Finding the closest Geometry in PostGIS

Your question can be also answered by a single (albeit complex) query like the following which returns the whole record and the distance to the reference geometry. Please note that if more than one record matches the min distance they are all returned.

SELECT 
  i.*,
  md.min_distance
FROM
  address AS i, 
  (SELECT 
     ga.address_geom,
     min( ST_Distance(
            ga.address_geom,
            gb.address_geom)
        ) AS min_distance
   FROM
     address AS ga,
     address AS gb 
   WHERE 
     ga.id <> gb.id 
   AND 
     ga.id = 3
   GROUP BY 
     ga.address_geom
  ) AS md 
WHERE 
  ST_Distance( i.address_geom, md.address_geom) = md.min_distance;

I have tested this query on table of addresses and it works. In the query above I am looking for nearest point to that with id=3.

George MacKerron has written a simple Nearest Neighbor function which I've found quite useful. This function returns the ID of the nearest neighbor to a given feature:

create or replace function 
  nn(nearTo                   geometry
   , initialDistance          real
   , distanceMultiplier       real 
   , maxPower                 integer
   , nearThings               text
   , nearThingsIdField        text
   , nearThingsGeometryField  text)
 returns integer as $$
declare 
  sql     text;
  result  integer;
begin
  sql := ' select ' || quote_ident(nearThingsIdField) 
      || ' from '   || quote_ident(nearThings)
      || ' where st_dwithin($1, ' 
      ||   quote_ident(nearThingsGeometryField) || ', $2 * ($3 ^ $4))'
      || ' order by st_distance($1, ' || quote_ident(nearThingsGeometryField) || ')'
      || ' limit 1';
  for i in 0..maxPower loop
     execute sql into result using nearTo              -- $1
                                , initialDistance     -- $2
                                , distanceMultiplier  -- $3
                                , i;                  -- $4
    if result is not null then return result; end if;
  end loop;
  return null;
end
$$ language 'plpgsql' stable;

Usage example:

SELECT id, nn(pt_geom,0.00001,2,100,'nw_node','node_id','node_geom') FROM my_point_table;

... selects the nearest node in nw_node table for every entry in my_point_table.

There is also a more generic function on the Boston GIS site.