Query question

Hi,

I have coded some improvements to phpPgAdmin that I think are pretty 
cool.  Basicaly, once you are browsing the results of an arbitrary 
SELECT query, you can still sort by columns, regardless of the 
underlying ORDER BY of the SELECT.

I do this like this:

SELECT * FROM (arbitrary subquery) AS sub ORDER BY 1,3;

Now, this all works fine, but I want to know if this is efficient or not.

Does doing a select of a select cause serious performance degradation?

Chris