unexpected result of union

[XuHuaiyu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

select greatest(cast("2020-01-01 01:01:01" as datetime), cast("2019-01-01 01:01:01" as datetime) )union select null;

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

mysql> select greatest(cast("2020-01-01 01:01:01" as datetime), cast("2019-01-01 01:01:01" as datetime) )union select null;

+---------------------------------------------------------------------------------------------+
| greatest(cast("2020-01-01 01:01:01" as datetime), cast("2019-01-01 01:01:01" as datetime) ) |
+---------------------------------------------------------------------------------------------+
| 2020-01-01 01:01:01                                                                         |
| NULL                                                                                        |
+---------------------------------------------------------------------------------------------+
2 rows in set (0.01 sec)

3. What did you see instead (Required)

tidb> select greatest(cast("2020-01-01 01:01:01" as datetime), cast("2019-01-01 01:01:01" as datetime) )union select null;

+---------------------------------------------------------------------------------------------+
| greatest(cast("2020-01-01 01:01:01" as datetime), cast("2019-01-01 01:01:01" as datetime) ) |
+---------------------------------------------------------------------------------------------+
| NULL                                                                                        |
|                                                                                             |
+---------------------------------------------------------------------------------------------+
2 rows in set, 1 warning (0.01 sec)

4. What is your TiDB version? (Required)

a414603

[XuHuaiyu]

This bug is introduced by #21150

[aytrack]

this issue can also reproduce in 5.1.0, 4.0.13, 5.0.3

[ti-srebot]

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