-- runs in 15 secs:
explain SELECT deviceid, MAX(eventtime) FROM log.event WHERE deviceid IN (3847, 3848) GROUP BY deviceid;
==>
QUERY PLAN
Finalize GroupAggregate  (cost=1354187.91..1354191.96 rows=16 width=16)
  Group Key: deviceid
  ->  Gather Merge  (cost=1354187.91..1354191.64 rows=32 width=16)
        Workers Planned: 2
        ->  Sort  (cost=1353187.88..1353187.92 rows=16 width=16)
              Sort Key: deviceid
              ->  Partial HashAggregate  (cost=1353187.40..1353187.56 rows=16 width=16)
                    Group Key: deviceid
                    ->  Parallel Seq Scan on event  (cost=0.00..1324178.67 rows=5801746 width=16)
                          Filter: (deviceid = ANY ('{3847,3848}'::bigint[]))


-- while this runs in 0,01 secs:
explain SELECT MAX(eventtime) FROM log.event WHERE deviceid IN (3847);
==>
QUERY PLAN
Result  (cost=3.28..3.29 rows=1 width=8)
  InitPlan 1 (returns $0)
    ->  Limit  (cost=0.56..3.28 rows=1 width=8)
          ->  Index Only Scan Backward using idx_event_deviceid_eventtime on event  (cost=0.56..20755559.59 rows=7630816 width=8)
                Index Cond: ((deviceid = 3847) AND (eventtime IS NOT NULL))