--Now that we have the PL/pgSQL procedural language (thanks to Jan Wieck)
--you can do almost every thing you desire with PostgreSQL in a easy way.
--For example; to emulate COBOL level 88, as you suggest you may create
--the DECODE function as in:

--COBOL level 88 using views and decode function------------------------------

drop function decode(text,text,bool);
create function decode(text,text,bool) returns bool as '
DECLARE
	stringa text;
	substringa text;
	code text;
	value text;
	i int2;
	z int2;
BEGIN
	stringa:= $2;
	loop
        	i:= position(''='' in stringa);
		if i = 0 then
			return $3;
		end if;
        	z:= position('','' in stringa);
		if z = 0 then
			z:= textlen(stringa)+1;
		end if;
		value:= substr(stringa,i + 1,z - i - 1);
		code:= substr(stringa,1,i - 1);
		if $1 = code then
			return value;
		else
			if z > 0 then
				stringa:= substr(stringa,z + 1);
			else
				return $3;
			end if;
		end if;
	end loop;
END;
' language 'plpgsql';


drop table customer, customer_view;
create table customer(
	customer_id	numeric,
	customer_name	varchar(30),
	customer_status	char(1),
	sales_rep	numeric
);
insert into customer values (1,'Pippo','A',20);
insert into customer values (2,'Pluto','H',50);
insert into customer values (3,'Topolino','a',10);
insert into customer values (4,'Paperino','P',30);

create view customer_view as
	select customer_id, customer_name, sales_rep,
		DECODE (customer_status,'A=TRUE,a=TRUE',FALSE) as active_client,
		DECODE (customer_status,'H=TRUE,h=TRUE',FALSE) as historical_client,
		DECODE (customer_status,'I=TRUE,i=TRUE',FALSE) as invalid_client,
		DECODE (customer_status,'P=TRUE,p=TRUE',FALSE) as potential_client,
		DECODE (customer_status,'A=TRUE,a=TRUE,P=TRUE,p=TRUE',FALSE) as current_client
       	from customer;

select customer_id, customer_name, active_client, sales_rep
from customer_view
where active_client;

select customer_id, customer_name, active_client, sales_rep
from customer_view
where not active_client;

--The other solution as you suggest is to have a function for every level 88
--this is a better choice to have high performance...

--COBOL level 88 using functions-----------------------------------------------

drop function current_client(text);
create function current_client(text) returns bool as '
DECLARE
	status ALIAS for $1;
BEGIN
	RETURN status IN (''A'',''a'',''P'',''p'');
END;
' language 'plpgsql';

drop function active_client(text);
create function active_client(text) returns bool as '
DECLARE
	status ALIAS for $1;
BEGIN
	RETURN status IN (''A'',''a'');
END;
' language 'plpgsql';

select *  from customer
where active_client(customer_status);

select *  from customer
where not active_client(customer_status);