Hint to read from tiflash doesn't work when the query is a PointGet

[lidezhu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

create table if not exists test.t(col_1 varchar(256), col_2 int not null primary key);
insert into test.t(col_1, col_2) values('test1', 555), ('test2', 666), ('test3', 777);
analyze table test.t;
explain select /*+ read_from_storage(tiflash[test.t]) */ * from t where col_2=555;

2. What did you expect to see?

The query is send to tiflash.

3. What did you see instead?

+-------------+---------+------+-------------------------------+---------------+
| id          | estRows | task | access object                 | operator info |
+-------------+---------+------+-------------------------------+---------------+
| Point_Get_1 | 1.00    | root | table:t, index:PRIMARY(col_2) |               |
+-------------+---------+------+-------------------------------+---------------+

4. What version of TiDB are you using? (tidb-server -V or run select tidb_version(); on TiDB)

Release Version: v4.0.0-beta.2-352-g5ede18f10
Git Commit Hash: 5ede18f
Git Branch: master
UTC Build Time: 2020-04-24 03:45:17
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[yudongusa]

@lzmhhh123 any update on this issue?