SELECT * FROM (
    SELECT messages.*,
        row_to_json(users.*) as user,
        row_to_json(channels.*) As channel
    FROM messages
    LEFT JOIN users ON messages.user_id = users.id
    LEFT JOIN channels ON messages.channel_id = channels.id 
    WHERE messages.channel_id = %s
    ORDER BY messages.timestamp DESC
    LIMIT %s
) AS subquery
ORDER BY subquery.timestamp ASC;