PostgreSQL aggregation function for Algebird's HyperLogLog monoid (serialized as a bytea)

hhl.sql

CREATE OR REPLACE FUNCTION sum_hll(a bytea, b bytea) RETURNS bytea as $$
DECLARE
	local_result bytea = a;
BEGIN
	IF get_byte(a, 0) <> get_byte(b, 0) OR get_byte(a, 1) <> get_byte(b, 1) THEN
		RAISE EXCEPTION 'HLL ERROR: FIRST 2 BYTES OF HLLs ARE NOT EQUAL. CANNOT SUM.';
	END IF;

	IF length(a) <> length(b) THEN 
		RAISE EXCEPTION 'HLL ERROR: HLLs LENGTH ARE NOT EQUAL. CANNOT SUM.';
	END IF;

	FOR i IN 2..(length(a) - 1) LOOP
		IF get_byte(a, i) < get_byte(b, i) THEN
			local_result := set_byte(local_result, i, get_byte(b, i));
		END IF;
	END LOOP;
	RETURN local_result;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;

CREATE AGGREGATE agg_sum_hll (bytea)(
	SFUNC = sum_hll,
	STYPE = bytea
);

This version is extremely slow... must move to a C implementation