EXPLAIN (ANALYZE, COSTS, VERBOSE, BUFFERS) SELECT pa.process_activity_id AS pa_process_activity_id FROM process_activity pa WHERE pa.app_id = '126502930200650' AND pa.created > '1970-01-01 00:00:00' AND EXISTS ( SELECT 1 FROM process_instance pi where pi.app_id = pa.app_id AND pi.process_instance_id = pa.process_instance_id AND pi.user_id = '137074931866340') ORDER BY pa.process_instance_id, pa.created limit 50;
-> Index Scan using process_activity_process_instance_id_app_id_created_idx on public.process_activity pa (cost=0.70..1061.62 rows=1436 width=32) (actual time=0.011..20.320 rows=23506 loops=2)
> Index Cond: ((m.process_instance_id = pi.process_instance_id) AND (m.app_id = '126502930200650'::bigint) AND (m.created > '1970-01-01 00:00:00'::timestamp without time zone))
I suppose during the nested loop the inner index scan could limit itself to the first 50 entries it finds (since the first two index columns are being held constant on each scan, m.created should define the traversal order...) so that the output of the nested loop ends up being (max 2 x 50) 100 entries which are then sorted and only the top 50 returned.
Whether the executor could but isn't doing that here or isn't programmed to do that (or my logic is totally off) I do not know.
