PostgrSQL: printf in PL/pgSQL

PostgreSQL wiki has entry about sprintf - is is quite simple approach (and isn't marked as immutable), the main drawback is iterating over all chars of format string. Here is a version use strpos to locate % in format string, and it's faster around 2 times:

CREATE OR REPLACE FUNCTION printf2(fmt text, variadic args anyarray) RETURNS text
LANGUAGE plpgsql IMMUTABLE AS $$
   DECLARE
      argcnt  int  := 1;
      head    text := '';     -- result
      tail    text := fmt;    -- unprocessed part
      k       int;
   BEGIN
      LOOP
         k := strpos(tail, '%');
         IF k = 0 THEN
            -- no more '%'
            head := head || tail;
            EXIT;
         ELSE
            IF substring(tail, k+1, 1) = '%' THEN
               -- escape sequence '%%'
               head := head || substring(tail, 1, k);
               tail := substring(tail, k+2);
            ELSE
               -- insert argument
               head := head || substring(tail, 1, k-1) || COALESCE(args[argcnt]::text, '');
               tail := substring(tail, k+1);
               argcnt := argcnt + 1;
            END IF;
         END IF;
      END LOOP;
   RETURN head;
END;
$$;