explain analyze select s.nazev, o.nazev, o.pocet_obyvatel 
  from (select nazev, array(select id from obce_pocet_obyvatel where okresy.id = okres_id order by pocet_obyvatel desc limit 3) as obceids from okresy) s 
    join obce_pocet_obyvatel o on o.id = ANY(obceids) order by 1, 3 desc;


postgres=# explain analyze select s.nazev, o.nazev, o.pocet_obyvatel from (select nazev, array(select id from obce_pocet_obyvatel where okresy.id = okres_id order by pocet_obyvatel desc limit 3) as obceids from okresy) s join obce_pocet_obyvatel o on o.id = ANY(obceids) order by 1, 3 desc;
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│                                                                           QUERY PLAN                                                                            │
╞═════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════╡
│ Sort  (cost=840.37..842.29 rows=769 width=24) (actual time=5.518..5.554 rows=227 loops=1)                                                                       │
│   Sort Key: okresy.nazev, ((obce.pocet_muzu + obce.pocet_zen)) DESC                                                                                             │
│   Sort Method: quicksort  Memory: 44kB                                                                                                                          │
│   ->  Nested Loop  (cost=4.09..803.51 rows=769 width=24) (actual time=0.132..4.735 rows=227 loops=1)                                                            │
│         ->  Seq Scan on okresy  (cost=0.00..1.77 rows=77 width=17) (actual time=0.020..0.071 rows=77 loops=1)                                                   │
│         ->  Index Scan using _obce_pkey on obce  (cost=4.09..10.29 rows=10 width=22) (actual time=0.007..0.018 rows=3 loops=77)                                 │
│               Index Cond: (id = ANY ((SubPlan 1)))                                                                                                              │
│               SubPlan 1                                                                                                                                         │
│                 ->  Limit  (cost=0.28..3.81 rows=3 width=8) (actual time=0.025..0.029 rows=3 loops=77)                                                          │
│                       ->  Index Scan using obce_okres_id_expr_idx on obce obce_1  (cost=0.28..95.43 rows=81 width=8) (actual time=0.024..0.027 rows=3 loops=77) │
│                             Index Cond: (okresy.id = (okres_id)::text)                                                                                          │
│               SubPlan 1                                                                                                                                         │
│                 ->  Limit  (cost=0.28..3.81 rows=3 width=8) (actual time=0.025..0.029 rows=3 loops=77)                                                          │
│                       ->  Index Scan using obce_okres_id_expr_idx on obce obce_1  (cost=0.28..95.43 rows=81 width=8) (actual time=0.024..0.027 rows=3 loops=77) │
│                             Index Cond: (okresy.id = (okres_id)::text)                                                                                          │
│ Planning time: 0.815 ms                                                                                                                                         │
│ Execution time: 5.693 ms                                                                                                                                        │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
(17 rows)