CREATE OR REPLACE FUNCTION decode(VARIADIC anyarray) 
  RETURNS anyelement LANGUAGE sql AS
$BODY$
with a as (select $1 as v),
     b as (select case when array_upper(v, 1)%2=0 
                  then v[2:array_upper(v, 1)-1] 
                  else v[2:array_upper(v, 1)] 
                   end as a,
             case when array_upper(v, 1)%2=0 
                  then v[array_upper(v, 1)] 
                   end as d,
                  v[1] as v
            from a)
select coalesce((select a[i+1] from (select generate_series(1, array_upper(a, 1),2)) i(i) where a[i] = v), b.d) from b
$BODY$;
SELECT h, 
       decode(h::varchar, '1', 'I', '2', 'II', '3', 'III', '10', 'X', '24', 'XII', 'zz'),
       to_char(decode(h%12, 0, 12, h%12), 'RN')
  FROM generate_series(1,24) t (h);

 h  | decode |     to_char
----+--------+----------------
  1 | I      |               I
  2 | II     |              II
  3 | III    |             III
  4 | zz     |              IV
  5 | zz     |               V
  6 | zz     |              VI
  7 | zz     |             VII
  8 | zz     |            VIII
  9 | zz     |              IX
 10 | X      |               X
 11 | zz     |              XI
 12 | zz     |             XII
 13 | zz     |               I
 14 | zz     |              II
 15 | zz     |             III
 16 | zz     |              IV
 17 | zz     |               V
 18 | zz     |              VI
 19 | zz     |             VII
 20 | zz     |            VIII
 21 | zz     |              IX
 22 | zz     |               X
 23 | zz     |              XI
 24 | XII    |             XII