nvl2 SQL vs pl/pgsql

test.sql

create or replace function nvl2(expr1 anycompatible, expr2 anycompatible, expr3 anycompatible) returns anycompatible as $$
	select case when expr1 is null then expr2 else expr3 end;
$$ language sql immutable;

create or replace function nvl2pg(expr1 anycompatible, expr2 anycompatible, expr3 anycompatible) returns anycompatible as $$
begin
	if expr1 is null then 
		return expr2;
	else
		return expr3;
	end if;
end;
$$ language plpgsql immutable;


explain (analyze,verbose)
select nvl2(custamt::text, 'set', 'unset') from tran;

--Append  (cost=0.00..18598.96 rows=493074 width=32) (actual time=0.019..129.488 rows=492929 loops=1)   
--  ->  Seq Scan on public.tran_initial tran_1  (cost=0.00..0.01 rows=1 width=32) (actual time=0.005..0.
--        Output: CASE WHEN ((tran_1.custamt)::text IS NULL) THEN 'set'::text ELSE 'unset'::text END    

explain (analyze,verbose)
select nvl2pg(custamt::text, 'set', 'unset') from tran;

--Append  (cost=0.00..141867.46 rows=493074 width=32) (actual time=125.141..681.455 rows=492929 loops=1)                      
--  ->  Seq Scan on public.tran_initial tran_1  (cost=0.00..0.26 rows=1 width=32) (actual time=0.008..0.008 rows=0 loops=1)   
--        Output: nvl2pg((tran_1.custamt)::text, 'set'::text, 'unset'::text)