optimizing simple math operator

["Ryan D. E nos" <renos(at)ucla(dot)edu>]

Hi,
I am an extreme postgres novice.  I occasionally use databases to handle 
large voter lists. 

I am running postgres on a Windows XP platform on a machine with 
Pentium(R) 4 CPU 3 GHz and 1GB of RAM.

I have a table with approximately 4 million rows that I am trying to 
update a single numeric column of with the following math operation 
using two integer columns:

update test
   set turnout1_cityblock = cityblock_election8/cityblock_regpop;

on the test table with approximately 45K rows, that update takes only 
about 4 seconds.  I have also tried casting the integers as numeric and 
that increases the update time to only about 10 seconds.

However, with the full table of 4 million rows, the update takes an 
unreasonable amount of time (I'm talking days).  There are no nulls in 
either column in the larger table.

Can anybody help me figure out why there is the inefficiency with the 
update in the large table?

thanks.
Ryan