cast real as string is not compatible with tidb/mysql

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table test.t(a float, b double);
insert into test.t values (3.4028234e38, 1.7976931348623157e308);
insert into test.t values (1.1754944e-38, 2.2250738585072014e-308);

// tiflash
set tidb_enforce_mpp=1; set tidb_isolation_read_engines='tiflash';
select cast(a as char), cast(b as char) from t;

// tidb
set tidb_enforce_mpp=0; set tidb_isolation_read_engines='tikv';
select cast(a as char), cast(b as char) from t;

// mysql
// execute in mysql
select cast(a as char), cast(b as char) from t;

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

// tidb
+-----------------+-------------------------+
| cast(a as char) | cast(b as char)         |
+-----------------+-------------------------+
| 3.40282e38      | 1.7976931348623157e308  |
| 1.17549e-38     | 2.2250738585072014e-308 |
+-----------------+-------------------------+

// tiflash
+-----------------+-------------------------+
| cast(a as char) | cast(b as char)         |
+-----------------+-------------------------+
| 3.40282e38      | 1.7976931348623157e308  |
| 1.17549e-38     | 2.2250738585072014e-308 |
+-----------------+-------------------------+

// mysql
+-----------------+-------------------------+
| cast(a as char) | cast(b as char)         |
+-----------------+-------------------------+
| 3.40282e38      | 1.7976931348623157e308  |
| 1.17549e-38     | 2.2250738585072014e-308 |
+-----------------+-------------------------+

3. What did you see instead (Required)

// tidb
+-------------------------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| cast(a as char)                                 | cast(b as char)                                                                                                                                                                                                                                                                                                                        |
+-------------------------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 340282350000000000000000000000000000000         | 179769313486231570000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000                  |
| 0.000000000000000000000000000000000000011754944 | 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000022250738585072014 |
+-------------------------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

// tiflash
+-----------------+-------------------------+
| cast(a as char) | cast(b as char)         |
+-----------------+-------------------------+
| 3.4028235e38    | 1.7976931348623157e308  |
| 1.1754944e-38   | 2.2250738585072014e-308 |
+-----------------+-------------------------+

// mysql
+-----------------+-------------------------+
| cast(a as char) | cast(b as char)         |
+-----------------+-------------------------+
| 3.40282e38      | 1.7976931348623157e308  |
| 1.17549e-38     | 2.2250738585072014e-308 |
+-----------------+-------------------------+

4. What is your TiFlash version? (Required)

master

The issue names no source file, test, or entry point. Start by running the SQL reproduction and compare TiFlash and TiDB output with MySQL; done means casting FLOAT and DOUBLE to CHAR produces the expected compatible formatting for the listed boundary values.