DDL set default timestamp value is not correct.

[Alkaagr81]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

SET timestamp = 1000;
drop table if exists t1;
CREATE TABLE t1 ( b INT );
INSERT INTO t1 VALUES (1),(2);
ALTER TABLE t1 ADD COLUMN a6 DATETIME(6) DEFAULT NOW(6) ON UPDATE NOW(6) FIRST;
ALTER TABLE t1 ADD COLUMN a5 DATETIME(6) DEFAULT NOW(6) FIRST;
ALTER TABLE t1 ADD COLUMN a4 DATETIME(6) ON UPDATE NOW(6) FIRST;
select * from t1;

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

mysql> SET timestamp = 1000;
Query OK, 0 rows affected (0.00 sec)

mysql> drop table if exists t1;
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE t1 ( b INT );
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t1 VALUES (1),(2);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql>  ALTER TABLE t1 ADD COLUMN a6 DATETIME(6) DEFAULT NOW(6) ON UPDATE NOW(6) FIRST;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>  ALTER TABLE t1 ADD COLUMN a5 DATETIME(6) DEFAULT NOW(6) FIRST;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>  ALTER TABLE t1 ADD COLUMN a4 DATETIME(6) ON UPDATE NOW(6) FIRST;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> 
mysql>  select * from t1;
+------+----------------------------+----------------------------+------+
| a4   | a5                         | a6                         | b    |
+------+----------------------------+----------------------------+------+
| NULL | 1969-12-31 19:16:40.000000 | 1969-12-31 19:16:40.000000 |    1 |
| NULL | 1969-12-31 19:16:40.000000 | 1969-12-31 19:16:40.000000 |    2 |
+------+----------------------------+----------------------------+------+
2 rows in set (0.00 sec)

3. What did you see instead (Required)

mysql> SET timestamp = 1000;
Query OK, 0 rows affected (0.00 sec)

mysql> drop table if exists t1;
Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE t1 ( b INT );
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO t1 VALUES (1),(2);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql>  ALTER TABLE t1 ADD COLUMN a6 DATETIME(6) DEFAULT NOW(6) ON UPDATE NOW(6) FIRST;
Query OK, 0 rows affected (0.01 sec)

mysql>  ALTER TABLE t1 ADD COLUMN a5 DATETIME(6) DEFAULT NOW(6) FIRST;
Query OK, 0 rows affected (0.01 sec)

mysql>  ALTER TABLE t1 ADD COLUMN a4 DATETIME(6) ON UPDATE NOW(6) FIRST;
Query OK, 0 rows affected (0.01 sec)

mysql> 
mysql>  select * from t1;
+------+----------------------------+----------------------------+------+
| a4   | a5                         | a6                         | b    |
+------+----------------------------+----------------------------+------+
| NULL | 2022-01-26 12:29:23.624460 | 2022-01-26 12:29:23.613093 |    1 |
| NULL | 2022-01-26 12:29:23.624460 | 2022-01-26 12:29:23.613093 |    2 |
+------+----------------------------+----------------------------+------+
2 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

| Release Version: v5.5.0-alpha-60-g8c70432b0-dirty
Edition: Community
Git Commit Hash: 8c70432b000d648e9ea26cc57a7fe79d2e67d341
Git Branch: master
UTC Build Time: 2022-01-26 16:56:34
GoVersion: go1.16.9
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |

[CbcWestwolf]

/assign

[mjonss]

Is this related to #29585 ?

[CbcWestwolf]

Is this related to #29585 ?

It is caused by missing implementation. SET timestamp = 1000; select now(); doesn't has this issue.