Autoid value is incorrect when field type is float or double

[nange]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

use test;
create table t1(id int primary key, n float auto_increment, key I_n(n));
create table t2(id int primary key, n double auto_increment, key I_n(n));

insert into t1 values(1, 1);
insert into t2 values(1, 1);
insert into t2(id) values(2);

2. What did you expect to see?

mysql> select * from t1;
+----+---+
| id | n |
+----+---+
|  1 | 1 |
+----+---+
1 row in set (0.00 sec)

mysql> select * from t2;
+----+---+
| id | n |
+----+---+
|  1 | 1 |
|  2 | 2 |
+----+---+
2 rows in set (0.00 sec)

3. What did you see instead?

mysql> select * from t1;
+----+------+
| id | n    |
+----+------+
|  1 |    0 |
+----+------+
1 row in set (0.00 sec)

mysql> select * from t2;
+----+-----------------------+
| id | n                     |
+----+-----------------------+
|  1 |                     1 |
|  2 | 4.6071824188000174e18 |
+----+-----------------------+
2 rows in set (0.00 sec)

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

./tidb-server -V

Release Version: v3.0.0-rc.1-290-g21d2590ac
Git Commit Hash: 21d2590ac37bf53e5e687b8a341d50105ddbffe8
Git Branch: master
UTC Build Time: 2019-07-06 12:56:31
GoVersion: go version go1.12.6 darwin/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false

[crazycs520]

Thanks for report and contribution.

BTW, the other case may have the same problem, I just test update:

use test;
create table t1(id int primary key, n float auto_increment, key I_n(n));
insert into t1 set id=1;
update t1 set n=n+1;
insert into t1 set id=2;

select * from t1;
+----+--------------+
| id | n            |
+----+--------------+
| 1  | 2.0          |
| 2  | 4.611686e+18 |
+----+--------------+

[nange]

I will take a look.