DOUBLE type can't keep precise

[lance6716]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

lightning's CI

create table precise_types (
    a BIGINT UNSIGNED NOT NULL,
    b BIGINT NOT NULL,
    c DECIMAL(21,1) NOT NULL,
    d DOUBLE(21,1) NOT NULL
);
insert into precise_types values (
    18446744073709551614,
    -9223372036854775806,
    99999999999999999999,
    18446744073709551614
);
SELECT a, b, c, d FROM precise_types\G

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

MySQL 8.0.19

mysql> SELECT a, b, c, d FROM precise_types\G
*************************** 1. row ***************************
a: 18446744073709551614
b: -9223372036854775806
c: 99999999999999999999.0
d: 18446744073709552000.0
1 row in set (0.00 sec)

3. What did you see instead (Required)

mysql> SELECT a, b, c, d FROM precise_types\G
*************************** 1. row ***************************
a: 18446744073709551614
b: -9223372036854775806
c: 99999999999999999999.0
d: 1.8e19
1 row in set (0.01 sec)

4. What is your TiDB version? (Required)

Release Version: v4.0.0-beta.2-1804-g06cd92e05
Edition: Community
Git Commit Hash: 06cd92e
Git Branch: HEAD
UTC Build Time: 2020-12-12 21:09:27
GoVersion: go1.13.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[ti-srebot]

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5. Affected versions

6. Fixed versions

[lance6716]

/unlabel type/bug

we didn't support DOUBLE(M,D) since https://dev.mysql.com/doc/refman/8.0/en/floating-point-types.html

[lance6716]

/label type/bug

since behaviour is not same with MySQL 8.0 (thought a deprecated feature), I still let this issue open

[tangenta]

I don't think this bug is fixed, here is the result in master(gcec1a9265):

mysql> SELECT a, b, c, d FROM precise_types\G;
*************************** 1. row ***************************
a: 18446744073709551614
b: -9223372036854775806
c: 99999999999999999999.0
d: 1.8446744073709552e19
1 row in set (0.00 sec)

got: 1.8446744073709552e19
expected: 18446744073709552000.0

[lzmhhh123]

I don't think this bug is fixed, here is the result in master(gcec1a9265):

mysql> SELECT a, b, c, d FROM precise_types\G;
*************************** 1. row ***************************
a: 18446744073709551614
b: -9223372036854775806
c: 99999999999999999999.0
d: 1.8446744073709552e19
1 row in set (0.00 sec)

got: 1.8446744073709552e19
expected: 18446744073709552000.0

1.8446744073709552e19 is equal to 18446744073709552000.0. For MySQL protocol, they are the same when received from the user.