Qury plan for sql function with security definer

Hello all,

I have two simple SQL function. The first is defined with the modifier
"security definer", while the second is not.

   1 create or replace function func_with_sec_definer(param text)
returns setof text as $$
   2
   3     select unnest(string_to_array(param, ',')) ;
   4
   5 $$ language sql immutable security definer;
   6
   7 create or replace function func_without_sec_definer(param text)
returns setof text as $$
   8
   9     select unnest(string_to_array(param, ',')) ;
  10
  11 $$ language sql immutable ;

When I look at the query execution plan, I see that the first function
uses the function scan, while the second is not. How can I do to make
the first function does not use function scan as well?

   =# explain select * from func_with_sec_definer('1,2,3');
┌───────────────────────────────────────────────────────────────────────────────┐
│                                  QUERY PLAN
         │
├───────────────────────────────────────────────────────────────────────────────┤
│ Function Scan on func_with_sec_definer  (cost=0.25..10.25 rows=1000
width=32) │
└───────────────────────────────────────────────────────────────────────────────┘

   =# explain select * from func_without_sec_definer('1,2,3');
┌────────────────────────────────────────────┐
│                 QUERY PLAN                 │
├────────────────────────────────────────────┤
│ Result  (cost=0.00..0.51 rows=100 width=0) │
└────────────────────────────────────────────┘


   =# select version();
┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│                                                   version
                                        │
├──────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
│ PostgreSQL 9.2.4 on x86_64-unknown-linux-gnu, compiled by gcc (GCC)
4.4.7 20120313 (Red Hat 4.4.7-3), 64-bit │
└──────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

--
Regards, Andrew G. Saushkin