As of versions 10.3.x and 10.4.7, functions ROUND() and TRUNCATE() convert temporal input to double and then return a rounded/truncated double value.

This behaviour is quite useless, because can return '60' at the end:

• ROUND() in double format can return '60' at the end by definition
• TRUNCATE() in double format can return '60' at the end because of lack of precision:

  TRUNCATE(TIMESTAMP'2001-12-31 23:59:59.999999', 1);

  +----------------------------------------------------+

  | TRUNCATE(TIMESTAMP'2001-12-31 23:59:59.999999', 1) |

  +----------------------------------------------------+

  | 20011231235960.0 |

  +----------------------------------------------------+

Therefore a rounded or truncated double value cannot be converted back to TIMESTAMP, DATETIME or TIME easily.

In order to fix MDEV-18153 easier, let's implement native TIMESTAMP, DATETIME, TIME support in ROUND() and TRUNCATE().

Let's change these functions to return:

• TIME in case of a TIME input
• DATETIME in case of a DATETIME input
• DATETIME in case of a TIMESTAMP input

Note, ROUND(timestamp) will return DATETIME for now. It should be fine in 99% cases, but won't work well near DST changes. Let's implement real native TIMESTAMP support in ROUND() in a separate change later.

Note, the DATE data type is out of scope of this task. ROUND() and TRUNCATE() will still return a double result for a DATE input. Native DATE support for ROUND() and TRUNCATE() will be implemented separately, when needed.