Active record is not releaed used memory

I have tried small ruby program like

require 'rubygems'
require 'postgres'
db = PGconn.connect('localhost', 5432, '', '', 'prdjnj','postgres',
res=db.exec("select * from users limit 300000")
puts "After clear result object"
while a!=2

Before running this program,Initial free memory is 640MB
The query("select * from users limit 300000") execution taken the time
is upto 110MB when i run this program .. So free memory is reduced from
640MB to 530MB..Now used memory is 110MB that released after the
execution of the query ie program reached the while conditions

I have followed same scenario in Rail's active_record instead of

puts "After clear result object"

Here Used memory is not released even after execute the query and clear
resulting object..Memory is released well when i use PGconn.. but its
not working in active_record

Please instruct me how to release used memory in Rail's active_record