notes=# select AsText(Envelope(ST_Union(the_geom))) from notes;
                           astext                                                                      
---------------------------------------------------------------
 POLYGON((-74.0071258544922 40.7674407958984,
          -74.0071258544922 40.8000373840332,
          -73.9178161621094 40.8000373840332,
          -73.9178161621094 40.7674407958984,
          -74.0071258544922 40.7674407958984))