postgres=# select i from (select i, pg_sleep(5) as sleep from generate_series(1,5) as g(i)) as _view;
e i
---
 1
 2
 3
 4
 5
(5 rows)

Time: 25006.180 ms
postgres=# explain (verbose, analyze) select i from (select i, pg_sleep(5) as sleep from generate_series(1,5) as g(i)) as _view;
                                                                 QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------
 Subquery Scan on _view  (cost=0.00..202.50 rows=1000 width=4) (actual time=5001.077..25005.338 rows=5 loops=1)
   Output: _view.i
   ->  Function Scan on pg_catalog.generate_series g  (cost=0.00..102.50 rows=1000 width=4) (actual time=5001.073..25005.317 rows=5 loops=1)
         Output: g.i, pg_sleep(5::double precision)
         Function Call: generate_series(1, 5)
 Total runtime: 25005.380 ms
(6 rows)

Time: 25026.864 ms

CREATE OR REPLACE FUNCTION trash.immutable_test(p_int int)
  RETURNS int AS
$BODY$
begin
        raise notice '%', p_int;
        return p_int;
END;

$BODY$
  LANGUAGE plpgsql immutable
  COST 100;

select q.i from(select i, trash.immutable_test(i) from generate_series(1,5) i) q