the index lookup push down returns wrong value

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

> create table t(id varchar(32) primary key, i int, v int, key i(i));
> insert into t values('a', 1, 1), ('b', 2, 2), ('c', 3, 3);
> select /*+ index_lookup_pushdown(t, i) */ * from t where id != 'a' order by id limit 1;

2. What did you expect to see? (Required)

> select /*+ index_lookup_pushdown(t, i) */ * from t where id != 'a' order by id limit 1;
+----+---+---+
| id | i | v |
+----+---+---+
| b  | 2 | 2 |
+----+---+---+

3. What did you see instead (Required)

> select /*+ index_lookup_pushdown(t, i) */ * from t where id != 'a' order by id limit 1;
+----+---+---+
| id | i | v |
+----+---+---+
| a  | 1 | 1 |
+----+---+---+

4. What is your TiDB version? (Required)

master