in postgres, there currently isnt a function to combine two arrays and remove duplicates - this will do that.

postgres-merge-arrays.sql

CREATE OR REPLACE FUNCTION mergeArrays (a1 ANYARRAY, a2 ANYARRAY) RETURNS ANYARRAY AS $$

	SELECT ARRAY_AGG(x ORDER BY x)
	FROM (
		SELECT DISTINCT UNNEST($1 || $2) AS x
	) s;

$$ LANGUAGE SQL STRICT;

doesn't seem to work when one of the arrays is empty.

It looks like it works to me - an empty array you must define as the kind of data type in the array, but it works.

https://dbfiddle.uk/?rdbms=postgres_9.6&fiddle=ae6385a5b138c262008de6f0b35986e5