The problem: I need to select, for each residential building in my table that has say at least 2 pharmacies and 2 education centers within a radius of 1km, all POIs (pharmacies, comercial centres, medical centers, education centers, police stations, fire stations) which are within 1km of the respective building. table structure->

building (id serial, name varchar )

poi_category(id serial, cname varchar) --cname being the category name of course

poi(id serial, name varchar, c_id integer)-- c_id is the FK referencing poi_category(id)

all coordinate columns are of type geometry not geography (let's call them geom)

here's the way i thought it should be done but i'm not sure it's even correct let alone the optimal solution to this problem

    SELECT r.id_b, r.id_p
    FROM (
         SELECT b.id AS id_b, p.id AS id_p, pc.id AS id_pc,pc.cname
         FROM building AS b, poi AS p, poi_category AS pc
         WHERE ST_DWithin(b.geom,p.geom, 1000) AND p.c_id=pc.id
         ) AS r,
         SELECT * FROM r GROUP BY id_b
         ) AS r1

     HAVING  count (
                       SELECT *
                       FROM r, r1
                       WHERE r1.id_b=r.id_b AND r.id_pc='pharmacy'

                 count (
                       SELECT *
                       FROM r, r1
                       WHERE r1.id_b=r.id_b AND r.id_pc='ed. centre'


Is this the way to go for what i need ? What solution would be better from a performance point of view? What about the most elegant solution?

  • 1
    In my opinion it would be better to rephrase the question for something like: "Advanced selection query in postgis" – com Jun 24 '11 at 11:05
  • 1
    @com i doubt this is really advanced selection.. i probably make it look advanced because of the way i'm complicating things but the problem seems rather simple to me and, i assume, the solution to the problem is much more simple than the one i have written (which i'm not sure is correct BTW) . – Dackster.ox Jun 24 '11 at 11:50
  • 1
    You should take a look at windowing function in postgresql doc if you are using PostgreSQL 8.4+. – Nicklas Avén Jun 24 '11 at 13:54
  • @ nicklas-aven i have postgres 8.3 currently installed – Dackster.ox Jun 24 '11 at 14:06

Based on the second example given on that page, I would try something like :

SELECT b.gid, b.name, [+ any fields] -- here you retrieve POI data
    FROM building b
        LEFT JOIN building f ON ST_DWithin(b.the_geom, f.the_geom, 1000) -- farmacy
        LEFT JOIN building ec ON ST_DWithin(b.the_geom, ec.the_geom, 1000) -- ed. center
        -- LEFT JOIN POI AND POI_CATEGORY tables + join conditions
    WHERE f.id_pc='pharmacy' AND ec.id_pc='ed. centre'
    AND COUNT(f.gid) >1 AND COUNT(ec.gid)>1;

Of course, it is not tested and the query is not complete but the logic would be to join tables regarding geometrical criteria. If that work, then add instructions to retrieve your POI data.

| improve this answer | |

I usually do these in plpgsql as I find it much easier to read and the results are usually more efficient because you can control the flow (exit early, etc):

br record;
phcount integer;
edcount integer;
FOR br IN SELECT id, name, geom FROM building LOOP
  SELECT count(id) from poi where ST_DWITHIN(br.geom, geom, 1000) and c_id = pharmacycateg into phcount;
  IF phcount < 2 THEN
     EXIT; --i think this is the equiv of continue, i.e. skip to next building
  SELECT count(id) from poi where ST_DWITHIN(br.geom, geom, 1000) and c_id = pharmacycateg into edcount;
  IF edcount < 2 THEN
  -- now select all POIS within 1KM and maybe return them as array, etc
$$ LANGUAGE 'plpgsql' strict;
| improve this answer | |

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