people who buy A, also buy C, D, E

The goal of my query is: given a book, what did other people who 
bought this book also buy?  I plan the list the 5 most popular such 
books.  In reality, this isn't about books, but that makes it easier 
to understand I think.

We have a table of customer_id (watch_list_id) and book_id 
(element_id).

freshports.org=# \d watch_list_element Table "public.watch_list_element"   Column     |  Type   | Modifiers
---------------+---------+-----------watch_list_id | integer | not nullelement_id    | integer | not null
Indexes:   "watch_list_element_pkey" primary key, btree (watch_list_id, 
element_id)   "watch_list_element_element_id" btree (element_id)
Foreign-key constraints:   "$2" FOREIGN KEY (watch_list_id) REFERENCES watch_list(id) ON 
UPDATE CASCADE ON DELETE CASCADE   "$1" FOREIGN KEY (element_id) REFERENCES element(id) ON UPDATE 
CASCADE ON DELETE CASCADE

freshports.org=#

I have a query which returns the needed results:
    SELECT W.element_id      FROM watch_list_element W     WHERE w.watch_list_id in (select watch_list_id from 
watch_list_element where element_id = 54968)  GROUP BY W.element_id  ORDER BY count(W.watch_list_id) DESC     LIMIT 5;

But performance is an issue here.  So I'm planning to calculate all 
the possible values and cache them. That is, given each element_id in 
a watch_list, what are the top 5 element_id values on all the lists 
on which the original element_id appears?

I'm having trouble constructing the query.  I'm not even sure I can 
do this in one select, but that would be nice.  Examples and clues 
are appreciated.

Any ideas?

Thank you.
-- 
Dan Langille : http://www.langille.org/
BSDCan - The Technical BSD Conference - http://www.bsdcan.org/  NEW brochure available at
http://www.bsdcan.org/2005/advocacy/