DESCRIBE FUNCTION hour;
DESCRIBE FUNCTION EXTENDED hour;

EXPLAIN
SELECT hour('2009-08-07 13:14:15'), hour('13:14:15'), hour('2009-08-07')
FROM src WHERE key = 86;

SELECT hour('2009-08-07 13:14:15'), hour('13:14:15'), hour('2009-08-07')
FROM src WHERE key = 86;