Postgres aggregator to get the first non-null value.

first_non_null.sql

-- Like the Postgres first_val() aggregator, exceppt that this returns the first non-null value.
CREATE OR REPLACE FUNCTION first_non_null_fun(numeric, numeric) RETURNS numeric
  AS 'SELECT case when $1 is null then $2 else $1 end;'
  LANGUAGE SQL IMMUTABLE STRICT;

CREATE AGGREGATE first_non_null ( numeric ) (
  SFUNC = first_non_null_fun,
  STYPE = numeric
);