SELECT * FROM agent_log t1
WHERE timestamp < now() - interval '7 days' 
AND timestamp < ( 
   SELECT max(timestamp)
   FROM agent_log t2
   WHERE t2.class=t1.class AND t2.promiser=t1.promiser
	AND t2.hostname=t1.hostname AND t2.ip_address=t1.ip_address
)
ORDER by class,timestamp;