time: parse datatime won't truncate the reluctant string

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

select cast('2020-05-28 23:59:59 00:00:00' as datetime);

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

+--------------------------------------------------+
| cast('2020-05-28 23:59:59 00:00:00' as datetime) |
+--------------------------------------------------+
| 2020-05-28 23:59:59                              |
+--------------------------------------------------+
1 row in set, 1 warning (0.01 sec)

3. What did you see instead (Required)

+--------------------------------------------------+
| cast('2020-05-28 23:59:59 00:00:00' as datetime) |
+--------------------------------------------------+
| NULL                                             |
+--------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

4. What is your TiDB version? (Required)

current master 8ef1031

[tangenta]

Why is it a critical bug? 😕

[aytrack]

Because "Bugs caused by type casts" are critical bugs. So I change the label to critical.

[ti-srebot]

Please edit this comment or add a new comment to complete the following information

Not a bug

1. Remove the 'type/bug' label
2. Add notes to indicate why it is not a bug

Duplicate bug

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2. Add the link to the original bug

Bug

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Example for how to fill out the template: #20100

1. Root Cause Analysis (RCA) (optional)

2. Symptom (optional)

3. All Trigger Conditions (optional)

4. Workaround (optional)

5. Affected versions

[v3.0.0:v3.0.20],[v4.0.0:v4.0.10][v5.0.0-rc]

6. Fixed versions

master