Zabbix frontend gets the oldest eventid in any cases using the next call:

5. event->get [events.php:390]
Parameters:
Array
(
    [source] => 0
    [object] => 0
    [output] => extend
    [objectids] => 
    [sortfield] => Array
        (
            [0] => clock
        )

    [sortorder] => ASC
    [limit] => 1
)
Result:
Array
(
    [0] => Array
        (
            [eventid] => 219
            [source] => 0
            [object] => 0
            [objectid] => 13628
            [clock] => 1401317490
            [value] => 0
            [acknowledged] => 0
            [ns] => 365969275
        )

)

query looks like:

• MySQL:

  SELECT e.* FROM events e WHERE EXISTS (SELECT NULL FROM functions f,items i,hosts_groups hgg JOIN rights r ON r.id=hgg.groupid AND r.groupid='7' WHERE e.objectid=f.triggerid AND f.itemid=i.itemid AND i.hostid=hgg.hostid GROUP BY f.triggerid HAVING MIN(r.permission)>0 AND MAX(r.permission)>=2) AND e.object='0' AND e.source='0' ORDER BY e.clock LIMIT 1 OFFSET 0
  

• ORACLE:

  SELECT * FROM (SELECT   e.* FROM events e WHERE EXISTS (SELECT NULL FROM functions f,items i,hosts_groups hgg JOIN rights r ON r.id=hgg.groupid AND r.groupid=:"SYS_B_0" WHERE e.objectid=f.triggerid AND f.itemid=i.itemid AND i.hostid=hgg.hostid GROUP BY f.triggerid HAVING MIN(r.permission)>:"SYS_B_1" AND MAX(r.permission)>=:"SYS_B_2") AND e.object=:"SYS_B_3" AND e.source=:"SYS_B_4" ORDER BY e.clock) WHERE rownum BETWEEN :"SYS_B_5" AND :"SYS_B_6"
  

  In this case we need only min clock to build time line.