		if (last_parent)
+		{
+			transvars_merge_context ctx;
+
+			ctx.child_appinfo = ch_appinfo;
+			ctx.target_rti  = last_parent->child_relid;
+
+			ch_appinfo->translated_vars =
+				(List*)expression_tree_mutator(
+					(Node*)last_parent->translated_vars,
+					transvars_merge_mutator, &ctx);
+			ch_appinfo->parent_relid = last_parent->parent_relid;
+			ch_appinfo->parent_reltype = last_parent->parent_reltype;
+		}
+	}
+}
+
+/*
+ * Replace Var nodes with corresponding child nodes.
+ */
+static Node *
+transvars_merge_mutator(Node *node, transvars_merge_context *ctx)
+{
+	if (node == NULL)
+		return NULL;
+
+	if (IsA(node, Var))
+	{
+		Var *oldv = (Var*)node;
+
+		if (!oldv->varlevelsup && oldv->varno == ctx->target_rti)
+		{
+			if (oldv->varattno >
+				list_length(ctx->child_appinfo->translated_vars))
+				elog(ERROR,
+					 "attribute %d of relation \"%s\" does not exist",
+					 oldv->varattno,
+					 get_rel_name(ctx->child_appinfo->parent_reloid));
+			return (Node*)copyObject(list_nth(ctx->child_appinfo->translated_vars,
+											  oldv->varattno - 1));
+		}
+	}
+	return expression_tree_mutator(node,
+								   transvars_merge_mutator, (void*)ctx);
+}
+
+/*
  * expand_inherited_rtentry
  *		Check whether a rangetable entry represents an inheritance set.
  *		If so, add entries for all the child tables to the query's
diff --git a/src/include/optimizer/prep.h b/src/include/optimizer/prep.h
index 0934e63..7a83938 100644
--- a/src/include/optimizer/prep.h
+++ b/src/include/optimizer/prep.h
@@ -49,6 +49,7 @@ extern Plan *plan_set_operations(PlannerInfo *root, double tuple_fraction,
 					List **sortClauses);
 
 extern void expand_inherited_tables(PlannerInfo *root);
+extern void collapse_appendrels(PlannerInfo *root);
 
 extern Node *adjust_appendrel_attrs(PlannerInfo *root, Node *node,
 					   AppendRelInfo *appinfo);