diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index 85c25d7d0b6..84b9c188fb5 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -942,6 +942,8 @@ infer_arbiter_indexes(PlannerInfo *root)
 		Form_pg_index idxForm;
 		Bitmapset  *indexedAttrs;
 		List	   *idxExprs;
+		List	   *idxNonVarExprs;
+		ListCell   *idxExprItem;
 		List	   *predExprs;
 		AttrNumber	natt;
 		bool		match;
@@ -1046,22 +1048,7 @@ infer_arbiter_indexes(PlannerInfo *root)
 		if (idxForm->indisexclusion)
 			continue;
 
-		/* Build BMS representation of plain (non expression) index attrs */
-		indexedAttrs = NULL;
-		for (natt = 0; natt < idxForm->indnkeyatts; natt++)
-		{
-			int			attno = idxRel->rd_index->indkey.values[natt];
-
-			if (attno != 0)
-				indexedAttrs = bms_add_member(indexedAttrs,
-											  attno - FirstLowInvalidHeapAttributeNumber);
-		}
-
-		/* Non-expression attributes (if any) must match */
-		if (!bms_equal(indexedAttrs, inferAttrs))
-			continue;
-
-		/* Expression attributes (if any) must match */
+		/* Simplify the index expressions the same way arbiterElems were */
 		idxExprs = RelationGetIndexExpressions(idxRel);
 		if (idxExprs)
 		{
@@ -1071,6 +1058,44 @@ infer_arbiter_indexes(PlannerInfo *root)
 			idxExprs = (List *) eval_const_expressions(root, (Node *) idxExprs);
 		}
 
+		/*
+		 * Build BMS representation of plain index attrs.  An index expression
+		 * that was simplified to a plain Var is treated as a plain attr too,
+		 * to match how arbiterElems are classified above.
+		 */
+		indexedAttrs = NULL;
+		idxNonVarExprs = NIL;
+		idxExprItem = list_head(idxExprs);
+		for (natt = 0; natt < idxForm->indnkeyatts; natt++)
+		{
+			int			attno = idxRel->rd_index->indkey.values[natt];
+
+			if (attno == 0)
+			{
+				Node	   *idxExpr;
+
+				if (idxExprItem == NULL)
+					elog(ERROR, "wrong number of index expressions");
+				idxExpr = (Node *) lfirst(idxExprItem);
+				idxExprItem = lnext(idxExprs, idxExprItem);
+
+				if (!IsA(idxExpr, Var) || ((Var *) idxExpr)->varattno == 0)
+				{
+					idxNonVarExprs = lappend(idxNonVarExprs, idxExpr);
+					continue;
+				}
+
+				attno = ((Var *) idxExpr)->varattno;
+			}
+
+			indexedAttrs = bms_add_member(indexedAttrs,
+										  attno - FirstLowInvalidHeapAttributeNumber);
+		}
+
+		/* Plain attributes (if any) must match */
+		if (!bms_equal(indexedAttrs, inferAttrs))
+			continue;
+
 		/* Check the arbiterElems against this index. */
 		match = true;
 		foreach_ptr(InferenceElem, elem, onconflict->arbiterElems)
@@ -1120,7 +1145,7 @@ infer_arbiter_indexes(PlannerInfo *root)
 		 * indexes redundantly repeat the same attribute, or if attributes
 		 * redundantly appear multiple times within an inference clause.
 		 */
-		if (list_difference(idxExprs, inferElems) != NIL)
+		if (list_difference(idxNonVarExprs, inferElems) != NIL)
 			continue;
 
 		predExprs = RelationGetIndexPredicate(idxRel);
@@ -1246,12 +1271,13 @@ infer_collation_opclass_match(InferenceElem *elem, Relation idxRel,
 		}
 
 		/* If one matching index att found, good enough -- return true */
-		if (IsA(elem->expr, Var))
+		if (attno != 0)
 		{
-			if (((Var *) elem->expr)->varattno == attno)
+			if (IsA(elem->expr, Var) &&
+				((Var *) elem->expr)->varattno == attno)
 				return true;
 		}
-		else if (attno == 0)
+		else
 		{
 			Node	   *nattExpr = list_nth(idxExprs, (natt - 1) - nplain);
 
diff --git a/src/test/regress/expected/insert_conflict.out b/src/test/regress/expected/insert_conflict.out
index b4c9bdb0e55..0dd8bc734a0 100644
--- a/src/test/regress/expected/insert_conflict.out
+++ b/src/test/regress/expected/insert_conflict.out
@@ -181,6 +181,33 @@ explain (costs off) insert into insertconflicttest values(0, 'Crowberry') on con
 
 drop index cross_match;
 --
+-- Index expressions that are simplified to a plain column can be inferred
+--
+create table insertconflict_nn (a int not null, b text);
+create unique index nn_expr_key on insertconflict_nn(coalesce(a, 0), lower(b));
+explain (costs off) insert into insertconflict_nn values(1, 'x') on conflict (coalesce(a, 0), lower(b)) do nothing;
+               QUERY PLAN                
+-----------------------------------------
+ Insert on insertconflict_nn
+   Conflict Resolution: NOTHING
+   Conflict Arbiter Indexes: nn_expr_key
+   ->  Result
+(4 rows)
+
+explain (costs off) insert into insertconflict_nn values(1, 'x') on conflict (a int4_ops, lower(b)) do nothing;
+               QUERY PLAN                
+-----------------------------------------
+ Insert on insertconflict_nn
+   Conflict Resolution: NOTHING
+   Conflict Arbiter Indexes: nn_expr_key
+   ->  Result
+(4 rows)
+
+-- fails:
+explain (costs off) insert into insertconflict_nn values(1, 'x') on conflict (a) do nothing;
+ERROR:  there is no unique or exclusion constraint matching the ON CONFLICT specification
+drop table insertconflict_nn;
+--
 -- Single key tests
 --
 create unique index key_index on insertconflicttest(key);
diff --git a/src/test/regress/sql/insert_conflict.sql b/src/test/regress/sql/insert_conflict.sql
index d119158549f..8539c49c939 100644
--- a/src/test/regress/sql/insert_conflict.sql
+++ b/src/test/regress/sql/insert_conflict.sql
@@ -78,6 +78,19 @@ explain (costs off) insert into insertconflicttest values(0, 'Crowberry') on con
 
 drop index cross_match;
 
+--
+-- Index expressions that are simplified to a plain column can be inferred
+--
+create table insertconflict_nn (a int not null, b text);
+create unique index nn_expr_key on insertconflict_nn(coalesce(a, 0), lower(b));
+
+explain (costs off) insert into insertconflict_nn values(1, 'x') on conflict (coalesce(a, 0), lower(b)) do nothing;
+explain (costs off) insert into insertconflict_nn values(1, 'x') on conflict (a int4_ops, lower(b)) do nothing;
+-- fails:
+explain (costs off) insert into insertconflict_nn values(1, 'x') on conflict (a) do nothing;
+
+drop table insertconflict_nn;
+
 --
 -- Single key tests
 --