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