Instead of :Advertising
WHERE cd='ca' ORDER BY l_postcode;
Write :
WHERE cd='ca' ORDER BY cd, l_postcode;
You have a multicolumn index, so you should specify a multicolumn sort exactly the same as your index, and the planner will get it.
---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ?
http://www.postgresql.org/docs/faqs/FAQ.html
