с версии 1.5 можно бует:

(1.5) ORDER BY clause can specify expressions and nulls placement
Nickolay Samofatov
The ORDER BY clause lets you specify any valid expressions to sort query results. If the expression is
consisted of a single number, it is interpreted as column (degree) number, as previously.
The ordering of nulls in the result set can be controlled using the nulls placement clause. Results can
be sorted so that nulls are placed either above (NULLS FIRST) or below (NULLS LAST) the sorted nonnulls.
Behaviour when nulls_placement is unspecified is NULLS LAST.
Syntax
SELECT ... FROM .... ORDER BY order_list ....;
order_list : order_item [, order_list];
order_item : <expression> [order_direction] [nulls_placement]
order_direction : ASC | DESC;