Performance issue when we use policies for Row Level Security along with functions

Hi,

I'm seeing a strange behavior when we implement policies (for RLS - Row level security)  using functions. 

table test  consists of columns  testkey,oid,category,type,description... 

Policy

create policy  policy_sel on test FOR SELECT to ram1 USING  (  testkey in (f_sel_policy_test(testkey))  );

pgwfc01q=> explain analyze select * from test;

RETURNS character varying

LANGUAGE plpgsql

AS $function$

Declare

            v_status character varying;

BEGIN

            if vpd_key = 'COMMON' then

                        return '''COMMON''';

            elsif vpd_key = ('COMMON_' || SYS_CONTEXT('ctx_ng', 'ctx_prod_locale')) then

                        return '''COMMON_' || SYS_CONTEXT('ctx_ng', 'ctx_prod_locale')||'''';

            elsif vpd_key = SYS_CONTEXT('ctx_ng_vpd', 'ctx_key_fil') then

                        return '''co'','''||SYS_CONTEXT('ctx_ng', 'ctx_testkey_fil')||'''';

            end if;

            return 'false';    

            exception when undefined_object then

                        return 'failed';

           

END;

$function$

If i replace the policy with stright forward without function then it chooses the index.   Not sure how i can implement with the function.   I thought of creating the policy with a lot of business logic in the function.  If i have the function then i notice going for full table scan instead of index. 

Please help me if i miss anything in writing a function or how to use functions in the policy.