Quantum Gis has been pissing me off for the last week with its slow rendering time. I probably should switch to something leaner. For the time being I will just export what I want as an image, for instance the convex hulls of the area codes that I want to keep in mind.
select area_code, ST_ConvexHull(ST_Collect(geom)) from stations group by area_code having count(*)>10;
The interesting thing here is that the Convex Hull function is not an aggregate, but the collect function is, as explained by the Postgis manual. I like the manual, it usually has the correct clarifications regarding functions so that I can make good decisions.