Proposal: DataType Timestamp remove the precision argument

[sundy-li]

Timestamp datatype now keep the precision which can be used to output the DateTime with a custom style.

For example:

MySQL [(none)]> create table t (a Timestamp(3), b Timestamp(6), c Timestamp(0) );
Query OK, 0 rows affected (0.008 sec)

MySQL [(none)]> insert into t values(now(), now(), now());
Query OK, 0 rows affected (0.020 sec)

MySQL [(none)]> select * from t;
+-------------------------+----------------------------+---------------------+
| a                       | b                          | c                   |
+-------------------------+----------------------------+---------------------+
| 2022-10-12 02:27:18.974 | 2022-10-12 02:27:18.975407 | 2022-10-12 02:27:18 |
+-------------------------+----------------------------+---------------------+
1 row in set (0.026 sec)

The storage stores the same value for now() which represents instants as the number of microseconds (µs) since 1970-01-01 00:00:00+00. The precision is only for output style, so it's useless to keep it in datatype, we can use other functions like: to_yyyy_mm_dd to handle the style.

[andylokandy]

Does it mean that the default timestamp format style will always be YYYY-MM-DD hh:mm::ss.6f?

[sundy-li]

Yes. We can:

1. Add a setting format_timestamp_precision
2. Use timestamp_format(ts, format) to control the format style.

I'll fill 2 after #8154 is finished.