diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c
index 2f6a6ff..ec777fd 100644
--- a/src/backend/catalog/heap.c
+++ b/src/backend/catalog/heap.c
@@ -2307,12 +2307,17 @@ MergeWithExistingConstraint(Relation rel, char *ccname, Node *expr,
 						(errcode(ERRCODE_DUPLICATE_OBJECT),
 				errmsg("constraint \"%s\" for relation \"%s\" already exists",
 					   ccname, RelationGetRelationName(rel))));
-			/* OK to update the tuple */
-			ereport(NOTICE,
-			   (errmsg("merging constraint \"%s\" with inherited definition",
-					   ccname)));
+
 			tup = heap_copytuple(tup);
 			con = (Form_pg_constraint) GETSTRUCT(tup);
+
+			/* If the constraint is "only" then cannot merge */
+			if (con->conisonly)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+				errmsg("constraint \"%s\" conflicts with non-inherited constraint on relation \"%s\"",
+					   ccname, RelationGetRelationName(rel))));
+
 			if (is_local)
 				con->conislocal = true;
 			else
@@ -2322,6 +2327,10 @@ MergeWithExistingConstraint(Relation rel, char *ccname, Node *expr,
 				Assert(is_local);
 				con->conisonly = true;
 			}
+			/* OK to update the tuple */
+			ereport(NOTICE,
+			   (errmsg("merging constraint \"%s\" with inherited definition",
+					   ccname)));
 			simple_heap_update(conDesc, &tup->t_self, tup);
 			CatalogUpdateIndexes(conDesc, tup);
 			break;
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 8473c9e..c615ac0 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -8950,6 +8950,10 @@ MergeConstraintsIntoExisting(Relation child_rel, Relation parent_rel)
 		if (parent_con->contype != CONSTRAINT_CHECK)
 			continue;
 
+		/* if ONLY constraint, cannot inherit */
+		if (parent_con->conisonly)
+			continue;
+
 		/* Search for a child constraint matching this one */
 		ScanKeyInit(&child_key,
 					Anum_pg_constraint_conrelid,
@@ -8977,6 +8981,13 @@ MergeConstraintsIntoExisting(Relation child_rel, Relation parent_rel)
 								RelationGetRelationName(child_rel),
 								NameStr(parent_con->conname))));
 
+			/* If the constraint is "only" then cannot merge */
+			if (child_con->conisonly)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+				errmsg("constraint \"%s\" conflicts with non-inherited constraint on child table \"%s\"",
+					   NameStr(child_con->conname), RelationGetRelationName(child_rel))));
+
 			/*
 			 * OK, bump the child constraint's inheritance count.  (If we fail
 			 * later on, this change will just roll back.)