-- from here: https://wiki.postgresql.org/wiki/Aggregate_Median
CREATE OR REPLACE FUNCTION _final_median(numeric[])
   RETURNS numeric
AS
$body$
   SELECT AVG(val)
   FROM (
     SELECT val
     FROM unnest($1) val
     ORDER BY 1
     LIMIT  2 - MOD(array_upper($1, 1), 2)
     OFFSET CEIL(array_upper($1, 1) / 2.0) - 1
   ) sub;
$body$
LANGUAGE sql ;
-- IMMUTABLE not accepted by pg migrate

CREATE AGGREGATE median(numeric) (
  SFUNC=array_append,
  STYPE=numeric[],
  FINALFUNC=_final_median,
  INITCOND='{}'
);