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This query can be used to solve this issue
SELECT m1.*
FROM messages m1
INNER JOIN
(
    SELECT t.v1 AS sender_id, t.v2 AS receiver_id, MAX(t.timestamp) AS maxTime
    FROM
    (
        SELECT CASE WHEN sender_id < receiver_id THEN sender_id ELSE receiver_id END AS v1,
               CASE WHEN sender_id < receiver_id THEN receiver_id ELSE sender_id END AS v2,
               timestamp
        FROM messages
    ) t
    GROUP BY t.v1, t.v2
) m2
ON ((m1.sender_id = m2.sender_id AND m1.receiver_id = m2.receiver_id) OR (m1.sender_id = m2.receiver_id AND m1.receiver_id = m2.sender_id)) AND m1.timestamp = m2.maxTime 
UNION ALL 
SELECT m1.*
FROM messages m1
INNER JOIN
(
    SELECT MAX(timestamp) AS maxTime
    FROM messages
  GROUP BY group_id
) m3
ON m1.timestamp = m3.maxTime
79156237
