Why indexes are not used when scanning from functions?

The table (script followed) contains more than 200000 records.
When I directly selecting records by the single select statement

select fio from patient_temp where fio like 'something%';

Postgres uses the index i have created and quickly returns the results. But if that query included into the function, slowest sequent scan starts. Is it a normal postgres behavior - not using indexes in functions? Or how to make indexes usable in functions?
I have tried PL/pgSQL, and have got the same result.

create table patient_temp(
code serial primary key,
fio varchar(80)
);

create index fio_patient_temp_ind on patient_temp(fio);

create function get_fio1(text) returns varchar
as 'select fio from patient_temp where fio like $1::text || \'%\';'
language 'sql';

Aidamir Lovpache <aidanet(at)yandex(dot)ru>