ddl: Update integration test of expr index (#10178)

tests/fullstack-test2/ddl/expression_index.test

@@ -0,0 +1,42 @@
+# Copyright 2025 PingCAP, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+mysql> drop table if exists test.t;
+
+# test for null expression express tiflash#9891
+mysql> CREATE TABLE test.t (id int);
+mysql> alter table test.t set tiflash replica 1;
+mysql> insert test.t values(0),(1);
+
+func> wait_table test t
+
+mysql> insert into test.t values (2), (3);
+mysql> set session tidb_isolation_read_engines='tiflash'; select * from test.t order by id;
++----+
+| id |
++----+
+|  0 |
+|  1 |
+|  2 |
+|  3 |
++----+
+
+mysql> CREATE INDEX idx_n ON test.t ((null));
+ERROR 3761 (HY000) at line 1: The used storage engine cannot index the expression 'null'
+
+mysql> drop table if exists test.t;
+
+mysql> CREATE TABLE test.t (id int, KEY idx_name ((null)));
+ERROR 3761 (HY000) at line 1: The used storage engine cannot index the expression 'null'
+mysql> drop table if exists test.t;