Fast fibonacci function in pure SQL

fibonacci.sql

# Fast fibonacci function in pure SQL
# Tested on Postgresql 9.5

CREATE OR REPLACE FUNCTION _fib (
    n integer, a bigint, b bigint
) RETURNS bigint AS $$
BEGIN
    IF n < 1 THEN
        RETURN b;
    END IF;
        RETURN _fib(n -1, b, a+b);
END;
$$ LANGUAGE plpgsql;


CREATE OR REPLACE FUNCTION fibonacci (
    n integer
) RETURNS bigint AS $$
BEGIN
    IF n > 91 THEN
        RAISE EXCEPTION 'Bigint overflow past fibonacci(91) [%]', n;
    END IF;
    RETURN _fib(n, 0, 1);
END;
$$ LANGUAGE plpgsql;