Return the correct collation for temporal results

[dbussink]

When functions like GREATEST, LEAST and COALESCE return a temporal result, the collation is not set to binary as expected.

The collation for temporal types normally is always binary which is the case for fields and for functions inheriting from Item_temporal_func.

The comparison functions mentioned here though don't inherit from that and can return various types. Internally they do set the numeric collation correctly, but at the protocol level (and for what the COLLATION function returns), it is translated to the binary collation.

This translation does not happen before this change for the given functions, so we add it here.

Before:

mysql> select collation(date'2010-01-01');
+-----------------------------+
| collation(date'2010-01-01') |
+-----------------------------+
| binary                      |
+-----------------------------+
1 row in set (0.00 sec)

mysql> select collation(greatest(date'2010-01-01', date'2009-01-01'));
+---------------------------------------------------------+
| collation(greatest(date'2010-01-01', date'2009-01-01')) |
+---------------------------------------------------------+
| latin1_swedish_ci                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

After:

mysql> select collation(date'2010-01-01');
+-----------------------------+
| collation(date'2010-01-01') |
+-----------------------------+
| binary                      |
+-----------------------------+
1 row in set (0.00 sec)

mysql> select collation(greatest(date'2010-01-01', date'2009-01-01'));
+---------------------------------------------------------+
| collation(greatest(date'2010-01-01', date'2009-01-01')) |
+---------------------------------------------------------+
| binary                                                  |
+---------------------------------------------------------+
1 row in set (0.00 sec)

This issue applies to all supported MySQL versions, starting from 8.0.x and later at least.

[mysql-oca-bot]

Hi, thank you for your contribution. Please confirm this code is submitted under the terms of the OCA (Oracle's Contribution Agreement) you have previously signed by cutting and pasting the following text as a comment:
"I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it."
Thanks

[dbussink]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

[mysql-oca-bot]

Hi, thank you for your contribution. Your code has been assigned to an internal queue. Please follow
bug http://bugs.mysql.com/bug.php?id=117543 for updates.
Thanks