Now
===

 postgres=# explain analyze select * from obce join okresy on obce.okres_id = okresy.id where okresy.nazev = 'Benešov';
                                                            QUERY PLAN                                                            
-----------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.28..15.19 rows=81 width=58) (actual time=0.067..0.246 rows=114 loops=1)
   ->  Seq Scan on okresy  (cost=0.00..1.96 rows=1 width=17) (actual time=0.025..0.053 rows=1 loops=1)
         Filter: (nazev = 'Benešov'::text)
         Rows Removed by Filter: 76
   ->  Index Scan using obce_okres_id_idx on obce  (cost=0.28..12.41 rows=81 width=41) (actual time=0.032..0.116 rows=114 loops=1)
         Index Cond: ((okres_id)::text = okresy.id)
 Planning time: 0.940 ms
 Execution time: 0.359 ms
(8 rows)

Alternative
===========


 postgres=# explain analyze select * from obce join okresy on obce.okres_id = okresy.id where okresy.nazev = 'Benešov';
                                       QUERY PLAN                                                            
------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.28..15.19 rows=81 width=58)
              (actual time=0.067..0.246 rows=114 loops=1)
   ->  Seq Scan on okresy  (cost=0.00..1.96 rows=1 width=17)
                           (actual time=0.025..0.053 rows=1 loops=1)
         Filter: (nazev = 'Benešov'::text)
         Rows Removed by Filter: 76
   ->  Index Scan using obce_okres_id_idx on obce  (cost=0.28..12.41 rows=81 width=41)
                                                   (actual time=0.032..0.116 rows=114 loops=1)
         Index Cond: ((okres_id)::text = okresy.id)
 Planning time: 0.940 ms
 Execution time: 0.359 ms
(8 rows)