KNN Function

CREATE TABLE building_height AS 
SELECT b.ogc_fid, geom, id, featcode,  
(SELECT h.prop_value FROM terrain h 
  ORDER BY b.geom <-> h.geom LIMIT 1)
FROM building b;

Useful blog posts

http://geeohspatial.blogspot.co.uk/2013/05/k-nearest-neighbor-search-in-postgis.html

http://boundlessgeo.com/2011/09/indexed-nearest-neighbour-search-in-postgis/