select min(clock) from events where events.source=3 and events.object=0 and not exists (select null from problem where events.eventid=problem.eventid or events.eventid=problem.r_eventid)

mysql> explain select min(clock) from events where events.source=3 and events.object=0 and not exists (select null from problem where events.eventid=problem.eventid or events.eventid=problem.r_eventid)\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: events
   partitions: NULL
         type: ref
possible_keys: events_1,events_2
          key: events_2
      key_len: 8
          ref: const,const
         rows: 15964577
     filtered: 100.00
        Extra: Using where; Using index
*************************** 2. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: problem
   partitions: NULL
         type: ALL
possible_keys: PRIMARY,c_problem_2
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 708846
     filtered: 19.00
        Extra: Range checked for each record (index map: 0x9)
2 rows in set, 3 warnings (0,00 sec)

mysql> explain select min(clock) from events where events.source=3 and events.object=0 and not exists (select null from problem where events.eventid=problem.eventid) and not exists (select null from problem where events.eventid=problem.r_eventid)\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: events
   partitions: NULL
         type: ref
possible_keys: events_1,events_2
          key: events_2
      key_len: 8
          ref: const,const
         rows: 15973658
     filtered: 100.00
        Extra: Using where; Using index
*************************** 2. row ***************************
           id: 3
  select_type: DEPENDENT SUBQUERY
        table: problem
   partitions: NULL
         type: ref
possible_keys: c_problem_2
          key: c_problem_2
      key_len: 9
          ref: zabbix.events.eventid
         rows: 1
     filtered: 100.00
        Extra: Using index
*************************** 3. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: problem
   partitions: NULL
         type: eq_ref
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 8
          ref: zabbix.events.eventid
         rows: 1
     filtered: 100.00
        Extra: Using index
3 rows in set, 3 warnings (0,00 sec)