Aggregate function weighted_avg for PostgreSQL

weighted_avg.sql

CREATE TYPE _weighted_avg_fraction
  AS (num double precision, denom double precision);

CREATE OR REPLACE FUNCTION _weighted_avg_step (acc _weighted_avg_fraction,
    val anyelement, pond anyelement) RETURNS _weighted_avg_fraction AS $$
BEGIN
  RETURN (acc.num + val * pond, acc.denom + pond);
END;
$$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION _weighted_avg_final (acc _weighted_avg_fraction)
    RETURNS double precision AS $$
BEGIN
  IF acc.denom = 0
  THEN RETURN NULL;
  ELSE RETURN acc.num / acc.denom;
  END IF;
END;
$$ LANGUAGE plpgsql;

CREATE AGGREGATE weighted_avg (anyelement, anyelement) (
  SFUNC     = _weighted_avg_step,
  STYPE     = _weighted_avg_fraction,
  FINALFUNC = _weighted_avg_final,
  INITCOND  = '(0, 0)'
);