Problem with complex query

Hello!

Query:

SELECT DISTINCT p.subsec_id
   FROM central cn, shops sh, districts d, positions p
      WHERE cn.shop_id = sh.shop_id AND sh.distr_id = d.distr_id
      AND   d.city_id = %d AND cn.pos_id = p.pos_id
      AND   cn.date_i >= current_date - '7 days'::timespan

While running postgres slowly eats all swap space (30 Meg) and aborts:

pqReadData() -- backend closed the channel unexpectedly.
        This probably means the backend terminated abnormally before or
while processing the request.

   Is it I just have not enough memory or bug?

Oleg.
---- 
    Oleg Broytmann     http://members.xoom.com/phd2/     phd2@earthling.net
           Programmers don't die, they just GOSUB without RETURN.