executor, expression: calculating the default value for datetime should consider the time zone

[zz-jason]

What problem does this PR solve?

The default value calculated for datetime column has not consider the timezone. This PR fixes it.

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch

[zz-jason]

@XuHuaiyu @tiancaiamao @eurekaka @lysu PTAL.

[zz-jason]

/run-common-test tidb-test=pr/620

[zz-jason]

@coocood Why?

[coocood]

@zz-jason never mind

CI failed, due to no database selected

[XuHuaiyu]

I think what @coocood mean is that:
When insert using current_timestamp, timezone should be take into consideration.
But when selecting we should not convert it using timezone, because that we do not know in which timezone the DATETIME value is inserted since DATETIME does not maintain the info of timezone.

[XuHuaiyu]

LGTM

[lysu]

LGTM

[eurekaka]

LGTM

[zz-jason]

Got it. After investigating, mysql document says:

MySQL converts TIMESTAMP values from the current time zone to UTC for storage, and back from UTC to the current time zone for retrieval. (This does not occur for other types such as DATETIME.)

And the behavior seems like MySQL converts DATETIME values from the current time zone to UTC for storage, but for retrieval, MySQL doesn't convert back from UTC to the current time zone.

[zz-jason]

/run-all-tests tidb-test=pr/620