A problematic query - Mailing list pgsql-novice

Hello,
Let's cut to the chase: 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 X km, all POIs (pharmacies, comercial centres, medical centers, education centers, police stations, fire stations) which are within X km 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)

so the structure would be : building, poi >----- poi_category

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  -- ST_DWithin (a,b, x) makes sure the distance between a & b is less or equal than x
         ) 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'

                    )>1
                 AND
                 count (
                       SELECT *
                       FROM r, r1
                       WHERE r1.id_b=r.id_b AND r.id_pc='ed. centre'

                    )>1

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?

Problem posted here also:  http://gis.stackexchange.com/questions/11445/postgis-advanced-selection-query
 but i received an answer that seems wrong or not optimal at the very least

And an example table that shows distances between :
      -- Ed. centers have poi.c_id  3 and and pharmacies have poi.c_id 1
building.id   1    1    1    1    1    1    1    1    1    2    2    2    2    2    2    2    2    2    3    3    3    3    3    3    3    3    3    4    4    4    4    4    4    4    4    4    5    5    5    5    5    5    5    5    5
poi.id           1    2    3    4    5    6    7    8    9    1    2    3    4    5    6    7    8    9    1    2    3    4    5    6    7    8    9    1    2    3    4    5    6    7    8    9    1    2    3    4    5    6    7    8    9
poi.c_id       1    3    1    2    3    4    1    2    3    1    3    1    2    3    4    1    2    3    1    3    1    2    3    4    1    2    3    1    3    1    2    3    4    1    2    3    1    3    1    2    3    4    1    2    3
distances   10 26  14  15 60  28  65  49  46  23 54 27   16 15  48  26  47 22 19  11   55  34 53  15  31  58  39 19  36  92  47  16 30  25  59  25  35  23 21 40 51 13   43  45  42
between
building
and poi
(i have also decorated the distances depending on wether they are for a pharmacy (bold) or an ed. center (underlined) )     
         
and the resulting table and the logic : I am looking for the buildings that have at least 2 pharmacies and 2 ed.centers within 30 respectively 25 distance units (DU)
So  building 1 has  2 pharmacies within 30 DU but only one ed. center within 25 DU
building 2 has  3 pharmacies within 30 DU and  two  ed. centers  within 25 DU
building 3 has  only 1 pharmacy within 30 DU so the rest doesn't matter
building 4 has  2 pharmacies within 30 DU and  two  ed. centers  within 25 DU ( one of them is exactly 25 DU away)
building 5 has  1 pharmacies within 30 DU , we don't care about the rest

The resulting table would then be:
building.id   2    2    2    2    2    2    2    2    2    4    4    4    4    4    4    4    4    4  
poi.id           1    2    3    4    5    6    7    8    9    1    2    3    4    5    6    7    8    9   
poi.c_id       1    3    1    2    3    4    1    2    3    1    3    1    2    3    4    1    2    3   
distances   23 54   27 16 15  48  26  47 22  19  36  92  47  16 30  25  59  25  -- without the distances column which i'm just printing to make it easier to understand the results
between



  Do help a fellow postgres user ;;)