cast: convert float value to json have different precision with MySQL

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a json);
insert into t values("323232323.3232323232");
select * from t;

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

MySQL

mysql> select * from t;
+-------------------+
| a                 |
+-------------------+
| 323232323.3232323 |
+-------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

TiDB

mysql> select * from t;
+--------------------+
| a                  |
+--------------------+
| 323232323.32323235 |
+--------------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

master

[djshow832]

Maybe it's duplicate with #22791

[xiongjiwei]

we have a document that says the difference

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.