text: CREATE FUNCTION ordered_set_transition(internal, anyelement)
    RETURNS internal ...;
  CREATE FUNCTION percentile_disc_final(internal, float8, anyelement)
    RETURNS anyelement ...;
  
  CREATE AGGREGATE percentile_disc (float8 ORDER BY anyelement)
  (
      sfunc = ordered_set_transition,
      stype = internal,
      finalfunc = percentile_disc_final,
      finalfunc_extra
  );
  