diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index aea0677d2b..33b62561ef 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -55,6 +55,7 @@ static RelOptInfo *generate_recursion_path(SetOperationStmt *setOp,
 static void build_setop_child_paths(PlannerInfo *root, RelOptInfo *rel,
 									bool trivial_tlist, List *child_tlist,
 									List *interesting_pathkeys,
+									List *groupList,
 									double *pNumGroups);
 static RelOptInfo *generate_union_paths(SetOperationStmt *op, PlannerInfo *root,
 										List *refnames_tlist,
@@ -399,7 +400,7 @@ generate_recursion_path(SetOperationStmt *setOp, PlannerInfo *root,
 								  &lpath_trivial_tlist);
 	if (lrel->rtekind == RTE_SUBQUERY)
 		build_setop_child_paths(root, lrel, lpath_trivial_tlist, lpath_tlist,
-								NIL, NULL);
+								NIL, NIL, NULL);
 	lpath = lrel->cheapest_total_path;
 	/* The right path will want to look at the left one ... */
 	root->non_recursive_path = lpath;
@@ -411,7 +412,7 @@ generate_recursion_path(SetOperationStmt *setOp, PlannerInfo *root,
 								  &rpath_trivial_tlist);
 	if (rrel->rtekind == RTE_SUBQUERY)
 		build_setop_child_paths(root, rrel, rpath_trivial_tlist, rpath_tlist,
-								NIL, NULL);
+								NIL, NIL, NULL);
 	rpath = rrel->cheapest_total_path;
 	root->non_recursive_path = NULL;
 
@@ -486,13 +487,18 @@ generate_recursion_path(SetOperationStmt *setOp, PlannerInfo *root,
  *
  * interesting_pathkeys: if not NIL, also include paths that suit these
  * pathkeys, sorting any unsorted paths as required.
+ * groupList: if not NIL, the SortGroupClauses (tleSortGroupRef = output
+ * column number) that define a group, e.g. the DISTINCT ON key of a UNION
+ * DISTINCT ON; the estimate below is then made over those columns only.
+ * NIL means the whole target list.
  * *pNumGroups: if not NULL, we estimate the number of distinct groups
  * in the result, and store it there.
  */
 static void
 build_setop_child_paths(PlannerInfo *root, RelOptInfo *rel,
 						bool trivial_tlist, List *child_tlist,
-						List *interesting_pathkeys, double *pNumGroups)
+						List *interesting_pathkeys, List *groupList,
+						double *pNumGroups)
 {
 	RelOptInfo *final_rel;
 	List	   *setop_pathkeys = rel->subroot->setop_pathkeys;
@@ -680,11 +686,26 @@ build_setop_child_paths(PlannerInfo *root, RelOptInfo *rel,
 			subquery->hasAggs)
 			*pNumGroups = rel->cheapest_total_path->rows;
 		else
+		{
+			List	   *groupExprs = get_tlist_exprs(subroot->parse->targetList,
+													 false);
+
+			if (groupList != NIL)
+			{
+				List	   *keyExprs = NIL;
+
+				foreach_node(SortGroupClause, sgc, groupList)
+					keyExprs = lappend(keyExprs,
+									   list_nth(groupExprs,
+												sgc->tleSortGroupRef - 1));
+				groupExprs = keyExprs;
+			}
 			*pNumGroups = estimate_num_groups(subroot,
-											  get_tlist_exprs(subroot->parse->targetList, false),
+											  groupExprs,
 											  rel->cheapest_total_path->rows,
 											  NULL,
 											  NULL);
+		}
 	}
 }
 
@@ -786,6 +807,7 @@ generate_union_paths(SetOperationStmt *op, PlannerInfo *root,
 		if (rel->rtekind == RTE_SUBQUERY)
 			build_setop_child_paths(root, rel, trivial_tlist, child_tlist,
 									union_pathkeys,
+									op->sortClauses ? groupList : NIL,
 									op->all ? NULL : &childGroups);
 		else
 			childGroups = rel->rows;
@@ -1167,12 +1189,12 @@ generate_nonunion_paths(SetOperationStmt *op, PlannerInfo *root,
 	 */
 	if (lrel->rtekind == RTE_SUBQUERY)
 		build_setop_child_paths(root, lrel, lpath_trivial_tlist, lpath_tlist,
-								nonunion_pathkeys, &dLeftGroups);
+								nonunion_pathkeys, NIL, &dLeftGroups);
 	else
 		dLeftGroups = lrel->rows;
 	if (rrel->rtekind == RTE_SUBQUERY)
 		build_setop_child_paths(root, rrel, rpath_trivial_tlist, rpath_tlist,
-								nonunion_pathkeys, &dRightGroups);
+								nonunion_pathkeys, NIL, &dRightGroups);
 	else
 		dRightGroups = rrel->rows;