unexpected result: casting real to string

[solotzg]

Bug Report

What version of TiKV are you using?

5.4 ~ master

What operating system and CPU are you using?

Steps to reproduce

ref pingcap/tidb#51109

CREATE TABLE `t` (
  `c` float DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
INSERT INTO `t` VALUES(4474.7812);
set tidb_opt_projection_push_down=1;
select c, cast(c as char) from t;

What did you expect?

mysql> select c, cast(c as char) from t;
+-----------+-----------------+
| c         | cast(c as char) |
+-----------+-----------------+
| 4474.7812 | 4474.7812       |
+-----------+-----------------+
1 row in set (0.00 sec)

What did happened?

mysql> select c, cast(c as char) from t;
+-----------+-----------------+
| c         | cast(c as char) |
+-----------+-----------------+
| 4474.7812 | 4474.7813       |
+-----------+-----------------+
1 row in set (0.00 sec)