AFAIK, sub select é sua melhor opção, da seguinte forma:
SELECT *
FROM mytable mt
JOIN ( SELECT MAX(timestamp) as max, event
FROM mytable
GROUP BY event) m_mt
ON (mt.timestamp = m_mt.max AND mt.event = m_mt.event);
SELECT *
FROM mytable mt
JOIN ( SELECT MAX(timestamp) as max, event
FROM mytable
GROUP BY event) m_mt
ON (mt.timestamp = m_mt.max AND mt.event = m_mt.event);