Custom Postgres function not working
Peter's quite right about the problem, of course. However, this entire function is unnecessarily slow and complex - that repeated string concatenation will be horrible for performance, and PL/PgSQL loops are best avoided when you can anyway.

You can do the same job with an ordinary SQL function using generate_subscripts:

CREATE OR REPLACE FUNCTION intarr_to_hex_string(integer[]) RETURNS text AS $$
SELECT 
  string_agg(
    lpad(to_hex($1[x]),2,'0'),
    ''
    order by x
  ) 
FROM generate_subscripts($1,1) x;
$$ LANGUAGE sql;


The above version expects an integer array, but you can accept text arrays with an overload:

CREATE OR REPLACE FUNCTION intarr_to_hex_string(text[]) RETURNS text AS $$
SELECT intarr_to_hex_string($1::integer[])
$$ LANGUAGE sql;


Note that no sanity check is done to make sure the hex string is two digits or less. You should really add a check where you use a CASE statement to execute a PL/PgSQL function that raises an error if there's bad input. Here's a complete example:

CREATE OR REPLACE FUNCTION raise_exception(text)
  RETURNS void LANGUAGE plpgsql AS
$BODY$ 
BEGIN 
   RAISE EXCEPTION '%', $1; 
END; 
$BODY$;

CREATE OR REPLACE FUNCTION intarr_to_hex_string(integer[]) RETURNS text AS $$
SELECT 
  string_agg(lpad(
    CASE WHEN $1[x] &lt; 256 THEN
      to_hex($1[x])
    ELSE
      raise_exception('Argument to intarr_to_hex_string contained value '||coalesce($1[x]::text,'NULL')||', expected all values in i[] to be 0 &lt;= i &lt; 256. Full argument was '||quote_literal($1)||'.')::text
    END,
    2, '0'),
  ''
  order by x) 
FROM generate_subscripts($1,1) x;
$$ LANGUAGE sql;