How? AFAIK a single join can be at most quadratic, and multiple joins should at most be polynomial, where the exponent is the number of joins. To go exponential, you'd need some kind of recursion or self reference, but I know no way to express such a thing as an ordinary join statement.

(of course quadratic performance is already prohibitively slow on large tables, so there is no need to go exponential in order to take "forever")